OK to Touch?


Europa? Enceladus?

Or a Tale of Missteps?

Copyright © 2017 by Robert Walker (UK). All rights reserved

This cover shows an astronaut searching for fossils on Mars. It's called "20/20 vision" and is by Pat Rawlings, courtesy of NASA. I've superimposed on it photos of two of the most interesting icy moons for the search for life, Jupiter's Europa (on the left) and Saturn's Enceladus. Both are thought to have subsurface oceans. Enceladus has geysers that erupt through its icy crust into the vacuum of space, and Europa probably does too. At lower left you see a Europa, released by NASA in 2014. It's the result of stitching together photos taken by the Galileo spacecraft in 2001. At lower right you see a detail from the geysers of Enceladus (taken by the Cassini spacecraft in 2007).

There's a higher resolution version of the cover here.

First published online and on kindle in January 2017. You can buy the kindle version on Amazon. For my other kindle books, see my author page on Amazon.com. You might be especially interested in my related books:

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We love to touch things. So what happens if humans touch Mars? Or more importantly, what happens to our trillions of microbe hitchhikers? Half of our body mass is microbes, and if anyone was sterilized like a robotic lander, to protect Mars, they'd be dead. Our history shows that we are capable of making mistakes, sometimes huge ones. This is a major decision that humanity will face, probably as soon as the 2020s or 2030s. Could native Mars life be vulnerable to Earth microbes? Well yes, if it is "RNA world cells" for instance. Perhaps they have not yet evolved DNA or proteins. They could go extinct before we know they are there. We can never reverse this, once the hardy spores of our microbes scatter in the global Martian dust storms.

Explorers in Star Trek and other science fiction stories seldom give this any thought. How wonderful it would be to take these stories as our guide, and say

"It's impossible to do harm to native Mars life by touching the planet ".

However we don't get to write the ending to our story this time. Is there an alternative and exciting future for humans in space that keeps all of our options open until we know more?

Let's try a thought experiment. Suppose that in the future, scientists announce that astronauts on Mars have found present day life. This leads to great excitement with the public - the whole program was justified by this discovery! Then, a few weeks later, they follow up with an announcement that what the astronauts found were some little known obscure Earth microbes that they brought there themselves, which have taken over the nearest microhabitat to the human base? Probably many people who never thought about it before will come out of the woodwork saying

“How could you do such a thing?”

It's far better to have that debate now, than when it is too late. In my experience most people, even keen space enthusiasts like myself, do care about planetary protection, if the situation is explained to them clearly, as is my aim in this book. It's just that so far, they don’t have the communicators, book writers, article writers and radio and TV presenters to give them the background information they need to realize what is involved. This book is meant as part of that process. It's to help fill what seems to be a gap in the literature, and to open up a debate that perhaps has been rather one sided for too long.

This book also looks into the search for life in our solar system, and the discoveries that could flow from it. I argue that this could be a "super positive outcome" as important for biology, medicine, agriculture etc as the discovery of DNA. It also looks at possible consequences of accidentally bringing Earth microbes to Mars, and also, to Europa, Enceladus and a few other locations that might be especially vulnerable to Earth life. It also looks at some of the other exciting destinations for humans in the solar system, and some of the ideas for unusual forms of biology we could find in our solar system.

I also look into a question that will be uppermost in the minds of space geeks and human spaceflight enthusiasts.

"What difference this would make to human exploration of the solar system, if we aim to keep our explorations biologically reversible for now, until we know more?"

Luckily, the Moon is a place we can go to with no major planetary protection issues. It also happens to be our closest neighbour, and by far the easiest place for humans to visit, and we know we can do it too, having been there already in the 1960s to 1970s. So we may be able to learn many lessons there first before we have to make decisions about further afield places like Mars. The Moon used to be the cool exciting place to visit for astronauts and space geeks in the 1960s. We'd look up, amazed, knowing that humans were walking there. I think it can be today as well.

It's the easiest place to visit for space explorers, tourists, and for unmanned telerobotic exploration from Earth. There's much there to interest scientists too, who could explore it directly from Earth, or from bases on the Moon like the ones in Antarctica. It's also a natural place for passive infrared telescopes at the poles, long wave radio wave telescopes on the radio quiet far side, and eventually, huge radio dishes and liquid mirror optical telescopes spanning its craters. They can also study the lunar geology, searching for ice, and precious metals like platinum, exploring the caves, and searching for meteorites in the polar ice for unaltered organics and even preserved life from early Earth and other parts of our early solar system. It's turned out to be far more interesting than we thought as recently as a couple of decades back.

Our Moon is also resource rich. The lunar poles particularly may be the easiest places to set up an astronaut's village in the near future, as suggested by ESA. It has sunlight available 24/7 nearly year round on the "peaks of almost eternal light", and ice also close by in the permanently shadowed crater. If you compare the Moon point by point with Mars, then the Moon actually wins over Mars as a place to live on just about every point, probably at least up to a population in the millions, and quite possibly further if we can build habitats in the vast lunar caves.

Also, it's an ideal place for early experiments in habitats that recycle as much as possible. Those are essential for explorers traveling on multi-year journeys through the solar system in the future. When we get to Mars, we can do much by "touching" it remotely via telepresence from orbit, in immersive HD 3D with haptic feedback, meanwhile exploring its two moons Phobos and Deimos close up. But Mars is not the only place we can send astronauts to. We can also explore the asteroids, Callisto, Venus Mercury etc. Once we have almost closed system habitats, the whole solar system will be open to us, and a voyage of a couple of decades will be a similar engineering and logistic challenge to a voyage of a couple of years.

This book is written as an "Op-ed". Whether or not you agree with my conclusions here, I hope that you will find the discussions and the ideas stimulating. I find it surprising how little is written about this, outside of the academic literature, for something that seems likely to become a matter of major practical importance in the near future. The consequences could impact on us all, on our expanding understanding of our solar system, and of the origins of life, and biology, not just those who are keen on space colonization.

Those who want to "colonize Mars" often barely mention planetary protection. Of course, it does get a paragraph or two in many books, and the occasional chapter, including the early books by Carl Sagan, a planetary protection pioneer. Also there are several entire books on various aspects of planetary protection, of a technical nature, written for specialists, by the National Research Council and the European Space Foundation. Then, of course, there are many technical papers and workshop reports on the topic. However, these are not written for the general public and even many interested scientists are seldom aware of this literature. The nearest to a popular book devoted to the topic that I know of is Michael Meltzer's When Biospheres Collide, which is a history of NASA's planetary protection programs, published in 2010. If that's right, this may be only the second book devoted to the subject that's intended for a general audience. If you know of any other, do say! I hope there will be many more such in the future. Perhaps this book can help to encourage other authors to take up the theme.

There is so much enthusiasm for space colonization, especially in the U. Added to that, we have the natural human inclination to touch things. It's got to the point where for many of the most enthusiastic, any suggestion that perhaps we might not wish to land humans on Mars at the first opportunity has become almost unthinkable. Even the planetary protection office of NASA, and the scientists with Planetary.org who have such a strong focus on science; also envision humans on Mars in the near future, possibly as soon as the 2030s, or if you go with Elon Musk's optimistic projections, perhaps even sooner. This is normal of course. People often do have many things that for them are unthinkable.

But let's look in this direction that seems so "unthinkable", and see what's there . Maybe there is more of interest than you'd expect. Including, too, maybe much more of interest to those keen on space exploration and perhaps human settlement too, than you might think.

The main sections in this book are

(skip to detailed contents)


So what does happen if humans touch Mars? Or more importantly, what would our microbes do on Mars? Our science fiction stories are based on their authors' experiences as writers of engaging and compelling narratives. Sadly, Star Trek,"The Martian", and other movies, TV series and books like them, don't give us any real practical experience of what will happen if we touch Mars.

The heroes and heroines of Star Trek and nearly all science fiction stories, movies and TV series never stop to give any thought to their microbial companions, when they visit a new planet.

Sometimes we make huge mistakes through inexperience. Nobody guessed that human visitors to the Lascaux caves would make the paintings moldy just by breathing. We take trillions of microbial hitchhikers with us wherever we go, in thousands of species. Mars now seems to be a place where hardy Earth microbes might possibly survive, either on the surface, or in water deep below, with connections to the surface. Astrobiologists can't yet say for sure that our microbes will play nicely on Mars.

The native Mars lifeforms, even if "just" microbes, would still be extra terrestrials. For instance, with one of the possibilities for Mars life, an early "RNA world" life, it wouldn't use DNA, and probably would not even use proteins. That would be more exotic by far than any of the lifeforms we can create by genetic manipulation, or tweaking cells to create an artificial form of life in the laboratory. An early form of like like that might also have no defences against modern Earth life. . What we discover there, even if only microbial, could lead to the next revolution in biology, medicine, agriculture, or even nanotechnology.

If we bring Earth life to Mars, accidentally, and it makes whatever is there extinct, how can we ever roll back? Yet, it is so easy and natural to forget all about the need for planetary protection, in our enthusiasm, as soon as the prospect of humans landing there enters into the picture.

News stories on humans to Mars normally don't mention planetary protection at all. Yet the literature is vast, written by many top scientists from Carl Sagan and Joshua Lederberg in the 1960s through to the present. The astrobiologists say that the three places in our solar system that we most need to protect from our Earth microbes are Mars, and two particularly special icy moons Europa and Enceladus with their subsurface oceans. They also say that we have to protect Earth itself from extraterrestrial microbes that we might bring back from these places. The literature covers many differing views about how this dovetails with human explorations of Mars, but just about all agree that planetary protection is a matter of great importance for the search for life in our solar system. We are also mandated to protect these places, as a matter of international treaty, in the Outer Space Treaty.

It's so good to see that at last planetary protection for human missions is getting a little attention and public discussion. In 2015 it was covered in one of the segments in the "Making of" section of the National Geographic Mars series, and also, in one of the segments of a Sky at Night episode about life on Mars. But even in those mentions it was still brushed away as if it was a matter of little consequence. You get the impression from these presentations that if humans land on Mars in the 2030s, then at last, with a collective sigh of relief, we will no longer need to protect the planet from Earth life. The idea that this could be a scientific disaster and a matter of great regret later on is barely considered at all.

We sterilize our Mars rovers carefully to protect Mars, so why would microbes on a human occupied ship get a special exemption to contaminate Mars as much as they please? Is it true, that we no longer need to protect Mars once humans get there, and if so, what is the reasoning behind this? There are interesting arguments on both sides, and I will look into this in detail in this book.

So, what if we decide not to touch Mars quite yet, or at least not in person? What happens to all our plans to explore the red planet? Well we can continue to "touch" it remotely, with our robotic hands and eyes on Mars, much as we explore the ocean bed. When we get a broadband connection to Mars in the 2020s, then we will have a much more immersive and direct way of "touching" Mars even from Earth. With a bandwidth of hundreds of gigabytes a day, we will be able to download 3D landscapes from Mars dozens of times a day. These will be so detailed that anyone on Earth can study rocks close to the rover, not just in 3D, but with the ability to zoom right in to observe them in microscopic detail. We could build in haptic feedback so that you can feel the texture of the rocks. Eventually we can also have humans orbiting Mars much as the ISS orbits Earth, who drive rovers over the surface with low latency, studying the landscape and doing experiments in real time from a shirt sleeves environment in orbit.

Meanwhile the Moon is the safest place by far to send our astronauts, and yet, it is still not an easy place to go. The Apollo astronauts made it look so easy, but they were veteran test pilots with a cool head. They could take huge risks without flustering. They also explored only for up to three days at a time, and always in the early morning of the two weeks long lunar day, when it's comfortable in temperature, and the angle of the light is ideal for illuminating the landscape for humans. They always landed at a time when the surface wasn't covered in shadows to such an extent that you can hardly see anything, and not illuminated from above with almost no way to judge the relief. They explored six landing sites close to the equator on the near side of the Moon, spending at most three days at each site, out of a total area larger than Africa.

There is so much more to discover about our "eighth continent", the second largest after Asia. We now know of ice in permanently shadowed craters at its poles, just next to its peaks of almost eternal sunlight. It's a perfect place for a base with a steady temperature, and solar power 24/7 almost year round (because the Moon's axis is almost perpendicular to the ecliptic, it doesn't have seasons like Earth). Its icy polar craters must have caught meteorites, much as ice in Antarctica does, and easy to spot in the ice. However, the lunar ice has been stable not just for millions, but for billions of years. Meteorites there could come from early Mars, early Venus, and right back to the earliest stages of life on Earth itself, probably with uncontaminated pristine organics preserved for all that time, for us to analyse. More recently, it could have fragments of ammonites and other creatures from the shallow seas at the time of the Chicxulub impact, possibly still with uncontaminated organics to analyse.

We also now know of cave entrances that lead to lava tubes which, in the low gravity, could be vast, tunnels, up to several kilometers wide, and some are thought to be over a hundred kilometers long. The geology is interesting too, including volatiles still being exhaled from its interior somehow (clear evidence of this from the argon in its atmosphere) - and with suggestive evidence of platinum and other metals which may have been splashed out from the interior core of the planetesimal that created the south polar Aitken crater when it hit the Moon.

Why rush our astronauts as fast as possible to the few places in our solar system where our microbial hitchhikers are most likely to cause major problems? There is so much of interest to explore right on our doorstep.

Plan for this book

Plan for this book

My plan for the rest of this book is to go through the main themes in a rather extensive introduction first, linking to later sections of the book to find out more. I'll then go through it in more detail in the book proper. The reason for doing this is that you may well have many questions, especially if you are a space colonization enthusiast, which will nag at you unless I go through them first, rather quickly. After that, once you see the basic plan of the book, hopefully you can then settle down to enjoy a more detailed and thorough survey of the same material.

The downside of this, is that there is more chance of repetition. However, though the same topics are covered, first quickly, then in more detail, hopefully the way I do it is entertaining enough and interestingly different enough in the detailed treatments to bear some repetition. The plus side of this is that when the same topics are covered again, in a different way, it can give another perspective on them, and perhaps a fuller, more rounded treatment of the subject.

I've designed this book for electronic formats, with the links, including internal links that let you jump to different sections, and external links in lieu of citations, videos, and so on, as an essential part of the structure. I have been asked if I have any plans to make a printed version of this book. I could do it easily with CreateSpace, which partners with Kindle, and publish on demand, but I don't see it working for a link rich format like this, So, no, I have no plans for a printed version of this book, at this time.

Introduction - How often have you seen this?

How often have you seen this scenario in movies, artist's renderings and science fiction? Bold and brave astronauts explore Mars, setting out from their base in pressurized rovers and spacesuits. They scale cliffs, adventure into caves, and dig deep. They search for past, and present day life. And one of them has just made a great discovery, a fossil!


Artist's impression of human astronauts exploring Mars, and discovering a fossil - credit NASA / Pat Rawlings

However there is another side to this picture. As these brave astronauts explore Mars, their spacesuits leak air. It would be easy to make a "light air-tight anthropomorphic balloon" as James Waldie puts it, but we need to be able to bend our arms and legs at the joints, and designers achieve that with flexible joints. These have tiny gaps between the moving parts, which leak small amounts of air constantly. Wherever they go, they will leave a trail of microbes.

This photograph of Alan Bean during Apollo 12 is often shared as an example of venting from his spacesuit, or perhaps, ice crystals from the ice sublimator. Actually,Kip Teague, author of the Project Apollo Archive is cited by NASA as saying that it's a smudge on the camera. It was blue when the central subject in the camera was bright and red in tint when it was dark. It was present in the photos from frames 6813 through to 6853 (this photograph is frame 6826).

The venting from the spacesuits wasn't so visible, but they did vent liters of air constantly on every EVA, as do the modern EMU units used for spacewalks on the ISS. If we use similarly gas pressurized spacesuits for Mars, then one estimate is that over 50 litres of human borne bacteria and other airborne effluent would escape through suit bearings and joints during each EVA, potentially contaminating soil, fossil and atmospheric sample" See this paper. Perhaps this can be reduced by measures such as using counterpressure spacesuits on the body pressurized in equilibrium with the Mars atmosphere with only the regions around the head pressurized to full pressure. Still, the experts say that some leakage is inevitable.

One of NASA's identified knowledge gaps for human EVA's on Mars is the amount of microbial leakage from their suits. But they say that some leakage from the spacesuits is inevitable. They also need to vent waste gases from inside the habitat from time to time. Also astronauts' bases and rovers would leak Earth microbes into the dust, every time they open an airlock. The experts say that if we have humans on Mars, then it is inevitable that we contaminate the landing site with Earth life. There is no way to design airlocks, EVA suits etc to prevent this altogether, at present.

Also, the ground they walk on is covered with a fine dust, as fine as cigarette ash, light and easily moved, even in the near vacuum winds of the Mars atmosphere. This fine dust can travel hundreds of kilometers in a few hours during the fast winds of the Mars dust storms. Every decade or so, these storms combine and spread to cover the entire planet, and then can last for weeks. These global dust storms block out the sun and turn day into night, and it takes months for all the thick clouds of dust to settle back out of the atmosphere.

Cover from Andy Weir's "The Martian" to illustrate the fast winds of the Martian dust storms. He uses a dust storm as a central plot point. He knew that in the thin vacuum of the Mars atmosphere, the winds, though fast, are also feeble. So that's a bit of poetic license on his part; they couldn't blow over an astronaut. Indeed you would probably not even feel them. But the winds are very fast, reaching up to sixty miles per hour in dust storms, and even faster in the tiny dust devils. They also do pick up the dust, forming dust clouds, exactly as in the illustration. The winds couldn't pick up most of the dust as we know it on Earth, but they can do that on Mars because the dust is so fine, as fine as cigarette ash. Indeed, they pick up so much dust that the dust storms block out the sun, turning day into night, sometimes for weeks on end. Potentially they could transport desiccation resistant dormant microbes and spores over distances of hundreds of miles a day, and throughout the planet. And all this time, much of the dust will be protected from the harsh UV rays of the sun.

NASA have this as one of their many knowledge gaps for human extraterrestrial missions to Mars: "Obviously, the current understanding of microbe survival in Mars dust environments remains uncertain and represents an important knowledge gap" (page 34 of this report).

Carl Sagan once remarked, that the iron oxides that make up these dust particles are perfect to shield a microbe from the sterilizing UV light of the unfiltered sun. Such a microbe, imbedded in a minute crack in a fine dust grain, could eventually fall to the surface undamaged by the harsh UV light, thousands of kilometers from its point of origin. After a human landing on Mars, billions of hardy microbial spores and desiccated microbes in other dormant states will stream out from their base and spread in the dust and winds. If there are any Mars habitats for them to find, some of them surely have a chance to get there eventually. For more on this see the section: How could this work on Mars with dust storms and a globally connected environment? (below).

Also, what is a mission that's in situ in very harsh difficult conditions for months on end going to do about trash? The ISS would generate many tons of trash every year, if it wasn't all burnt up in our atmosphere. See Trash, on the Moon (below) . And what if someone dies when they are on the surface? Cremate the body?

So far, you might wonder if it is possible to contain our microbes somehow. They do exactly that with rover computers, which can't be totally sterilized either at present. They enclose the core box and some sensitive electronics with high efficiency filters (like HEPA filters) to keep even particles as small as microbes inside. Could we somehow do something similar for humans on Mars? Keep everything humans touch inside of giant filters to keep the microbes away from the surface of Mars? Could we perhaps somehow include filters in the design of our astronauts spacesuits, in every joint, and vent the spaceships and airlocks etc through such filters? And recycle everything with no trash or waste gases, everything is contained? It might be hugely challenging - but it is possible?

Well, so far the answer seems to be no. The experts say that they would design the missions and equipment to limit the contamination as much as they could, but that it it doesn't seem possible to keep our microbes totally within impermeable barriers when astronauts are exploring another planet. As Rick Davis put it in a press conference,

“Human beings are, roughly speaking, about 50 percent microbes by mass. We’re basically, if you will, big sacks of microbes. And so keeping that segregated from the Martian environment when humans get there is probably impossible.”

However, this reaches a whole new level of challenge once you start to think about the possibility of an "off nominal" mission. Suppose that instead of a safe landing, they crash on Mars killing everyone on board? After all, the space shuttles Columbia and Challenger crashed, and the landing on Mars is particularly challenging. It's far harder to land there than on Earth, and even with the best efforts to make it safe, a crash of an early crewed mission to Mars has to be fairly high in probability. I cover this in Why do spacecraft crash so easily on Mars? (below) .

Elon Musk warns that early settlers must be prepared to die in the attempt. After a crash like that, minute fragments of the astronauts bodies, food, air, water and the spacecraft itself would spread in the dust, infused with trillions of microbes, and this is likely to irreversibly contaminate Mars with Earth life. The space shuttle debris fields spanned hundreds of miles

Columbia disaster debris field in East Texas - it spans about 250 miles

With Elon Musk's supersonic retropropulsion, the rockets come in at great speed. They don't have parachutes but just use their rockets fired in reverse, assisted with atmospheric resistance, to land vertically on the surface of Mars. His plan is to use the same technology they use to lands the Falcon 9 first stages on a barge in the sea, to land on Mars. He has only done it with first stages so far, which is less of a challenge, but he seems confident that they will achieve vertical landings of the second stages as well, all the way from orbit around Earth, back to our surface. On Mars, they can still come in too hard, and topple over even on Earth, as happened with his first attempt at a landing on a barge. But there's an additional hazard on Mars. The astronauts' spacecraft has to skim really close to the surface to get enough resistance from the atmosphere to slow down enough to land. They skim so close to the surface that they can't land on the higher mountains and the Martian highlands, because the air up there would be to thin for this technique. When he says that early settlers have to be prepared to risk their lives, he is not exaggerating. Perhaps the NASA mission might be less risky, it's hard to say, especially since they have no detailed designs yet for their landing craft, but any human landing on Mars is bound to be dangerous. When Curiosity landed on Mars, they didn't call it "Seven minutes of terror" for nothing.

Do let me know if any of you know how to work out how far the debris would spread on Mars immediately after a SpaceX or NASA crash during re-entry, as I can't find anything about it yet. But however small or large the debris field is, the remains would get mixed up in the dust, and perhaps it doesn't make that much difference. The Mars dust storms would surely, in time, spread the finer particles and microbe spores from it throughout Mars. For more on this, see the sections below: How could this work on Mars with dust storms and a globally connected environment? , Why do spacecraft crash so easily on Mars, and Elon Musk's fun but dangerous trip to Mars.

Could we design spacecraft in the near future so that even in a crash on Mars, they will contain all the microbes in a human occupied spacecraft? For some ideas, see Could we send humans to the Mars surface in a biologically reversible way? (below) . At any rate we don't have any realistic near future spacecraft that can do this.

If something irreversible like that happens to Mars, it impacts on not just the explorers and not just the US, and not just the twenty first century. It impacts on all nations on Earth with an interest in exploring the planet, and also our descendants, and all future civilizations in our solar system, for as long as Mars continues. If this happens, then for the entire billions of years future of Mars, nobody will ever have the same opportunity again that we have right now, to study the present day pristine planet.

Also if we do eventually introduce Earth life to Mars, it may well make a big difference when and how we do it. If we take this fast and risky approach of sending humans there as soon as possible, the entire future biology of Mars could depend on the chance event of some microbe that got there accidentally in the first crash of a human occupied spacecraft on the planet. Can this be the right way to introduce Earth life to Mars, even if that's the decision we make eventually? For more on this see Why bringing Earth life to Mars could be like making wonderful yoghurt - or bad smelling gone off milk

There are plenty of other places we can go to do our very first experiments with spreading Earth's biosphere into space. We can start on Earth itself - continue with the line of experiments for Biosphere II. Then we can go into space and build small scale habitats of cubic meters, then eventually, cubic kilometers, in originally lifeless environments such as caves on the Moon or domed greenhouses at the lunar poles. We can make our first mistakes in a reversible way, in short term projects that take only decades to complete rather than the thousands of years of a terraforming project. There are plenty of other things we can do right now. There is no urgency to take on the huge challenge of transforming an entire planet.

See these sections (below):

Why don't explorers in science fiction have these problems when exploring other worlds?

The explorers of other planets in Star Trek, "The Martian", Kim Stanley Robinson's Mars trilogy, and the many movies, books and TV series don't seem to have any of these problems when they explore other worlds. Why is that?

Perhaps it is because these stories are fiction and are the result of the authors' imaginations. They aim to entertain, after all. The result is a collaboration of authors, script writers, directors and sometimes ideas from the actors themselves. Much of it is based on a need for drama and easy story telling rather than accuracy. It's like the "Coconut effect" - modern audiences expect horses galloping over grass, or a sandy beach, to sound like two half coconuts hit together. Of course they only sound like that when galloping over hard surfaces. Well the same is true of many on-screen science and science fiction effects.

So, for instance, we expect highly radioactive materials in movies to be bathed in a sickly green light. In reality if there is any light at all from radioactivity, it's blue from Cherenkov radiation, or else orange, for some materials that get hot from the waste heat of their own radioactivity. But if a director showed radioactive materials as blue or orange, many viewers would think it was a mistake, or just get confused and not recognize what they are supposed to be. This is an example of "Technicolour science", It's like a visual shorthand that serves as an instant cue to the audience that the scene is bathed in deadly radiation.

These tropes are often things which help move the plots forward, and make the stories more visually dramatic. For instance, most science fiction fans would expect a Mars dust storm to have strong winds with dramatic effects, and Andy Weir's novel "The Martian", as we just saw in the last section runs with that idea. It starts off with a dust storm on Mars strong enough to blow over a spaceship, or an astronaut. The winds are indeed fast, but feeble, in the thin atmosphere. An astronaut would barely notice the strongest winds, standing right in them. Even its very fastest winds in the dust devils, as fast as a hundred miles per hour (162 km / hr) could barely stir an autumn leaf in the near vacuum of its "atmosphere". The discarded parachutes from our landers remain in the same position on the surface for year after year, without moving at all in any of the winds.

This is not a mistake on his part, but a bit of artistic license, as he explains 14 minutes into this interview : Triangulation 163: Andy Weir from Triangulation (MP3). He had other ways to start the novel. But those other, more scientifically accurate scenes, didn't have the drama he felt he needed for that so important first scene of a novel. He needed to grip the reader right away and get them involved in the story. Most of the novel has enough hard science to delight the most demanding space science geek, and as a result, we are ready to overlook this dust storm scene, and the idea that wind can blow away pieces of equipment, for the sake of a dramatic story.

So, some of our science fiction is based on these tropes built up purely as dramatic devices to help with plots and visual effects. These may have no connection to reality. Some is based on deliberate number fudging as poetic license for dramatic effect (e.g. Andy Weir's Mars storm). Some is based on hard science extrapolated well beyond anything we know about. But none of this vivid and entertaining story telling is based on any actual experiences of exploring other worlds.

If we look at early twentieth century science fiction projections of their future, they are a mix of the far sighted, like stories about television-like devices long before it was invented, the almost correct like H.G. Wells prediction of atomic weapons in 1911, as devices that have no more force than ordinary high explosives, continuing to explode with a half life of seventeen days, and the bizarrely dated.

When science fiction got it right - watching baseball on a television of the future, cover picture from Science and Invention magazine, July 1922. Idea used in stories by Hugo Gernsberg. such as Ralph 124c 41+: A Romance of the Year 2660, published in 1911. (The Hugo Awards for science fiction writing are named after him).

Asimov's early "hard science fiction" stories about a supercomputer Multivac (made of vacuum tubes) are especially dated now, exciting and visionary as they must have seemed at the time. He depicts it as so large, that in one story it filled Washington DC right out to the suburbs, accessed by terminals world wide. His stories reflect accurately how computing was done at the time, right through to the early 1970s, when I first learnt programming. Your programs ran on a "main frame" computer which filled an entire room, and you never saw it. Just as in his stories, the computer itself was maintained by a team of technicians. Asimov, and his readers at the time would have thought that surely, as computers get more powerful, they will get larger and larger, needing more and more technicians to keep them going. Eventually, the world's fastest super computer would be so huge, and so expensive to build and maintain that it would surely be the only such computer in the world. Indeed perhaps there wouldn't be any other computers at all, even small ones, as you could do everything so much faster by connecting to Multivac. Why build your own?

All of that was a natural and logical extrapolation to Asimov and his readers at the time. He used the best scientific understanding of his day for his stories (he was a trained scientist with a PhD in biochemistry). It was not unreasonable either, after all if it had been a story about particle accelerators, he'd have been right. To this day the largest accelerators are expensive to build and maintain and need large teams of technicians. We have only one Large Hadron Collider.

When science fiction got it wrong - back of one of the panels of ENIAC, an early computer, packed with vacuum tubes. In his early stories, Asimov imagined that by now we would have a huge supercomputer several stories high and a half a mile long built of vacuum tubes like these. In another story it is the size of Washington DC – with many interior corridors, with the one supercomputer serving the entire world, or an entire country. All the famous hard science fiction writers have had epic fails like this, as well as, sometimes, astonishingly accurate predictions.

Two of the programmers of ENIAC in a photograph from 1945-1947. Back in the early days of computers, all the programmers were women, while men were more involved in the engineering, building the computers. Left: Betty Jennings (Mrs. Bartik) Right: Frances Bilas (Mrs. Spence) setting up the ENIAC.

Asimov envisioned a future with a vast LHC sized "Multivac" computer with corridors running throughout and many technicians and programmers to keep it going. He envisioned it as such an expensive endeavor that there would probably only be one in the entire world (again like our Large Hadron Collider). It all made perfect sense at the time, to both him and his readers. Now it seems bizarrely dated.

If you wanted a portable calculator at the time, you used a slide rule. So, also, logically enough (at the time) an early story to feature "faster than light" travel, perhaps the first story on the topic, in 1938 has explorers using slide rules to navigate their faster than light spaceships. (Full text).

Arthur C. Clarke was another one of our hard science fiction writers, striving for accuracy, with a first class degree in physics and maths,. Yet, though he had many "future prediction" successes, he also had many "fails". One of the most notable is in his "Fall of Moondust" book which described a Moon covered in thick drifts of dust, deep enough to swallow a dust skimming "cruiser". It was shown to be false within a decade.

Low resolution photograph of the cover of the first edition of Arthur C. Clarke's 1961 book "A Fall of Moondust". It was published in the very early days of space exploration, just after the first mission to impact on the Moon Luna 2. He describes a surface covered in deep layers of dust which tourists explored with a dust skimming "cruiser" as shown in the book cover. One of them sinks 15 meters into the fine dust, leaving not a trace on the surface. It's carefully written hard science based on the best knowledge of his day, and exploring many ramifications of their predicament. But it described a future that was shown to probably be false, only five years later with the first soft landing of Luna 9. It remains a well written exciting "hard science fiction" story that still delights science fiction geeks to this day (though some of its attitudes to women, are dated and even offensive to us nowadays). But we now know that our Moon has only thin surface layers of dust.

Sometimes the "predictions" of our best "hard" science fiction authors are astonishingly accurate, and sometimes they fail badly. The problem is that we don't have any way to know in advance which is which.

For a fine example of an accurate prediction, Hal Clement in 1956 wrote a short story called "Dust Rag" in Astounding Science Fiction about levitating dust on the Moon, two astronauts find their faceplates get covered in dust:

“All right. There are, at a guess, protons coming from the sun. They are reaching the Moon's surface here — virtually all of them, since the Moon has a magnetic field but no atmosphere. The surface material is one of the lousiest imaginable electrical conductors, so the dust normally on the surface picks up and keeps charge. And what, dear student, happens to particles carrying like electrical charges?”

“They are repelled from each other.”

“Head of the class. And if a hundred-kilometer circle with a rim a couple of kilos high is charged all over, what happens to the dust lying on it?”

The astronauts didn't get their faceplates obscured by dust. But they did observe mysterious bands and streamers at sunrise and sunset while in orbit around the Moon. And the Surveyor spacecraft photographs of the Moon showed a not quite sharp horizon with a slight haze. These were finally explained in 2005 as due to electrostatic levitation, nearly half a century after his story. Apparently some of Hal Clement's dust gets elevated high enough to be seen from orbit around the Moon!

Few science fiction authors have tackled the theme of forward contamination of other parts of our solar system by Earth microbes, but there's one poignant sad story, again by Arthur C. Clarke, "Before Eden". This story was published in the same year as "A Fall of Moondust", in Amazing Stories, June 1961. Back then, though they knew Venus was hot, scientists thought it was still possible that Venus could have water on its surface, perhaps at the top of its mountains.

One of the covers for Arthur C. Clarke's "Before Eden" -a poignant sad story about forward contamination of Venus, published in 1961 at a time when surface life there was still a remote scientific possibility. You can hear the complete story read as an audio book here.

These adventurers are exploring a completely dry Venus, or so they think. Up to then (in the story), everyone thought Venus had no water, and was sterile of life. That was a natural thought, because the temperatures they encountered were always above the boiling point of water. But the heroes of the story are stranded near the not quite so hot South pole, and find mountainous cliffs there. On those mountains they find a dried up waterfall - and then - a lake!

“Yet for all this, it was a miracle—the first free water that men had ever found on Venus. Hutchins was already on his knees, almost in an attitude of prayer. But he was only collecting drops of the precious liquid to examine through his pocket microscope.... He sealed a test tube and placed it in his collecting bag, as tenderly as any prospector who had just found a nugget laced with gold. It might be – it probably was – nothing more than plain water. But it might also be a universe of unknown, living creatures on the first stage of their billion-year journey to intelligence....”

“...What they were watching was a dark tide, a crawling carpet, sweeping slowly but inexorably toward them over the top of the ridge. The moment of sheer, unreasoning panic lasted, mercifully, no more than a few seconds. Garfield’s first terror began to fade as soon as he recognised its cause....”

“… But whatever this tide might be, it was moving too slowly to be a real danger, unless it cut off their line of retreat. Hutchins was staring at it intently through their only pair of binoculars; he was the biologist, and he was holding his ground. No point in making a fool of myself, thought Jerry, by running like a scalded cat, if it isn’t necessary. ‘For heaven’s sake,’ he said at last, when the moving carpet was only a hundred yards away and Hutchins had not uttered a word or stirred a muscle. ‘What is it?’ Hutchins slowly unfroze, like a statue coming to life. ‘Sorry,’ he said. ‘I’d forgotten all about you. It’s a plant, of course. At least, I suppose we’d better call it that.’ ‘But it’s moving! ’ ‘Why should that surprise you? So do terrestrial plants. Ever seen speeded-up movies of ivy in action?’ ‘That still stays in one place – it doesn’t crawl all over the landscape.’ ”

“‘Then what about the plankton plants of the sea? They can swim when they have to.’ Jerry gave up; in any case, the approaching wonder had robbed him of words... ”

“... ‘Let’s see how it reacts to light,’ said Hutchins. He switched on his chest lamp, and the green auroral glow was instantly banished by the flood of pure white radiance. Until Man had come to this planet, no white light had ever shone upon the surface of Venus, even by day. As in the seas of Earth, there was only a green twilight, deepening slowly to utter darkness. The transformation was so stunning that neither man could check a cry of astonishment. Gone in a flash was the deep, sombre black of the thickpiled velvet carpet at their feet. Instead, as far as their lights carried, lay a blazing pattern of glorious, vivid reds, laced with streaks of gold. No Persian prince could ever have commanded so opulent a tapestry from his weavers, yet this was the accidental product of biological forces. Indeed, until they had switched on their floods, these superb colours had not even existed, and they would vanish once more when the alien light of Earth ceased to conjure them into being...”

“...For the first time, as they relaxed inside their tiny plastic hemisphere, the true wonder and importance of the discovery forced itself upon their minds. This world around them was no longer the same; Venus was no longer dead – it had joined Earth and Mars. For life called to life, across the gulfs of space. Everything that grew or moved upon the face of any planet was a portent, a promise that Man was not alone in this universe of blazing suns and swirling nebulae. If as yet he had found no companions with whom he could speak, that was only to be expected, for the lightyears and the ages still stretched before him, waiting to be explored. Meanwhile, he must guard and cherish the life he found, whether it be upon Earth or Mars or Venus. So Graham Hutchins, the happiest biologist in the solar system, told himself as he helped Garfield collect their refuse and seal it into a plastic disposal bag. When they deflated the tent and started on the homeward journey, there was no sign of the creature they had been examining. That was just as well; they might have been tempted to linger for more experiments, and already it was getting uncomfortably close to their deadline. No matter; in a few months they would be back with a team of assistants, far more adequately equipped and with the eyes of the world upon them. Evolution had laboured for a billion years to make this meeting possible; it could wait a little longer.”

“...For a while nothing moved in the greenly glimmering, fog-bound landscape; it was deserted by man and crimson carpet alike. Then, flowing over the wind-carved hills, the creature reappeared. Or perhaps it was another of the same strange species; no one would ever know. It flowed past the little cairn of stones where Hutchins and Garfield had buried their wastes. And then it stopped. It was not puzzled, for it had no mind. But the chemical urges that drove it relentlessly over the polar plateau were crying: Here, here! Somewhere close at hand was the most precious of all the foods it needed – phosphorous, the element without which the spark of life could never ignite...”

" ... And then it feasted, on food more concentrated than any it had ever known. It absorbed the carbohydrates and the proteins and the phosphates, the nicotine from the cigarette ends, the cellulose from the paper cups and spoons. All these it broke down and assimilated into its strange body, without difficulty and without harm. Likewise it absorbed a whole microcosm of living creatures—the bacteria and viruses which, on an older planet, had evolved into a thousand deadly strains. Though only a very few could survive in this heat and this atmosphere, they were sufficient. As the carpet crawled back to the lake, it carried contagion to all its world. Even as the Morning Star set its course for her distant home, Venus was dying. The films and photographs and specimens that Hutchins was carrying in triumph were more precious even than he knew. They were the only record that would ever exist of life’s third attempt to gain a foothold in the solar system. Beneath the clouds of Venus, the story of Creation was ended.”

How sad it would be if future explorers on Mars get glimpses of early forms of life on Mars, and then they go extinct soon after they are discovered. Or indeed, even before, maybe they are extinct before anyone finds them. It would be great to be able to say that humans on Mars will cause no problems. It's what most of us want to be true, and we love to read science fiction stories, and watch movies, based on this idea. If you say this, you are bound to be popular with space colonization enthusiasts and science fiction geeks, and your work will probably get widely shared.

But our actions on Mars will have real world consequences, and won't just lead to popular acclaim and book or movie sequels. We don't get to write the script for what happens next. We need to take a careful and thorough look at what might actually happen before we act. Let's look beyond the widely shared optimistic stories reassuring us that nothing can go wrong.

Could we get a future news story: "Debate over Moldy Mars is a Tale of Human Missteps"?

We have made so many mistakes on Earth, already. I will start this book with an example of the many things that went wrong during our attempts to preserve the Lascaux cave paintings. Could the same happen some day with Mars? Might we some day read an article in the Washington Post,or New York Times, similar to a recent one about the Lascaux caves, but this time it says: "Debate over Moldy Mars is a Tale of Human Missteps"? Like this:

The last page of my series of fake newspaper stories in a (hopefully) "alternative future" in which humans accidentally introduce Earth life to Mars, then regret what they did. For the complete story, see the section Prestige or dishonour, first footsteps on Mars (below)

The Lascaux cave painting photo is by Prof Saxx.

If so, is this something we can foresee in advance and prevent?

At least nowadays scientific news stories about Mars sometimes mention these issues. But still, it's too often brushed over quickly. almost as an afterthought. Let's take an example from the scientifically highly respected The Sky at Night late night television program in the UK (hosted for many years by Patrick Moore until his death). A recent episode, Life on Mars aired in November 2016, briefly covered the need to protect Mars from Earth life. They also talked about the impossibility of keeping Mars pristine, with humans on the surface. But they treated it as a minor matter. The discussion starts about sixteen minutes into the program. The presenter, British geneticist and broadcaster Adam Rutherford ended by saying (around twenty minutes in)

"So, that's the balance of the argument, extreme caution to protect the pristine Martian environment, versus our desire for the most important scientific discovery of all time. If it were up to me, I think the scientific benefits outweigh the contamination costs.

Maybe none of this is going to matter, in a few years time. Last month president Obama announced a human mission to Mars by the 2030s. Elon Musk wants to get there much sooner, with hundreds or even thousands of people forming permanent Martian colonies. Now, humans are messy, leave trails of cells, and DNA wherever we go. So when that happens, who is going to really care about a few bacteria?"

(The episode is no longer available to watch for free even in the UK, and sadly, there no longer seems to be any option to buy previous Sky at Night episodes since they closed the BBC store)

In other words, the idea is that our present situation is frustrating. Once we send humans there. we will no longer need to be bothered about protecting the planet, because the die will be cast. With Mars irreversibly contaminated with Earth microbes, then you get the impression that with a huge sigh of relief, at last, we can go about exploring Mars much as we explore Earth (though in spacesuits of course).

That argument may seem convincing to you. Who cares about a few bacteria when there may be far more exciting discoveries to be made there? Indeed even many scientists think this way, as this shows. If you listen to this, and you haven't read the vast literature on planetary protection, it would be easy to think "end of story" at that point . Kudos to the BBC for raising the issue at all however, as the idea of planetary protection is so often ignored completely, as soon as the discussion turns from robotic to human missions.

Another recent video raising these issues is this one from VSauce Is it okay to Touch Mars? which they did for the National Geographic series on humans to Mars. I got the idea for "OK to Touch" in the title of this book after listening to their video. This book covers some of the same issues that they cover (starting nine minutes into that video), but there is so much more to be said.

For a slightly different view, here are a couple of quotes from the planetary protection officers for NASA and the ESF, interviewed by Larua Poppick for the Smithsonian.

Gerhard Kminek, planetary protection officer for the European Space Foundation:

“If you do it badly once, that might be enough to compromise any future investigation related to life. And that’s why there is strong international consensus making sure there are no bad players around.”

And Cassie Conley, NASA's planetary protection officer:

“For certain types of Earth organisms, Mars is a gigantic dinner plate. We don’t know, but it could be that those organisms would grow much more rapidly than they would on Earth because they have this unaffected environment and everything is there for them to use.”

It all rather depends on what you expect to find on Mars, and what you think our Earth life could do there. First, a little background on planetary protection.

Planetary protection - researches by Sagan and Lederberg onwards - and Zubrin's arguments

The first scientists to write papers on planetary protection were Carl Sagan, and Joshua Lederberg, in the 1960s. Joshua Lederberg got his Nobel prize for pioneering work on microbial genetics, and started to think about these issues already in 1957, making him perhaps the first to give it serious consideration. He wrote to the eccentric mathematician, geneticist and all round brilliant scientist J. B. S. Haldane in 1959. Recalling his visit to see him on November 6th 1957, he writes:

" I recall this was the night of a lunar eclipse, and there was also some excitement about the expectation that there might be a demonstration moonshot marked by the deposit of some visible powder! It must have been around this time surely that I began to think of the scientific consequences of lunar and planetary probes. At any rate, since then, it has become very plain that planetary exploration is close enough to realization that tangible plans must be made for it, and I have been particularly exercised at the possibility that irreparable harm might be done before our biological colleagues woke up to this and attempted to exert some influence. I have in mind the quite tangible possibility of contamination by terrestrial organisms of the surfaces of Mars and Venus, unless stringent precautions are taken to sterilize any vehicles sent there...."

Letter by Joshua Lederberg, 2nd February 1959

Already by February 14th, 1960 COSPAR had considered problems of extraterrestrial contamination and decided to set up a "Working Group on Standards for Space Probe Sterilization". Here is a copy of the letter inviting Joshua Lederberg to join the group.

Their work did have some effect, as it lead to planetary protection measures for the lunar landings for Apollo. Indeed it lead to lots of internal discussions and elaborate precautions taken. However,, NASA only published the details of the precautions they planned to take on the day that Apollo 11 launched to the Moon, as CFR Title 14 Part 1211, which they did deliberately to avoid extensive public debate (such rushed through legislation would not be permitted today). They weren't even given clear legislative authority to produce such guidelines either. (See page 452 of When Biospheres Collide).

So Carl Sagan, Joshua Lederberg and others had no chance to comment on the guidelines. The Moon was never thought to be a likely location for life, even before the Apollo astronauts landed there, which is just as well, as those planetary protection measures turned out to be little more than a gesture in their effect, despite the millions of dollars spent on them, and many hours of debating time, congressional hearings and so on. (For details, see Chapter 4: Back Contamination: The Apollo Approach, in When Biospheres Collide). They did not really protect Earth at all, and are most useful for us now as examples of things that can go wrong, and what to avoid in future planetary protection measures. There were also several major lapses in their implementations of the measures they did propose (see Example of Apollo sample return - learning from our mistakes in the past below).

Luckily the Moon didn't have any life. So, although there was no way for them to know that at the time, there was no risk to Earth from the Moon, and in the forward direction, no lunar life or potential habitats, to be contaminated by Earth microbes.

Mars is different though. It's got clear evidence of rivers, lakes, even seas in the past and had all the conditions necessary for life to evolve as far as we know. Also, though it is very inhospitable now, the atmosphere is still just about thick enough for salty brines to exist, exposed to the surface atmosphere. It can even have fresh water trapped beneath thin layers of ice at the poles. So, if there was any life evolved there in the early solar system, then, it's possible, relics of that early life may still linger on (much like the Venusian life in Arthur C. Clarke's story) in favoured places. We've had numerous workshops and detailed studies since then, and they all come to the same conclusion, that this Mars life, if it exists, could potentially be vulnerable to Earth life. Also, in the other direction, all the studies have come to the conclusion that there's a probably small, but not zero, possibility that the environment of Earth could be vulnerable to life returned from Mars. So we need to take care in both directions.

Jim Rummel, senior scientist for astrobiology at NASA and former planetary protection officer for NASA, puts it like this in his foreword to Michael Meltzer's "When Biospheres Collide":

"We are bathed in Earth organisms, which makes finding our own kind of life palpably easy and detecting indigenous life on other worlds much more difficult. We are not exploring the solar system to discover life that we have brought with us from home, and we are aware that Earth organisms (read: invaders) could very well erase traces of truly extraterrestrial life."

"Likewise, we don't know what would happen if alien organisms were introduced into Earth's biosphere. Would a close relationship (and a benign one) be obvious to all, or will Martian life be so alien as to be unnoticed by both Earth organisms and human defenses? We really have no data to address these questions, and considerate scientists fear conducting these experiments without proper safeguards. After all, this is the only biosphere we currently know - and we do love it!"

That “Would a close relationship (and a benign one) be obvious to all, or will Martian life be so alien as to be unnoticed by both Earth organisms and human defenses” is the crucial sentence. If the returned life was so alien that Earth organisms defenses don’t notice it, then it would be able to overwhelm us, live inside or on us, and our bodies would mount no defenses against it. That was Joshua Lederberg’s insight originally which he expands on in a couple of papers, as we'll see later in Why we can't prove yet that Mars life is safe for Earth (below) .

There are many other ways that microbes could harm us, without attacking any Earth life directly. The harm could just be an accidental result of something they do. Here is another of my "future fake news" stories to illustrate the idea. It uses an example of how life from another planet could harm Earth given by Chris Chyba:

Title: Lake Eyrie Blooms: Algae from Mars. Body: The algae from Mars, accidentally released into our biosphere last year, have now spread to lake Eyrie. It is experiencing its worst blooms for decades. These are not just harmful to native fish. They also produce a liver toxin which can cause sudden death in cattle in hours, and also often kills dogs swimming in the water, and is a skin irritant for humans. Scientists are trying to contain the outbreak The main concern at present is that the Martian algae may be able to spread to the sea, as they are pre-adapated to salty conditions on Mars. (Future Fake News)

The photograph there is a detail from an algal bloom of Lake Eyrie in October 2011 during its worst cyanobacteria bloom for a long time. The cyanobacteria produced microcystins which is a liver toxin and can cause sudden death in cattle within hours, also often kills dogs swimming in the water and is a skin irritant for people.

The algae is not "keyed to the hosts" in any way, and it is no advantage to an algae to kill cattle or dogs. It's used as an example of one way that life from another planet could harm our biosphere. For more about this and this example, see Many microbes harmful to humans are not "keyed to their hosts" (below)

I made this “future fake news” story with this online Newspaper generator

In the wider scientific community, there are differing ideas about how much care we need to take to protect Mars, or in the reverse direction, Earth, but not that many by way of dissenting voices about whether we need to protect them at all. But one of the dissenters is famous, Robert Zubrin, a famous space engineer and leader of the Mars Society, and a keen advocate for Mars colonization. He is a formidable debater, and often takes on astrobiologists in debate. Here is one of his latest debates with Andy Spry of the Planetary Protection Office and SETI.

His arguments for saying we don't have to protect Earth are:

His arguments for saying that we don't need to protect Mars are:

These arguments may seem convincing at first, even knock down arguments perhaps. His followers are ready to be convinced of course, as they are so keen for humans to land on Mars and they have wanted this for decades, some of them. These arguments are widely shared and used in discussions on planetary protection. You might even wonder why scientists continue to take care to protect Mars from Earth life. But these arguments are actually easy to demolish if you come to them without that perspective.

I will do so later in this book, in detail, but before we get there, you might like to have a try at debunking them yourself.

Here are a few questions to get started.

For samples returned to Earth you can ask some of those same questions again, and a few additional questions:

If you have been persuaded by these arguments, this book may have much that will surprise you, and perhaps also, help you to understand why so many scientists say that we should continue to protect Mars. For the answers to those questions, and for detailed debunking, in the direction from Earth to Mars, see Demolishing Zubrin's arguments (below) . For the direction from Mars to Earth, see: Zubrin's arguments in: "Contamination from Mars: No Threat" (below) and Some highlights from the rebuttals of Zubrin by astrobiologists in "No Threat? No Way" (below).

Zubrin's arguments are somewhat beside the point anyway, as all the space faring nations are in agreement that we have to protect Mars, and also to protect Earth in the case of a sample returned from Earth. Both are protected by international treaty as we'll see. It doesn't seem likely that we will drop all planetary protection measures any time soon.

However there is a great deal of variety in views about what we need to do to protect Mars from Earth microbes. NASA's planetary protection office and the Planetary Society both agree that a human landing will lead to irreversible contamination of Mars by Earth life, so long as there are habitats there for them to colonize. They both base their plans on the idea that what we need to do is to keep Earth life away from sensitive regions on Mars long enough to make scientific discoveries there (hopefully). It's basically a delaying tactic. However they have different ideas about how best to do that.

NASA's approach is to land humans on Mars as soon as practical. They still plan to do the best they can to keep Earth life away from the most sensitive areas for as long as possible, to give them time to make scientific discoveries. Their approach is to set up a corral type area around the landing site which they permit to be contaminated. The humans have to remain within this "corral" which spans many square kilometers. Then they have robots which the astronauts will send further afield to study pristine areas not yet contaminated with Earth life and return samples to their base for them to look at. Once humans land on Mars, there is probably no realistic way to prevent nearby RSL's etc from being contaminated eventually (if habitable to Earth life) but they do their best to delay the event of this happening for as long as possible. I cover this in Land humans quickly - to explore nearby regions kept free of Earth life - NASA's approach (below)

The Planetary Society has a similar approach, but a different emphasis. They strongly recommend that we study Mars as much as possible from Earth and with humans in orbit around Mars, controlling robots on the surface via telepresence, in an earlier mission, before landing there. But they still think in terms of an end date, when we send humans to the surface of Mars. They usually suggest a date for humans landing on Mars in the 2030s, but Bill Nye gives an optimistic projection of humans in orbit in 2023 and on the surface in 2025 (see When will we know enough about Mars? - below). From then on it is like the NASA idea. On that timetable, the human landing would be only two years after the telerobotic visit, which doesn't give much time for a biological study of Mars. Also, if I understand their vision right, they think we should do that, no matter what we have discovered by then about life on Mars, and no matter how incomplete our understanding of the Mars surface conditions are by then. So, it is the same idea as NASA really, a delaying tactic, but with addition of an extra mission in Mars orbit first before the humans land on the surface. I cover this in Explore Mars from orbit by telepresence first - but still land on Mars towards the end of the 2030s - The Planetary Society's approach (below)

It is beginning to seem even quite likely that there are some surface habitats there that Earth life could colonise (see Habitats for life on the surface of Mars below). If that's so, they are both of the opinion that it is impossible to keep Earth life away from Mars indefinitely once we have humans on the surface. They have given up on the idea of a biologically reversible exploration of Mars once humans land there. If that's right, it would seem that we are pretty much bound to contaminate Mars irreversibly with Earth microbes, eventually, following those policies. However, both are agreed that humans should land there in the near future, probably in the 2030s, possibly even sooner.

In this book I'll suggest that we shouldn't set a date for a human landing on Mars at all, at present, but should explore it from Earth and from orbit around Mars instead. I will suggest that we should continue to explore Mars in this way, from Earth and from orbit, until we feel we have a reasonable understanding of whether there are habitats there, and what the effect would be of landing humans on the planet. Using the example of the Lascaux caves, I argue that we need to do this, I think, to have a decent chance of making wise decisions in the future, and to avoid what might turn out to be huge mistakes, like the moldy Lascaux cave paintings, written large over an entire planet.

At any rate, whatever your views on that, before we can make the right decisions for the future, it is essential that we have a clear understanding of what the issues are. We have a long way to go by way of raising awareness of these issues, and I hope to help with this book.

Fossil optimists and early life enthusiasts

As you read this book,you may be surprised to learn

The main reason for this difference, I think, is that many of us, without thinking about it, are "fossil optimists" as I like to put it. After all, we are used to learning about past life from fossils on Earth, so it's not too surprising that we expect the same to happen on Mars. Enthusiasts, including scientists, even search Opportunity and Curiosity photos for what they think may be fossils of past Martian life. They search for these fossils in what they know to be lake beds on Mars that have been dried up for more than three billion years. Nearly all Earth's macro fossils date from the last half billion years of our geological history. There are older macro fossils, but they are rare and consist mainly of hard to recognize stromatolites and others that are equally ambiguous. It took a lot of proof before they were accepted as life.

These optimists searching for fossils in Gale crater are hoping that Mars life had at least a two and a half billion year head start over Earth. Actually, this fossil optimism is not absurd; indeed you can come up with some interesting reasoning in favour of it. Evolution on Mars could have been accelerated greatly compared to Earth. But the case for it isn't strong either, as we will see. You can as easily argue the case in the opposite direction, that Earth life is likely to be ahead of Mars life, and that Mars life, if it exists, is quite likely to be an earlier form of life than Earth life. That could be true even of present day life on Mars. Especially when it comes to life from three billion years ago, we may well find early and primitive forms of life there that no longer survive on Earth.

Many professional astrobiologists are "early life enthusiasts". They design all their instruments to search for life similar to whatever existed on Earth over three billion years ago. Perhaps even life so early that it predates DNA. The microbes may be so small that you can't see them at all, not just with a magnifying glass, but even with the best of optical microscopes. They don't expect to find easily recognizable macrofossils. Instead they pin their hopes on the cold and dry Mars conditions, which could preserve organics for billions of years.

They are optimistic that they could find this early life eventually, if Mars did ever have life, but they expect this signal to be weak, degraded, mixed in with organics from other sources including (large quantities of organics from meteorites and comets hitting Mars), and only present in a few rare locations. That's why they think it unlikely in the extreme that samples returned from Mars right now, with our present knowledge of Mars, will be enough to solve the major questions in their field. They are likely to be as inconclusive for the search for life on Mars as the organic rich meteorites from Mars we already have in our collections.

We just don't know where to look yet, or what to return to Earth. They also expect that they will need to drill to depths of several meters to find the clues they are looking for. This is what makes it so important to do in situ searches with sensitive biosignature detectors - so sensitive that they can detect a single amino acid in a sample. Even then it may also require a fair bit of detective work before we are sure that what we've found is evidence of past life on Mars.

They also expect present day life to be microbial, and elusive, for different reasons.

Dallas Ellman fine tunes a component of the astrobionibbler. It uses ideas from the larger UREY instrument, using high temperature high pressure subcritical water as a solvent for non destructive extraction of organics. However, with advances in technology, it's now miniaturized to a "lab on a chip". During his summer internship at JPL in 2014, he helped discover and replicate the conditions the Astrobionibbler team needed to extract and detect amino acids from Martian regolith. As a result it is now so sensitive that it could detect a single amino acid in the sample.

Asrtobionibbler was developed from an earlier and larger instrument UREY. This was developed in the US and then was approved for the European Space Agency (ESA)'s ExoMars but was descoped when NASA pulled out of the partnership. It was too heavy to launch to Mars on Russian rockets.

After that, ESA approved the lighter Life Marker Chip, mass of 4.7 Kg, another very sensitive life detection instrument, which uses polyclonal antibodies to detect biosignatures, but it was later descoped. Another version of it, LDChip300 was tested in the very dry core of the Atacama desert and was able to detect a layer of microbial life at a depth of 2 meters below the surface from analysis of less than half a gram of material. This was a habitat that no-one had ever discovered before. Sadly, this also was descoped from ExoMars, so won't fly quite yet.

The target mass for astrobionibbler is 2.5 kilograms, a quarter of the mass of UREY. We have many other instruments especially designed by astrobiologists to search for life "in situ" on Mars, but so far, none have flown since the Viking landers in the 1970s.

To find out more about these tiny, and exquisitely sensitive instruments, see the section In situ instrument capabilities below. So far we haven't sent any instruments to search for life directly on Mars since the two Viking missions in the 1970s

With that background, the tiny microbes are the very thing you are looking for. They plan such sensitive searches that a single amino acid in the sample could be a major clue to the puzzle. With such sensitive searches, introducing Earth microbes and organics could be disastrous for ones hopes of finding out about life on Mars, either early life, or elusive present day life.

So what then is the role of humans in this vision? Well, to start with, the robots on Mars themselves are our outposts there, our ways of exploring Mars, our mobile eyes and hands on Mars. We can do more in situ Mars exploring with our rovers and the pace should increase once we have a stream of data coming back, and more capable and more autonomous rovers, and many of them on Mars. Enthusiasts and experts on Earth will be able to explore the surface of Mars in great detail in the comfort of their own homes and faculties. But humans have value in space too. Eventually I think we will have humans in orbit around Mars and on its moons exploring it via telepresence.That may be a step too far at present just for safety reasons, as we'll see. There's an obvious place to begin though, closer to home.

Value of humans in space - and fossil hunters on the Moon?

In my other kindle books and booklets, and my articles, I've written a fair bit about the value of space resources, and the many ways that humans can contribute in situ to exploring the solar system. I also argue strongly for the Moon as the obvious place to get started with human exploration. It's not just as a stepping stone to Mars. Planetary scientists have proposed calling the larger gravitationally rounded moons in our solar system "Moon planets" (paper here). The Moon is of great science interest, and not just for the polar ice deposits and the lava tube caves. It's even still geologically active, with recently formed small cliffs, trenches (formed as recently as 50 million years ago) and enigmatic almost crater free patches of strangely patterned terrain like the Ina depression that formed on its surface, some as recently as fifty million years ago, There have been many surprises, as I explored in the Moon science surprises section of my Case for Moon First.

But it is also of astrobiological interest too, as we'll see. It is a place of great interest in its own right, nearby, and far easier to visit than Mars. It also has little by way of planetary protection issues to deal with, as the Moon is Category II:

"… where there is only a remote chance that contamination carried by a spacecraft could jeopardize future exploration”. In this case we define “remote chance” as “the absence of niches (places where terrestrial microorganisms could proliferate) and/or a very low likelihood of transfer to those places.”

Could we find habitats for life on the Moon which might lead COSPAR to revise this guideline? Well in short, the chance remains "remote". There's ice at the poles but it's far too cold for life. Any warm ice or liquid water would sublimate into the atmosphere at tens of meters per year. There is currently no evidence at all for liquid water on or below the surface of the Moon. It's not theoretically impossible as water could be trapped below a layer of high molecular weight organics, and the Moon does have volatiles that escape from its interior, but the chance is so remote that I have not come across a single paper suggesting this as a possibility in recent times (it was suggested as a possibility in a paper by Carl Sagan before Apollo 11). For details see Life or prebiotic chemistry on the Moon (below)

At any rate, so far, there are no restrictions on lunar explorer astronauts.We just have to document clearly what we do. There might be issues with trash, and with organics spreading in the dust, but we'll get to that in a minute (see Trash, rocket exhausts and microbes on the Moon - testing ground for planetary protection measures for a human base (below).), but it's nothing compared to the issues for Mars.

The Moon can also help bring us with the biological search for early life, rather surprisingly, through remains of life that landed there in meteorites. It has extraordinarily cold conditions at the lunar poles. We might find fossils also, after a simulated impact on the Moon, fossil diatoms are still recognizable, and indeed the smallest ones are intact, complete fossils. There must be a lot of material from the Chicxulub impact on the Moon, which may also contain fragments of larger creatures such as the ubiquitous ammonites of Cretaceous seas. Perhaps the Moon will be one of the best places for fossil hunters in our solar system, outside of Earth.

Artist's impression of Cretaceous period ammonites, courtesy of Encarta. The Chicxulub impact made these creatures extinct. It hit shallow tropical seas and the ejecta could have sent fragments of Cretaceous period sea creatures such as ammonites all the way to the Moon. Fragments in the cold polar regions may even have the organics preserved.

The Moon must have meteorites from Mars too, for us to pick up, also from early Venus, from before its atmosphere became as thick as it is now. That's especially exciting as early Venus might have had oceans and might have been as habitable as early Earth and Mars. However, most of that record is probably erased even if we get to explore the surface of Venus. It was resurfaced by volcanic processes around 300 hundred million years ago. It doesn't have continental drift, and the leading explanation for its young cratering record is that its entire surface turns over from below from time to time. We have something similar on Earth, the superplumes. These are huge but very very slow motions deep below the surface, for instance, perhaps a superplume beneath the Pacific drives the volcanic activity around the Pacific "ring of fire". Venus may have had superplumes so large they resurfaced the entire planet.

Anyway for whatever the reason, Venus' surface is geologically young. It's atmosphere is so thick that no meteorites get from Venus to Earth right now, and anyway, a modern meteorite or a sample return would tell us nothing about early Venus. That leaves any Venus meteorites on the Moon as our best, and maybe only way to find out about early Venus, including any biology from those times. That's especially so if they have the organics preserved. For more on this see Search for life from Mars, Venus, or the Earth - on the Moon in Meteorites! below.

The Moon also must have collected organics from the comets and asteroids of the early solar system that bombarded Earth, so it can help give us an inventory of the organics that lead to kick starting life on Earth. For more about all this, see also Charles Cockell's paper: Astrobiology—What Can We Do on the Moon?

The Moon also is a far safer place to start our human exploration of space. The ISS has "lifeboat" spaceships attached at any time, with enough seats to take the entire crew back to Earth within a few hours in an emergency. We can have similar lifeboats on the Moon. They won't be able to get the astronauts back to Earth in hours, but they can take the entire crew back to Earth within a couple of days. That's not too much of a challenge, as they can be kept supplied at all times with fuel and food for the journey. On Mars or in Mars orbit it can be up to two years to get back in an emergency, which may be a step too far right now. You'd need something rather more substantial for a lifeboat to withstand two years of travel through interplanetary space.

The main problem there is life support. You can't test life support intended for a zero g environment on the ground, not properly. The ISS has had numerous life support issues which were only fixed due to resupply from Earth. See this list of some of them. None were immediately dangerous, and some were relatively minor but some of them would have been fatal on a timescale of months. If issues like that arose on a spacecraft like the ISS as far away as Mars many of those issues would have lead to the entire crew dying as they could never have got their spaceship back to Earth in time. The same would be true of other issues that arise over long timescales only, e.g. damage to equipment or to hull integrity from a micrometeorite - or else on a long mission, some problem with the packaging of the food for instance and you find most of your food has gone off and you don't have enough left to survive the journey back, or some problem with the water, or microbe films building up in a way that is harmful. Or a fire, or release of harmful chemicals, in either case damaging vital equipment for life support, or essential provisions.

The Moon is not only safer, it's also a natural place to begin to develop reliable technology for multi-year missions throughout the solar system. If we can achieve that then the cost of human missions to the Moon will go down dramatically to a fraction of the normal cost. Imagine what a cost saving it would be if we could send a crew to the Moon for two years with no resupply from Earth, as if it was an interplanetary mission to Mars? We need these shake out cruises close to Earth first. The Moon or the L1 or L2 positions are ideal places to do them, close to Earth, and also of great interest in their own right. Once we have biological closed systems working on the Moon, then missions throughout our solar system that last for a decade or more could be as easy to support as ones that last for a couple of years or less. Once we have that capability, we can go to Venus, Mars and beyond, even to Mercury, the asteroids and Jupiter's Callisto, with no worry about narrow safety margins. Or for earlier missions, using ISS type life support to get to Mars or Venus, then we could do shake down cruises in the Earth Moon system to make sure everything is working well before we send the crew on their interplanetary missions, so far from help from Earth, or any possibility of aborting back to Earth.

The Moon is also a place to explore some of the less glamorous sides of space exploration.

Trash on the Moon

Even the Apollo crew left their share of trash on the Moon, though not that much as they were only there for up to three days. The ISS produces huge amounts of trash every year.

The ISS discards that much trash into our atmosphere every few months. Like the Apollo 11 footprints, any trash you leave on the surface of the Moon will still be there thousands, and probably millions of years into the future. So will we get huge trash heaps build up around any lunar base, and if so, how will the astronauts handle this on the Moon?

Buzz Aldrin standing near a leg of the lunar module - notice how many footprints they left on the Moon?

Every time crew land on the Moon and take off, they will leave the descent stage behind on the Moon, so that's trash too. Spacesuits need to be replaced after a number of EVAs, so that's trash. Also there's packaging for equipment, equipment that fails and needs to be replaced, dirty clothes and socks, the list goes on and on. The amounts of trash they'd leave would eventually be enough to fill caves or craters around the base. So what would they do with it all? It won't degrade , rust away, and mix into the landscape. Also surely they wouldn't use precious rocket fuel to send it back to Earth?

Also, how far do organics spread in the dust? You might think hardly at all, but it turns out, that fine dust particles, large enough to carry spores and other organics, can spread out levitated at a height of a meter or more above the ground by effects of UV radiation and plasma hitting the Moon's surface, through electrostatic levitation. A few spores spreading out over the surface won't do much to confuse science results kilometers away, but they could be significant if the base is close to an area of special interest, such as volatiles at the poles. Luckily there are large areas of volatiles, probably, but still, it may be a consideration, as this review paper from 2007, the authors suggest that perhaps we might need to set up "organic special regions" on the Moon that need to be kept free of organics.

The historical lunar landing sites may also need protection from contamination by Earth microbes, as "valuable and limited resource for conducting studies on the effects of humankind’s initial contact with the Moon" (quote from page 774 of this paper) They are also decades long unplanned experiments in the interplanetary cruise stage of panspermia - the ability of microbes to remain viable in the conditions of interplanetary space for transfer from one body to another.

Another planetary protection question for the Moon (in its broadest sense) is whether our landers would change the Moon's very tenuous "atmosphere" or exosphere with rocket exhausts. We have a golden opportunity right now to observe its atmosphere "as is". The rocket exhausts from Apollo added nearly half as much again to the mass of the lunar atmosphere, for a month. But this should have dissipated long ago. Amongst other things we can study the movement of water vapour in the lunar atmosphere and see where it comes from.

When the Chinese Chang'e 3 landed on the Moon on 14th December 2013, NASA's LADEE was in orbit, and, surprisingly, they found that, if it modified the lunar atmosphere at all, the changes were beyond their detection limits.

However the rocket exhausts could have significant local effects. This is a model of the effects of the Apollo 17 landing exhausts on the lunar surface near their landing site:

Figure 28 from this paper showing their modeled rocket exhaust contamination of the lunar surface from Apollo 17 superimposed over Google Moon. The contaminated area spans 522 kilometers of the lunar surface. The red range rings contain 50% and 67% of the total contamination respectively.

This modeling suggests we may need to take care about the effects of rocket exhausts from spacecraft landing in the vicinity of the lunar village, especially once the larger rockets start landing with astronauts on board. The authors of the paper looked at ways the contamination can be reduced, including pointing the engine over the horizon during braking maneuvers with the exhaust gas velocity much more than the lunar escape velocity.

I have another suggestion, to use Hoyt's Cislunar tether transport system. His lunar tether masses only seventeen times the payload mass, and once it is in place, you would no longer need to use rockets to land on the Moon or take off from it. For details see the Exporting materials from the Moon section of my Case for Moon First.

In short, the Moon is an ideal place to study these issues before we have to deal with them further afield. Even in a mission to Phobos, then the regolith contains a valuable record of impact ejecta from Mars, right back to the early solar system when it may have had life or pre-biotic chemistry. So, it might be rather important to keep the surface of Phobos free from trash and organic contaminants during a human mission there. If so, our early experiences on the Moon could help us devise suitable precautions, ones that we know work.

This is a summary of my sections: Trash, rocket exhausts and microbes on the Moon - testing ground for planetary protection measures for a human base and Rocket exhausts, microbial spores and organics mixing with levitating lunar dust (below) which go into more details with cites.

Alternative visions for the future

Here are my two online and kindle books where I go into detail about the value and interest of the Moon and also various challenges we'd face there. The aim is to sketch out an alternative positive and exciting future for humans in space with planetary protection and the science value of our explorations there as core principles.

"MOON FIRST Why Humans on Mars Right Now Are Bad for Science", available on kindle, or Read on my website (free)

The cover of that book shows the result of replacing the black night skies of the Apollo photographs with a blue one. This helps to bring out how "Earth like" the Moon really is, in many ways that might surprise you, a theme I expand on in the book.

Case For Moon First: Gateway to Entire Solar System - Open Ended Exploration, Planetary Protection at its Heart on kindle or Read on my website (free).

This is the first book I wrote on the Case for Moon first. In a point for point comparison with Mars, I was as surprised to find that Earth often wins hands down over Mars in terms of the resources available, the ease of using them, and compatible conditions for humans to build bases. The thin carbon dioxide atmosphere of Mars is not much of a benefit, when you realize that carbon dioxide is normally a problem gas to get rid of. Plants need only a few kilograms of carbon dioxide in a habitat greenhouse atmosphere, in trace quantities, to grow. Meanwhile the lunar vacuum is actually a benefit in many ways. There's pure iron on the surface, not oxidized, mixed in the dust, nanoscale iron. This makes it as easy to melt a kilogram of dust with microwaves as to boil a kettle. The hard vacuum also has many benefits, including solar cells that are easier to make on the Moon than on Earth. The close to 24 hour day of Mars is actually a major issue because of the huge temperature swings from day to night, at day time up to well above 0 C, and at night, for 100 days of the year, it's cold enough for dry ice to form and precipitate out of the atmosphere as the Martian frosts (mixed in with ice). Meanwhile temperatures at the lunar poles are stable, within 10 C or so, 24/7 and year round. The lunar caves also have stable temperatures. This makes it much easier to design the base and keep the temperatures regulated. At the poles, then the sun always comes from close to the horizon, so you can track it with a vertical solar panel or mirror spinning slowly once a month on a single vertical axis. Meanwhile heat rejection is easy, with horizontal radiators around the base, that are always in shadow, radiating the heat upwards.

That's just a few of numerous comparisons where the Moon scores over Mars. For a bulleted list of some of the advantages of the Moon over Mars, see Why the Moon is best for humans right now (below)

It's also close enough to Earth for there to be at least some possibility of commercial exports such as platinum as Dennis Wingo has suggested in his books on lunar colonization. It beats Mars on almost everything. At least, it does, for as long as you are thinking of populations of up to a few tens of thousands. If the caves can be made into habitats too, then the Moon could have populations of many millions, and it may be the easiest place in our solar system to do this, with economic support from Earth from exports from the Moon, tourism, etc. Similarly, if "city dome" type habitats become practical, they could be habitats for millions.

Longer term, using resources from the asteroid belt then we could have off world populations of trillions, with a total habitable surface area a thousand times that of Mars. There is no need to colonize Mars, in either the near or more distant future, no matter how keen you are for humans to set up habitats outside of Earth. The key to all this, I think, is maintenance. If a large city sized habitat can be closed system, producing all its own food, air and water, and low maintenance, once built, perhaps hundreds of thousands of dollars per inhabitant, then it may even be easier to live there than it is on Earth, at least once you have paid off the huge costs of building the colony in the first place. If the maintenance costs are high, costing millions of dollars per inhabitant per year, then I don't see how it can work at all. (See Asteroid Resources Could Create Space Habs For Trillions; Land Area Of A Thousand Earths)

In those books I also argue that with our lunar adventures, we will learn what humans can and can't do in space, and how to stay healthy there. We can also learn how to be self sufficient for months and then years at a time, without resupply from Earth. If we can do that on the Moon it will reduce costs hugely. Once we've done that, it will also be much more practical and safe to send humans not just to Mars but to the Venus clouds, Mercury, asteroids and further afield. Even Jupiter's Callisto, which orbits just outside its dangerously intense radiation belt, is less than two years journey away on a fast Hohmann transfer orbit from Earth (see Sending humans to Callisto or Ganymede (below) ) . Once we know how to keep humans healthy in space for years on end, then Callisto also should be within reach of Earth.

Once it is safe to send humans to Mars orbit, we can use this to explore the surface in an immersive way. This is similar to exploring a three dimensional virtual world in a computer game, but this time the world explored is real. Telepresence like this may be a great way to explore the Moon too, so we can gain experience of this on the Moon first. This virtual way of touching Mars, especially when combined with haptic feedback, is in some ways more immediate than touching in a spacesuit, and is an exciting and adventurous alternative vision for humans in space. It is also safer, and has none of the irreversible and possibly devastating consequences for science of landing on the Mars surface directly, with all the microbes that inevitably accompany us.

I think it's best to say all that from the outset as I've found in the past that my readers sometimes see my articles as an attempt to stop humans from exploring space. Far from it, I'm a science fiction geek and long term enthusiast for humans in space since the time of Apollo. As a teenager I found those missions exciting and followed them keenly.

Humans on Mars are not the problem. The problem comes with the microbes that accompany us, in the air, in our water and food, and indeed on and in our bodies too, trillions of them, that can't be removed or we'd die. These include microbes capable of living in extreme environments, since many extremophiles retain their ability to survive in the most ordinary conditions. So, though capable of living in conditions extreme as Mars, they can also manage just fine on and in our bodies, and on the surfaces of our spaceships. Even the organics that make up our bodies, and our food, human wastes etc could be a problem in the event of a crash on Mars, as we'll see. They could confuse those astrobiological searches for elusive degraded organics, with instruments sensitive to a single amino acid in a sample.

If we explore Mars via telepresence, from orbit, we can be there in person without these possibly devastating consequences of touching Mars.

12th April 2011: Cady Coleman takes pictures of the Earth from inside the cupola.- I've "photoshopped" in Hubble's photograph of Mars from 2003 to give an impression of the view of an astronaut exploring Mars from orbit. One of the orbits suggested for exploring Mars is particularly exciting, the sun synchronous Molniya orbit. It comes in close twice a day, approaching opposite sides of Mars, always with Mars lit up by full sunshine. It's an orbit that skims close to the ice caps of Mars on the way in and out each time, much like the view in this photograph. It flies low over the surface of Mars when closest to the planet. Then as it recedes, Mars dwindles to a distant planet, and the cycle repeats twice a day. It would provide great views of Mars, continually changing, in an exciting orbit.

The approach would be the same as for the Moon. Just as the Moon is of great interest in its own right, and doesn't need to be justified as a "stepping stone" to Mars, in the same way also, a Mars orbit optimal for telepresence exploration would be a destination for humans of interest in its own right. The immediate aim would be to find out more about Mars and its moons. It doesn't need to be justified as a "stepping stone" for humans to the Mars surface. I think we need to leave the future open. Who knows what future it might lead to, if we leave our options open after that?

Should we return samples from Mars right now?

Carl Sagan, Joshua Lederberg and others didn't just warn about the planetary protection risk in the forward direction, the potentially harmful effects of Earth life on Mars. They were also deeply concerned about the potential for Mars life returned to Earth to harm us, or the biosphere of Earth. What we discover on Mars, in some of the most interesting scenarios, could be microbial ETs. Though only microbes, they could be far beyond anything we could discover through genetic modification or adding extra bases to DNA or even using a different biopolymer from DNA to carry the genetic information in a cell.

However, this also means that what we return could be unlike anything we know about or can make in a laboratory as an artificial lifeform, or even describe in detail. Even if we replace DNA with a different biopolymer in our lab experiments, the result is just a form of Earth life, with the same genetic code, translation tables, and cell machinery. Only one (very important) part of the cell would be changed. We simply don't know enough yet to try anything more original than that. Life from Mars could be far more exotic than that, with numerous details of how the cells work different in fundamental ways throughout. So is it safe to return something to Earth that could potentially have such a revolutionary biology as that (if we find it)? If so, how can we do this safely?

Joshua Lederberg put it like this:

"If Martian microorganisms ever make it here, will they be totally mystified and defeated by terrestrial metabolism, perhaps even before they challenge immune defenses? Or will they have a field day in light of our own total naivete in dealing with their “aggressins”?

from: "Paradoxes of the Host-Parasite Relationship"

We don't know the answer to that. Also, there is more to it than that. The Martian life doesn't have to attack us or any other terrestrial life directly. There are many ways that microbes can harm us indirectly, with examples such as botulism, ergot disease, BMAA etc, see Many microbes harmful to humans are not "keyed to their hosts" (below) . Or they could harm us just by changing the environment. For the most famous example of this, the first photosynthetic life on Earth, which may have caused an early mass extinction on Earth, didn't directly attack anaerobic life. It just produced oxygen as a byproduct which made much of the biosphere poisonous to it, including the ocean. Could life returned from Mars, Europa etc do something similar, if perhaps on a smaller scale? Could it transform parts of our ecosystem just by creating different byproducts from Earth life, and so make it inhospitable to some forms of life native to our planet? I cover an example of this sort of thing in the section Invasive diatoms in Earth inland seas, lakes and rivers (below) .

Given Mars’ history then it's a reasonable hypothesis that whatever life there is may be an early form of life based on what we have so far. But it could also be more highly evolved than Earth life. One way this could happen is if the harsh conditions early on stimulated rapid evolution. Could it have had eukaryotes, the cells with a nucleus which are the basis for most modern complex life, already there 3 billion years ago? Anyone who searches optimistically for easily recognizable fossils in Curiosity images is hoping that Mars life is at least 2.5 billion years ahead of Earth life in evolution - or at least, was that far ahead of us up to 3 billion years ago. So that raises the possibility of life on Mars that is harmful to the biosphere of Earth as a result of being more evolved than Earth life. Or, at least, life that is as far evolved as Earth life, and with more robust adaptations than Earth life for some particular type of habitat.

A more evolved microbe from Mars might not necessarily cause problems right away, or cause problems to humans. It might not cause any problems at all in quarantine facilities or laboratory tests. For instance suppose it is better at photosynthesis, or in some other way, is a a form of microbe that takes over from some key component of our biosphere? Or suppose it is harmless to humans but harmful to some other animals or plants on Earth, in a way that's not obvious until it gets into our biosphere? Also, maybe it needs to adapt to some challenge on Earth, then causes a problem, something microbes can do rapidly because of their short life span and rapid evolution. Or it might transfer the capabilities to Earth microbes through horizontal gene transfer, via gene transfer agents, or in the other direction, our Earth microbes might give the Mars microbes the capabilities they need to thrive here via GTAs.

I can imagine that perhaps in the vastness of our universe there are places where there are planets like Earth with intelligent species living on them, and other planets or moons nearby, which unlike our Moon, have indigenous life of their own. The intelligent species on these planets, when they develop space travel, are likely to bring back life by accident, as we would surely have done if there had been native life on the Moon. Perhaps in most of the cases no harm is done, or it is a minor nuisance, like problem species of plants returned from another continent. But perhaps in a few cases the life brought back may severely degrade their biosphere and make it less habitable for them. Who can say, perhaps in a very few cases, especially if done as early as Apollo when their technology isn’t very sophisticated, perhaps they go extinct as a direct result of their first mission to a nearby Moon or planet? Perhaps looking at it this way, as something that might actually happen, or have happened to other extra terrestrials, could help put it in perspective?

These concerns don't make a sample return impossible, but they greatly complicate it, in ways which the proponents of a sample return may not have fully taken account of. If we knew what we were returning, we could handle it much as we do for comet and asteroid sample returns. If we can assess what it is, and show that it is harmless (supposing it is), then the process will be easy. Or if there is a known risk, then we can work out how to deal with it. If we knew that a sample canister contains anthrax spores, as Carl Sagan once put it, we would take great care. But we'd also know what measures to take to return it safely. So similarly if we know that the Mars life is hazardous, but know what the hazard is, we can plan accordingly.

What makes it really tricky is that the plan is to return a sample long before we know what is in it, so we can't assess it's capabilities prior to the return. In that situation, everything gets so much more complicated. We end up having to design a complex facility to deal with any conceivable form of microbial exobiology. How confident can we be in such a facility, when we don't yet have a single example of any non terrestrial life to base the precautions on? For instance we wouldn't know how small the life can be or what its capabilities might be. The facility ends up having to be hugely over engineered, to return less than a kilogram of material that may not even include any life at all, and even then it's hard to have total confidence in it, with the experts adding more and more requirements to the specifications with each new study, based on new scientific discoveries about possible capabilities of tiny forms of life. And then there's the ever present risk of the canister being breached in some way before it reaches the facility, through accident, or even malicious actions of some sort, or of the precautions being bypassed by forgetful human operatives.

Just the legal processes involved may involve much more than most of you would expect, if you haven't looked into it. Margaret Race did, and found out that there are with numerous domestic and international laws to be passed and probably also involving domestic laws to be passed in other nations as well before it can go ahead. Our legal situation is far more complex than it was in the 1960s, to the extent that if NASA was really serious about doing a sample return in the 2030s they probably should have started on the process already, to have a chance of completing the legal process in time. The 1960s regulations were inadequate, but anyway, they have also been repealed so can no longer be used "as is". I cover this in Legal complexities (below)

I think that when faced with the "sticker shock" of the cost of the sample return facility (most recent estimate was half a billion dollars), combined with the complexity of the legal situation, it's possible that NASA might end up either sterilizing their Mars sample, or returning it to somewhere outside of Earth. Both of these ideas are legally and practically far simpler to achieve. I suggest a good place to return it would be to above GEO. For the reasons in detail, see the section: If likely to be of greater astrobiological interest - return samples to above GEO

The cost is a not insignificant factor also, NASA is "betting the ranch" on an extremely expensive plan to return a half kilogram of samples of rock from Mars in the 2030s at a cost of millions of dollars per gram. We already have samples from Mars in the form of Mars meteorites. Many astrobiologists have written papers saying that in their professional opinion, this is a very long shot. They expect these samples to be as controversial and hard to interpret as the meteorites we already have. They say that what we need to answer many of the central questions in their field is more in situ research with our exquisitely sensitive and light weight modern life and biosignature detection instruments, and that the money set aside for this mission could be used much more productively for in situ life detection on Mars.

I cover this interesting controversy in detail. I also ask if this emphasis on returning samples so soon could end up as a huge embarrassment for NASA in the 2030s, if the mission returns with samples that are ambiguous, as the astrobiologists are predicting, and don't answer any of their questions about life on Mars.

Body: Are these tiny structures life? NASA are embarrassed when the samples they returned at a cost of around $10 million per gram prove to be controversial In a rerun of the ALH84001 controversy, astrobiologists say that they simply can't tell if these structures are evidence of past life. They say to NASA that they warned about this possibility in many papers decades previously.

The image here is a detail of one of the less well known close up electron microscope photographs of ALH84001, the controversial meteorite that was first announced as the potentially the first discovery of life on Mars, but later the announcement was withdrawn as premature. It remains controversial to this day, with astrobiologists arguing both sides of the case.

I created this fake newspaper story using this online free newspaper generator.

I cover this in the sections:

Human settlement and exploration - hugely positive or hugely negative - it all depends how it is done

Of course many of my readers will be keen on human settlement in space. Though that's not the focus of this book, I should just touch on it briefly, in a few pages, because if I don't many of you will have this as one of the top questions on your mind. I want this book to have a positive message, and to do what I can to help us to find the way to a future that is inspiring and worthwhile.

So, first I argue in my Case For Moon First: that the Moon has huge advantages over Mars, in almost all respects, as a place to send humans. For a bulleted list of some of them, see Why the Moon is best for humans right now (below)

Now, that doesn't mean that the Moon or Mars, or indeed anywhere else is a good place to colonize right now. They are just so very inhospitable. Whatever you think about the feasibility of Elon Musk and Robert Zubrin's ideas for colonizing Mars, perhaps you can see that if you are in a desert anywhere on Earth, and there is sea nearby, and a breathable atmosphere, you have resources beyond the wildest dreams of a “Mars or Moon colonist”. A desert or a patch of our sea, with breathable air what's more, would seem an absolute paradise on Mars or the Moon.

Here where I live on the West coast of Scotland we have numerous uninhabited islands that are thought to be not worth living on because they don't have any sources of fresh water. Their owners only visit them for a few days a year, and for the rest of the year, at most, they may put a few sheep on them to graze. The same must surely be true in many places. Even islands that do have fresh water are often uninhabited here too, because it is just too inconvenient to have to take a boat every time you want to go to the shops or the post office, and maybe sometimes not be able to travel at all because of storms.

So never mind uninhabited deserts, we have many uninhabited islands, just because of the inconvenience of travel, or lack of fresh water. These are places surrounded by the sea, which you could turn into fresh water with a desalination plant, and indeed, produce salt as a byproduct. Also with plenty of rain that you could collect with water catchment areas. These issues would be comparatively easy to solve with modern technology, but nobody is bothered to do it because there are far easier and less expensive ways to live on Earth. Similarly, there are vast areas of Canada, Siberia etc, far more habitable even than our deserts, which are almost totally uninhabited. Hardly anyone is interested in colonizing these places.

It's the same also if you are floating in the sea on a floating platform or boat. The resources available to you just from sea water and sunlight, the air to breathe, and perhaps a few rocks from the sea bottom, would be far far beyond anything you could expect anywhere in space outside of Earth. That's without fishing, or using the sea in any way at all, just using the sea water itself.

Idea of the Seasteading Institute to set up floating islands, gardens floating in the sea. They would be rather like space habitats in how they function. No need to fish or exploit the sea at all, but instead in a sustainable way just use the air from our atmosphere and sea water from the sea to sustain their population along with solar power from the sun. It's like a space habitat but without the need for radiation shielding or environment control and life support.

An artificial island, floating on the sea, using nothing but sea water and air to sustain itself, would be an absolute paradise compared to Mars, and far far easier to make self sustaining.

We could also use ideas from space habitats to support ourselves in deserts, in a self sustaining way, using only the desert sands, sea water and the air. This may seem a little idealistic and perhaps bordering on fantasy to some. But compared to the plans of Mars colonization, this is easy peasy. We could easily support several times the Earth's population just from a small patch of the Pacific ocean in a sustainable way in these floating sea cities. It would require far far less by way of resources and technology than supporting a similar population on Mars.

This doesn't make off world habitats impossible. But you need some other strong motivation to be there - also some financial benefit that can't be obtained more cheaply with robots or telerobots controlled from Earth. In the near future - that suggests bases for explorers, tourists, and research scientists. It would work much like the bases in Antarctica. Longer term - if large habitats can be made so self contained that the maintenance drops almost to zero, then perhaps they can become as easy to live in as Earth. But that prospect seems a long way away at present.

I cover this in more detail towards the end of my An astronaut gardener on the Moon in Why Humans on Mars First are Bad for Science.

Mars fantasies and realities - a sobering dose of common sense

If you've only read the articles and books, and listened to Mars colonization enthusiasts, as they wax lyrical in realms of fantasy about future Mars cities and a terraformed Mars, you may not realize that there are others who are profoundly skeptical about it all, bringing a perhaps sobering dose of common sense. Paul Spudis, senior staff scientist at the Lunar and Planetary institute in Houston, and author of The Value of the Moon: How to Explore, Live, and Prosper in Space Using the Moon's Resources. is particularly scathing about these ideas of a Martian colony in the near future. If you haven't come across these views before, his Delusions of a Mars Colonist may give you an interestingly different perspective from the stories that get widely publicized extolling the virtues of Mars colonization.

"So aside from the inconvenient facts that we don’t know how to safely make the voyage, how to land on the planet, what the detailed chemistry of the soil is, or if we can access potable water, whether we can then grow food locally, or how to build habitats to shield us from the numbing cold and hostile surface environment, don’t know what protection is needed due to the toxic soil chemistry, or how to generate enough electrical power to build and operate an outpost or settlement – in spite of these annoying details that make this idea prohibitive, the creation of a Mars colony within a decade is marketed to the public as if the plans had already been drawn up."

..."With flashy artwork depicting futuristic cities, sleek flying cars, and lush green fields resplendent under transparent crystal domes (in startling contrast to the red-hued surrounding desert of the martian surface) it is simply assumed that a human colony on Mars will evolve into some kind of off-Earth utopia."

"But how will these future Mars inhabitants make a living? And by that, I mean what product or service will they offer that anybody on Earth will want? If you think that the answer is autarky (complete economic isolation and self-sufficiency), then you are imagining an economy (and likely, a political state) in which North Korea is a free market, pluralistic paradise by comparison. People who migrate to Mars need more than food and shelter – they will need imports from Earth, material and intellectual products designed to enrich and refine life on the frontier. What will they have of value to trade or to sell for these imports?"

..."Much is made of the possible economic value of “information,” but it is not clear that Mars is particularly rich in factual data marketable to those back on Earth, although a martian pioneer might have desperate need of it – which would make them their own “customers” and exacerbate the economic disparity of the colony to an even greater degree."

The Mars enthusiasts' plans get particularly sketchy when they cover the economics of a Mars colony (while Moon firsters tend to cover lunar economics in great detail). There is only one short, and perhaps not very convincing chapter on this in Zubrin's Case for Mars. In this chapter, he relies on exports of intellectual property rights by the inventive Mars colonists as one of the most important ways to pay for the colony.

I have to admit to being very skeptical that a "Mars colony" could come to anything, apart from planetary protection considerations. Elon Musk's idea that you could sell your house for $200,000, buy a ticket to a "new world" on Mars, and set up home there, particularly, seems bordering on fantasy. You've sold your house on Earth - to pay for your trip - but you still need somewhere to live on Mars. Is he going to provide free houses on Mars for all his colonists? Surely not. A house on Mars would be vastly more expensive than one on Earth. He would no longer be making a profit on every colonist, but rather, an immense loss. Even Elon Musk couldn't sustain a business shipping a hundred colonists to Mars at a time while making a loss of millions of dollars per colonist.

Also, it's not much use being on Mars without an EVA spacesuit. There are two main kinds of spacesuit, the IVA suit you wear inside a spacecraft, e.g. during launch, designed to protect you if you get a loss of pressure, and the EVA suit which protects your for missions outside your spacecraft or habitat (Extra Vehicular Activities). You would need both, but the EVA suit is the most expensive of the two. It's a little hard to get hold of unit cost estimates for a spacesuit, as they are hardly consumer items yet, and you hear varying estimates on the cost. For instance, according to one rough estimate, it will set you back $2 million as the approximate cost of making a spare EMU for the ISS. That's not including the design cost. It is just the cost for someone to make it, after the design is completed. It requires about 5,000 hours of work and would take someone who had all the necessary skills about two and a half years to build, given supply of all the parts and materials needed, a long job involving many complex intricate components. That may surprise you, but a spacesuit is not an "off the shelf" item. Building one is not unlike building a spaceship. Basically it is a very small mobile spaceship with its own independent life support. It currently costs $100,000 per astronaut just to fit the airtight bladder inside their gloves to help reduce the risk of them losing their fingernails as a result of the stiffness of the gloves, and to make the gloves a bit more comfortable. So, that's half the cost of your house, and so half the cost of your trip to Mars, already blown on this airtight bladder.

For some other examples, to give an idea of the total cost of a suit including design, a 1998 Washington post report says that NASA paid $10.4 million per suit for it's initial order of twelve EMU suits (that's about $15.6 million in 2017 dollars), and the Chinese EVA spacesuits are reported as costing $4.4 million each. The Apollo spacesuits cost less per person but were less capable and only needed to last for three EVA's each. Your suit also will need to be maintained and repaired, which itself is a tricky job, and it has a finite life too, the 1998 EMUs were certified for 25 space walks each before they needed to be returned to Earth for expensive overhaul. A Mars suit would need to have a longer design life than that surely. At any rate, the cost would surely be over a million dollars for your suit, and it would need to be replaced or have expensive overhauls at regular intervals.

We don't really have the technology of a durable, low maintenance deep space suit capable of doing large numbers of space walks yet, which is likely to require many new innovations. So, our reality is a fair bit away yet from the spacesuits of science fiction.


Suitsat - a Russian Orlon suit that reached the end of its useful life, discarded as a satellite experiment. With current technology at least, your "Mars suit", as complex as a small spaceship, would probably cost over $1 million to build, would need a lot of maintenance, and after using it for a couple of dozen EVA's, it would need to be discarded and replaced by a new one, or sent back to Earth for reservicing. Is it true that Mars colonists could pay for their spacesuits, and everything else they need, through their inventions and other intellectual property, which they sell back to Earth?

Then, to survive in your habitat for any length of time, you need complex life support for that too. You need to have oxygen supplied all the time.But as well as that, you need to have carbon dioxide scrubbed all the time too,as we can't survive long if levels build up to as high as 1% of the atmosphere, which doesn't take long in a small enclosed habitat. Many other noxious gases like hydrogen sulfide and sulfur dioxide will build up in the habitat too, like "sick building syndrome" to the nth degree. You can't just open a few windows to air your house, so all those have to be scrubbed. Then you have to filter out potentially harmful build ups of microbes too.

How are you going to pay for all that technology, which also is likely to need a fair bit of servicing? Then as well as that, you need solar power or some source of power to run all the equipment. You need batteries, or nuclear power to survive dust storms that blot out the sun. Then you have to have a habitat that can hold in the atmosphere at a pressure of ten tons per square meter outwards pressure. You also need radiation shielding meters thick covering it to protect from cosmic radiation and solar storms. How much does that kind of a "house" cost to build? You can't build it on Mars, except the shielding. All the rest has to be imported from Earth. Also if it is anything like the ISS, your habitat, which is now your only home, has a finite lifetime. After a few decades you will need to import a new "house" to replace the old one which is now aged so much in the harsh space environment, surrounded by vacuum, huge temperature changes every day, that it is no longer worth repairing.

Nothing grows there. You are suddenly in the middle of a desert, with no water, maybe ice but it has to be melted to be used, a few rocks, and most difficult of all, no air to breathe. You never needed to think about how to get air to breathe when you were back on Earth, and no Earth colonist has ever had to give this even a moment of thought. Now it is your topmost priority, your main pre-occupation. Without a pressurized spacesuit you can't even go outside to repair your habitat, so the spacesuit is vital too. The average temperatures are the same as Antarctica, but it's much worse than that sounds, because the temperature swings are so extreme between day and night. It's so cold that carbon dioxide freezes out as dry ice / water ice frosts in the morning for 100 days of the two Earth year long Mars year even in the tropics. This also puts extreme thermal stress on your habitat. You get dust storms every two years which sometimes blot out the sun completely for weeks on end. If you somehow could take one of the coldest driest deserts on Earth, the Atacama desert, and elevate it to a height of 30 kilometers on Earth, you'd have the same atmospheric pressure as the lowest points on the Mars surface, that is still far more habitable than Mars (still a little oxygen in the atmosphere, more sunlight, no dust storms, easy access from Earth, ozone layer and magnetosphere to protect you somewhat), The top of Mount Everest (at 8.848 km above sea level) is far far more hospitable than Mars.

And how do you pay for it? Elon Musk's idea is that the colonists pay for all of this through inventing things.

The ISS is funded at around a billion dollars per year per astronaut for NASA’s contribution, just the operating costs (three billion dollars a year - and normally has three people in it at any time, sometimes goes up to six but rarely more than that in recent times, see how many astronauts are in space right now). The ISS itself cost around $150 billion including the cost of building it, and that's in LEO (see also my "Is the ISS the most expensive single item ever built"). We don’t yet have any example of anyone who has lived anywhere in space without immense costs like that, and it's not at all clear how they could.

Suppose Elon Musk manages an order of magnitude reduction in costs, to the extent that it costs a tenth of the current costs to LEO to get the same cargo all the way to Mars. That seems wildly optimistic to me, erring on the optimistic side. But suppose he achieves this, then it is still going to be of the order of a hundred million dollars per astronaut per year to support them and five billion dollars per astronaut to set up a habitat to last for a few decades on Mars. Yes, perhaps you can use plants to grow food, and generate your oxygen and filter and purify the air. This is not yet tested in space. Yes, it is likely to make a huge difference for long duration missions, but it only becomes economic to do so for missions of over two years. For details, see my Sending humans to Mars for flyby or orbital missions - comparison of biologically closed systems with ISS type mechanical recycling (also relevant to long duration lunar missions). Also, it does nothing to pay for spacesuits or equipment repair. The million dollars cost for the spacesuit doesn't include transport costs to get it to your habitat.

Perhaps, as he says, Mars would have a labour shortage with jobs in short supply - but what job is going to pay you an upfront cost of billions of dollars for your habitats, millions of dollars for spacesuits and other essential equipment on Mars and on going costs of perhaps (optimistically) a hundred million dollars a year per astronaut for their maintenance and repair and replacements when they wear out? What exports will Mars have that they use to pay for all those imports?

Well, Elon Musk shares Robert Zubrin's ideas that the Martian colonists in such tough situations will be so inventive they will invent a stream of inventions that transform life on Earth and earn them huge amounts of money to pay for their colony. I suppose it is understandable that he'd find this idea compelling ,considering his own inventiveness. It's based on analogies with the technological inventiveness of early settlers in the US. Again this seems bordering on fantasy to me. Surely it will be mainly the other direction, that with all their complex technology, which they will need just to survive at all, they will depend tremendously on the many discoveries we made on Earth?

Even Elon Musk with all his inventiveness and business nous would not be able to pay to support everyone in a Mars colony, and he hasn't suggested that he hopes to do so. They are on their own. If they have to pay for everything themselves, it would seem they all have to be as successful as Elon Musk and indeed more so, just to have a chance of surviving there.

Earth best for a backup - maybe with a small knowledge and seeds library on the Moon with caretakers

Also Elon Musk and Stephen Hawking motivate space colonization by the idea that we have to make a "backup" of Earth. There are plenty of other reasons to go into space, but this one just doesn't stack up in my view. Let me explain why.

We live in a quiet suburb of our galaxy around a long lived stable star. The Earth is at no serious risk from an asteroid impact large enough to make an adaptable omnivore such as ourselves extinct with only minimal knowledge of technology. It's true that some think that humans in sub-saharan Africa were reduced to 2,000 around 70,000 years ago. But that was in a world with Neanderthals and Denisovians in other continents, and though homo sapiens may or may not have had clothing, it was long before humans had thought of growing crops or keeping animals, which didn’t happen until 10,000 BC onwards: Neolithic Revolution .

Modern humans are the least at risk of just about any of our species of going extinct. Even after a dinosaur extinction type event, even if there were only a few percent of species left, there would be something for at least millions of humans to eat and cultivate. Perhaps we'd get reduced to eating shellfish, the staple of many early hominids. Or seeds, roots, fish, animals, birds, fruit, and nuts. We are lucky to be omnivores able to eat almost anything. Also, unlike dinosaurs we can detect and deflect asteroids, evacuate impact zones, and prepare supplies and seed banks. Indeed we have had many surveys detecting asteroids already with a great deal of success, and, we already know all the ten kilometer diameter asteroids that could potentially hit us. None come anywhere close for centuries. At present only one in 147 of the objects that fly past Earth are comets, and that makes it a 1 in 147 million chance of a 10 km comet impact per century, a probability so low we can ignore it.

There is no chance at all of Earth getting hit by the larger asteroids like the ones that made the Hellas basin on Mars, and the larger craters on our Moon, as those impacts date back to the late heavy bombardment soon after the formation of the Moon. There have been no impacts that large in the cratering record of Mars, the Moon, Earth, Mercury or what we have of the history of Venus, for well over three billion years. Apparently Jupiter protects us from the largest asteroids and comets (and models confirm this).

It also doesn't make a lot of sense to go to Mars to create a habitat that will be safe from asteroid impacts. After all, Mars gets ten times the influx of asteroids over Earth, it also has no atmosphere to protect against the smaller ones that cause fireballs on Earth but could easily destroy a habitat on Mars, and if your habitat is damaged, there is no air to breathe outside of it. If you are worried about indirect effects of asteroid impacts, it's true that an asteroid impact on Mars would have mainly local effects, but that's because the whole planet is uninhabitable without very advanced technology. What asteroid impact could so devastate Earth as to make it as uninhabitable as Mars? None in the present day solar system.

We can't predict when a star will go supernova exactly, but the only stars that can go supernova are ones that are at a particular stage in their life, and they have to be massive too, for Type II supernovae, and for type Ia it needs a white dwarf companion. Our sun can't go supernova at all, it's too light. There are no nearby supernova candidates of either type

The Type Ia are the hardest to spot and it used to be that we didn't know if there were any of those close enough to be hazardous. But now we do know, through much more detailed star surveys. The nearest Type Ia candidate is IK Pegasi which at 150 light years away.That's far too far away to harm us. It’s moving away from us and the scientists think it won’t go supernova for several million years, by which time it will be perhaps 500 light years away. It would need to be within 30 light years to be harmful. The nearest type II candidates, such as Betelgeuse, are thousands of light years away and though they would become bright stars in our sky, briefly, they are of no conceivable threat to Earth.

We can also identify gamma ray burst candidates and the only candidate that seems likely is WR104, 8,000 light years away, and we now know that it is tilted at an angle of 30° - 40° (possibly as much as 45°) which would mean it would miss. See WR 104 Won't Kill Us After All - Universe Today. This is the only “Wolf Rayat star” we know of which we are facing more or less along its axis, so we are okay for those too for at least of the order of thousands of years.

The sun will eventually get too hot for humans, but this is so far into the future, that humans could evolve once more from the smallest microscopic multicellular lifeforms before it happens. There may be technological solutions, such as to block out some of the excess sunlight with orbiting sheets of mylar or similar in space, or even techniques proposed for moving the Earth slowly outwards in its orbit. And if not, well there is plenty of time. Indeed ideas to partially "terraform Mars" now, if they worked, would only be temporary, creating atmospheres that would be stripped by solar storms over those long timescales of hundreds of millions of years. So to attempt to terraform Mars now may well make it less useful for that distant future when humans might perhaps really need Mars. Also would they be humans by then? Maybe they have different environmental requirements from us.

As for issues due to our own technology, then they are as likely to arise in space colonies, as anywhere, the most technological settlements ever envisioned. When it comes to science fiction scenarios, then it's as easy to come up with a story in which space colonists endanger Earth as one in which they help Earth. Also a global nuclear war would not make Earth uninhabitable either. It would cause massive hardship of course, but it would tip Earth into a nuclear autumn, not a nuclear winter. That's based on the revised models developed after the spreading smoke from the Kuwaiti fires during the gulf war proved that the original models predicting a nuclear winter were false. The entire southern hemisphere is a nuclear free zone, and in the northern hemisphere, most of the radiation effects are over especially after airbursts, within weeks. It does leave hot spots (especially from ground strikes) that we would need to keep clear of for longer periods of time of decades or more, but it doesn't make everywhere so radioactive as to be uninhabitable. Of course we have to avoid global nuclear war, for many reasons, but this doesn't risk making us extinct.

Climate change can't make Earth uninhabitable either. The research so far says that we don't risk Earth tipping into a hot Venus, even if we follow business as usual and burn all the extractable hydrocarbons. We would need to burn at least ten times the amount available. The next step down is the moist hot greenhouse with an average surface temperature of 57 °C too hot for human habitability, and again, it seems we don't have enough hydrocarbons to burn to get to that state. However, if we followed "business as usual", it does seem possible that we could end up with a world with the wet bulb temperatures above 35 °C over much of the Earth, leaving only the higher latitudes habitable for humans without technology. That would happen at a temperature increase of 7 - 10 °C above pre-industrial. But even that would not make us extinct.

Anyway, we aren't following "business as usual". Even with the US out of the Paris agreement, still many in the US are doing their bit to reduce greenhouse gas emissions. Outside the US, then the commitment is stronger than ever to do something about it. The pledges to the Paris agreement so far should keep temperatures within 3.4 °C by 2100. That's plenty of time for many new administrations in the US. We can move to a carbon neutral world by 2100, hopefully well before then. For more about this, with cites and quotations from papers and experts, see my Hawking Says Trump Could Tip Earth To Hot Venus Climate - Is It True? What Can Earth's Climate Tip To?

I think it does make sense to do a backup of knowledge and data, even a seed bank in space, and the Moon is the obvious place for that. It's passively cooled, ideal for a seed bank, stable low temperatures, and geologically stable. For the idea of a backup of knowledge, seed, and a small settlement of caretakers on the Moon see Backup on the Moon - seed banks, libraries, and a small colony, also in my Case for Moon First book.

As for humans, the best place to backup humans is surely on Earth. We need to protect and cherish our Earth as the only place in our solar system where humans can survive without advanced technology. So, I don't see this as a good motivation for sending humans into space. Rather, it's a motivation for setting up backups on Earth, if you think this is a serious risk, and also using space technology to protect Earth and move industry from Earth into space and such like. For more about all this, with details and cites, see my: Earth is the best place for a backup. in my Case for Moon First book.

Why the Moon could have a commercial case

The Moon is a bit different from Mars. It might actually have a commercial case. Though life would be very expensive there also, as for Mars, there is more chance of an income stream to pay for it. Authors like Paul Spudis etc pay a lot of attention to the commercial side of things in their plans. The big advantage the Moon has over Mars is its nearness to Earth, making exports far easier and tourism possible. It's not quite a "day trip" to get there with current rocket technology, but you could visit it, and be back within a week. Also there are various ideas that could reduce costs of transport from the Moon to Earth hugely, which wouldn't work for Mars. It's only two days travel to get there, also, with easy access any time of the year (not just every two years). It's also far far easier to get back in an emergency, which makes it much safer for humans. It's also far easier to leave the surface than for Mars, reducing export costs. Only half of the loaded (wet) weight of the rocket has to be fuel to export materials to lunar orbit. Also there's the possibility of ice at its poles, combined with solar power available 24/7 year round as a source of abundant power. Paul Spudis and others believe it will be economic to supply this ice as water and rocket fuel to LEO, outcompeting water sent from Earth. Water is vital to humans in space and very expensive to send to orbit from Earth, at present.

So, the "Moon firsters", though they do tend to be rather optimistic at times about the commercial value of the Moon, also tend to be far more realistic than the "Mars firsters" in my experience . They are not so involved in these ideas that seem to belong more in science fiction and fantasy than in real life, of just setting up home as if you could build a log cabin on Mars and live off the land. You may be interested in my Is there a fortune to be made on Mars, the Moon or anywhere else in space? in my "MOON FIRST Why Humans on Mars Right Now Are Bad for Science" (it was also featured as an article in Forbes magazine). It compares the economic case for Mars and for the Moon.

In “We Need to Stop Talking About Space as a ‘Frontier’.” by Lisa Messeri she suggested that language helps and that perhaps we need to stop thinking about space as a "Frontier" with its unfortunate connotations of damage to the environment of North America, and the destruction of American Indian peoples and cultures.

"Comparing outer space to the frontier is so prevalent that it’s sometimes hard to remember that it is a metaphor, not an accurate portrayal of what lies beyond Earth. The commercial space industry prides itself on newness and novelty, and yet the reliance on the same old metaphor both limits the imagination of humans in space and glosses over the social and historical problems of imagining a frontier that is empty and beckoning."

..." But mobilizations of the frontier metaphor from Turner to today don’t just ignore the historical reality of war, disease, and environmental destruction. The Americanness of the frontier metaphor is also at odds with the need for international cooperation in the new era of space exploration. While the frontier might inspire Tumlinson and his fellow American baby boomers, does it have salience more broadly? As we try and move from a model of space competition to space cooperation, does the frontier, which necessarily pits “us” against “them,” undermine the peaceful expansion many imagine?"

Steven Lyle Jordan put it rather well, I thought, in his blog post: Space is not a frontier, commenting on her article - why not refer to space as our "environment" rather than our frontier?

"There is lots of room for expansion in the Environment… but absolutely no guarantee that we can, in fact, expand beyond this oasis and thrive. Most of the Environment is downright hostile to us. Intelligence might allow us to figure out a way… but the uncontrolled elements of that vast Environment may eventually doom us to non-existence anyway. Once more… we have no way to know. But there’s nothing stopping us from trying; only the incredible difficulty and unlikelihood of succeeding."

"The word “environment” embodies the knowledge of science and nature, the desire to experience it and learn what is learnable… but not to desecrate, strip-mine or destroy it for personal gain. If that’s not a noble-enough reason to explore new environments, I don’t know what is."

"This way of thinking about space probably gives us the best and most accurate image of the universe and our place in it. It will also serve us best in imagining our future activities in space: How we should treat the vast Environment; and how we should act when or if we discover others out in the Environment. (It probably wouldn’t have hurt if we’d considered Earth this way, instead of seeing it as empty spaces to exploit. Just saying.)"

So, this focus on colonization for its own sake really narrows our vision, I think. Everything we do becomes a step on the way to the aim of eventually attempting to colonize a place with freezing temperatures, frequent dust storms, water only in the form of ice, and a near vacuum for an "atmosphere". I don't think that's even going to work as a long term inspiration for space exploration, once the reality of the situation kicks in. Well that's how I see it at least.

So, I don’t see us colonizing any of these places for their own sake, any time soon. Rather there has to be some other reason to be there. The Moon is the most likely place to provide such a reason because it is so close to Earth and also has so little gravity, so with a low escape velocity. Books on the Moon settlement have many chapters about the economic value of the Moon, unlike books on Mars that skim over this in a single chapter typically with rather sketchy ideas about how it just possibly might be economically worth while if .... Also, the lunar lava tube caves could potentially give huge low maintenance enclosed spaces. If we build closed system habitats like that, eventually, perhaps they could even be as economic to live in as Earth through economies of scale and because the Moon has no weather to speak of and is tectonically very quiet. But that's a fair way into the future.

Mars could provide such a reason too, for scientific study, search for present day life or past life, and its two moons also. But contaminating Mars with Earth life could destroy much of the most interesting motivation for studying it. Its two moons don't have those same issues. so they may be a better starting point for a scientifically orientated advance human base, rather like the ones in Antarctica.

Lockheed Martin looked into Phobos and Deimos as intermediate destinations for their "Stepping Stones to Mars" and they remain destinations of great scientific interest, both in their own right, and as a base for studying Mars from orbit. Deimos also may be a valuable resource too, as it is a type of meteorite that often has water, though this is not yet confirmed for Deimos. They are tiny worlds so we also need to consider the potential of negative scientific impact of humans building a base on them, the problem of trash, and rocket exhausts as for the Moon, but hopefully that can be worked around. Perhaps we might eventually have settlements there of some sort too. I cover this in detail in I cover this in Interesting flyby and orbital missions for Mars (below) .

Anyway I argue strongly that Moon is the obvious place to start our experiments in sending humans to somewhere else other than Earth, for safety reasons and nearness to Earth too as well as all the other reasons.

We could eventually build dome cities and settlements in lava tube caves, and Stanford Torus type settlements. For those to work we need a way to build large structures that cover a lot of habitable space and are low maintenance. For instance if a lunar lava tube cave is indeed as large inside as an O'Neil cylinder, then it might be possible to turn it into a reasonably strong and maintenance free habitat. If we can also manage closed system recycling, and solve the problem of provide light in the lunar night for crops - perhaps we could have a habitable volume in space that is actually somewhat more habitable than Earth in some ways - no hurricanes, earthquakes, volcanoes, deep below the surface protected from most meteorite impacts also. I could imagine that actually working some decades or centuries into the future though there are rather a lot of "if"s there to fill in before we can get there.

Detail of lunar colony showing a greenhouse inside a base. Detail from image from NASA, 1989. This was for the Lunar Oasis proposal for a ten year program to establish a self sufficient science outpost on the Moon to act as a test bed for space settlements. The larger the habitat, the less surface area for the enclosed volume, so - perhaps you also have less maintenance to do per inhabitant. If eventually we can make habitats that are kilometers in scale, perhaps they can be so easy to maintain per inhabitant that it is as easy to live there as on Earth? Especially with the advantages of the Moon of greater tectonic stability, and no storms, volcanoes, earthquakes etc.

I cover this topic in Maintenance for habitats in free space and city domes, and Greenhouse construction - comparison of the Moon and Mars in Case for Moon First, and my An astronaut gardener on the Moon in Why Humans on Mars First are Bad for Science.

Why the Moon is best for humans right now

I'm an unashamed Moon firster :). This is based on careful arguments however and I have no hesitation saying that the Moon simply is "the best" without qualification as my personal view on this. See what you think about my reasons. Here are a few of them:

I cover many of these points, and many more, in my "Case for Moon First". For instance, here is a link to the section on lunar and Mars dust.

I do think there are good reasons to have humans in orbit around Mars once we can do it safely and for less than enormous expense, for telerobotic exploration of Mars, and for exploration of its two Moons. Also there is a possible commercial case for habitats in Mars orbit or on its moons. Perhaps there is some chance of paying for them by exports of water from Deimos, if it does have reserves of water there. However, I think that's less likely to be competitive for supply to the Earth Moon system, if the Moon has easily extracted ice reserves.

Before I wrote "Case for Moon First", I had totally bought into this idea that Mars was best for humans, and the Moon was a poor second cousin for humans to live. This gets repeated so much, you come to believe it just through repetition. The news stories and articles can seem so convincing, and it also seems to make sense as Mars looks rather more Earth-like in the white balanced photographs. Also Robert Zubrin puts what at first seems a compelling case in his Case for Mars. I argued that we shouldn't send humans to the Mars surface yet for planetary protection reasons but thought I was facing an uphill struggle in conversations with those who advocate colonization as rapidly as possible, given what seemed to be obvious advantages of Mars over the Moon for humans.

However, as I wrote the book, and researched into this topic in detail, and read the books written by "Moon firsters" carefully, I realized I'd got it all wrong. The Mars colonization enthusiasts rarely try to make a direct comparison with the Moon. When you do that, i one point after the other the Moon wins just about every time in a comparison with Mars as a place for humans to live. But you have to look at it with a lunar rather than a Mars perspective. Of course solutions designed for Mars probably won't all work "as is" on the Moon. For instance you can't use hydrogen feed stock to make methane fuel on the Moon. But then, you have water ice probably at the poles, useful for fuel. You also have abundant sunlight, and much easier transport back and forth from the Earth. Also there is no need to make fuel "in situ" on the Moon, just to get back to Earth, as you would have to do on Mars. The Apollo astronauts managed this rather easily back in the 1960s. This idea of generating fuel from hydrogen feedstock in a carbon dioxide atmosphere is an ingenious Mars motivated solution to a problem you don't really have on the Moon. It's the same for other things. The apparent advantages of Mars all seem to just melt away when you look at the Moon in its own terms.

Further into the future, if we have millions in space - can we be one of the"wise ET's"?

More generally, looking further into the future, habitats on the Moon would probably be just a first step. Suppose we do find a way to have millions living in space - I argue in my Moon First books that settlement in space has the potential to be hugely positive but it could also be hugely negative. It depends very much how it is done, and it may well turn out to be a good thing that we are likely to have comparatively few humans in space to start with.

Though I'm keen on humans in space, I'm no advocate for sending large numbers of us there as fast as possible (except as explorers and tourists). After all think what the consequences would be if we had the likes of ISIS and North Korea as space colonies? North Korea claim to have space aspirations and have put a satellite into orbit. Right now their space program is not really credible, and most consider it to be a cover for ICBM research. But in the future with improved technology world wide and millions in space, then perhaps such governments will have the ability to set up their own colonies. Or, it may not be an extremist state or group on Earth that gets into space, it could just be that in any group of millions of people you may start to get some with strange and even destructive and violent ideologies. If this happens in space then they automatically also have space technology far advanced over ICBMs. We may get many peaceful, positive, vibrant "feel good" ideologies in space, but others might turn out to be as extreme as anything we have on Earth. If that happens, then how can it last for long, with the habitats in space so fragile to any violent action, even lobbing a rock at them at a few kilometers per second.

Once we have hundreds of thousands, and millions of people in space, if we do succeed in this, we can't restrict space colonization to the "good guys or gals" whoever we think those would be.

Longer term the difference between positive and negative future outcomes may become even more stark, if you start to think of a "civilization" like ours spreading to fill the galaxy, with the ability to modify their own genes, make self replicating machines, cyborgs etc, and the most rash and aggressive able to spread through the galaxy most quickly. How can we make sure that such a future is reasonably peaceful? How can we stop it from turning into never ending waves of destruction in a future galaxy filled with remote cousins many times removed, with bizarre ideas and unfathomable technology approaching at close to light speed from thousands of light years away?

This might be a significant and important future challenge that we have to find a way through. Perhaps all Extra Terrestrial Intelligences that develop space travel encounter these issues eventually.

Actually I'm optimistic there, especially if we are not the first extra terrestrial space capable species in our galaxy. If there have been others like us before, then I think that the Fermi paradox "where are they all" can give us hope, that they have found such a solution. Otherwise the chaos in our galaxy from battling ETIs would be plain to view, and it is hard to see how it could end as it seems that they could never go extinct. Indeed nowhere would be untouched. Once an aggressively expanding civilization has reached its nearest stars, it will fill the galaxy within a million years. And I mean totally fill it, in a population explosion. They'd have taken over Earth long ago, as they would need to use all the resources they could find to cope with their constant wars and exponentially increasing populations. How could our Earth and solar system remain untouched? I think this shows that civilizations mature and that our future civilization at some point will no longer be subject to the extremes of violence that we encounter today.

Imagine if you gave the nineteenth century people present day technology. How long would the blue whales last, how much chaos would they cause to their environment, what kinds of wars would they fight with modern weapons, including the capability for chemical and biological warfare? Theirs was a simpler time and there are many things nineteenth century humans took for granted, that would be unacceptable today. Not just that, their ideas and habits would also cause utter chaos in the modern world with our technology. So we have surely changed and learnt a lot already, slow though the progress seems on a year to year basis. So where is this trend headed, a couple of centuries into our future?

We can be one of the wise ETs. I think, there is evidence that we may be wiser than the most reckless ETs possible, which might destroy themselves in space wars pretty much as soon as they begin on spaceflight. Carl Sagan refers to this as "the intrinsic instability of societies devoted to an aggressive galactic imperialism".

Though we have stumbled a lot, we have made many good decisions, such as dealing with the problems of DDT and CFC's, human rights (a lot of progress though much still to do), preventing chemical and biological warfare (even in the almost all out conflicts of WWII neither side used the chemical weapons of WWI such as mustard gas, even though they stockpiled them and issued their civilians with gas masks). The Geneva protocol banning many forms of biological and chemical warfare came into force on 8th February 1928. I know there have been exceptions but most wars don’t use them. Imagine how different a world like ours would be, if it was inhabited by ETIs who were so aggressive and short sighted in their thinking, and so unable to negotiate that they couldn't agree to such a protocol at all? All their wars would use those kinds of weapons and probably worse ones too.

Similarly, we've developed nuclear weapons, and yet, for decades we haven't used them. Indeed Carl Sagan suggests that maybe weapons of mass destruction are the deciding factor here. After talking about our own efforts to deal with nuclear bombs he then goes on:

"If every civilization that invents weapons of mass destruction must deal with comparable problems, then we have an additional principle of universal applicability. Weapons of mass destruction force upon every emerging society a behavioural discontinuity: if they are not aggressive they probably would not have developed such weapons; if they do not quickly learn how to control that aggression they rapidly self destruct. Those civilizations devoted to territoriality and aggression and violent settlement of disputes do not long survive after the development of apocalyptic weapons. Long before they are able to make any significant colonization of the Milky Way, they are gone from the galactic stage. Civilizations that do not self-destruct are pre-adapted to live with other groups in mutual respect."

He goes on to say that because we have only just reached this stage then this future scenario of mutual respect may seem unlikely because of our short term perspective. He suggests that the required changes may take a thousand years or more, for us to reach maturity as a species. From Carl Sagan's "The Solipsist approach to Extraterrestrial Intelligence",

Actually, I don't think that nuclear weapons by themselves would be enough, at least not for creatures similar to us. Even after an all out exchange of all of our nuclear weapons, in the cold war, it would not have made Earth uninhabitable or even significantly radioactive except locally. To do that we'd have had to use Cobalt 60 bombs, with deliberate aim to make the surface of Earth radioactive and uninhabitable, and lots of them, and even then probably a few humans would have survived. Same also with chemical and biological weapons, easy to target large numbers of people, not so easy to get everyone on Earth! And nobody was crazy enough to do that in our civilization. For details see my Doomsday Debunked - Nibiru Is Nuts - What About Nuclear War, Asteroid Impacts, Runaway Warming,... and on the resilience of human beings, and indeed our basic understanding of science, to mass destruction, see my Why Resilient Humans Would Survive Giant Asteroid Impact - Even With Over 90% Of Species Extinct.

So I think yes ETIs in a similar position to us, could have destroyed their civilization already through weapons of mass destruction, but to have seriously set back their civilization by more than a few decades, an ET would have to be far more aggressive than we were. They'd need to be of such a character as to fanatically keep on using weapons of mass destruction of all sorts, when they can see that Earth's population has been decimated and all hope is lost. Otherwise they'd keep knocking themselves back perhaps but restore their civilization within a few decades or centuries, a bit like the story of John Wyndhams "The Chrysalids". That would be just a blip on a geological timescale.

However in the future with space habitats, so fragile, I can see that depending how quickly our society adapts, we could easily keep destroying our space capabilities every time we start to have millions of us in space, and indeed create clouds of debris around our Earth of all the exploded satellites if we had wars in space. Perhaps an aggressive immature culture coupled with space technology could eventually make itself extinct that way. At any rate I think our current prospect is quite hopeful, that we aren't headed for a future like that.

We’ve prevented starvation with the often forgotten Green Revolution between the 1930s and the 1960s, stopped the birds eggs thinning scenario of Silent Spring, the thinning of the ozone layer, stopped nearly all whale hunting, done lots of work to preserve species and environments etc, developed many international agreements that stop the worst of biological and chemical warfare and been able to prevent nuclear warfare for many decades. If you compare our present world with what it could have been without all those initiatives - I think it gives room for optimism for the future too. And I think we’ve made an excellent start on peaceful use of space with the Outer Space Treaty.

Although it’s frustrating that we don’t have warp drives or even the Star Trek “Impulse drive”, and don't yet have easy low cost ways to build habitats in space, I actually think it helps, that space is so hostile. Hopefully by the time we figure out how to live sustainably in space habitats, we will also have figured out how to do it peacefully, or reasonably so. With competition of course, but more like the Olympic Games than WWIII. Hopefully we can become more forward looking as we continue to colonize space. Perhaps the increased resources from space can help us to become more peaceful if we can handle it right.

If so we might well eventually have a chance to explore even our entire galaxy peacefully, and without harmful consequences to ourselves and other intelligent species that may exist in our galaxy. And if we meet ETs, the ones that still retain space technology, then they also I think would be ones that have figured out how to explore the galaxy in a similarly peaceful way.

Surely amongst the many ETIs (if we do have neighbours) there will be those that are competitive, as with the Olympic games, innovative, eccentric, genius. But we need to find a way to embrace the good sides of those qualities, that which also works in the universe we live in without being a nuisance to ourselves and the other species we share our galaxy and universe with. Let’s be one of the civilizations in our galaxy and universe that flowers like a beautiful flower.
- Amazonian giant water lily.

I cover all this in detail in my Case for Moon First in these sections:

I suggest also that settlement can have hugely positive consequences if done well. It can help protect and sustain Earth, move heavy industry into space, and provide power and resources that may help us in the future. It can also help support our explorations and discovery throughout the solar system. Eventually it can also open up the Moon to tourism, giving the opportunity for many people to see the Earth from space and get a different perspective by visiting the Moon.

I suggest that human exploration without settlement can also be either hugely positive or hugely harmful. We don't have to motivate it by settlement. Nobody is interested in settlement of Antarctica, yet there's a lot of interest in the continent. Let's just go into space and then find out by doing, what it is that humans want to do in space, what's worthwhile for us, and what works.

This book is about the especial case of the impact of in situ human exploration of the solar system on the scientific search for life. Mars is the one place in the inner solar system most vulnerable to Earth microbes. The same issues also apply for Jupiter's Europa and Saturn's Enceladus with their deep ice covered oceans connected to the surface, so I will cover those as well, also the Venus clouds, and some more exotic places we could search for life, such as Titan, Io, and Triton even (some of which perhaps have no planetary protection issues, at least in the forward direction, because they are so different Earth life can't survive there).

The main focus is on Mars, as there are no plans to send humans to any of those other places in the near future. However, I also cover Europa and Enceladus in some detail, because though there are no near future plans for human visits, we may get our first Europa lander in the 2020s, and eventually scientists are keen to drill into their deep subsurface oceans and to send submarines there. We have never done close up exploration of an environment like that, even robotically. An ocean world with an icy crust and probable geysers and communication between the subsurface ocean and the surface provides particularly acute difficulties for planetary protection. Do we know enough to "touch" Europa so closely, even remotely in this way, via a robot?

Humans can probably help a lot by being close at hand, for in situ exploration, because of our ability for fast and accurate on the spot decision making. But we have to be careful to look at the downsides as well as the upsides of humans "on location" in the solar system. We need to understand what could go wrong, as well as right, to decide how best to plan our explorations. We need to continue to take care with our robots as well, to make sure we understand the implications and possible effects of them "on location" as well. By doing this we can make best use of both humans and robots, and preserve the science value and interest of the places we explore.

So let's get on to the book. What are the possible consequences and ramifications if humans touch Mars? Or Europa, or Enceladus, or other places in our solar system?

Sources and structure of this book

Just about everything in this field is available to read on line, so I give my sources through hyperlinks rather than footnotes. These take you straight to high quality press releases, astronomy and science news stories, detailed studies, and technical papers. Nearly all of these are open access ( I search for an open access paper whenever there is a choice). This makes it easy to click through to read more. Some of the techy papers are behind paywalls but there's nearly always a good summary of what they say that is open access, or I can link to a well written popular article about the work.

I also strongly recommend the Google Scholar button for the Chrome browser for anyone who is interested to follow through to the scholarly articles. It lets you highlight the title of any paper cited in the paper you are currently reading and jump straight to it. What's more, it also includes a link to the full article, whenever it is available as an open access paper. Many papers that are behind paywalls when you go to the journal that publishes them are also available separately as open access papers for instance as a preprint on arxiv.org, and in many other online open access depositories of papers. The Google scholar button finds these open access versions of the paper for you automatically.

I've made the sections of this book self contained as far as possible, so that you can just click through to anything that interests you and read it. This requires a small amount of repetition, of a few sentences, for instance you'll find a quick summary of the three main types of photosynthesis several times, first introduced in Surprising distant cousins (oxygen, and sulfur based, and using a similar method to the way our eyes see light). For any large sections, for instance the oxygen rich atmosphere for early Mars , or the instruments designed for situ searches on Mars, I cover the topic in detail once, and provide links to click through elsewhere.

Sometimes I cover the topic in detail several times, but each time with a different slant, so that the sections build up on each other. To take an example, I set out a theme ih Instead of terraforming Mars in a multi-millenium project, why not terraform a lunar cave in a multi-decade project? which I come back to again in Best places to introduce Earth life right now - while continuing with biologically reversible exploration of Mars - and then again, with another slant on the topic, in Plenty of places to experiment with sending life to other places in our solar system - Asteroid belt resources, NEO's, caves on the Moon

Sometimes I go into techy details, or supply calculations. I will indent these. That way, they are easy to skip, while still readily available for those who are interested in such details.

This is an op-ed, not an encyclopedia or literature survey

This book is an opinion piece, not an encyclopedia, or a literature survey. So, along with all the views of the various experts that I cite here, I won't hesitate from expressing strong opinions of my own, which I hope will stimulate discussion.

There are plenty of books and articles that promote the ideas of geologists, chemists, physicists, space engineers, and space colonization advocates, on how to search for life on Mars. I think it may be interesting to have a book that focuses more on the ideas of the astrobiologists on how to do it, as set out in their research papers on the topic. You may be surprised, maybe even shocked, at how these differ from the received wisdom of the NASA road maps and the decadal review. This is especially striking when it comes to their views on sample return and on in situ searches on Mars, as I touched on already in the introduction in Should we return samples from Mars right now?.

According to the astrobiologists, the NASA sample return program is a hugely expensive attempt to return samples that are likely to be as controversial and ambiguous for their discipline as the Mars meteorites they already have. They want to send life detection instruments on rovers to explore in situ. These have been in development for decades now, designed to overcome the issues that beset the Viking instruments. These have never yet been sent to Mars.

As Chris McKay put it, in this interview

If we’re going to search for life, let’s search for life. I’ve been saying this to the point of exhaustion in the Mars community. The geologists win hands down as they are entrenched in the Mars program...

...Right now, as far as I’m concerned, there is no alignment between the Mars strategy and astrobiology.

He is one of the few astrobiologists to see some value in a sample return. But he doesn't motivate it by any idea that it would settle central questions in astrobiology either. He thinks it is worth doing it for the general science community - a low cost mission to Mars to just grab a sample of dirt and return it to Earth. For his reasons: Chris McKay's view - just grab a sample of dirt as a technology demo, and return it - one day on the surface, no rover

Amongst many other differences, the astrobiologists also think it is very important to be able to drill at least several meters below the surface, ideally 10 meters. That's not a top priority for the geologists, for most of their work, it is enough to be able to remove the surface layer on a rock and scratch beneath its surface to see the underlying geology. None of the NASA missions to Mars to date have been able to drill to any depth, and Mars 2020 will be limited in this in the same way as Curiosity (though ESA's ExoMars will be able to drill 2 meters). Their only proposal for a drill is for the Insight Lander and that's a stationary lander and a purely geological mission, with a heat probe that will drill to a depth of 5 meters.

Another difference is that the geologists, space engineers, and human spaceflight advocates, tend to think of the search for life on Mars as a search for visually recognizable microfossils, or even macrofossils. Astrobiologists are acutely aware of how ambiguous and controversial these minute structures can be, after their experiences with Mars meteorites and also as a result of the many debates over attempts to find microfossils in the oldest rocks on Earth. It's the same story for the controversial putative fossils of the most ancient stromatolites on Earth. They are interested in microfossils, yes, but organic microfossils with still recognizable biosignatures, preserved in the extreme cold of the Mars surface environment.

Then as well as that, geologists tend to think that if you detect organics, perhaps with an isotope ratio that suggests it could be life, then that will be enough to decide if a sample is worth returning for the astrobiologist to analyse. Many astrobiologists with experiences of attempting to analyse the organics in Martian meteorites are far more skeptical that this would work, and think the chance that it will help resolve any of the central questions in their field is very low. Most organics on Mars will be from meteorites, comets and other abiotic sources and they can also mimic biological isotope ratios too (see Tissint meteorite - a great example of what we might get in a sample return from Mars). For them, the way ahead is to search for clear unambiguous biosignatures, or multiple simultaneous biosignatures, in situ on Mars. They say that the time do do a sample return is after you identify life on Mars, or after we have exhausted what we can do via in situ biological searches.

So far, we haven't even started on those searches (apart from Viking). And our capabilities have improved vastly since Viking with many low mass sophisticated instruments, including chips that just a decade or two back would have required equipment filling a lab, that could do amazing feats of analysis for biology if sent to Mars.

I think myself that it is high time we let the astrobiologists try out their own ideas for how to search for life, and see what happens. You wouldn't ask an astrobiologist to design a rocket, so why expect space engineers and geologists to be the ones to make all the decisions about how to search for life on Mars?

So, I make no apology for devoting a large part of this book to the views of the astrobiologists. There are plenty of books and articles presenting in great detail the views of the geologists and space colonization advocates and space engineers on how they think we should search for life on Mars. There is so little on the views of astrobiologists on this topic, outside of their specialist papers. So, I hope this may be of interest, especially to those who haven't yet heard much from the astrobiological side of this debate.

As well as writing this as an op ed to stimulate debate, I also wanted to write a book that was lively, and that gets the reader thinking for themselves. As part of that, I have included many sections with fun speculations about things we have no answers to yet. They are clearly labeled as such.

I will be delighted if this book helps to stimulate critical and open ended thinking, and wide ranging debates on the subject of planetary protection. The aim is always to stimulate thought, and never to try to convince others to take on my views, no matter how carefully and thoroughly I argue for them. Please think through the arguments for yourself and come to your own conclusions.

I'd like to share a video by Andrew Maynard, a physicist, who runs the risk innovation lab at ASU. He ran a plenary session for the Astrobiology Science Conference (AbSciCon), April 2017. A couple of quotes. From a member of the audience who didn't give her name, at 38:30 in, who put the astrobiology case rather starkly (emphasis mine):

"I wanted to say about forward contamination being this big issue for astrobiologists, is that a lot of the public desire for space exploration is to send humans and if we send humans anywhere we will contaminate the planet, because we are dead if we are sterile, so there is a huge conflict between trying to understand microbial life anywhere and human exploration in particular."

To which Andrew Maynard responds

"That is a really big value disconnect. Talk about risk. You have got the people that are really dedicated to getting humans on other bodies see people who don't like that idea because of contamination as a risk to their mission. On the other hand if you're interest is in alien life, or the evolution of life or life like systems in other systems, the idea of putting humans on another body is a risk to what you think is incredibly important. So how do we begin to find the common ground between those."

This gives an idea of the sort of issues we are facing here. Andrew Maynard referred to these issues, seventeen minutes into the talk as "Wickedly complex". He went on to say, slightly paraphrasing:

"There are no silver bullets, no easy solutions to moving forward. In the social sciences they call it a "Wicked problem". That's one without definite solutions, and what's more, as you progress to try to implement solutions, this changes the nature of the problem, so you are always chasing after a moving target."

Another good point he makes (51 minutes in):

"The compromised science represents this community, and there is a lot of consensus of what is important here, but it's also important to realize that there is some diversity and perspective here within this room as well, and it's always relevant to understand the value and validity of that diversity of opinion"

That's the spirit within which I wish to present this book. I put forward my own views for discussion, I present other views as clearly as I can understand it. I see it as especially important to recognize the value and validity of our diversity of opinions.

Here is Andrew Maynard's presentation and debate: in full, in the Plenary session on planetary protection.

There are many other videos from the conference here.

Perhaps I can highlight another section of his presentation. At the end (58 minutes in) he says (emphasis mine)

"We are all part of this society that has a stake in what we do in the solar system and beyond. So whether you agree with people or don't, or whether people understand the science or don't, or whether they understand the politics or the policy or the broader conditions and situations, everybody has got some stake, some voice in this. And so it is really important that we actually engage with different members and sectors of the public, different institutions and organizations, if progress is going to be made."

That's really what this book is about. It's an attempt to engage with the public. These are major decisions. They are going to be made in our name in the future, one way or another, and we all have a huge stake in them, whether we realize it or not. Ourselves, or our descendants are potentially going bo be impacted in major ways by the outcomes of these decisions. So by writing this book, in as fun and entertaining a way as I can manage, I hope to help bring these issues to the general public, and to help everyone to engage with them.

Attribution for the research, opinions, and fun speculations

This is the convention I follow in this book:

When it comes to attribution, the sheer number of names would soon become confusing if I attributed everything inline. With a book designed for online reading, I felt that it worked better to do the attributions as linked text rather than footnotes, so I use the links to the sources to credit them. It's easy to click through to find out who did the research. So, sometimes I'll give the author, especially if it has a single author, or someone particularly notable, or I'm discussing their paper or papers at length. Other times I just link to the paper and don't give the authors here.

I want to engage the reader in the process of trying to think through innovative ideas for themselves. So I don't hesitate also to discuss new ideas not in the academic literature. Some examples include the suggestion that Mars astronauts, if we ever have any on the surface, would wear sky blue swimming goggles, in Could we see green on Mars? and the idea of Thistledown light planes with low take-off speeds for the thin atmosphere of Mars. Perhaps these ideas are mentioned somewhere but I haven't found them anywhere yet, and there are several other examples like that. I label the ideas clearly if they are speculative thoughts of this nature based on informal discussions, so there should be no difficulty recognizing when I'm referring to established research, and when I'm referring to speculative thoughts to stimulate the imagination.

I also sometimes do what on wikipedia they would call "synthesis" - develop ideas that follow fairly immediately from ideas in the literature, but in cases where I don't actually know of anyone else who drew those conclusions.

Here are three favourite examples which I use often in this book:

They are natural extrapolations from the literature, so surely someone else has used them before in planetary protection discussions, or so it seems to me. But so far, I haven't yet found a clear example for any of these. Whenever I say something like that, as a synthesis, I will cite the sources that I have used, and I will certainly cite the literature if I know of anyone else who said the same thing. Do say if you know of good cites for any of these, so I can mention and discuss them, thanks!

Some sections of this book are entirely speculation based on just a few sources, such as Would photosynthetic life on Mars be green - or could it be other colours such as red, purple, orange, yellow, brown or black? - I don't know of any articles that speculate on this, but felt it was an interesting topic, relevant to the question of how easily an astronaut could spot photosynthetic life on Mars visually. I based this section on a study of possible colours of photosynthetic life on exoplanets, and various ideas about how the colour of vegetation evolved on Earth and colours of various forms of photosynthetic life here, including the brown seaweeds, and the purplish pink haloarchaea. And then I took off from there in a section I label clearly as synthesis and speculation, for the reader's enjoyment. I do something similar also in the section Could oxygen generating photosynthetic life set up an "anti Gaia" feedback on Mars? I do this in a few other sections also, clearly labeled. I hope these sections may be fun for the reader, and of interest, and stimulate discussion, and who knows, maybe new ideas.

Again do say if you know of any published papers on these topics. And more generally, if you spot any mistake in any of this, however small, please be sure to let me know. Thanks!

About me

Perhaps some of you might like to know a little about me, and how I came to write this book? Also you might find it helpful to know where I'm coming from. My background is that I'm trained as a mathematician, with a good first class degree in maths, from York university, and I have had a long term interest in science, space exploration, astrobiology etc, which dates back to at least since I was a young teenager, before the astronauts first landed on the Moon in the late 1960s. I was inspired to an interest in astronomy since a young child by Patrick Moore amongst others. Though I was already keen on astronomy, loved books about stars, galaxies and planets. I can't remember where the interest came from originally - seems like it is something that always fascinated me.

I also have an M.Hum - on paper it's a masters degree, but the content of the masters was a second undergraduate degree in philosophy, which I completed in two years of study (skipping the first year). I then did postgraduate research for several years at Wolfson College Oxford, with Robin Gandy as my supervisor, in the foundations of mathematics and particularly, the logic of maths (mathematical logic), and the philosophy of maths. That's when I learnt how to do rigorous research into the literature, to check sources, and follow up the sources cited in those sources too. I was keen to follow them all the way through to the original research to check what they say too, and I'd find that even peer reviewed academic sources sometimes get details wrong when they report other people's research.

I think if anything, that is my "trademark approach" as it were, it's to follow through to the sources, and the sources for the sources, and read them carefully, perhaps more so than most do. I also have always been interested in thoroughly understanding the maths, science and physics, at a fundamental level,much more interested in understanding how things work than in memorizing rules and methods. That also was the nature of my research at postgraduate level - rather than ordinary maths research, which I could have done easily, it was research into the foundations of maths, what makes maths "tick" at a fundamental level. I had a special interest in work on developing new axiom systems - and the philosophical motivations for them. That's always been the sort of thing that interests me most of all, getting right down to the basics, to the simplest concepts behind anything.

I've also developed a special interest in planetary protection which I've read up on extensively over the last several years. I've engaged in many discussions about these issues with others expert in exobiology, space engineering etc. I am not a trained in biology or astrobiology, or in space engineering either. But perhaps that has its advantages too? This is such a vast field that nobody could be expert in everything.

The positive side to all this is that I have no personal investment in any of the research, or the planetary protection measures currently being used, or in future planetary protection measures. I am not affiliated to any organization, or university. I don't have any particular research interests to promote or astrobiological life detection instruments that I favour because of my research. Nor do I have a research interest in some particular hypothesis for the origins of life, or anything like that. I am not a spokesperson for the NASA planetary protection office, or the planetary society, or the Mars society or any of these other organizations that have various established priorities and programs and views in the topic area. I don't need to worry that I'm going to embarrass my establishment or team by presenting outspoken views on the topic that diverge from their own established policies and approaches.

All this perhaps gives me an opportunity to come at it from a new perspective, and to say challenging things.The maths has trained me in logical reasoning, analysis, and looking for underlying themes. The philosophical training helps with dealing with the fuzziness of planetary protection issues, and situations with no single right answer, with many valid opinions that can't be proved or disproved by scientific experiments. It's also helped me to learn to present other people's views "as is" without adding personal prejudices to my summary of what they said. My blogging and answers on quora have helped me develop the literary skills to present the material clearly, and in a fun way, to the non specialist general public, using helpful analogies. And my many online debates on the topic have given me experience in the views of the Mars colonization advocates, and to understand some of where they are coming from. I also have the background of a keen interest in space exploration and humans in space myself, from my own side, both from the human exploration programs we've had so far and from a keen interest in science fiction. The Mars colonization enthusiasts are saying things that I myself would have said just a decade or two ago, before I became aware of the many ramifications of the planetary protection issues. Back then, though I was not a space colonization activist (we are not so strong on colonization here in the UK as in the US), their approach certainly had my sympathies, and if the topic came up, then back then, I'd have been cheering the Mars colonization initiatives on with the best of them. I'd have had no idea there could be any planetary protection issues with it.

I was invited to give a presentation on planetary protection to the Icy Moons symposium held in one of the Oxford colleges during their summer break, as a blogger on space exploration and astrobiology. I have written many op-ed type planetary protection posts for my blog on Science20, along with many other topics, often answer questions on this topic on Quora, and have been invited as guest to David Livingston's "The Space Show" several times, usually to talk about planetary protection issues.


Touching Mars

We love to touch things. If you put a sculpture in an art gallery and say "please touch", you can guarantee it, that both children and adults will do so. So it's natural that we want to touch Mars too, and other planets, if we can. But there are plenty of things we can't touch on Earth. Not just sculptures and works of art in art galleries. The Lascaux cave paintings for one,

Photograph of the Lascaux paintings by Prof Saxx.

Many of us would love to touch these paintings, as the original painters did, and feel the texture of the rock they are painted over. But not only are we not permitted to touch them - we have to take care even about going into caves like these at all. The warmth, humidity and carbon dioxide from our breath have taken their toll. Fungi and black mold are attacking the ancient cave paintings.

The purple markings in this photograph show some of the damage we've caused, not directly, but through our breath and in other ways, unintentionally.

The cave was found by four children, out with their dog in the 1940s after a tree blew down exposing a hole in the ground. It was opened to the public immediately after WWII, when the owners of the land, the La Rochefoucauld family, enlarged the entrance, added steps and replaced the sediment that covered the cave floor with concrete. This venture was wildly successful, with 1,500 visitors a day, but the humidity, carbon dioxide and warmth of all the visitors took their toll. This lead to microbes, fungi and black mold colonizing the cave. They eventually closed down the cave and made a copy of it for the visitors, known as Lascaux II, recreated using the same techniques and pigments, as best they could . Even though the original cave has been closed for some time now, to all except occasional specialists, it is already too late to restore it back completely to its original condition.

Scientists' attempts to fix the many issues arising from human visitors have often made things worse,with one more misstep after another. For instance, after a white fungus spread over the floor and up the walls, the scientists took great care to photograph every single painting in detail, to keep track of the damage. It seemed an eminently sensible thing to do. What they didn't realize is that the bright lights they needed for their photographs were damaging the cave paintings, by encouraging the growth of a black mold. This is now a major issue there with black spots spreading over the paintings. For details see the Washington Post article: Debate Over Moldy Cave Art Is a Tale of Human Missteps. In a recent conference, climatologists said that it is possible to restore the original environmental conditions of the cave. But the microbiologists said that it is not possible to restore the pre year 2000 microbial conditions. They say that the only way forward is to just accept that we can't do anything about the new species of microbes we've brought there. Instead, we have to try to find a new equilibrium. Trying to destroy the new invasive microbes will only make things worse.

Will we some day see a similar headline?

"Debate over Moldy Mars is a Tale of Human Missteps?"

Enthusiasts who are keen for humans to land on the Mars surface as soon as possible tend to brush these concerns aside.

"We are going to Mars, that's what humans do. We always push beyond frontiers, whatever they are and wherever they are ".

You ask them, "What about planetary protection from Earth life? They say (I'm paraphrasing):

"Oh, that will get sorted out, the scientists will find a way. We will go there in the 2020s or 2030s."

"We care about protecting Mars and will do whatever they ask us to do. But we must not be stopped or delayed. The scientists just have to find a way to make it work for us. They have to find a way to protect Mars, while at the same time permitting humans to land on the surface."

The idea that scientists might ask them to delay their landing, or not to land on Mars at all is something they may dismiss or even find outrageous, as I've found in many conversations. Yet there are places on Earth where humans can't go. We can't go into the Lascaux caves without great care. When new cave paintings or etchings are discovered nowadays, the cave is immediately closed off to the general public and only a few scientists can visit.

New cave etchings in the Iberian peninsular, as much as 14,500 years old. They were discovered in May 2016, and immediately closed off to the general public to preserve them. They will use technology instead to give us the best view of them possible without directly visiting them.

Then there are other places on Earth where humans can't go at all. Even if you desperately want to visit lake Vostok in Antarctica, kilometers below the surface of the ice, you can't go there. Even if that is the one place you most want to visit; even if you hoped to do it for your entire life; you wouldn't be allowed to go there. Even if you are a billionaire, even if you raised your fortune just to go there, and fund the expedition entirely yourself, none of that is sufficient. You still would not be permitted to go down in a sub and explore it looking for hydrothermal vents and whatever unusual lifeforms live there. If you did that, you'd introduce surface life to the lake. This would confuse the scientific study of a body of water that has been cut off from the surface, perhaps for millions of years. The scientists themselves would dearly love to mount such an expedition to explore this lake, but they haven't yet found a way to do it that preserves its science value and interest, in the way they would like to.

Microbial ethics and biorespect

So, could we harm Mars, as much as we did the paintings in the Lascaux cave, or perhaps more so, just by visiting it? The debate about this often centers around ideas of "microbial rights" and microbial ethics. Of course, nobody would say we have to take account of the rights of an individual microbe. But if we discover life on Mars, in whatever form, does that form of life, that species, not perhaps have the right to evolve undisturbed by interference from humans? Might we even decide to restore early Mars conditions, to help the life there to evolve undisturbed by us?

Arguing along lines like that, some will say that microbial life on another planet deserves a "biorespect" from us independent of whether we can actually make use of it, and independent of whether we find it of value to ourselves. The astrobiologist Charles Cockell has written extensively about this. For instance see what he says about it in "A Microbial Ethics Point of View". Also his Planetary protection—A microbial ethics approach. He says that what matters is whether our actions make the microbes extinct. So contamination may be fine so long as it doesn't lead to extinctions. He writes (these quotes are from Planetary protection—A microbial ethics approach)

"Microorganisms have specific metabolic requirements and quite narrow physiological ranges of pH, temperature, chemical tolerances, etc., which are unlikely to be met on a planetary scale, thus disrupt ecosystems on global scales. More likely, even if a microorganism can grow, its effects will be local.

... The ethical approach suggests that contamination is acceptable. The shedding of microorganisms from human habitats and space suits, microorganisms which themselves originate from unsterilizable humans, is acceptable provided that they do not destroy the integrity of indigenous microbial communities."

He mentions that if there is a globally connected system, of similar habitats then the situation may be different. He uses Europa as an example:

"One potential and important exception might be the Jovian moon, Europa. This moon has a subsurface ocean. The invasion of a globally connected subsurface ocean by an organism that can grow in the oceanic conditions might stand a chance of causing planetary-scale contamination. In the case of Europa, more information is needed on the extent of homogeneity of the ocean conditions to assess the potential environmental impact of contamination."

He goes on to draw a difference between biorespect and planetary protection, by outlining a situation where we might deliberately introduce new microbes in order to protect native life.

"Consider a hypothetical community of Martian microorganisms in an oasis underground. They are localised and running short of nitrogen compounds in the soil. Soon they will go extinct. Planetary protection guidelines would counsel against contamination. However, suppose we have access to another microorganism that can fix nitrogen from the Martian atmosphere. If injected into the oasis in their trillions these microorganisms will provide nitrogen to the indigenous organisms and the latter will not go extinct.

"Furthermore, suppose we can show that the non-indigenous organisms will have no detrimental effect on other organisms or processes. A respect for microbial life might allow us to introduce these organisms. We do not have to do this, but if we respect microorganisms and believe that they have instrumental and intrinsic value, then we could do it as a manifestation of a respect for them."

These papers are from 2005. We now are in the situation where we have found clear (though indirect) evidence of present day liquid water on the Mars surface in the form of transient brines.We don't know yet if they are habitable as they could be too salty or too cold for life, or both, but many scientists think that there may be habitats for life on the surface of Mars. We also have some species that could potentially survive in these habitats just about anywhere on Mars such as the polyextremophile chroococcidiopsis. Also microbes could be transferred in the dust during global dust storms.This makes the situation on Mars rather more like Europa, than like Mars as most scientists envisioned it in before 2008. We are now in a situation where life introduced at one point on Mars may well be irreversible, and can potentially have global effects, whether this happens quickly or over a longer period of decades or longer. This doesn't mean that introduced Earth life will necessarily have a global effect on Mars or adversely impact on Mars microbes if it does. But we need to be very sure, when so much may depend on the outcome.

Also, what if what we have on Mars is some very early form of life, made extinct on Earth by DNA based life? It could be widespread and occurs as many different species, in different habitats,perfectly adapted to Mars over hundreds of millions and billions of years. But the same surely was the case for early life on Earth before DNA, that it was perfectly adapted to Earth, probably in numerous species too. If that is what we have on Mars, is it not possible that modern Earth life could make all those species on Mars extinct quickly, just as it made early pre-DNA or maybe even pre-RNA life extinct on Earth?

I think there is much of interest in this approach of biorespect. And it's an interesting idea, that once we find out more about Mars, and depending what we find, perhaps we may consider doing things such as introducing new lifeforms to protect the native Mars life and do actions to optimize Mars for the micromartians. Though of course this has to be done with a lot of care, even more so than with species introductions on Earth.

Biorespect is not universally accepted by everyone. Robert Zubrin has an argument against the need for biorespect for Mars which he presented in the "Making of" episode 0 of season 1 of the National Geographic series Mars.

"I would say that we have not only the right, but the obligation, to go and establish ourselves on Mars. We are the creatures with all of our flaws that the Earth's biosphere has evolved to allow itself to reach out and establish itself on additional worlds. And we will take this nearly dead world and we will create a fully living world there. And so there'll be new species of birds and fish and plants. And it will be magnificent. No-one will be able to look on it and not feel prouder to be human."

Do you find that convincing? After all we can build habitats in the lava tube caves of the Moon, or Stanford Torus type habitats in space using materials from the asteroid belt. There are plenty of places in space where we can set up habitats with Earth life in them. The smaller asteroids, and the Moon also, are not just "nearly dead" but completely dead. We can fill those with all sorts of creatures and they could be magnificent. Or just let them evolve on Earth. We don't need to go to Mars to create conditions where new forms of Earth life can evolve.

Also, our Earth's biosphere has no plan or foresight, as there is no being there who decided to evolve humans in order to get into space. So we can't ask our biosphere why exactly it evolved us and what we are expected to do in space. It also has no way of getting into space without us. We provide a way of getting into space, yes. But we also at the same time provide our biosphere with the intelligence, foresight, and deep scientific and ethical understanding to guide that exploration. We are the Earth's biosphere's guiding intelligences in space, and that may be one of our main roles. It's us who have this responsibility, and we can't delegate it to any other creatures in our biosphere at present. We are the ones who have to work out what to do with this capability, and what its value is. For instance one of the main reasons for going into space may be to protect Earth from hazards (such as asteroids), or to find resources for use on Earth, or to increase our understanding of ourselves, and of science, biology and the universe, or indeed, as a place for adventure and recreation. We may find many reasons for being in space, indeed have already found many benefits, already, through our satellites in Earth orbit.

It's not automatic that what anything humans find inspiring and want to do is going to work, and is going to be harmless to ourselves or to others. That's like saying that there is no need to protect the Kakapos, flightless parrots in New Zealand because the dogs and cats we introduce that kill them will be magnificent. I think part of the reason for this tension is the idea that Mars is the only possible destination for humans in space. But we have the Moon right on our doorstep, far more interesting than previously expected, and a natural first destination for humans in space. Also, the solar system is vast with many other places of great interest where we can visit, and even set up home, without risk of contaminating them. See Searching for a non confrontational way ahead (below) .

But whatever ones views on these ideas of biorespect, our current motivation for protecting planets and moons in our solar system from Earth life is much more practical.

Planetary Protection, Outer Space Treaty and protecting the science value of Mars

The current policies are not based for biorespect, but rather on the wish to protect the science value of Mars, Europa and so on for us. The legal basis is this phrase in the Outer Space Treaty

"Article IX: ... States Parties to the Treaty shall pursue studies of outer space, including the Moon and other celestial bodies, and conduct exploration of them so as to avoid their harmful contamination and also adverse changes in the environment of the Earth resulting from the introduction of extraterrestrial matter and, where necessary, shall adopt appropriate measures for this purpose."

All space faring states have signed it along with all those with space faring aspirations. Nearly all have taken the additional step of ratifying it (formally indicating its consent to be bound by the treaty, a process that varies according to the country but for most democracies involves passing a bill in parliament). The only states with space faring aspirations who haven't ratified it yet are the United Arab Emirates, Syria and North Korea. It's ratified by 104 states so far in total.

There's no sign that anyone wants to evade these provisions, and indeed even those who haven't ratified the treaty are keen to abide by the provisions. Cassie Conley said recently on the Space Show that she was approached by the UAE who have ideas for a robotic mission to Mars, asking for advice to make sure they comply with the planetary protection provisions of the OST. Also, it already has the status of customary international law because of the consistent and widespread support of its fundamental tenets, and because it is based on a 1963 declaration that was adopted by consensus in the UN National Assembly. This means that it is binding on all states, even those who have neither signed nor ratified it. See page 220 of this paper.

The central phrase here is "harmful contamination". All of our planetary protection policies are based on interpretations of that phrase. The currently widely accepted customary interpretation is that

“any contamination which would result in harm to a state’s experiments or programs is to be avoided”.

This is interpreted in detail by COSPAR, a group of scientists that meet internationally, every two years. The current COSPAR policy is based on this policy statement:

“Although the existence of life elsewhere in the solar system may be unlikely, the conduct of scientific investigations of possible extraterrestrial life forms, precursors, and remnants must not be jeopardized. In addition, the Earth must be protected from the potential hazard posed by extraterrestrial matter carried by a spacecraft returning from another planet or other extraterrestrial sources. Therefore, for certain space-mission/target-planet combinations, controls on organic and biological contamination carried by spacecraft shall be imposed in accordance with directives implementing this policy.”

We may be on the point of making the greatest discovery in biology, perhaps since discovery of evolution, and the helical structure of DNA. It just makes sense not to make this hard for ourselves, or even impossible, by introducing Earth microbes first, to confuse the search.

So, why would microbes confuse the search for life on Mars? For the search for past life, surely we just go and look for fossils, which will be easy to spot? Also, won't present day Mars life be easy to distinguish from Earth life too?

Let's start with the fossils. I get to present day life later in Searching for present day life on Mars in the popular imagination.  I'll go into Zubrin's arguments in detail in What are Zubrin's arguments? But we need some background first to set the scene. So first, let's look a bit closer at this idea that we can find life by searching for fossils on Mars.

Searching for fossils on Mars in the popular imagination

In the popular imagination, this is probably how most would think we would search for life on Mars. Pick up rocks, crack them open, and find fossils.


Painting "20/20 vision" by Pat Rawlings, courtesy of NASA illustrates search for life on Mars. I've used a detail from this painting for the cover of this book.

Original caption: "Did life ever exist on Mars? If so, the best evidence may be fossils preserved in the rocks. Geologists and biologists will one day explore Mars, piecing together the history of the planet and perhaps its ancient life".

After all that is how fossils of earlier lifeforms were first found on Earth. For instance, here is Mary Anning - the Victorian fossil hunter who is described in the popular tongue twister

"She sells sea shells on the sea shore"

Sketch of Mary Anning by De la Beche, gathering fossils. Her hammer is made of wood clad in iron. It's displayed in Lyme Regis’s Philpot Museum. Details from page 78 of this World Heritage assessment of the Dorset and East Devon fossil beds.

More about her in this video:

She used to dig up fossils of ammonites and the squid like belemnites and sell them in her fossil shop at Lyme Regis. And indeed, if we found something like this, the search would probably be over, after all, could anything take this form except through life processes :)


Fossil ammonite from Lyme Regis museum, photo by Kimtextor.
Or if we saw this, well what else could it be but a past lifeform?

Pen and ink drawing of a Plesiosaur by Mary Anning, from 1824. This and more photos and video on the BBC Mary Anning famous people site for children. She also found a two meters long (6.5 foot) skull of an ichthyosaur, and fossil dinosaur faeces and pterodactyls, amongst some of her most notable discoveries there.

Mary Anning's ichthyosaur

One of her pterodactyls

There would be no question about what we had found if we found something like this on Mars

But how likely is it that we find fossils like this on Mars? It was only as habitable as Earth for the first few hundred million years. After that it got more and more hostile for life over much of its surface. So did it ever develop plants or creatures large enough for us to see as fossils?

Oxygen rich atmosphere for early Mars

Well there is at least one thing in favour of this suggestion that Mars could have fossils for us to discover there. Mars may well have had an oxygen rich atmosphere early on, over three billion years ago, long before Earth did. On Earth this happened gradually over hundreds of millions of years with the main spike starting a billion years ago. The Gale Crater deposits are between 3.3 and 3.8 billion years old. While exploring them, Curiosity found manganese oxides. These can only form in highly oxygenated water.

The dark patch on the rock in this photograph, cleared of dust, is made up of manganese oxide. It filled a fracture and was resistant to erosion. The three pale circular dots in a row in the bottom left enlargement are drill holes made by Curiosity to analyse the material. Manganese oxide can only form in highly oxygenated water. So, the early Mars water would have had plenty of oxygen for ammonites and indeed fish and pleisorus. Such oxygen rich water isn't so very astonishing, as after all, the reason Mars is red is because the iron on the planet rusted long ago - but it was unexpected even so.

We don't know how the oxygen got there. It could be the result of ancient Mars microbes, if they developed photosynthesis very early on. After all that is how some of the manganese deposits formed on Earth. But Mars has another way to make large quantities of oxygen over geological timescales. It has no protection from solar storms, because it has almost no magnetic field, not any more anyway. It probably did have one originally but all remains of it now are some patches of magnetic rock. Without protection from a magnetic field, the solar storms can split water vapour in its upper atmosphere. The lighter hydrogen escapes into space making its atmosphere more and more oxygen rich. See How a weird Mars rock may be solid proof of an ancient oxygen atmosphere

Magnetic map of Mars courtesy of NASA / University of California, Berkeley. Earth's magnetic field is more than 40 times stronger than Mars' field. Mars must once have had an internal dynamo like ours to support its magnetic field but it shut down long ago and these patches are the remnants of its ancient magnetism.

With no magnetic field, it has no protection from solar storms and so water vapour in its upper atmosphere gets split into oxygen and hydrogen and the hydrogen escapes. This may be how it developed an oxygen rich atmosphere early on.

Earth has another way to make manganese deposits too. According to one theory, the vast manganese deposits of the Kalahari in Africa, which form 80% of the economic reserves in the world, may have been the result of UV interactions with ice forming hydrogen peroxide during snowball Earth. Hydrogen peroxide freezes just one degree below ordinary water ice. The result is that repeated freezes and meltings can collect and concentrate it in snowball Earth conditions, and that concentrated hydrogen peroxide could lead to manganese oxides forming and precipitating out of the water.

So, though you might think at first that this evidence for oxygen on Mars is pretty good evidence for past life, it's not at all as conclusive as you might think. All we can say is that if Mars did have photosynthetic microbes, they might be responsible for these oxygen rich waters. But they could easily be the result of non life processes.

However, whatever the source of the oxygen, it did make the water very habitable for multicellular life and fish if there were any such around at that time. On Earth the Great Oxygenation Event seems to have triggered evolution of multicellular life, by giving access to a very rich source of energy for their metabolisms. So could it have done the same on Mars, at a much earlier stage?

Even before the Cambrian explosion, the increasing oxygen levels may have encouraged evolution of cells with a nucleus (eukaryotes) which later became the basis for complex multicellular life here on Earth.

This paper for instance suggests that the last common ancestor of the eukaryotes may have lived between 1.855 and 1.677 billion years ago. That's at a time when the oceans were only moderately oxygenated. Most of the varieties (clades) of eukaryotes diverged before 1 billion years ago, probably before 1.2 billion years ago. But the huge diversity we have today within those clades only started 800 million years ago when the oceans started to change to their modern chemical state.

So could the oxygen on Mars have triggered the same thing, or something similar, to happen, two or more billion years earlier than on Earth? Well the oxygen rich oceans are promising, but what about the other conditions at the time?

Mars geological periods - Noachian, Hesperian and Amazonian

There are three main periods of Mars geology, the Noachian period with a thick atmosphere and seas, the Hesperian with extensive flooding and a brief appearance of a second sea, and the Amazonian which continues to the present day with localized flooding, with the atmosphere getting thinner and thinner, and the surface cold and very dry. In a little more detail:

  • The early Noachian and pre-Noachian periods - the entire northern hemisphere was covered in an extensive sea (though possibly often ice covered). The early Noachian, is also when the large basins such as the Hellas basin formed. Many huge asteroids hit Mars. The earlier pre-Noachian is when Mars itself formed, as well as the northern lowlands. Pretty much the entire northern hemisphere is lower in altitude than the mountainous southern hemisphere,which is why the sea filled the northern hemisphere.
  • The Hesperian period of volcanic eruptions and extensive flooding. It also had a second sea, briefly, three billion years ago. This is also when most of the great rift valley of the Valles Marineres formed. It had the beginnings of continental drift, but after part of the surface pulled apart, forming the Valles Marineres, it stopped.
  • The Amazonian period (which continues through to today),- no seas, but occasional massive localized flooding. Mars has had three billion years of cold dry conditions with occasional massive localized flooding with outflow channels. These show that the flooding rushed to fill craters and then soon broke through their walls and drained away in dramatic fast moving flows of water. The climate is quite variable depending on the tilt of Mars, the eccentricity of its orbit, volcanism, and local effects of impacts. At times the atmosphere is thick enough for pure water to be liquid. Right now, with the tilt nearly vertical, almost the same angle as for Earth coincidentally, there is no chance for large areas of water or rivers. The ice everywhere on the surface is either already at boiling point as soon as it melts, or close to it. But this will change in the future as the tilt of the planet's axis changes, also the eccentricity of its orbit.

    If you want to find out more, Emily Lakdawalla has a good overview of these periods, written in 2013. It's reasonably up to date still. The main difference since then is that we now have lots more evidence for its early seas, including clear deltas and a shoreline all the way around the northern lowlands, so this is generally accepted as something that happened.

Curiosity is exploring deposits in Gale Crater that formed during the Hesperian period over three billion years ago. Mars may have had plenty of oxygen back then, but what about higher lifeforms complex and large enough to form fossils?

Simulated oblique view of Gale Crater as it would have looked three billion years ago. This is where Curiosity is exploring right now. Though it was very habitable in the past, that was long before there were any easy to spot fossils on Earth.

Fossil optimists and early life enthusiasts

Did Mars have life capable of forming easily recognizable fossils so early on? There are quite a few obstacles in the way.

  • Did life ever evolve on Mars at all? We don't know.
  • Was it ever abundant? It's quite possible that it evolved around hydrothermal vents, say, and never developed photosynthesis or any other way to spread any further.
  • Did it ever develop to macroscopic life? Most fossils from Earth which are large enough for us to see date back to the last half billion years, out of over four billion years of evolution. The earlier fossils of stromatolites and microbial mats are similar to the effects of non life processes and would be hard to prove to be life on Mars.
  • If it did develop macroscopic life, did it do it so early, more than two and a half billion years before it did so on Earth? The oxygen rich conditions would seem to make it possible that it did. But that's a long way from proving that it actually happened.

So, is it possible that multicellular life got off to a much faster start on Mars than on Earth? Well, yes and no, you can argue both ways. Life may have had a much tougher time on Mars, but does that slow down or speed up evolution? What was the climate like on early Mars?

Tougher conditions for life on Mars - sometimes - and at other times much easier

You'd expect Mars to be colder because it's further from the Sun, and gets about half the light of Earth. And on the whole it was, but the question is more complex than that. Its orbit is much more variable than Earth's, varying hugely through the influence of the other planets. Currently its orbit is close to circular and it is cold all the year round. When its orbit is at its most eccentric, it gets moderately warm every two Earth years when it is closest to the Sun.

It would still be very cold and it's somewhat a mystery, how it had liquid water at all in the early solar system. However, there is plenty of evidence that it did, especially with the discovery of features such as deltas feeding into the ancient oceans.

Confirmed ancient delta on Mars, left ,compared with delta on Earth to the right. This is amongst the strongest evidence that Mars had an ocean in the Northern Hemisphere. We can trace a shoreline all the way around the Northern Lowlands, with rivers and deltas flowing into it. How it managed to have an ocean, is still something of a mystery as it would seem to be too far from the sun to be warm enough for this, even with a thick atmosphere.

If we could somehow magically transfer Earth's atmosphere to Mars, it would still not be nearly warm enough to keep an ocean, or lakes liquid. Yet we know that early Mars did have oceans and lakes. Even a pure carbon dioxide atmosphere with several times the atmospheric pressure of Earth wouldn't be able to keep Mars nearly warm enough for large areas of liquid water. The average surface temperature would be still be around -40 °C (-40 °F) (see figure 2 in this paper).

Meanwhile, the lake in Gale Crater seems to have been liquid at a time with at most a few tens of millibars of carbon dioxide. That's even more of a challenge to explain, yet Gale crater doesn't show any sign of features you'd expect from an ice covered lake such as ice wedges. It seems to have been not only liquid, but warm enough to be ice free most of the time. How could that be?

Ice wedges in Sprengisandur, Iceland - the ice penetrates deep into the ground in these ice wedges as a result of repeated melting and refreezing. There are no signs of any ice features in Gale Crater so it probably never froze over for long. So how did it stay liquid?

Greenhouse gases on early Mars

Carbon dioxide just isn't warming enough, so what if Mars had much stronger greenhouse gases than carbon dioxide? There are quite a few of these, including sulfur dioxide, hydrogen sulfide, methane, and ammonia. Surprisingly, even hydrogen, which is usually not a greenhouse gas at all, can be warming when it's mixed with a heavier gas such as nitrogen, through "collision induced absorption".

Techy aside about how greenhouse gases work, and how the symmetrical molecules of hydrogen and even nitrogen can be greenhouse gases through "collision induced absorption":

Most greenhouse gases like sulfur dioxide and water vapour have molecules which are asymmetrical. This lets them interact with electromagnetic radiation through a permanent "dipole moment", a charge separation as a result of their asymmetry, in just the right way to trap photons in the far infrared.

Carbon dioxide is symmetrical, but it can also bend and stretch in a way that makes it sometimes asymmetrical which is how it is able to absorb photons. This light then causes it to change how fast the entire molecule is spinning - and those transitions happen in the far infrared, ideal for trapping heat. (techy details here)

Hydrogen, methane and nitrogen are not only symmetrical molecules, they also have such stable symmetrical structures that they can't bend or stretch either, not in a gas consisting all of the same type of molecule. So there is no way for them to create an asymmetry. Indeed methane on its own is a slightly "anti-greenhouse" gas because it absorbs incoming light in the near infrared. (see section 3 of this paper) while it is transparent in the far infrared so lets the heat out. So you'd think that these can't possibly be greenhouse gases, and indeed they can't when they are on their own, in a gas consisting of a single type of molecule

However, when a heavier molecule such as nitrogen hits a hydrogen molecule then it distorts it momentarily in a way that makes it able to absorb light over quite a broad part of the spectrum, and so it can absorb heat also, more easily. Technically it does it by giving it a "dipole moment", an uneven charge distribution. The hydrogen in Titan's atmosphere keeps it warmer than it would be otherwise through this process. Titan is especially interesting because it has both a "greenhouse effect" and an "anti greenhouse effect" - because of its smog layer which reflects heat away.

You can also get a similar effect when two nitrogen atoms collide and stick together momentarily to make a temporary "super molecule" which can be asymmetrical and absorb light.
This nitrogen collisions processes actually has a significant warming effect in the far infrared for Earth, Titan and early Mars.

Carl Sagan suggested a mix including hydrogen or ammonia as a way to warm up early Mars or Earth in a letter to Nature in 1977. However both of those suggestions have drawbacks. Hydrogen is lost rapidly. Ammonia gets decomposed by UV light, and we don't know of a way that Mars could have made large enough quantities of ammonia to keep it warm.

Volcanoes can produce sulfur dioxide, which is a greenhouse gas, and a study of the sulfur content of our Mars meteorites suggests that early Mars might sometimes have had enough sulfur dioxide to keep it warm. Mars has had many episodes of volcanic activity in the past. So the idea is that though it would normally be far too cold for liquid water, it would warm up from time to time after those episodes. This sulfur dioxide approach is still a strong contender, but it's no longer the only way that early Mars could be warm enough for liquid water.

Mars would also produce hydrogen sulfide, but this is much less effective as a greenhouse gas, with a third of the temperature change for the same partial pressure, Also, sulfur dioxide's effect is amplified by water vapour, especially in a dense atmosphere, while water vapour actually reduces the greenhouse effect for hydrogen sulfide, see table 3 of this paper).

One new idea is to have a thick carbon dioxide atmosphere mixed with a small amount of both hydrogen and methane. Collisions of carbon dioxide with the methane and hydrogen jostle the molecules, temporarily changing their state in a way that makes them more absorbing of some frequencies of light. This has a much greater warming effect than any of these three gases separately. New research (published 24th January 2017) shows that when you add in these effects, the greenhouse effect can be strong enough for liquid water on early Mars. This graph shows how the collisions help fill in the gap in the carbon dioxide absorption spectrum.

Here the grey line shows how carbon dioxide traps sunlight. It's got a window in the region shown in grey in the infrared, which lets heat out and cools the planet. The red and blue lines show the optical depth for collisions of carbon dioxide with methane and hydrogen which are both strong in the gap. The dotted lines show the effects of collisions with nitrogen. Visible light extends from around wavenumbers 14,000 to 25,000. So this figure shows a region in the far infrared. This is figure 1 from their paper.

The authors of the paper found that adding just 3% of hydrogen and 3% of methane to a carbon dioxide atmosphere raises the surface temperature by a rather dramatic 40 °C. That's enough to reach a temperature of zero degrees centigrade, averaged over the Mars surface, which means many regions will have temperatures above zero. For details see their Figure 2 and discussion. This makes it more than enough to permit liquid water in the form of lakes and seas, given local variations in climate.

It's a neat idea, but there are quite a few problems with this model. See the discussion section of their paper which I'll summarize.

First, their model can't work for present day Mars as its surface is highly oxidizing today. The methane and hydrogen would soon be removed from the atmosphere. It could work for the early Mars surface, but for that to work, its surface would need to be very different, reducing, rather than oxidizing. (A reducing atmosphere is one with methane or hydrogen etc which removes oxygen and reactive oxidized materials). That's reasonable enough. Atmospheres don't have to be oxidizing, with Titan as an example of a moon that still has high levels of methane, in a nitrogen atmosphere.

However, even with an early Mars that's compatible with their model, with a reducing surface, and reducing atmosphere, you still need a continuous source of hydrogen to keep the atmosphere hydrogen rich. Volcanoes produce carbon dioxide as the main gas, both on present day Earth and on Mars. However, that can change, as it depends on whether the mantle is oxidising or reducing. So, one way the early Mars could have a hydrogen rich atmosphere s if the volcanoes produced hydrogen instead of carbon dioxide. So far, fine but the problem then is that hydrogen is not warming by itself, it needs the carbon dioxide in the atmosphere to collide with. So how do you get the carbon dioxide into the atmosphere, to collide with the hydrogen? It is hard to get volcanoes that produce both carbon dioxide and hydrogen in large quantities at the same time. If the mantle was reducing enough to outgas hydrogen then it would tend to retain carbon in the melt, and so wouldn't produce carbon dioxide in any quantity.

So that then becomes the central question with this model. How can you get enough hydrogen into a carbon dioxide rich atmosphere to act as a greenhouse gas, with these high percentages of 3% each of hydrogen and methane? Well one way is through serpentization. Even with a non reducing mantle, with volcanoes producing carbon dioxide as they do on Earth - you can get hydrogen from serpentization. That's the reaction of the rock olivine with water to produce hydrogen, and it is something that happens on Earth locally in hydrothermal vents. The problem is that on Earth this happens only over small parts of its surface. However, if 5% of the Mars surface was rich enough in olivine for serpentization it might create enough hydrogen for as long as it stayed like that. That would work, but that's a massive amount of serpentization.

The hydrogen and methane could also be created during huge meteorite impacts, through the heating of the atmosphere and reactions caused by the impact itself. Of all their ideas about how it could happen, perhaps this impact generated hydrogen has most in its favour. Mars had numerous really huge impacts in the early solar system at just the same time that it had its oceans and lakes. If this is right, then the picture is one of episodes of warmth after and during a time of massive impacts, rather than a continuously warm climate in early Mars, similarly to the sulfur dioxide and the volcanic eruptions idea.

So, in short either sulfur dioxide or hydrogen and methane could warm up early Mars.But we don't have proof yet that either of these things happened. If Mars did have strong greenhouse gases like that, then in both cases they are likely to have been temporary. They might have kept it warm enough for liquid water for short periods of time, perhaps after volcanic activity (for the sulfur dioxide) or large meteorite impacts (for the hydrogen and methane), or maybe both were factors.

Oceans that are only liquid part time as Mars' tilt and orbital eccentricity change

Another idea is that Mars had only a "part time liquid" sea in every two year orbital cycle, when Mars was closest to the Sun. This does away with the need for super powerful greenhouse gases like sulfur dioxide, hydrogen and methane in a nitrogen atmosphere, and would let it have liquid seas mainly when its orbit was at its most eccentric. The details of its climate would also depend on its tilt too, which would change which hemisphere gets warmest when Mars is close to the Sun, and by how much. The tilt varies a lot - Earth's hardly at all. And Earth's orbit stays close to circular, for billions of years, while Mars' orbit constantly changes in eccentricity too.

This is a matter of ongoing research. Since the early seas always formed in the northern hemisphere, because most of the low lying land is there - the best times for liquid water seas might be when Mars is closest to the sun during its northern summer. Mars' axis precesses, just like Earth's axis, sometimes with the northern hemisphere tilted towards the sun when it is closest to the sun and sometimes with the southern hemisphere tilted towards the sun, so the northern oceans would be liquid only at times when the northern hemisphere is tilted to the sun.

The tilt of its axis varies hugely too, sometimes almost vertical, sometimes so tilted over that it is coldest at its equator instead of its poles. This turns out to be probably one of the biggest effects on its habitability in the models.

Variations in the tilt of Mars' axis. At present it is tilted by 25 degrees, similar to Earth. But with no stabilizing Moon, its tilt varies wildly. Sometimes it tilts so far that its equator is colder than its poles as shown at top right. Other times it is almost vertical. When it is almost vertical, then ice migrates to its poles creating large ice sheets that trap it there, and it probably never gets warm enough for the ice to melt. The best time for liquid water - and for global oceans in the past - is probably when the equatorial regions are coldest, because that drives ice and snow to lower latitudes where they are more likely to melt.

So, to find out when Mars is most habitable, we need to look at how the tilt of its axis varies. The tilt of the axis of Mars is actually chaotic in the mathematical sense of "chaos theory". This means you can't predict it exactly over long timescales. This also means we can't retrodict - work out what it must have been in the past based only what we know about Mars' orbit and spin axis in the present. Perhaps we may get ground data to sort out the past history, but meanwhile, we have no way to retrodict precisely. Instead we have to try out different possible past histories and compare possibilities to see what sorts of things could have happened in Mars' past.

This next graph may look complex, but it isn't really. The black, red and green lines here show three different possible pasts from different runs of the model. They could have gone on to calculate many more possibilities. It starts with the present day on the left, running to the past as you follow the graph to the right.

At present, Mars' axis is tilted by 25 degrees. When the tilt is at least 40 degrees then it may get warm enough for water to stay liquid. Early Mars with a thicker atmosphere could have had liquid seas at those times, the times when the graph goes above the blue horizontal line. Figure from page 4 of this report.

You also need to take account of the eccentricity of its orbit, as it needs to be reasonably eccentric to have liquid water. When they took account the eccentricity of its orbit as well, they got this

These show three equally likely possible pasts for Mars. The blue peaks show availability of liquid water. The black line shows the atmospheric pressure, and the red line shows the variation in the tilt.

During the times shown with liquid water in these diagrams, Mars would still be frozen with no liquid water, and so largely dry, for some of the time every two years when furthest from the sun. They count Mars as continuously habitable if it has liquid water for at least part of every Mars year (two Earth years). In their simulations, the longest continuous reasonably habitable period was 60 thousand Earth years.

Study of sediments on present day Mars back up these conclusions.

Sedimentary layers in in an unnamed crater in Arabia Terra, Mars.

Caltech researchers studying these layers in a 3D stereographic projection found evidence of variation in climate with each layer formed over a period of about 100,000 years when conditions were favourable for forming them. Though they can't say in detail how they formed, there's clear evidence that they formed due to variation in the climate of Mars which would also correspond to variations in habitability.

These ideas suggest that early Mars could have changed in habitability frequently, sometimes more habitable, and sometimes less habitable, first in two year periods and also over longer timescales. It might have been almost completely dry for most of the time, alternating with periods of a few tens of thousands of years with liquid water. Impacts also might have made a big difference to habitability, both by creating liquid water and destroying life. Especially in the very early solar system, when Mars was most habitable, it would have had many large impacts.

It is still changing in habitability frequently. In the last five million years it has gone through forty ice ages, so on average once every 125,000 years, with its ice sheets melting away and moving to its equator, then back to its poles.

This also leads one to wonder - how unusual is Earth, which has been continuously very habitable for billions of years? Our Moon may have a lot to do with it. Mars might be more representative of what we will find with habitable exoplanets than Earth is. Though an investigation into early Venus shows that it had a much more stable axial tilt than Mars, similar to Earth's so that gives another approach which means you might not have to have a stabilizing Moon. Still, it's possible that Mars is the most normal case for an exoplanet. If so, this is just one of many ways in which we may have just been lucky which is the thesis of Mark Waltham's "Lucky Planet"

Anyway whatever the situation for exoplanets, it seems likely that Mars was very different from Earth with these many brief periods of oceans alternating with much drier colder conditions. The greenhouse gas models also suggest pulses of warmth and liquid water depending on amounts of volcanic activity, and so frequent changes of habitability. So, whether you rely on axial tilt and changing eccentricity, or greenhouse gases, or both, it seems that the same picture emerges, of a planet with constant changes in habitability.

What are the effects of these frequent ups and downs in habitability?

So Mars may have had long periods of time with permanently frozen oceans and other times when the oceans melt every two years. It might have had more liquid water if it had greenhouse gases to assist with the warming. But still it would still have a lot of climate variability with liquid water only after major asteroid impacts or episodes of volcanic activity.

What would all those changes in habitability do to evolution? That's very different from anything that happened on Earth, apart perhaps from the time of the Snowball Earth hypothesis, 650 million years ago, so we don't have much to go on by way of an analogy.

And what also about the solar storms and cosmic radiation? Again we don't have those on Earth, which is protected by its thick atmosphere and the Earth's magnetic field. Also what about the frequent meteorite impacts? Mars is closer to the asteroid belt and still gets about ten times the number of asteroid impacts for the same area as we do. Also, it had many more large impacts than Earth in the very early solar system at the times of its oceans, which is the very time when it had an oxygen rich atmosphere and was most habitable for multicellular life. These impacts would boil the oceans and melt the rock, at least locally (and the earliest impacts, perhaps globally) so if evolution did get started, what did that do to the life that was evolving there?

I think everyone would agree that Mars was a tougher place for life to evolve. But what would it do to the pace of evolution? That's much harder to answer, with only the Earth as an example to base all our reasoning on.

It might have accelerated evolution, especially after it stopped getting impacts large enough to boil its oceans, with life continually faced with new challenges to overcome. Some think that the Cambrian Explosion was a result of a previous snowball Earth, so that might back up the idea that it accelerated evolution.

On the other hand these variations in habitability might have kept knocking evolution back so that it never evolved far, keeping life on Mars at an early stage. The evolution would have to be accelerated hugely to have multicellular life there already three billion years ago, and especially with it perhaps having only brief periods of tens of thousands of years of liquid oceans at a time. If you are optimistic about macro fossils on Mars, however, you could go with the hypothesis of hugely accelerated evolution on Mars, accelerated further by its oxygen rich oceans, to back up your hopes.

If evolution on Mars proceeded independently of Earth evolution, it would be a great surprise if it has reached exactly the same stage of evolution as life on Earth. However it's rather amazing how large the differences are between these different views on the possibilities for past and present day life on Mars. If you are a fossil optimist, and expect to find fossils in the Hesperian age deposits on Mars such as Gale Crater, easily recognizable as fish, or plants or similar - that means that you think that Mars had its equivalent of the Cambrian explosion more than three billion years ago. The Cambrian explosion is a short period of a few tens of millions of years when life diversified hugely, including some with unusual shapes like opabinia.

Opabinia - if Mars evolved creatures as advanced as this already in the Hesperian period, it's evolution would be about two and a half billion years ahead of Earth evolution. We had many "experiments" with creatures that seem quite bizarre to us now, and if you are a fossil optimist you might wonder if ancient Mars had creatures as diverse as this too.

Also we are used to the idea that photosynthesis lead to multicellular life. But multicellular animal life on Earth only evolved in the conditions created by photosynthesis, with the photosynthetic life mainly important because it created an oxygen atmosphere. The animal life didn't evolve from the photosynthetic life itself. So could Mars have had multicellular animal life before it evolved photosynthesis? Might it even have had multicellular life early on, yet never developed photosynthesis at all? Is our idea that photosynthesis comes first just a bias that results from living on a planet with a strong magnetic field, so that photosynthesis is the only way to create an oxygen rich sea? Or is photosynthesis one of those things that is bound to happen fairly early on?

There was a huge diversity of strange creatures back then on Earth. Only a few survived and continued to evolve into present day life. If Mars had a head start over us for the prokaryotes of two and a half billion years. this might lead one to wonder, what happened next after that on Mars? What will Earth life be like two and a half billion years from now? And, what would happen to Earth life, after two and half billion years of evolution in Mars like conditions starting from now? If Mars life continued to have an accelerated evolution, through the equivalent of our pre-cambrian period, and then beyond, it might be the equivalent of many billions of years ahead of us by now, as compared to Earth's slower pace of evolution, evolving very rapidly in the brief moments of greater habitability.

But you can argue just as convincingly for Mars life to be at a much earlier stage of evolution. With so many setbacks, it might have proceeded much more slowly. Perhaps early life evolved multiple times then went extinct and had to start again from scratch. If it was unable to develop photosynthesis, and hadn't yet developed highly resistant dormant states or spores, perhaps you could imagine even multicellular life evolving only around a few hydrothermal vents in an earlier hospitable ocean, and then being made extinct by an asteroid impact or the vents stopping, or the sea freezing over so that no more oxygen could reach the water. Perhaps Mars had multiple genesis of life, with completely different forms of biochemistry that evolved one after another, or even simultaneously in geographically separated parts of the ocean or surface.

Indeed with all those obstacles to evolution, you can also argue that Mars could still be at a pre-biotic stage with cell like structures that resemble Earth life with some of the qualities of life, but perhaps they don't yet reproduce exactly. Or what we might find there in the most habitable conditions might be self replicating chemicals like the "RNA ocean" hypothesis, with no cell walls at all. Both of these of course would be very vulnerable to introduced Earth life.

Or perhaps life on Mars evolved in pace with Earth life, more or less, so that it had early forms of life in the Noachian and the Hesperian periods, similar to Earth life, and evolved to later forms of life roughly in step with us. You can argue convincingly for this possibility too. Perhaps evolution has a steady pace that it follows almost irrespective of what happens, so long as there are habitats sufficient for life to evolve in. It may have got off to a slightly earlier start than us, as the Earth - Moon system formed quite late, but then that was followed by many ocean boiling impacts on Mars which may have leveled the playing field.

If so then we might, just possibly, find fossil microbial mats, stromatolites and acritarch's in the Hesperian deposits. Stromatolites are boulder like structures that are made from algae if left to grow undisturbed for long periods of time in the sea. Though made of single cells, they combine together to form these larger structures which then form fossils.

Modern stromatolites in Shark Bay, Western Australia - if Mars had stromatolites in the Hesperian era then its evolution was similar to Earth life as the earliest possible stromatolites date back to 3.7 billion years ago on Earth, 1-2 cm high putative stromatolites found in Greenland.

However ancient stromatolites are hard to identify conclusively. If Mars does have stromatolites, there might be much debate and further research before they are accepted as such

Then Acritarch's are a general term for ancient microscopic patches of organics which seem to be associated with algae but nobody is quite sure what they are. More generally, this refers to any ancient organics that we don't properly understand.

Acritarch - these organic microfossils are also very ancient , date back to between 1.4 and 3.2 billion years on Earth. The name was coined by Evitt in 1963 and means "of uncertain origin" and the term is used for any microscopic organic fossils that can't be assigned to any other classification. They may be associated with green algae, some kind of a cyst or resting state. Since nobody is sure what they are then they are classified by their structure instead. For instance as prismatic, spindle shaped, egg shaped, spiky like a thorn bush, etc. See also wikipedia article on Acritarch.

If Mars evolution reached a similar stage to Earth evolution then we might find similar organic microfossils on Mars. If so, there might be a lot of debate about what they are. We could expect similar announcements to these about Mars: "Organic-walled microfossils in 3.2-billion-year-old shallow-marine siliciclastic deposits" or "Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia" followed by much discussion of what they were, and indeed, about whether they were life or not. That's often a matter of considerable debate for early Earth putative microfossils. Are they the products of life or not? It would be even more a matter of debate on Mars, because Mars has many impacts by meteorites with organics in them, and it has many ways to create organics by inorganic processes on the planet itself.

So, in short, if there are easily recognizable macrofossils in Hesperian deposits on Mars, like Gale crater, then evolution there has to have been at least two and a half billion years ahead of Earth life when the multicellular life first evolved. If it reached an equivalent stage of evolution to Earth life, neither ahead nor behind us, we might find the equivalent of those ambiguous acritarch and stromatolite fossils from 3 billion years ago on Earth.

If evolution on Mars evolved much later than on Earth, with many setbacks, which is also a distinct possibility, then we might find very early life there, so early that even stromatolites and acritarch fossils might be unlikely. This is an especially interesting possibility, because we know so little about early life on Earth, with nothing that's survived to help fill in the big gap between pre-biotic chemistry and modern life. On the plus side the stable geology of Mars without continental drift, and the extremely cold conditions there, may make early life easier to study, even without clear large fossils. This early life might also still survive on Mars to the present day. If so, that would put present day evolution on Mars three to four billion years behind Earth, and this would be very exciting for astrobiologists, as a way to peer into the processes of a planet in early stages of evolution.

So, evolution on Mars could be anywhere between three to four billion years behind Earth, and billions of years ahead of us. It could also have never happened at all, so all we find is pre-biotic chemistry, another very interesting and intriguing possibility.

I don't think we can distinguish between these and many other possibilities on the basis of what we know so far about Mars. And even that much is based on a lot of speculation and assumptions about a similarity between evolution on Earth and Mars. We only know of evolution on Earth, so we have no idea even if the pace of evolution here was typical for the universe as a whole. Perhaps life got off to an unusually rapid start on Earth, or perhaps evolution here was unusually slow compared to other analogous planets in our galaxy. We can speculate endlessly, but without at least a bit more data it's hard to draw any definite conclusions quite yet.

What about thick deposits of life on Mars, like our oil rich shales?

Could Mars have meters thick layers of ancient life in some form or another? Could it have organic deposits like our oil rich shales, or the equivalent of chalk, thick deposits made up entirely of shells? Well, if it did evolve those forms of life, it did also have enough time for this to happen. It probably had hundreds of millions of years of relatively stable conditions, in the very early solar system, and continued to have seas and lakes (probably intermittently) for over a billion years. That would be plenty of time to build a thick deposit of oil shale in ideal conditions. The whole of the 5.5 km high Mount Sharp consists of sediments (for the lower layers) and wind blown deposits.

If we found something like this, even without the multicellular life fossils, in deep meters thick beds of organics, our task would probably be easy, with plenty of organics to analyse to see if it was formed through life or non life processes:

Fossils in Ordovician oil shale (kukersite), northern Estonia (Ordovician period)

However we haven't found anything like this yet. Maybe conditions on Mars were never favourable for creating thick deposits of organics (caused by life or otherwise). Or, it could be that they were washed out by the later floods, and what's left was destroyed by surface conditions. Or maybe Mars still has deposits like this, many meters below the surface beyond the reach of the cosmic radiation, Any surface deposits of organics, even meters thick, would soon be degraded to just water vapour and other gases by the cosmic radiation over these very long billions of years timescales, which breaks the bonds in complex organics. So we'd only spot them if they were unearthed in the recent geological past, perhaps by crater impacts.

At any rate if those deposits exist, we don't know where to look for them yet. There is no sign of them from orbital observations, and our rovers haven't spotted anything like this yet either.

Even multicellular fossils would be hard to find

Even with accelerated evolution, if Mars had birds, and fish, with its equivalent of a Cambrian explosion in the early solar system, the chances are that we wouldn't have found any signs of them yet.

This picture shows Archaeopteryx. It was hard to find. They had to search through tons of quarry material to find a few thin flakes with Archaeopteryx preserved.

You could send a rover to Earth and set it to explore rock formations in our desert regions, at the slow pace of our current generation of Mars rovers, for decades, and it might never spot a single fossil, depending where you send it. Or it might find a layer of chalk or similar with hundreds of them, and discover them right away.

How would we recognize fossils on Mars?

The other problem is that we don't know what to look for on Mars. If we are lucky enough to find a fossil archaeopteryx or a fish it would be obvious. Even a fossil multicellular plant. But for billions of years,as we've just seen, the only macro fossils on Earth were microbial mats, stromatolites, and the acritarchs. So, what if we find these?

These are now known to be early stromatolites from 3.4 billion years ago. But it took a lot of work and evidence, particularly the evidence of organics caught up in the material of the stromatolite fossil itself, before they were accepted as such. The later stromatolites were easier to identify but these very early ones were particularly challenging.

There are many formations on Earth that look for all the world as if they were some fossil lifeform, such as this.

Baryte Rose from Cleveland County, Oklahoma, photograph by Rob Lavinsky

If Curiosity found this on Mars, I'm sure many people would be convinced it was a fossil. But no. It's a "Desert rose" - a crystal like structure that can form in desert conditions. Enthusiasts have found many strange shapes on Mars that they think may be fossils. For some remarkably compelling examples, see for instance “Mars Fossils, Pseudofossils or Problematica?”, by Canadian scientist Michael Davidson.

So, what's the answer, how do we cope with this conundrum? We have to use the Knoll criteria to evaluate them. It's not enough that they look like fossils:

"The Knoll criterion is that anything being put forward as a fossil must not only look like something that was once alive -- it must also not look like anything that can be made by non-biological means.”

Oliver Morton, author of Mapping Mars: Science, Imagination, and the Birth of a World

This criterion is named after Andrew Knoll, author of “Life on a Young Planet" a book about past Earth life, who is on the Curiosity mission science team. We will be very lucky indeed if we find a lifeform on Mars that we can conclusively identify as living just by its physical shape. Even if it turns out that the planet had stromatolites, or even multicellular fish and birds, in the past, the problem is finding them. We are more likely to find something like this - these are potential fossil signs of past life found on Curiosity photographs by geobiologist Nora Noffke

To her expert eye these look like trace fossils of microbial mats. But another geobiologist Dawn Sumner thinks they are just the result of normal erosion processes. See Follow Up - Signs of Ancient Life in Mars Photos?

To add to the difficulties, Mars has radically different geological conditions from Earth in many ways, which could lead to many structures forming on Mars that would be impossible on Earth:

  • Perchlorates, chlorates, sulfates and hydrogen peroxide instead of chloride, and sulfides. The Mars surface chemistry is highly oxygenated and reactive
  • Dry ice geysers and avalanches caused by dry ice - we don't have any geological formations on Earth to compare with them.
    Even though temperatures in Antarctica and other places get cold enough for dry ice, the partial pressure is far too low for it to freeze out (about 0.04%), indeed even at a temperature of -86 °C, dry ice will sublime rapidly in an Earth atmosphere. The depositional temperature for dry ice on Earth under 1 bar of pressure is −140 °C (−220 °F) [14]. The lowest temperature reached in Antarctica is −94.7 °C (−138.5 °F) recorded in 2010 by satellite
  • Low gravity which lets the wind sculpt shapes that are impossible on Earth because they are unstable here.
  • Near vacuum atmosphere, and in present day Mars the fastest winds are only barely strong enough to move an autumn leaf. This again permits many strange structures to form that would be unstable on Earth, because the wind would just blow them away before they can form
  • If there is no life there, then geological processes may step in to form structures that only form through life processes on Earth (the "blueberries" are an example, see next section).
  • Even very small meteorites make their way to the surface. No continental drift, but lots of impact gardening. This turns the surface to a "soil" or regolith to depths of several meters, which is something we have no analogue of on Earth.
  • Much finer dust than we have on Earth, similar in consistency to cigarette ash
  • Much larger temperature variations from day to night

Mars is such a different world, with such different geological processes, that it won't be surprising at all if we find unusual hard to identify geological formations on the Mars surface. So, no, it's not very likely that an astronaut could pick up a fossil on Mars and identify it as such, if it is just an ancient stromatolite or fossil microbial mat. They might suspect that it is formed by life, but proving that would be another matter altogether, and they might easily be mislead and think it is life originated, by analogy with similar structures on Earth, when it is not.

"Blueberries" - Mars equivalent of Moqui marbles - are they signs of past life on Mars?

Opportunity's blueberries, to take another example, are made of iron oxides. That may not sound very life like. But on Earth similar nodules form in the presence of life.

Blueberries - photographed by the Opportunity Rover of the "berry bowl" in Eagle crater, near to Opportunity's landing site on Mars. This is how you would see it with naked eye, approximately true colour. But the Mars light is a reddish gray because of all the dust in the air. When it is colour balanced the berries look somewhat blue. These so called "blueberries" are made up of hematite and so must have formed in water. Similar

Here is a colour balanced example which brings out their blue colour:

Cluster of hematite rich spherules (blueberries") photographed by Opportunity at its Eagle Crater landing site - image from exhibition in the Smithsonian institute in 2014.

They are very similar in both structure and composition to the Moqui marbles

Interior of a Moqui marble from the Navaho desert to show its structure.

Moqui marbles which are left behind as sandstone erodes, in the Navaho desert. Photo by Brenda Belter, University of Utah

They originally formed in wet conditions and then got embedded in the sandstone, then eroded out of it. The Mars blueberries seem to have been formed in a similar way. Though there are many mysteries still to be unraveled about how exactly the Moqui marbles formed, with competing ideas.

So anyway what makes this interesting is that on Earth there is some evidence that microbial life played an active role in the formation of the metallic oxide outer coatings of the marbles - the evidence is based on looking at structures that resembled micro-organisms in the marbles, and on the carbon 13 to carbon 12 ratio (living beings preferentially take up a bit more of the lighter isotope).

So, is it possible that the blueberries on Mars were made in a similar way, by past life? Well it's rather circumstantial evidence, and the jury is out on that. We can't say that they must have been formed in the same way just because they look so similar - as Mars is so different from Earth.

Need to search for biosignatures if there are no easy to identify macrofossils

So, if there is life on Mars, and it doesn't form fossils or the fossils are ambiguous, how will we find it and recognize it? Well one way is to look for biosignatures. After all, that's how the ancient stromatolites on Earth were eventually proven to be fossils rather than geological formations. Once we prove that some particular type of formation is the result of life then we may be able to identify them by their shape too. For instance if we manage to use biosignatures to show that the blueberries are the result of life processes, they will be easy to find after that. For all we know, our rovers may be driving past fossils of ancient Mars life every day, such as biomats, concretions caused by life, stromatolites, etc, but we just can't prove that they are fossils yet. In the future, we may be able to confidently identify many of them as fossils. Astrobiologists, when devising life detection experiments for Mars, focus almost exclusively on various biosignatures. For instance, the chirality of the amino acids - whether it uses the molecule in one form, or its mirror image

That sounds straightforward enough, but it's not as easy as it sounds. The past life on Mars may have been destroyed long ago except in a few favoured patches. These may have only a few trace amounts of organics left from the past mixed with organics from meteorites, volcanoes, and various non life processes on Mars, degraded by cosmic radiation and reactive chemicals from the surface, washed out by floods, and easily masked by just the minutest traces of present day Earth life contamination. We may have to dig deep and drill in many spots to find this faint signal which might need a lot of detective work to sort it out. For more on this see What if Mars has really tiny cells - like the structures in the Mars meteorite ALH84001? (below) .

With the exobiologists keen to detect even a single molecule biosignature from past life, how can that work if samples of past life get contaminated by modern Earth life?

Searching for present day life on Mars in the popular imagination

In the popular imagination, at least to judge by TV and movies, life on Mars will be easy to find as soon as we send humans there. For instance, as dramatized in the National Geographic Mars TV series, the astronauts need to take no precautions to avoid contamination of Mars with Earth life. They even bury dead members of their crew in the Mars soil. In season 1, episode 6, Crossroads, they drive up to a potential habitat that they think could have present day Mars life and walk right up to the habitat to search for life there (they say that it is an RSL so a patch of dark streaks caused by flowing thin films of salty water just below the surface, though in the movie it just looks like a small outcrop amongst sand dunes).

The astrobiologist Marta Kamen, then walks along the outcrop with her two companions inspecting it visually. She picks up a rock and looks at it and turns to her companions, sighs, "not what I'm looking for", and they walk on dejected until suddenly she spots a small patch of reddish soil in a crack in the rock which she picks up with a pair of tweezers:


Somehow she and her companion astronauts immediately know that this is what they are searching for.

She returns it to their lab for inspection, where she sees a network of interconnected purple strands.

She recognizes it as native Mars life by feeding it a nutrient with a pipette and noticing that it moves slightly in response.

Can it be? Dawning realization

Yes, there is life on Mars!

Though that's obviously a dramatic simplification, it's pretty much how Robert Zubrin thinks it would happen, the founder of the Mars Society, space engineer, and author of Case for Mars. He thinks that Mars life would not be harmed in any way by introduced Earth life and would be easy to spot, and easy to distinguish from Earth life if it is present. He thinks that introduced Earth life would not harm the search for life on Mars in any way, and that human astronauts on the Mars surface would greatly speed up the search for life there. So, in this story, the National Geographic series presents in a simplified dramatized form how the search for Mars life would go according to Zubrin's ideas.

Why it's likely to be hard to search for present day life on Mars

What's the problem with this? I will go into this in a lot more detail later, but let's just introduce some of the main difficulties with searching for preset day life on Mars.

First, if it is anything like life in similarly harsh areas of Earth, then it's likely to be microbial. Or at most, lichens perhaps, huddled in cracks in rocks for protection from UV light. That seems likely to be the case even if life on Mars did get off to a great start, with rapid evolution and multicellular life already there in the oxygen rich atmosphere three billions years ago. The conditions now are very harsh for present day multicellular life. If we can judge Mars by comparison with Earth then the closest we have to such harsh conditions are the very dry (hyperarid) core of the Atacama desert and the McMurdo dry valleys in Antarctica. In the most cold and dry conditions on our planet, even with our rich variety of multicellular life and the oxygen rich thick atmosphere, all we find are small populations of microbes, in thin films, which live very slowly, and for multicellular life, at most some lichens.

Also, in such harsh conditions, even photosynthesizing microbes tend to be hidden from view below the surface of rocks, beneath the soil surface and in salt pillars. It's also likely to be patchy with some places that have life and others apparently identical right next door that don't.

Researchers in Beacon Valley in Antarctica, one of the most Mars like regions of Earth. There's no snow or ice, kept dry by fast winds blowing off the Antarctic plateau, and there is little moisture, just some limited melting around the edges of the valley and thin films of brine around permafrost structures.

In cold dry areas like this, typically there are no visible signs of life. It's hidden in the soil, beneath the surface of rocks, inside salt pillars. Some places have life and other identical patches nearby don't. This type of terrain corresponds very roughly to the most habitable regions of Mars. It's only a partial analogue. Yet there is life here, very slowly metabolizing, in small quantities in scattered locations.

So, present day life is going to be hard to spot by eye on Mars. Unless, that is, it is obviously novel, say a purple lichen that you can be pretty much certain never got to Mars on our spacecraft.

Lichen P. chlorophanum on a Mars analog substrate for the DLR Mars simulation experiments. - colour adjusted to a dark purple. If we saw a lichen or other multicellular lifeform on Mars, especially if coloured in some unusual non Earthly colour, it would be convincing evidence of native Mars life which we could spot visibly. At least, it would be, in early stages of human exploration (if we send humans to the Mars surface). In a Mars that's been settled by humans for some time, perhaps an unusually coloured lichen could be an adaptation of introduced Earth life, maybe partly through gene exchange with native Mars life.

Generally, multicellular life on Mars, large enough to see visually would be easier to spot, and it would be easier to show that it is from Mars, if it is. Also we'd be less likely to have accidentally introduced multicellular life than accidentally introduced microbes, in early stages of Mars exploration, if we do send humans to the surface. However most of the suggestions for searches for life on Mars focus on microbes. If these microbes are mixed in with the dust or in the rocks, thinly distributed, and perhaps are reddish in colour, similar to the Mars surface, or a darker black in colour - how would we spot them visually?

Also, if we go to the most habitable areas, we'd be lucky to find a total area of a few square meters of sparse, slowly metabolizing life, in an exploration region of several square kilometers. If there are liquid brines, say, some parts of the RSLs may be more habitable than others. Some may be too salty, too cold, have harmful reactive chemicals in them. Some of the dark streaks may have life and some not. Or we might find a few lucky spores in the dust, but perhaps have to examine a fair bit of dust to find them. So you are talking here about searching for something in sparse populations and hidden just beneath the surface, quite possibly dark in colour, or so spread out that it hardly has any colour at all or is hidden from view.

Robert Zubrin argues forcefully that Earth life can't harm the scientific exploration of Mars in any way at all, either the search for past or present day life. It would be so wonderful if that was true. It would make things so much easier, as there would then be no need to do anything special to protect Mars. We could just explore it looking for unusual life forms, past and present, much as we do on Earth. But is that really the situation? We need to be sure here, since so much will depend on getting this right.

I'll go into how we would search for life on Mars in some more detail in a moment, but first, let's look at Zubrin's arguments.

What are Zubrin's arguments?

The enthusiasts who want to send humans to Mars right away, as soon as we have the capability to do it, tend to brush off all of these concerns, and say either

"No need to worry, Mars life will be identical to Earth life so it doesn't matter what we bring there."
Or they may say:
"No need to worry, it will be easy to tell the difference between Mars and Earth life, so it doesn't matter what we bring there."

And then

"No need to worry, Earth life can't survive on Mars or vice versa. It's like sharks trying to survive in the African Savannah."

They get these arguments from the Mars colonization enthusiast and space engineer Robert Zubrin, author of Case for Mars, and head of the Mars Society. He will often bring up all three of those arguments in the same talk, as different reasons why we don't need to worry about introducing Earth microbes to Mars. His audience of fellow human spaceflight enthusiasts find these arguments very persuasive and clap him enthusiastically.

I think that perhaps they feel he has covered all bases. Either Mars is so inhospitable to Earth life that it's like sharks surviving in the savannah; or it is so similar that Earth life not only would fit right in, but has already got there on meteorites; or if neither of those apply then Earth life would be as easy to distinguish as anthrax by genetically sequencing it. But actually, as we'll see, those are just three of numerous possibilities and indeed they are all rather unlikely ones at that.

He also talks about the advantages of human astronauts over robotic rovers on the surface, citing as an example, a fossil discovery that he and a team of others made in Arizona in a Mars exploration simulation. He comments on how they were able to find petrified wood and a fossil bone fragment within two days. Here is a quote from his log book a the time of the discovery:

"There is a lesson in all of this for those who think that robots represent a superior way of exploring Mars. With a human crew on this site, impaired by all the impedimentia of spacesuit simulators with the cloudy visors, backpacks, thick gloves and clumsy boots, our crew found petrified wood and a fossil bone fragment within two days. But to do it we had to travel substantial distances, and climb up and down steep hills from which we could take views and map out new plans. We had to search the sites we visited, processing the equivalent of millions of high-resolution photographs with our eyes for subtle clues. We had to dig. We had to break open rocks and take samples back to the station for detailed analysis. In short, we had to do a ton of things that are vastly beyond the capabilities of robotic rovers.

"Sojourner landed on Mars and explored 12 rocks in 2 months. Today we explored thousands. If a robot had been landed at the position of our hab, it would have spent months examining a few uninteresting rocks in the immediate vicinity of the station. It would never have found the fossils."

This turned out to be an interesting discovery, a new place to search for fossil dinosaurs: Scott Williams of the Burpee Museum of Natural History rates it as one of the nations best places to search for Jurassic era fossils. That might seem rather convincing, if human astronauts could find something like that so quickly, would that make it far faster than using robots? If you find his arguments convincing, that's not surprising at all. After all, we've seen human exploration like this dramatized in countless movies and works of fiction. It's also how we are used to exploring Earth itself. It's just what you would expect.

But, as I said in the introduction, nobody has actually explored a planet like Mars before, or indeed any other planet. It's something completely new and out of our experience altogether. Do these Earth analogies actually work for Mars? Zubrin is such an accomplished debater that even when he has a one on one debate with a planetary protection expert, as he does frequently at Mars Society conferences, he is generally seen as winning the debate by his target audience. They come away remembering all of his vivid points, and none of the responses to them by his distinguished planetary protection expert debating partners.

I covered this in the introduction, under Planetary protection - researches by Sagan and Lederberg onwards - and Zubrin's arguments. So here are his main arguments in summary::

  • Earth life can't compete with native life on Mars - it would be like sharks competing with lions in the Sahara, because the planets are so different
  • Any Earth life that could survive on Mars is already there, transported on meteorites
  • It will be easy to distinguish Earth life from Mars life by sequencing its DNA, as we can even tell which lab a strain of anthrax comes from
  • Humans on the surface can explore Mars much more quickly than any robot and do things in minutes that would take years for our robots

Could you see any flaws in them? Now is the time to look at that in detail. If you are one of those convinced by them, perhaps this may give some pause for thought?

Demolishing Zubrin's arguments

It may seem so simple after listening to him, Why does anyone continue to think about planetary protection at all? There would be no need to write the rest of this book, or for scientists to continue to research into planetary protection. But sadly, as you'll see, the arguments are easy to demolish.

Sharks in the savannah or rabbits in Australia?

  • Yes, Earth life on Mars could be like sharks in the savannah. Or it could be like rabbits or cane toads in Australia. Or like invasive plants like Kudzu or Himalayan Balsam

The Mala or shaggy haired wallaby, considered as creation ancestors for the Anangu Aboriginal people - are in competition with the introduced rabbit.

Would Earth microbes on Mars be more like sharks competing with lions, or rabbits competing with wallabies? We can't decide this by using colourful analogies.

We have many examples on Earth of native species that have gone extinct due to rabbits, rats, cats, cane toads etc. The wallaby is perfectly adapted to Australia, and the rabbit isn't; it's a generalist. However, the rabbit happens to be better at living in Australia than the marsupials that evolved there originally. And this is not just an issue for higher animals.

It's not just multicellular life that is affected though. Much to the surprise of microbiologists, you get invasive microbes on Earth as well, especially in fresh water ( fresh water lakes are often isolated from each other). The Great Lakes in the US have over 180 species of invasive microbes, and New Zealand is trying to eradicate an invasive diatom accidentally introduced to it from the northern hemisphere, probably on damp recreational diving gear. It is causing problems in its lakes.

For more analogies, see the section Examples of invasive Earth life (below), and for details about invasive microbes, see single cell diatoms, and concerns about invasive microbes in Antarctica,Invasive diatoms in Earth inland seas, lakes and rivers

The same could happen for Earth microbes competing on Mars. They don't need to be adapted to attack Mars microbes. They just need to be able to

  • Outcompete them, like the rabbits in Australia,
  • Smother them, like invasive plants, block out the sunlight or deny the Mars life access to essential nutrients
  • Change the habitats there in ways that make them no longer so habitable to the native Mars life like the invasive diatoms in New Zealand cold fresh water lakes.

The same could also happen the other way around for Mars life returned to Earth. The. Mars life could be like the invasive rabbits and Earth life could be like the wallabies. More about this in Safe return of an unsterilized sample below.

As Cassie Conley, NASA planetary protection officer, put it in a recent interview with Space.com,

"It's unfortunate so many people don't seem to understand that transferring potentially biohazardous material between Mars and Earth could be problematic for life on both planets. There are lots of biohazards on Earth … Do we really want to bring them to Mars indiscriminately?"

Yes, in some places in our solar system, this analogy of sharks in the savannah works just fine, and Earth life would have no chance, like a fish out of water. Titan is like that, with temperatures of -180 °C, in liquid ethane and methane (see Life in the oceans of ethane and methane on Titan - below). Earth microbes need temperatures above- 20 °C to complete their life cycle, with some metabolic activity down to -26 °C and perhaps lower. So, surely, this -180 °C of the Titan oceans is way too low to be habitable for Earth life. The environment is also different in many other ways, particularly, that it has non polar ethane and methane in place of liquid water, but the low temperatures by themselves are enough reason to be confident that Earth life can't survive there. In the other direction, we can't really prove that Titan originated life could not survive on Earth, but it would need to be extraordinarily versatile to have a chance of survival in such a different environment. If neither can live in the other's habitat, that would indeed be like the sharks and lions in Zubrin's analogy.

Mars could have habitats like that too. Many of the potential habitats on Mars may be either too cold or too salty for Earth life, or both. It's possible that the only native Mars life survives at extremely low temperatures, down to -80 °C, laced with antifreeze mixtures of perchlorates and other salts, and mixed with hydrogen peroxide. All Earth life has water mixed with sea salt inside their cells, but in such different conditions, the Mars life could have water mixed with perchlorate salts and hydrogen peroxide inside them. Such life probably wouldn't be able to survive on Earth, and Earth life couldn't survive in the habitats that it favours either. See. Life that uses hydrogen peroxide, or perchlorates, or both, INSIDE the cells (below).

There's at least one other potential "Sharks in the Savannah" type habitat on Mars. It could also have Life in liquid ("supercritical") CO2 at depths of 100 meters below the surface. Again, Earth life introduced by astronauts would have no chance of surviving there, and unless the life that lives there is extraordinarily versatile it couldn't survive on the Earth's surface. So again, compared with Earth life, any life in such habitats might well be like Zubrin's lions and sharks.

But there are many potential surface habitats that would be habitable to Earth life if they exist. Indeed, Earth life has rather a lot in its favour when it comes to surviving on Mars, mainly because we have somewhat Mars like conditions on Earth, especially in the McMurdo dry valleys in Antarctica and in the Atacama desert. Zubrin actually recognizes this himself in his other argument where he says that microbes transferred on meteorites could survive on Mars. In this meteorite argument, he even suggests that Mars will have all the same identical species in the same conditions. In other words not at all like sharks competing with lions as almost none of the multicellular species that share a habitat with sharks will be able to survive in the savannah. Some of the advantages of Earth life for survival on Mars include

  • Earth life comes from an oxygen rich atmosphere, so is pre-adapted to deal with oxidative stress. The surface of Mars is highly oxygenated with perchlorates, and hydrogen peroxide. But because of our oxygen rich atmosphere, this should be no problem for some hardy microbes. We even have microbes that use perchlorates as food.
  • Some Earth microbes are well adapted to UV light. That's especially so if they are adapted to deserts and also survive in ordinary conditions. For instance Chroococcidiopsis which has a wide distribution can handle the Mars levels of UV just fine in conditions of semi-shade. And many of the potential habitats are beneath the surface of rocks or protected from UV light by dust or in other ways protected from UV.
  • Earth microbes have hardy spores that could probably survive for long periods of time on Mars until they find a habitat.
  • Some Earth microbes have mechanisms to repair their DNA . They evolved this probably to deal with desiccation, but they also turn out to be effective methods to repair damage from cosmic radiation and solar storms.
  • Although Earth microbes can't reproduce below -20 °C, many of them are not destroyed or harmed by much lower temperatures. They can survive very low temperatures no problem by going into a glassy state. Mars life might be limited in the same way and have the same coping strategy.

There are many suggested habitats for Earth life to survive in on Mars, including the potential for photosynthetic life to survive almost anywhere using just the humidity which reaches 100% at night on Mars due to the huge temperature swings from day to night, and often leads to morning frosts even in equatorial regions. See Habitats for life on the surface of Mars (below).

We simply don't know for sure what the situation is on Mars, until we have a chance to study the potential Mars habitats close up and find out what is in them and what the conditions are there. Once we do that, then yes, we will know for sure if Earth life can survive in them, and if there is life there, and if it has the same limitations and adaptations as Earth life or different ones.

If Earth life can survive on Mars then the correct higher animals analogy to use for Earth life on Mars is rabbits competing with wallabies or any other similar example of species that can survive in the same habitats. Which of course could also include non invasive species that co-exist happily. Then the question is, which are the rabbits and which are the wallabies in the comparison, or are they just happily co-existing?

We can look at this from the point of view of the evolutionary status of Mars life. As we saw, the life might well be at a much earlier stage in evolution than Earth life since it has had such an interrupted difficult past for evolution. Or it might be equally evolved, or more evolved than Earth life, or more evolved in some directions. So some of the possibilities are:

  • If it is some feeble RNA world life for instance, it might just be a walk over for Earth life with perhaps just about every single Earth microbe that could survive there becoming an invasive species on Mars, as in Arthur C. Clarke's Venus story in the introduction (in the section: Why don't explorers in science fiction have these problems when exploring other worlds? (above).
  • If the life on Mars is at a similar level of development to Earth life, then some Earth life might be invasive on Mars, and some Mars life might be invasive in the other direction back to Earth, in a situation analogous to invasive species of higher animals and microbes transported between continents on Earth.
  • Mars life could be so highly evolved that it is almost universally invasive and defeats Earth life. In that case, though it might be safe to send astronauts to Mars as far as Mars is concerned, with no risk of forward contamination, it might not be so safe for the astronauts to return to Earth.
  • The Sharks and the Savannah analogy might be correct, and there are no habitats for present day Earth life on Mars. Perhaps Earth life can't survive there, and there is Mars life but it can't survive here either. That is still possible at present.
  • Or his last possibility might be correct, that Mars and Earth life survive in the same habitats but play nicely together.

We can't settle any of this by using vivid analogies. The only way to find out is to learn more about Mars. Vivid analogies may well help convey our understanding, once we do know what the situation is there. Zubrin's "Sharks and the Savannah analogy could be a great way to explain the situation to the general public, if that is indeed what Mars is like. However, we don't have that understanding yet.

(N.B. I think this evolutionary way of looking at the planetary protection issue for present day Mars may be my own idea. It would seem to follow logically from things the researchers say. If Mars had early forms of life early on, as was suggested, for instance, for ALH84001, and there was no significant evolution since then, and it survived to the present, it would follow that it still has an early form of life there now. The suggestion of a "shadow biosphere" of RNA world life on Earth also suggests this idea of a co-existing present day early form of life, which if it is possible even on present day Earth, surely is a possibility on present day Mars, that early RNA life survives to the present day. Then in the other direction, if it had multicellular life already three billion years ago, then it must have been far ahead of us in evolutionary terms. But I'm not sure if anyone puts it quite in these terms, asking whether present day Mars life is evolutionarily ahead or behind Earth life or at the same stage. It seems an obvious way to look at it once you set it out like this, so surely someone else has done it, but I just don't happen to know of any papers that put it quite like this. It just kind of dropped out, when working on this book, as a neat way of organizing the material, but I can't remember reading it anywhere. Do say if you know of someone who presents it like this, so I can cite them, and discuss their ideas, thanks.).

How do we use a DNA sequencer to prove that a microbe is from Mars? Can you guess the percentage of microbial species sequenced?

  • Yes a well known microbe like anthrax on Mars would be easy to detect. So in some special cases like that we can prove that a microbe is from Earth. A more likely species to find on Mars could be chroococcidiopsis which is a well studied microbe on Earth. If that got to Mars from Earth we could recognize it by its DNA sequence.

    But how do we use DNA sequencing in the other direction to prove that a microbe is from Mars? When you try to flesh this out, it gets hard to see how it could work.

If we find life on Mars, and have a portable DNA sequencer and it matches Earth life, great, we know it was introduced by us in our spacecraft. That is, unless of course it was already there, brought there on a meteorite which is a whole other dish of worms to deal with, but we will come to that later in the meteorite argument section.

But suppose we have already introduced Earth life to Mars, perhaps accidentally, and then we find clear proof of life in one of the habitats there. We find biosignatures, or even see the microbes swimming. And then suppose you find DNA and it doesn't match a known Earth species - would that count as a proof of native Mars life?

In the introduction, I suggested you try to guess what percentage of Earth microbe species have had their DNA sequenced? 10%? 1%? 0.1%? 0.01%? Well to a microbiologist, the most striking thing is the amount that we don't know about Earth life. Hardly any Earth microbes have been sequenced, and even fewer have been cultivated.

This is the problem of what microbiologists call "Microbial dark matter" by analogy with dark matter in astronomy. It's a rather close analogy. These "dark matter" microbes probably account for most of the Earth's entire biomass and biodiversity, but we know nothing yet about their most basic metabolic or ecological properties (paraphrasing from this paper). Of an estimated one trillion species, only ten million have been identified and catalogued. Of those only about 100,000 have classified sequences, and only 10,000 have ever been grown in the lab.

That makes it only 0.00001% of all microbial species on Earth that have been sequenced to date. Then, of that 0.00001%,, 90% can't be cultivated in the lab, and are the result of sequencing a single isolated cell using new techniques which reached maturity in the last three years. See Largest ever analysis of microbial data (May 2016).

It's even more striking if you look at bacterial phyla. That's a very "broad brush" classification. Humans belong to the phyla of chordates, which is, one step above creatures with a backbone, Of the 89 bacterial phyla known, half don't have a single cultivated species as yet. So that's many entire phyla which have none of their species cultivated. We just have gene sequences. We know almost nothing about these microbes, and what their capabilities are, as we have no way to cultivate them and study them to see what they do. That's the situation for half of the bacterial phyla discovered to date.

In any spaceship occupied by humans, sent to Mars, then just as for any other habitat containing Earth microbes, then nearly all the species on board will not have been sequenced, and of the few that have been, most won't have been cultivated. Even spacecraft assembled in clean rooms have numerous microbe species that are not sequenced, and their properties are not well known. These include archaea, though they seem to be under represented, and it's not clear if they are viable. Most of them are bacteria, eukaryotes, and surprisingly, also many viruses, all of which are still viable. But a human occupied spacecraft can't be sterilized in this way, so would have vast numbers of species of unidentified microbes of all types.

Also, with so few species sequenced, then it would be no surprise if those 89 known phyla are just the tip of the iceberg. The number of phyla still to be discovered may be as many as 1,500. If that is correct, we only know a bit short of 6% of the bacterial phyla. As for the ones we can cultivate and study in the laboratory, that would mean we only have example cultivable species from 3% of the phyla. That doesn't mean that those 3% are thoroughly understood at all. Many may have only one or a few cultivable species. It just means that for three percent of the phyla we have at least one known species that we can culture and study.

With this background, you can see that it would be easy to discover not just a new species on Mars but even an entire new phylum, and have no idea whether that entire phylum is indigenous, or got there on our spaceships. There would be no way to use gene sequencing to resolve this, not right away. We'd then have to do a massive search on Earth to see if we can locate the same phylum here. If we can't find it, then that would just make it inconclusive, whether it is a hard to find phylum that got to Mars, or is indigenous.

So, if a wide range of species of Earth life was introduced accidentally to a Mars habitat, for instance after a human spaceship crashes on Mars, then typically only the tiniest fraction of a percent of the species there could be recognized as definitely coming from Earth. For the rest, you'd just have to say you don't know where they came from, even if nearly all or all of them actually came from Earth originally. It would be the same situation indeed if nearly all of them are indigenous Mars life. We might never find out, or the debate might continue for decades before we get some resolution of the question.

So in short, if Mars life has common ancestors with us, even as long ago as over three billion years ago, there may well be no way to recognize it as Mars life after contamination from Earth. You'd have to prove that a particular species or phyla couldn't have come from Earth. But how would you do that?

Photomicrograph of the anthrax microbe, Bacillus anthracis using Gram-stain technique. This is one of 100,000 species of microbes that have been genetically sequenced out of possibly a trillion species to be discovered, and its phyla is one of the 89 known phyla of bacteria out of perhaps 1,500 yet to be discovered.

Robert Zubrin is fond of using anthrax as an example in his talks. Yes if we found a microbe like this, we'd be able to tell that it is from Earth. But after contaminating a Mars habitat with Earth life, for instance, after a crash of a human occupied spaceship on Mars, we can expect to be unsure about the origins of 99.99999% of the species that actually originally came from Earth. So genetic tests can't tell us for sure which lifeforms on Mars originated there and which came from Earth, after contamination by Earth microbes.

Also many of the exquisitely sensitive tests that astrobiologists want to send to Mars, able to find the faintest of biosignatures or to detect just a few microbes metabolizing, would be just useless once there are Earth microbes widely spread over the Mars surface. Whenever you detect a biosignature, you'd assume it was from Earth. Even if the sample had abundant Mars life in it, you'd detect biosignatures, true, amino acids, maybe even more complex chemicals like chlorophyll, but that would not prove that it was indigenous life.

The only way to identify life as from Mars, after the Earth life contamination of a human occupied spaceship crashing there, or irreversibly introducing Earth life to the planet, would be to use gene sequencers, unless it has unusual biochemistry.

Then on top of that, there's an additional complication. If Mars life has a common ancestor, even billions of years ago, it will exchange gene fragments with Earth life very readily via gene transfer agents. This is an ancient mechanism that permits gene transfer between totally unrelated bacterial phyla, and in the right conditions, for instance in the sea, so also perhaps in salty habitats on Mars, this can happen overnight, within hours. So after introducing Earth life to Mars, as a result of all this horizontal gene transfer, you are likely to end up with a hodge podge of Earth microbes that incorporate Mars gene sequences, and Mars microbes that incorporate Earth gene sequences. Imagine how hard it would be to disentangle all that and work out which of the fragmentary gene sequences in these hodge podge microbes are from Earth originally, and which from Mars in this situation? And how much harder will that be, when, in addition, most of the microbes, whether from Earth or Mars, can't be matched with any known Earth microbe gene sequence.

Also, even if Mars has no DNA in it, the introduced Earth life could cause major complications, making it hard to distinguish. For instance suppose we find what seems to be early RNA based life and we are only able to extract RNA from the cells - this result would probably be highly controversial. It could just be a mistake that for some reason the instrument failed to identify the DNA. Even if that seemed extraordinarily unlikely, you'd still probably get many people arguing that that must be what happened. This would become especially hard to establish, if you had DNA from other Earth microbes in the habitat mixed up with it.

So if Mars and Earth life have a common ancestor at any time since the origin of DNA, then the DNA sequencing test would probably be useless as far as identifying native Mars microbes. If there is no common ancestor, or the ancestor predates DNA based life, then it would make it far harder to identify Mars life, if the only way we can do it is by proving that it doesn't have DNA. It would be far harder to find life and prove it is not from Earth than to just identify it as life by the biosignatures, especially if it has a lot in common with Earth microbes, such as RNA, and similar cell walls and other cell components.

In short, it's hard to see how introducing Earth microbes to Mars could avoid hugely complicating in situ searches for present day native Mars life, even if it doesn't make it extinct.

Well, so it seems to me. What do you think? Do give this careful thought, as we need to be very sure here. It could easily be case of gambling with what could be the most important discovery in biology and astrobiology of this century, or even, the most important discovery in biology perhaps of all of human history to date (maybe with the exception of Darwin's original theory of evolution).

Comparison of spacesuits on Mars with shirt sleeves exploration from Mars orbit and Earth - with a hundred new multigigabyte 3D landscapes streamed back each day

  • Advantages of human astronauts for fossil hunting on Mars - first - would they be fossil hunting in the first place? This all depends on Mars life having easy to recognize fossils. As we've seen, even early stromatolites are likely to be tiny (one or two cm in size), hard to recognize, and controversial once found.

    If the search is for organics, then how would the astronauts distinguish between organics from meteorites and organics from early life, just by strolling over the landscape and looking at it by eye and breaking open rocks.

    And anyway - in practice the NASA plans for Mars astronauts are to restrict them to a contamination zone around their base. They wouldn't roam widely. They would leave that for the robots.

The astronauts could spot things like the blueberries easily- but how much use is that if they can't tell if they are the result of life processes or not? Also, our rovers can find them anyway. It would work for dinosaur bones, petrified wood, and other macrofossils, which they can recognize to be life just by their visual shape - but how likely are those on Mars? When it comes to the search for organics, robots have the huge advantage that they can be sterilized and search in situ.

With NASA's plans, humans would have to explore within a zone permitted for humans, and can only explore regions that could potentially have present day life telerobotically. They have to wait for the robots to return with the samples for them to analyse. They would have to stay well away from any sensitive areas with present day life or organics exposed to the surface.

In this scenario, seems to me, that it might be quite a challenge to keep the samples from being contaminated when the robots return them to the human base. After all that's a major issue for Mars 2020 even with all the facilities we have on Earth. See Difficulty of keeping returned sample free of contamination from Earth - Mars 2020 will have a permitted 1 part per billion of Earth originated biosignatures (below) . How much harder will it be to do this for multiple expeditions by rovers away from a human occupied habitat on Mars? Yet, they would have to do better than that, to have clear incontrovertible results, especially if the signals are faint, as most experts expect them to be. How could they keep them sterile before they send them out, and to keep the samples clean even of slight traces of amino acids, when they recover them too - could they do that?

And if they can do that once, how do they resterilize the robots to better than Viking standards to send them back out again to look for a new sample to return? For more about NASA's plans,see NASA's plan for safe zones - based on finding Mars life easily (below) .

It might be better to just send the robots to Mars independently, to do in situ searches, sterilized back on Earth. Although we haven't had any robotic in situ searches for life on Mars since Viking, we can get an idea of how the search could be done robotically from the in situ tests of LDChip300 (part of the SOLID project). This is an organic biosignature detector designed by astrobiologists for in situ searches on Mars. It uses 300 different antibodies - which together can be used for exquisitely sensitive tests for organic biosignatures. They tested it in the very dry "hyper arid" core of the Atacama desert, drilling into the extremely salty "hypersaline" subsurface. From in situ analysis of just half a gram of a sample, it found a previously undiscovered microbial habitat two meters below the surface. Humans had explored the desert for decades and never found it.

Microbes in salt crystals two meters below the ground in the Atacama Desert, discovered using LDChip300, part of the SOLID program. SOLID is one of several extremely sensitive instruments, low mass, and low power, which astrobiologists hope one day can be sent to Mars for in situ search for life. Image credit Parro et al./CAB/SINC. For many more such instruments, see In situ instrument capabilities below

Future rovers would also be more robust and capable than Sojournor, Spirit, Opportunity or even Curiosity. Curiosity has made many improvements in autonomy over its predecessors, and ESA's next rover, ExoMars has made many more with its SAFER approach for autonomous driving tested in 2013 in the Atacama desert. (Video of the tests speeded up 16 times here). They have also worked on ways that it can make intelligent decisions for itself about what to photograph - something that Curiosity already does with its photographs of dust devils (for instance), extending that to other things like photographing interesting details from rocks without being asked to do it first.

But this is just a start. With better communications they could also be far more mobile. Even in the 1970s the lunakhod traveled as far as Opportunity did in a decade, in a few months. Most of that difference in speed is not so much because of the light speed delay, as because of the low bandwidth to Mars. At present the Curiosity team typically exchange data with Mars once every Martian sol, for about eight minutes, and it transmits around 200 - 250 megabytes during that window of opportunity. With that 24 hour turnaround time, they could drive it nearly as quickly if it was studying a Kuiper belt object many times the distance to Neptune.

Before we send humans to Mars, whether in orbit or on the surface, we would need almost continuous broadband communications back to Earth. NASA plans a new relay satellite for the 2020s which will increase the bandwidth to 800 gigabytes of information a day, a dramatic difference.

That by itself would make a huge difference to the pace of exploration, even with robots controlled from Earth. We'd be able to stream back millions of images, or hours of realtime HD video, every day. Back on Earth, these would be reconstructed to make a 3D landscape that we can then explore at leisure. We could even fly over the virtual landscape, and don't have to trudge around in clumsy spacesuits. Also anyone on Earth could explore it and use their expertise to look for things.

Here is a four gigapixel image made by Curiosity over a period of 14 days from sols 136 to 149. I show a series of snapshots, gradually zooming in on a single point in the image. The first in this sequence is itself a detail in a 360 degree image.:

Click here to go to the scene itself and try zooming in yourself

By the mid 2020s, our rovers will have enough bandwidth to "phone home" with two hundred of these full 360 degree four gigapixel landscapes every day.

An image that currently takes 14 days to return from Mars could be returned every four minutes throughout the twelve hour day. Or they could stream them back in binocular 3D, in which case they could stream back one hundred of these multi-gigapixel landscapes each day (one every seven and a half minutes).

Curiosity was going to have binocular 3D 15:1 zoom capabilities for its Mastcam - but that was descoped to save money, and instead it has two cameras, one fixed in the zoomed in position, and one permanently zoomed out. Mars 2020 will have a binocular zoom with its Mastcam Z though with only a 3.6: 1 zoom, giving 3-4 cm details 100 meters away. Surely future rovers will continue to have more impressive 3D zoom capabilities.

These future rovers will surely have zooms right in to microscopic detail. There are several examples on the GIGAmacro website which is a commercial system for producing automated images of this sort. This shows a zoom in on one of their images of a geological core specimen using their automated gigapixel macro images.

First zoom in:

Zoomed in to

Then to

You can try it out here with other examples on their website GIGAmacro. That's a 1.44 gigapixel image so by the 2020s we can return 555 of those a day, or in binocular 3D, 277 of these a day.

For more examples see this zoom in on a museum specimen of a Splendid-necked Dung Beetle one of several in a microsculpture exhibit by Levon Biss. More about this project here. Also for some wonderful multigigabyte photographs of microbiomes, for instance of fungi and lichens on branches, where you can zoom in to almost microscopic detail in Mathew Cicanese's Gigamicrobiome project.

With images streamed back like that, you'd be able to not just explore the surface of Mars, but also, to zoom in on any rocks close to the rover as if you were able to examine them with a geological hand lens.

Once that satellite is in operation, then every day we will get more data back than we got for the entire 500 Gb for the New Horizons flyby of the Pluto system. It would also mean we get the same amount of data that we currently will get from Curiosity after eight years and ten months, streamed back, every single day! Think how much more they could do with 800 gigs a day and with communication back and forth every eight minutes throughout the day, when Mars is closest to Earth? With these improvements, and more autonomy for the rovers, a Mars rover could travel tens of kilometers per day or even in an hour, like the lunar rover, streaming back high frame rate binocular HD video of everything it sees, wherever it goes.

Also, there's a third possibility which can combine the best of both approaches, of astronauts and of robots. We can have astronauts in orbit around Mars. They could explore it via telepresence with binocular vision, haptic feedback so that they can feel the rocks on the surface and pick them up easily, white balanced to make the landscape easier to read, and as before, stream everything they see back to Earth, where, as for the robotic missions, we can explore everything they found at our leisure and find things that they missed.

This shows how researchers on Earth or in Mars orbit could explore a Mars environment with virtual Avatars and even talk to each other within the virtual environment.

With broadband streaming of data back from Mars we could build up these environments rapidly, many times a day. Indeed with continuous HD binocular video streaming back from Mar,s we could have continuously expanding 3D VR landscapes developing in real time as the rover travels over the surface, and image the landscape in 3D to hand lens resolution around the rover wherever it goes. The same idea could also be used for teleoperation and telepresence exploration from orbit above Mars. This could build on techniques currently in use for exploring the ocean depths, for telesurgery and for computer games. See Almost Like Being There.

You wouldn't be able to actually turn over the rocks yourself. But you'd be able to roam throughout the landscape, much like a geologist on Earth, and without the encumbrance of a spacesuit. You could then mark points of interest and get those future highly mobile landers to drive, or even fly up to the site of interest, photograph it closer up, drill, or turn over the rocks.

So - how much of an advantage is it to be able to physically turn over the rocks yourself, to drill into them rather than schedule a drilling for the future, and to be able to conduct the experiments in real time? Well, though we wouldn't expect to find easily identifiable fossils, still, there's lots that a human could do to speed things up if there "in situ". However those advantages might not be as great as you'd think, if you compare the present day exploration of Mars with humans there.

Remember, that you have response times of eight minutes, between you seeing an image, and you telling the rover to do something in response to something it saw eight minutes earlier. That's a whole lot better than the current delay time of 24 hours. The rovers also would be able to travel at great speed, kilometers per hour. We just don't have any experience of exploring Mars, or even the Moon, in conditions like that, which will be common place probably by the mid 2020s.

Then there are various other ways to speed things up, especially, the idea of "artificial real time". Finally though, there's the possibility of telerobotic exploration of Mars from Mars orbit. Hardly anyone ever tries to compare in situ exploration with humans on the surface with humans exploring Mars from orbit via telepresence. And when it comes to drilling, often presented as a major advantage of humans " on site", it turns out that in the near vacuum of the Mars atmosphere in clumsy spacesuits, that drilling is probably best done by automated moles, especially since you also want to keep the drillings clear of any possibility of contamination by Earth microbes, again.

More on all this later in the section What should our objective be for humans to Mars? including the subsections What should our objective be for humans to Mars? and Telerobotics as a fast way for humans to explore Mars from orbit. Other sections devoted to this discussion include:

Meanwhile, for now, let's look at one other topic. You might think that a human being can spot photosynthetic life easily on Mars because on Earth typically you crack open a rock or salt in a desert and you see a green stain which is a sign of photosynthetic life here. Is this not a huge advantage of humans, that an astrobiologist on Mars could wander around the surface just cracking open rocks looking for life visually? Anyway, would that work on Mars? Let's just look briefly at how easy it would be to see green on Mars, and what colour Mars life might be.

First there's the question of whether you could see the colour green at all. That's not as much of a problem as one might think at first. But then there's the question of what colour life would be on Mars. Would it be easy to spot like the (often) green photosynthetic life on Earth?

Could we see green on Mars?

Green is a rare colour on Mars. Even purple is. So, you might think it would be easy to spot on Mars. You might have to break open a rock, but after that it would be easy to see. Here for instance is what green photosynthetic life looks like in the core of the Atacama desert.

Figure 2 from a paper on distribution of scytonemin, a UV protecting pigment, in cyanobacteria in the Atacama desert. Cyanobacteria typically are green, as are many (but not all) photosynthetic lifeforms on Earth. You'd think this would be easy to spot on Mars.

However, would Mars life be as easy to spot as this? There's one thing to look into briefly first. Colours such as green are likely to be hard for the human eye to spot on Mars. The problem is that light on Mars has a permanent reddish brown cast to it, because most of the blue light is filtered out of the sunlight by dust. Even if there were green rocks there, you wouldn't notice them, probably The pictures we see are all white balanced to help geologists on Earth recognize rocks. This for instance is what the Curiosity Mars dial looked like on Earth.

Curiosity Mars dial as photographed on Earth before attaching it to Curiosity.

This is what the same dial looks like on Mars. It hasn't faded. It's the same colours as before, but the reddish brown Mars light dulls out most of the variation of colours.

Curiosity's "Mars dial" colour calibration target as photographed on sol 69. Raw unprocessed image. This gives a rough idea of what it would look like to human eyes on Mars with the natural ambient light there.

The green patch there is barely distinguishable from the gold and even the bright red. To human eyes everything in the Mars landscape, however brightly coloured, will seem to have subtle shades of brownish gray.

The amount of blue light removed isn't that much actually; it's only reduced to between 42% and 62% of the original according to one study which was based on taking photographs of the calibration target (the "Mars dial" above). Even the red light is absorbed to between 60% and 80%. However, that imbalance is enough to change the balance of the colours significantly to human eyes.

This shows the amount of light transmitted through the atmosphere for global sunlight from 400 nm to 1000 nm as measured using photographs of the Mars Exploration Rovers calibration target (compared with its pre-flight calibrated reflectance). This was at a time when the sky was reasonably clear of dust, though not at its clearest (in a scale according to which the optical depth 1 is more or less opaque, Opportunity had an optical depth about 0.94 for Opportunity and Spirit had a depth of 0.93 - in winter when the air is much clearer Spirit's optical depth goes down to 0.2). Shows the linear visible spectrum superimposed.

Here is the original image of the cyanobacteria in the Atacama desert adjusted with the red reduced to 80%, green and blue to 60%, and then increased in brightness (corresponding to the way our eyes auto balance the brightness of images). It's very approximate, just to give a rough idea of how the colours would change with Mars illumination.

That's why nearly all the images shared with the public here are white balanced - colour adjusted so that the brightest colour in the image is white. That's also presumably why the National Geographic Mars TV series had so many bright bright colours in them, on the suits, buildings, and the landscape much brighter and colourful than it would actually be. If they had shown the colours realistically as they would appear to human eyes, the scenes would have been dim and hard to make out.

So a visual search on Mars based on colour would be hard to do without enhanced vision. But this may not be too hard to fix. An easy way to deal with it would be to tint their spacesuit visors cyan (close to sky blue) to let through more blue and green and block out some of the red light. The landscape would be a bit darker but they could pick out colours more easily because with reduced red, the green and blue colours in the landscape would be more obvious.

Cyan is the colour of white sand seen through shallow water because the water filters out red, leaving green and blue. Astronauts with cyan tinted goggles would see the Mars landscape with much of the red filtered out, in a similar way which could make it easier for them to see colours such as green and blue in the landscape.

Am I the first person to suggest that astronauts on Mars would have cyan, or perhaps "sky blue" coloured spacesuit visors :). I expect they would have cyan tinted habitat windows too, if we do ever have astronauts on Mars. Or even simpler, they could just wear sky blue goggles inside the suit and inside their habitat to colour correct the landscape to Earth vision.

Something like this might be a vital piece of equipment for future astronauts on Mars if we do ever send humans to the surface, worn inside their spacesuits and inside the habitats to make the landscape look much bluer and so more like an Earth landscape. Or alternatively, they have all their spacesuit visors and habitat windows tinted "sky blue" or cyan. Image public domain from Pixabay.

So, this is a nuisance but it's not a big deal as we can just use cyan coloured spacesuit visors to fix it. This would make the landscape a bit darker, but especially if searching with the sun high above the horizon in the tropics then you should be able to see colours fine. When searching in low light conditions, these tinted spectacles would dim the light further, so then, digitally enhanced vision would help. When searching with robotic spacecraft or telerobotically then we can use digital enhancement automatically.

So, if the photosynthetic life on Mars is indeed green, it might be easy to spot so long as you wear sky blue tinted glasses. Then indeed, you could split rocks open to find it, just as on Earth.

What colour would photosynthetic life be though, on Mars?

Would photosynthetic life on Mars be green - or could it be other colours such as red, purple, orange, yellow, brown or black?

What colour would Mars life be? H.G. Wells in War of the Worlds speculates that it could be a vivid blood-red.

"Apparently the vegetable kingdom in Mars, instead of having green for a dominant colour, is of a vivid blood-red tint. At any rate, the seeds which the Martians (intentionally or accidentally) brought with them gave rise in all cases to red-coloured growths."

Of course we now know that Mars doesn't have vegetation like Earth, but it could still have photosynthetic lichens and microbes. What colour would they be? I can find almost nothing specifically on this question, in the literature, no articles on the possible colours of photosynthetic life on Mars. The closest I could find was some material on the possible colours of photosynthetic life on exoplanets with low light levels. So I invite you to a section of synthesis and speculation.

Actually, we'll see, H.G. Wells' red or rust coloured does have some advantages on Mars, but there are many other possible colours too. It's actually a bit of a puzzle why Earth photosynthetic life is green. After all the green colour of Chlorophyll means that it absorbs mainly blue and red light, It rejects light in the strongest part of the solar spectrum. Why aren't leaves dark red, or dark purple, or indeed black?

Perhaps it is all just a historical accident. Leaves might be green because they get plenty of light, need to reflect some light away to resist dehydration, and there is no advantage in changing to black or purple. Or perhaps early life was indeed purple, absorbing green, like the modern haloarchaea, and photosynthetic green microbes originally evolved to take advantage of the light that the purple microbes rejected? That's the "Purple Earth Hypothesis" (abstract here). Or perhaps green is reasonably optimal anyway, once you take account of other effects?

At any rate, whatever the reason why so much Earth vegetation is green, yes, green photosynthetic life does work in Mars simulation experiments, for instance in the DLR lichen experiments (see Lichens and cyanobacteria able to take in water vapour directly from the 100% night time humidity of the Mars atmosphere (below) ). So Mars could potentially have green photosynthetic life. But this is hardly the most efficient way to make use of the spectrum on Earth, and it would be even less efficient on Mars with half the light levels of Earth, and with the dust filtering out much of the light towards the blue end of the spectrum.

Solar radiation for direct light at the top of the Earth's atmosphere, and at sea level. Shows the linear visible spectrum superimposed.As you can see, we get more green light than any other frequency, yet for some reason, most of our vegetation is green. Would Mars photosynthetic life be green too? The sunlight at the top of its atmosphere would be identical, though half the intensity.

Well actually, even on Earth we have many photosynthetic microbes that are are red, purple, or pink.

Lake Hillier in Western Australia, a "pink lake". It's pink partly because of the purple haloarchaea, and partly because of red carotene accumulating in a green algae dunaliella salina.

For instance Mars life could be like the purple haloarchaea which are able to survive in very salty conditions, and use Bacteriorhodopsin and Halorhodopsin for photosynthesis which resemble the pigment rhodopsin that we use for vision. Bacteriorhodopsin is a purple and absorbs green light most efficiently.

Lake Hillier is also pink because of ordinary green algae which have made carotenes, the same pigments that make a carrot red. These pigments absorb UV, violet and blue light, while scattering red and orange light. They dissipate some of this light as heat so protecting other organics such as proteins and membranes from the damaging effects of UV light, which would be useful of course on Mars. However, they also transmit some of the energy they receive to chlorophyll.

Pigments like carotene that can transfer energy to chlorophyll like this are known as "antenna pigments" and they do it by dipole to dipole coupling (the process is called Förster resonance energy transfer). There are many other antenna pigments which photosynthetic life on Earth uses, and so Mars life could probably use them too, or something similar.

For instance, Mars life could have the equivalents of the yellow pigments Chlorophyll b and xanthophylls (which colour egg yolks and autumn leaves yellow) which have similar roles. Another example is Lycopene which makes tomatoes red. Cyanobacteria and red algae also have phycocyanin and allophycocyanin which absorb orange light, and a red pigment phycoerythrin which absorbs green light. Any of these could be useful on Mars

Deep dark red of algae in the crater lake of Mount Simba volcano at a height of 5,900 meters in the Altiplano, Chile. The microbes have developed special pigments to cope with extreme levels of UV. A few years ago the researchers measure what remains to this day the highest levels of UV measured in the world. Image credit SETI Institute/ NAI High Lakes Project

Life uses lots of different pigments to capture light in many areas of the spectrum. This next diagram shows how it works. For instance, Chlorophyll a can only absorb a narrow band of light in the red part of the spectrum (688) and Chlorophyll b in the blue part of the spectrum. But as you can see, there are other antenna pigments that help it take advantage of other parts of the spectrum, and Mars life could have these pigments, or similar ones, coloured purple, orange, pink etc in colour.

Part of Figure 1 from this study of the colour of life on Earth and exoplanets.

This graph doesn't show the purple "retinal pigments" such as bacteriorhodopsin, just chlorophyll and antenna pigments that work with it.

However, the most efficient way to do photosynthesis is to capture all the light that lands on a plant. And actually, photosynthetic life that evolved in low light conditions might need to be much more efficient at absorbing light than it is on Earth. Jack O’Malley-James of the university of St Andrews, Scotland, has suggested that life which evolved around red dwarf stars, especially binary star systems, could be dark in colour, or black, because it would receive far less visible light than Earth life does, so would need to make use of as much of it as possible. We do have black plants on Earth though they are rare.


Examples of Earth vegetation with black flowers or leaves. When levels of light are low, then plants may become dark in colour or black to use as much of the spectrum as possible.

Possibilities for plant life under low light levels around other stars - perhaps in low light conditions the vegetation would be black.

We have dark coloured microbes too, as well as the black plants, and dark coloured seaweeds too. Some red algae and brown algae are nearly black, and grow in depths around 270 meters where the light is less than 1% of surface light. Life on Mars might experience not dissimilar conditions. That's especially so, since any photosynthetic life on Mars is likely to be hidden in cracks or beneath thin layers of dust or underneath the crust of rocks in order to shelter from UV light, so it might well be evolved to gather as much light as possible in dark conditions like that. The red iron oxides of the Mars surface are especially good at filtering out UV light so photosynthetic life might well use it as protection from sunlight, so that takes us back to H.G. Wells' red plants idea. Perhaps a rusty red could be useful to protect against UV.

This likely semi-shade habit, huddling in cracks for protection from UV, cuts down the available light more. Dust storms then would cut it down even further. So Mars:

  • Already gets half the visible light of Earth life because of distance
  • Blue is reduced further because of the dust in the air
  • Photosynthetic life likely to huddle beneath the crust of rocks or in the dust in semi-shade for protection from UV.
  • Photosynthetic life on Mars would receive far less during dust storms, less even than for a red dwarf star, and the storms can last for weeks.

The next few diagrams show how Mars life will have to cope with low levels of light during dust storms:

This shows photographs taken by Opportunity during a dust storm from sols 1205 to 1236 (one month). Each horizon view has been compressed horizontally (but not vertically). By the end of this period it reached a visual optical depth tau 4.7 which means that 99% of the sunlight was blocked. However that is for direct light.

Of course the dust will also scatter a lot of light, and if you include ambient as well as direct light then the figures are not quite so extreme. Here are a couple of graphs to show the distinction between light levels from ambient light combined with direct light, and the light levels from direct light alone, during a Martian dust storm.

This shows theoretical prediction of the combined direct and ambient light for different optical depths on Mars. Here optical depth is a number which is defined in such a way that, the larger it is, the more the light is blocked out. Tau 4.7 corresponds to around 99% of direct light blocked, which can happen during a dust storm on Mars. However, much of that light is scattered so adding to the amount of ambient light.

The curves in this graph show the amount of the combined direct and ambient light. The numbers 10, 20, 30 etc show the angle of the sun from the zenith - so with the sun at 90 degrees from the Zenith it's horizon skimming with very little light, and if there is an dust it goes through thick layers of dust, with almost no illumination.

So for instance, to find out how much light you get on the surface at the height of a typical dust storm go to around 4.7 in this diagram. Of the original 600 watts you still get around 200 watts when the sun is vertically overhead, at its zenith, even during a dust storm. However, you get it greatly reduced when the sun is closer to the horizon, as you'd expect, almost to nothing.

This shows the irradiance for direct sunlight. Very much less. During a Mars dust storm there is almost no direct sunlight with the sun at any angle from the zenith, even if it is directly overhead. Graphs from this 1999 paper. Our ideas of Mars have changed a bit since then but not enough to make these significantly out of date I think, at least for our purposes here. If you know of more recent graphs do say.

So, during a dust storm there is almost no direct sunlight. The amount of indirect sunlight is cut to a third even when the sun is directly overhead, at the tropics, and much more at other times of day even in equatorial regions (and much more so at higher latitudes). So, this is just my own suggestion here. Might some Martian life have very dark photosynthetic life in order to take advantage of as much light as possible during dust storms? Kelp for instance and other forms of seaweed that are adapted to the lower light levels in the sea are often brown in colour, to absorb as much sunlight as possible.

Giant kelp is brown to absorb as much sunlight as possible. It's coloured brown because of the accessory"antenna" pigment Fucoxanthin which absorbs light in the blue-green to yellow-green part of the spectrum. Some photosynthetic microbes are dark in colour too. In the conditions on Mars with dimmer light and the dust storms, might photosynthetic life on Mars be dark like this to use as much of the sunlight as possible for photosynthesis.

On the other hand, unless it is well sheltered by dust, or within rocks, it would also need to reflect away UV light for UV protection and may need to prevent desiccation. Perhaps another likely colour could be purple?

"Plants would appear darker under much dimmer, redder stars that emit more infrared than visual wavelengths of light but the color could vary widely"

Colour adjusted photograph by Tim Pyle of Caltech to illustrate possibilities for vegetation around other stars. Red dwarf stars particularly would have much more light in the red and infrared and photosynthetic life could have evolved to take advantage of it.

Or indeed, if it used iron oxides somehow for protection from UV light, it might be rust red in colour. Or perhaps it absorbs red light too, for the maximum amount of photosynthesis during dust storms, but it rejects blue, which is only present on Mars in small quantities, making it a dark blue in colour when seen with our sky blue tinted glasses (from the last section)?

So from those examples, it's hard to know what colour photosynthetic life on Mars could be. It might well not be green. It could easily be various shades of red, yellow, pink, purple, dark blue, or indeed almost black, amongst other possibilities.

What about non photosynthetic life. Actually, we don't know if Mars does have photosynthesis. If it doesn't, then we will be looking for life depending on chemosynthesis, in salty brines perhaps, just below the surface. Also, unless photosynthesis is common everywhere on the surface of Mars, those might well be the first lifeforms we find, protected from the surface UV by a cm or so of dust, and relying on chemistry rather than photosynthesis as their source of energy. If so, even if it doesn't photosynthesize, it can be damaged by UV if it is ever exposed to the sunlight. So, again, it's likely to use the red coloured surface rusty iron oxides for UV protection, or if it uses carotene for UV protection, it's again likely to be reddish in colour. By way of example, one of our most ionizing radiation resistant microbes, not a photosynthetic lifeform, Radiodurans ranges in colour from red to pink. That's because it uses carotene for UV resistance. It is thought to have got its ionizing radiation resistance incidentally as a desert species from desiccation resistance (which has similar DNA damaging effects). Radiodurans requires oxygen (it's an obligate anaerobe), so it's not a candidate microbe for present day Mars (except perhaps as part of a community with photosynthetic life to provide oxygen for the microhabitat?). But it's an example to show that UV resistant non photosynthetic life could easily be reddish in colour on Mars. Life doesn't have to be in the form of algae or lichens to take advantage of red colouration for protection from UV.

So, even non photosynthetic UV resistant life might well be red or purple on Mars. This is one thing the National Geographic sequence got right. It seems quite possible that Mars could have purple lifeforms, or red, or pink, though it could also be many other colours such as black, dark blue, orange, yellow, etc as well as green.

Rust coloured or red life would be hard to spot amongst the iron oxides even with filtered vision. And dark or black life, hidden beneath a layer of dust, or in the shadow of a crack, might be almost impossible to see by eye. Green photosynthetic life is still possible however, and would be easy to spot with sky blue tinted visors.

You couldn't split rocks like that in the VR landscapes streamed back to Earth. Though you could also do this from orbit via haptic feedback. You could also mark a bunch of rocks you want the rovers to split, from Earth, leave it to them to split them all, then go back and look at them in VR from Earth as it splits them, and when closest to Earth, you could mark a bunch of rocks for splitting, and then start to see the result of your instruction as soon as eight minutes later. It's not as streamlined and as easy as it would be if you were there in situ, but you could learn to work around the issues and make efficient use of what you can do. However, to confirm that it is life, you'd have to analyse it and search for organics and other biosignatures.

Why is it that, decades after the Viking landers, we still know so little about life on Mars?

We can also get some insight into the difficulties of searching for life on Mars with humans from our experiences with the Viking spacecraft, which helps bring another perspective to Robert Zubrin's arguments. You might wonder why we know so little about Mars biologically, even after several decades. Is it because we haven't sent humans there yet? Not really.

First, it's hard to send landers to look at the best places on Mars for present day life because of the planetary protection issues. Curiosity is just a few kilometers away from some dark streaks on Mount Sharp, which may be possible habitats for present day life there. It would be wonderful to go close up to check them out, but it can't go up to look at them because it isn't sufficiently sterilized. It may be able to take photos of them from a few kilometers away.

The fictional astronauts in the Mars National Geographic TV series can just walk straight up to a possibly habitable Recurring Slope Lineae (RSL) in their spacesuits, and pick out a sample of life with tweezers. But in real life, Curiosity can't approach them, although sterilized to have less than 300,000 spores over its entire surface on leaving Earth, and with far fewer spores than that surely by now after the journey to Mars and the UV radiation on Mars itself. Even with such a sterile rover, the risk of introducing Earth life to the RSL is thought to be unacceptably high if it approached them.

What's more, even the Viking sterilization levels weren't designed for a rover to actually contact habitable liquid water directly. Just a few viable spores might be enough if it actually touches the water and those spores happen to be pre-adapted to be able to survive in those conditions. I'd actually argue that we should aim for 100% sterilization, when exploring potentially habitable liquid water in our solar system. Though we can't do it quite yet, I think such levels are not impossible to achieve in the near future. We need to create conditions that a microbe can't survive, but our rover can, and with modern technology, that's actually within the bounds of feasibility, indeed there's been a serious proposal already for a 100% sterile lander for Europa. I discuss this in detail later in this book, see Can we achieve 100% sterile electronics for an Europa, Enceladus, Ceres, or Mars lander? and Design change suggestions that could make a lander close to 100% sterile.

We've known about the RSLs since August 2011. But we haven't yet sent a rover to look at them. Nor do we have any plans to do so in the near future either. It's at least partly because of these planetary protection issues, how do we sterilize them sufficiently? The Viking methods can't be used with modern spacecraft because it would damage the sensitive electronics, put delicate instruments out of alignment, etc. So we have to use new methods which are still being developed though some have reached maturity already. They are also on very steep slopes. Far steeper than in the National Geographic movie, 1 in 2 slopes, or about 33 degrees. This doesn't mean that we have to send humans to explore them. Indeed, humans are more vulnerable than robots. An astronaut could easily stumble, fall, and perhaps die, on a 1 in 2 slope. It would seem much steeper to someone trying to walk up it. It would also be a slow and hard slog in cumbersome spacesuits. Or they could rappel down from above - but robots can rappel as easily as humans.

Here is one way it could be done, with a rover which drives to the top of a cliff with RSLs and then lowers another rover over the edge using a cable which then can explore the RSLs.

AXEL Rover - Mars Cliffbots - this is one way a rover could explore the RSLs, to rappel down the steep 33 degree slope, much like a human astronaut would do.

There are many other ideas for ways we can explore Mars telerobotically, to access hard to reach places such as cliffs, caves etc and to expand the search radius of our rovers. We can also use these approaches robotically from Earth. Most of these ideas have never been tried out, though there is an idea to add a helicopter to Mars 2020. For more on this see Small planes and entomopters etc. See also the Origami rover which is capable of climbing a 45 degree slope.

So, with that background, how can it be right to send a human expedition with telerobotically operated rovers that set off from a human base on Mars to explore the RSLs? I don't get it. Why not do it from orbit?

Then, as well as that, even if a rover was sufficiently sterilized and sent right up to a potential habitat, rappelled over the cliff and down to the RSL to look at it, it still might be difficult, even very difficult, to spot any life there. At least, our only attempt to search for life to date, in the 1970s with Viking suggested that it is likely to be difficult to do. If you crack open a rock on Mars, or scrape away some dust, and the life is rust red, or dark brown say,, mixed in, in small quantities, with rust red or gray soil, how would you recognize it as life? How could you tell that it is not, for instance, organics brought there on meteorites? Which might also be the same colour? You need some way to detect it as life, in situ, it would seem.

This is not a problem we face on Earth. If you find organics, it's almost certainly from life. Not necessarily alive of course. It may be from present day life but decomposed long dead. Ii may be from past life, long ago, but it's a clear sign of life in most circumstances, a clue that this is an interesting place to do a biological investigation. The only likely chance of finding organics from meteorites is if you actually analyse a meteorite fragment, because any other organics that reaches the surface is quickly scavenged by Earth life. Even if you analyse meteorite fragments, you have to get to it quickly after its fall, or find it in some environment with hardly any life, like Antarctica, to have a chance of finding unmodified meteorite or comet organics in it. Generally you can just forget about meteorite organics. Unless you already know it's a meteorite, if you find organics, it's from life, present day, or from the distant past.

But that's not the case on Mars, not at all. Curiosity has detected organics on Mars, but it is thought to have come from meteorites. Similarly, in the future, if Mars 2020 detects organics, or indeed ExoMars or any other future expeditions to Mars, the first assumption will always be that it comes from meteorites or comets, unless you have a clear indication that it doesn't. It's the other way around from Earth, where you can assume organics come from life originally unless you have clear indication that it's from some other source.

That's because Mars has a constant rain of meteorites that hit the surface bringing fairly large quantities of organics to the planet, and meanwhile, if it does have present day life there, it's not likely to produce much by way of organics, and any past organics, unless buried deep below the surface, is likely to have been destroyed by cosmic radiation over timescales of hundreds of millions or billions of years.


Detail from the drilling which provided Curiosity's first detection of organics. The organics seem to come from meteorites. Past and present day evidence of life on Mars is likely to be like this, soil or mud that has organics in it. Organics doesn't normally mean life on Mars, as many meteorites have organics in them and the main puzzle for Mars is to understand why it has so little organics. It should have a lot more and there is some process actively destroying it.

If there was no degradation of the organics, Mars should have 60 ppm of organics from organics deposited into the regolith, averaged over its entire surface to a depth of a hundred meters (see page 10 of this paper). So, if we find organics there, the first guess is that it is organics from meteorites that has escaped destruction from the surface processes that clearly destroy both organics from life and from meteorites and comets. So, to tell if there is life there, it doesn't do at all to just detect organics. That's likely to mean nothing. We need to use life detection instruments. Even then it needs care as there are potential biosignatures such as a chirality imbalance, or nitrogenous organics, or complex sugars, that can be mimicked by organics from meteorites. For more about this, see the section Organics created on Mars by non life processes (below)

Now, a few people actually think that the two Viking landers may have found life on Mars already in the 1970s. If this is so, it might be easy to find present day life. We might just need to send sensitive life detection instruments to check the sand dunes in similar locations to the Viking landers.

So did it find life? Well, even with all the extra information we have today after several more rovers, landers and orbital missions, we still don't know enough to rule it out (or confirm it). We can't say, totally for sure, whether Viking found life or not in the 1970s. Indeed some new research has actually, if anything, increased the chance that Viking actually did discover life in the 1970s. What makes it so tricky to tell is that Levin's labeled release experiment on Viking was extraordinarily sensitive for its day. It remains by far the most sensitive experiment we have ever sent to Mars, indeed the only one we've sent that had a chance of detecting life in such small quantities. Technology has moved on of course, and we could easily send more sensitive life detection experiments to Mars, or send the equivalent of the Viking labeled release as a much smaller payload. But all the instruments since then have focused on geology and habitability rather than a direct search for life.

The Viking labeled release was ahead of its time in sensitivity, as it was able to detect microbial respiration from just a few cells, even if they couldn't reproduce in the culture. It just detected whether any of the radioactive carbon in the organic "food" gets exhaled as a gas. Our detectors can be very sensitive to radioactivity, so it doesn't take much by way of microbial respiration to generate a signal. His experiment also assumed very little about Mars life. It depends on the Mars life being able to survive in the culture medium and not be poisoned by it, but there is no need for it to reproduce and grow. So it doesn't have to be "cultivable". We don't need to know how to create conditions for it to reproduce. Just not kill it, and create conditions where it can take up food and respire.

So, the idea was that even if Mars life has a different biochemistry from Earth life, even if it doesn't use DNA or proteins, still, if it works anything like Earth life, it is likely to exhale carbon in some gaseous form when it metabolizes organics. If it does that, that's all that's needed for his experiment. In preliminary tests before the mission, published in 1976, the experiment detected life in all the samples from Earth except for a few samples from Antarctica that might have been sterile. So, if there is any life like that and it starts to metabolize the organics into gases containing carbon, and is not killed by the culture medium, then Viking's labeled release would detect it.

This sensitive experiment did seem to detect respiration, just as it was designed to do, but it could have been confused by the chemistry of the Mars soil. The main reason for skepticism about this possible "detection of life" is that Viking didn't find any organics. However, as the scientists discussed the results, they found out that the organics detection experiments on Viking also may well have been confused by the same chemistry that confused the labeled release - what we now know to be perchlorates and other super-oxides in the soil. Also the Viking organics instruments were not as sensitive as the labeled release, and would not have been able to pick up the small levels of organics you find in cold dry deserts on Earth that we know contain life.

Meanwhile, later researchers have also found ways that the complex chemistry of Mars that we now know about, with the perchlorates, could have mimicked some at least of the characteristics of life, enough perhaps to have fooled the labeled release experiment. So the jury is out on this.

The Viking landers did find extraordinarily harsh conditions in the equatorial regions where they landed, so harsh that life there may seem to be between unlikely and impossible. At the time that was another reason for skepticism about the labeled release results. However, it's not such a strong reason now as there are a few ideas now for ways that life could be possible there, including:

Also, Viking could have found dormant spores in the dust and sand, which then revived. It didn't have to sample a habitat directly to get a positive signal. The spores would just need to wake up and start metabolizing. So, in one way or another, it's not impossible that it did find life.

Some of the 1970s labeled release results are puzzling for life. However other results are just as puzzling for the non life explanations. Also, there's been some renewed interest after another researcher re-analysed the old Viking data and spotted patterns characteristics of circadian rhythms (metabolic cycles) significantly offset from the temperature variations. This is especially hard to achieve through complex chemistry without life.

Patterns characteristic of circadian rhythms in the Viking labeled release data. The interesting thing is that they are significantly offset from the temperature variations, which to an expert on circadian rhythms who spotted this, strongly suggested life rather than non life processes. More on this in the section Rhythms from Martian sands - what if Viking detected life? (below) and following

With all these ambiguities in the results, and many still unresolved puzzles, we have no way to answer this question definitively at present. Probably most scientists who specialize on Mars would still say that they don't think Viking found life, though if not, it surely did find some form of unusual and rather complex chemistry in the Martian soil not yet fully understood in every detail. A few think that it is possible that it did find life. You can't say at present that those who think it found life are definitely wrong in those views.

This example of Viking helps highlight how hard it is likely to be to search for life on Mars. But our experiences with Martian meteorites show that it is equally hard to search for life using samples returned from Mars. It just is a tricky thing to do, which seems likely to need great care and multiple lines of investigation.

The obvious thing to do for Viking was to send a follow up mission to find out what happened, and settle the question. Not to settle the question of whether Mars has life of course, as this is only one of many possibilities for life there, but to settle the question of whether Viking discovered life in the 1970s. But the problem with this is that you need to get your instrument approved for launch to Mars when competing with many other instruments with other objectives. There's a rather simple update of the Viking experiment which has the potential to solve this question quickly, in a positive way, actually confirm life on Mars. It could also rule out many possibilities if it came out blank.

Many organic molecules come in two mirror image forms, such chemicals are called "chiral". Modern Earth life can only eat one of them - for much the same reason that all DNA spirals the same way. The mirror image chemical is inedible. Astrobiologists think that even unrelated life on another planet is likely to work in a similar way.

So the natural follow up experiment is to feed the sample with amino acid "food" of only one type, then try the mirror image chemical in its place and see if there's a difference. If whatever is in the Martian soil can only respond to one of these chemicals, then what Viking discovered surely must be life, or something else approaching the complexity of life. While if it is just chemistry it should make no difference which of the two mirror image chemicals you feed it. This is the "chiral labeled release experiment".

A chiral labeled release would not be totally conclusive as there are some ideas for biochemistries that are chirality indifferent, especially for early life and prebiotic "almost life" (see Joyce's ribozyme and "autopoetic" cells (below) ). Conceivably the Viking labeled release could have detected some non chiral form of life or an interesting chirality indifferent life precursor. So, it can't disprove the detection of life. However, if Viking did detect life, the chiral labeled release has a good chance of a positive result. And if there is no chiral dependent response, the chance that Viking detected life gets significantly reduced because non chiral life would seem quite unusual.

So, this is the obvious next follow up for the Viking results. If this was an experiment on Earth then it would have been totally non controversial. They would have done a chiral labeled release right away. Depending on what it found this could confirm that Viking found life, or, it could give more information to constrain non biological explanations. Or indeed it could still be inconclusive, in the remote chance that perhaps what Viking found was chirality indifferent biochemistry. But that's how science goes. Depending on what it found, you'd then work on more follow up experiments to address the remaining questions and ambiguities, until you understand the situation well enough to announce a result, either way. If Viking did discover life, it's possible that this simple experiment could have confirmed it decades ago.

However, sending instruments to Mars is of course immensely expensive, and something that only NASA has been able to do successfully since Viking. The main problem is that NASA changed its emphasis after Viking and stopped looking for past or present day life on Mars, instead focusing on past habitability. It's a matter of emphasis, more than an absolute rule, but the outcome is that the chiral labeled release was an excellent fit for its Viking old remit to search for life directly, but it isn't such a good fit for NASA's new remit to search for evidence of past habitability. Specialized life detection instruments get beaten by other experiments that focus on their main goal. The only way to do life detection with a decent chance of approval would be to send an experiment that can do both - search for habitability, and also do biosignature detection as well. However, sadly, the chiral labeled release is specialized for life detection. It's not going to help with the search for past habitability.

As a result, none of NASA's rovers since then have been equipped with instruments anything like as sensitive as the labeled release experiment, either for organic biosignatures in such low concentrations or to find signs of metabolism. We have many such instruments now, and some were actually approved for a launch and then descoped. But we just have never flown them in space. Russia does not have the same policy as NASA, and was interested to send Levin's chiral labeled release experiment to Mars in 1996, but it was descoped and then the launch failed. So for now, nobody can say for sure whether Viking discovered life on Mars or not. Also NASA was interested to collaborate with ESA to send UREY to Mars to search for biosignatures, and UREY was actually developed in the US. So it's not like they have no interest at all in this. But they pulled out of this deal, leaving ESA to find another partner, Russia, for ExoMars. This then reduced the payload capacity for ExoMars leading to UREY being descoped.

I go into this in much more detail in the sections Rhythms from Martian sands - what if Viking detected life? (below) and following.

The main point right now though is that this experience with Viking shows how hard it can be to search for life on Mars. It is easy to get ambiguous results, as Viking did. Then when you do, with transport to Mars so expensive, it's hard to do the obvious follow up experiments. If Viking did find life, perhaps we confirm this with the first follow up mission to search for it directly. But if not, well we are going to want to do similar life detection experiments in many other locations on Mars until we find what we are looking for (if it is there to be found). So, to get back to Zubrin's arguments, imagine trying these sensitive Viking experiments with humans there as well who contaminate everything around them with Earth life. Now imagine that you get similarly ambiguous results, which is surely rather likely in your first searches for life there...

Contamination by Earth life would make the types of experiments that flew on Viking far harder to operate, and perhaps impossible. If we do ever send the chiral labeled release experiment to Mars, it is so sensitive that a single Earth microbial spore that gets into the nutrient and finds it to its liking, would be plenty enough to confuse the results and make it no longer useful. There are many other modern experiments that are as sensitive as this, that we could fly in the near future, and some that are much more sensitive to contamination. We have experiments sensitive enough to pick up not just a few microbes, but just a single molecule of a biosignature in a sample.

Microchannels like this can be used in a "lab on a chip" to move minute quantities of liquid about for analysis. The Astrobionibbler is an end to end instrument design which drills into the Martian soil, takes a sample, mixes it with water at around 180 C kept liquid at high pressure. The water at this temperature can dissolve out organics because it turns into a non polar solvent. The organics are then labeled with fluorescent dyes. The resulting system is so sensitive it can detect a single amino acid in a gram of sample, and the whole system, including its sample collection, weighs only 2.5 kilograms.

With instruments like this we can increase the sensitivity for in situ searches for organics hugely over Viking. With the chiral labeled release we can also send exquisitely sensitive metabolism tester to Mars which would be able to detect life so long as it metabolizes (doesn't have to grow at all) even slowly and so long as it has a preference for one amino acid over its mirror image, as most life probably does according to astrobiologists.

These experiments are exquisitely sensitive, designed for the extremely challenging conditions we now know exist on Mars, and would be confused by even the smallest traces of life from Earth.

Viking may or may not have found life. However, whatever the situation about that, astrobiologists now think that our experiences with it serve as a useful precedent. We are likely to face similar problems wherever it is that we search for present day life on Mars. In such cold, harsh conditions, any life is likely to be sparse, slow growing, and require exquisitely sensitive instruments to detect it. Just a few organic molecules from Earth life could confuse the biosignature detection, and a few Earth microbe spores would throw them off completely.

Any discovery of life there that's native to Mars will be such a major discovery. Extraordinary discoveries require extraordinary evidence. If there is only one chance in a million that somehow a microbe spore or dormant state from Earth has got into the Mars sample, then there will be doubters who will say "perhaps it is just Earth life". Just the possibility of such contamination, whether it happened or not, would be enough to cast the whole thing in doubt and greatly complicate the process of confirming life on Mars, even if the Earth life doesn't make it extinct.

In short, modern life may be scarce and hard to find and we may need sensitive instruments to find it, and to try a wide variety of different ways of testing for the presence of life there. With such harsh and unfamiliar conditions and such sensitive measurements, a few stray spores from Earth might well be all that was needed to hide the signal altogether, even if Earth spores don't reproduce there.

I agree, Zubrin's arguments may seem persuasive at first, especially if you are keen for humans to touch Mars. But once you reflect on those points, they may not seem quite so compelling. For more on this see also How a human spaceship could bring microbes to Mars - Zubrin's arguments examined in my MOON FIRST Why Humans on Mars Right Now Are Bad for Science. So anyway I leave that as something to reflect on.

Meanwhile, let's turn to the last, and for many, the most persuasive of Zubrin's arguments.

What about Zubrin's meteorites argument?

This may seem one of the most convincing arguments of them all at first. If life gets to Mars from Earth on meteorites anyway, what does it matter if we take it there in our spaceships? So I'd better go into this in some detail, as again, some readers will think that there is no need to read any further if I don't answer this.


This one takes a while to answer properly. Can life be transferred to Mars from Earth anyway? And if so, could so many species be transferred that Mars and Earth have essentially the same ecosystems? And if the two ecosystems did turn out to be essentially the same, would that then mean that there is no reason to take any care about whether we introduce Earth microbes to Mars?

I'll look at whether life can be transferred to Mars from Earth in the next few sections. But first, supposing Robert Zubrin was right here, and we did find that Mars life is nearly identical to Earth life. That by itself would be a hugely surprising discovery to many, and it would actually make our job of protecting Mars from Earth life harder, if anything. As Cassie Conley (NASA's planetary protection officer), said about this argument, quoted by National Geographic:

“It becomes more difficult and more important to prevent Earth contamination if Mars life is related to Earth life. If we're totally different, then it's easy to tell the difference. If we're related to each other and we want to study Mars life, then we really need to make sure that we don't bring Earth life with us.”

She is an astrobiologist and microbiologist and the current planetary protection officer for NASA.

If Mars life is related, it would give us a wonderful opportunity to learn about how easily meteorites can introduce life to another planet, and about what happens to life which is transposed to a planet as different as Mars is from Earth. For instance, does it evolve in a different direction, under the very different conditions on Mars. Or is Robert Zubrin right, does it just keep identical in lock step with evolution on Earth, to such an extent that you can't tell which planet you are on from studying the microbes? If so, why and how does that happen?

Alberto Fairén and Dirk Schulze-Makuch put forward this meteorite argument in some detail, in "The Over Protection of Mars"., published in Nature Geoscience in June 2013. Many humans to Mars enthusiasts will have heard of this paper, which is popular amongst them of course, because of its message that we don't need to take any precautions. Naturally enough, a paper so optimistic that humans on Mars would lead to no planetary protection issues is going to get shared far more widely than one that suggests that there are potential planetary protection issues.

This is an example of our natural "Confirmation bias". Though it's been known throughout history, this term was first coined by the English psychologist Peter Watson on the basis of an experiment in 1960, where he found participants often made up hypotheses and then ignored data they were presented with that went against their hypothesis. For details see the history section of the wikipedia article, which is good on this topic. As Wikipedia puts it, it's

"a tendency to search for, interpret, favor, and recall information in a way that confirms one's preexisting beliefs or hypotheses."

As a result many of you may not know of its rebuttal, which is so seldom shared, that hardly anyone knows about it. It was rebutted in a follow up article "Appropriate Protection of Mars", published in Nature Geoscience just one month later, in July 2013, by the current and previous planetary protection officers Catherine Conley and John Rummel. The two papers are summarized in The Overprotection of Mars? published in NASA's online astrobiology magazine, and also in Overprotection may be hampering hunt for Mars life in New Scientist.

One of the things that might surprise you, if you have been convinced by Robert Zubrin's arguments, or by this "Over protection of Mars" paper, is how hard it can be for a microbe to transfer between planets. It has to withstand the tremendous shock of ejection after the impact, and the heat caused by rushing through Earth's atmosphere at kilometers per second (it has to do this in both the forward and the backward direction). Then once it is in interplanetary space, the fragment of rock is spinning in the extreme cold and vacuum of interplanetary space, with hot sterilizing UV rich sunlight bathing the sunlit side whenever the sun spins into view, and it has to withstand the penetrating cosmic radiation of interplanetary space, and solar storms. It hasn't got a rocket engine, and can't steer towards Mars or Earth. Most rocks will take millions of years, and many flybys of Earth, Mars, and possibly other planets before they hit anything. Many eventually are ejected from the solar system, or hit the Sun or Jupiter, and a few of the rocks that are ejected from a major impact on Earth will eventually hit planets and moons even further afield, to Saturn or beyond. See Could Europa or Enceladus have DNA based life? for details.

Out of all those rocks, perhaps around a fifth of a percent will get to Mars eventually, though that does amount to hundreds of thousands of fragments of rock. Most of those take millions of years and arrive there thoroughly sterilized by cosmic radiation to depths of meters. The fastest ones get to Mars after about a century. That is still a long time for a microbe to spend exposed to cosmic radiation, in interplanetary space. When your rock gets to its destination, it then has to withstand the shock of impact, and then any life on it has to find somewhere to live on the new planet, and it has to be pre-adapted to survive there.

Indeed, when researchers started to publish papers saying that it might be possible for some hardy microbes to get transferred between planets after a meteorite impact, this came as quite a surprise to specialists. The most likely time for microbes to get from Earth to Mars is in the early solar system, over 3.75 billion years ago, soon after formation of the Moon. It was challenging even then. Most microbes could not survive the shock of being ejected from Earth at velocities high enough for the rocks to punch all the way through our atmosphere and exit it at escape velocity. It's easiest with the very largest impacts from the early solar system, as after a huge impact, some of the rock hit by the impactor that are shocked less than most on the way into space, and also the very largest asteroids, hundreds of kilometers in diameter, will punch a hole in the atmosphere making it easier for some of the debris to get into space. We don't get impactors that large any more. The larger the impactor, the larger these regions of less than average shock, and the less the shock in them. But these regions of rock with minimal shock are also well below the surface, meters down, so not so likely to have abundant life in them. Through repetition, the general public I think have forgotten quite how difficult this journey is. We are so familiar with the idea now, that we forget the formidable obstacles that are still in the way of a microbe traveling between planets on a small interplanetary rock.

By far the easier direction for microbe transfer is from Mars to Earth, because the shock of ejection is less, but that direction also is still a huge challenge for most lifeforms. Also (just as for the Earth to Mars direction), the least shocked rock, and most likely to get into space as well, comes from some meters below the surface of Mars. The most habitable regions on Mars are probably close to the surface, in the top few centimeters, or else, too deep underground to be reached by these impacts. Microbes in those types of habitat couldn't get into a meteorite headed for Earth. Habitats also may be rare on present day Mars. It's true that some species seem to be capable of surviving the transition. However, most species can't survive this either, or is handicapped in this respect. Most photosynthetic life, to take a significant example, does not seem to be robust enough to get from Mars to Earth easily (it might just about be possible at the lower end of the range of shock), and it's even harder for it to get from Earth to Mars (almost impossible). I cover this in detail in Case study - can photosynthetic life be transferred from Earth to Mars or vice versa? .

With this background it would be an extraordinary discovery to find that Mars has identical life to Earth. If that happened, we would need to re-evaluate all this research which seems to say clearly that many species couldn't get there. We would surely want to keep Mars free of contamination from Earth life until we found out more about how something so amazing happened. And the bottom line is that, though we have lots of theory, we have zero empirical data on this topic. We don't yet have a single example of a species that has been transferred from one planet to another by meteorite impact.

Let's look at this argument in more detail.

Microbe stowaways on meteorites traveling from Earth to Mars

So, yes, it's true, meteorites get from Earth to Mars. On average tons of them must get there every century. But that's an average over timescales of billions of years. The numbers fluctuate hugely. We get meteorites here on Earth from the Moon, from Mars, possibly from Mercury, but so far we haven't found a single confirmed "Earth meteorite" originating from our own planet, in modern times. That is to say, we haven't found any that actually left Earth, and then came back here again some time later. This suggests that Earth has already cleared its orbit of all the "Earth meteorites". So, probably, similarly, there are no meteorites from Earth arriving on Mars right now either.

This is not too surprising as impacts large enough to create such "Earth meteorites" are rare. That is apart from tektites, which are small rocks millimeters to centimeters in scale, that just head off into space in suborbital trajectories and immediately return to Earth.

Two splash form tektites, dumbbell and teardrop. Image credit Brocken Inaglory. Most large meteorites will send fragments of rock into space on suborbital trajectories, but they immediately return to Earth as tektites, and have no chance of hitting Mars.

The impactor that created Meteor crater in Arizona wasn't anything like large enough to send ejecta all the way through the atmosphere with escape velocity.


Meteor crater Arizona, slightly over 1 kilometer in diameter, result of an impact by a meteorite around 50 meters across. This was nowhere near large enough to send material through our atmosphere all the way to Mars.

You need a rather huge impact to do that, like the Chicxulub meteorite impact 66 million years ago. Earth clears its orbit of asteroids over a period of twenty million years. So all the material ejected by that impact is probably gone by now. With that background, it's not such a surprise that we have no "Earth meteorites" hitting Earth right now. Probably we'll find many of them on the Moon, but they are long gone from interplanetary space.

The last meteorite to get from Earth to Mars may have got there over forty million years ago. Then, even if there are rocks from Chicxulub that have somehow survived to the present, still traveling through interplanetary space, that have not yet hit any planet, the solar storms and cosmic radiation would sterilize a meteorite that spent twenty million years in space to a depth of several meters. The best time for a meteorite to get to Mars is in the first century after the impact on Earth, as that's when the first ejecta gets there, A small amount can get there even sooner, perhaps as soon as ten years after the impact.

So, yes, the simulations suggest that many meteorites got to Mars even as soon as one century after the Chicxulub impact on Earth.

Artist's impression by Don Davis of the Chicxulub meteorite impact into a warm tropical ocean. A huge impact like this could send debris all the way to Mars through our thick atmosphere, and the first rocks would get there even in the first century after the impact on Earth..

Remarkably, experiments suggest that some very hardy extremophiles could survive this travel time of a century in the vacuum, cold, and solar radiation of space. But then look at the obstacles in the way of a microbe before it can get to Mars by this route. First, to get to Mars, it has to be able to:

  • Withstand the heat of ejection from Earth and impact on Mars. It has to travel through the thick Earth's atmosphere so quickly that it is still traveling at over 11 kilometers per second as it exits our atmosphere, so the outside would get very hot indeed and ablate away.
  • Withstand the shocks (extreme acceleration) of ejection from Earth, also (not so much) of impact on Mars.
  • Survive deep inside a rock - because anything on the surface would be ablated away during the passage through Earth's atmosphere, and also be sterilized by the external heat, and then also probably by the hard UV radiation in space.
  • Withstand the hard vacuum of space.
  • Withstand the extreme cold of space - so after the heat and shock of he ejection, it has to survive freezing well below the freezing point of water (so it has to vitrify, not freeze, as freezing causes ice crystals to damage the cell).
  • Survive the cosmic radiation and the solar storms of the journey to Mars. These break apart the molecules of DNA and RNA and other organics in the cell.

But that's just the beginning. It then has to find its way to a habitat on Mars and then survive there.

  • It has to find its way to a habitat. Remember that to have survived so far, it is deep inside a rock. So it's not likely to be dispersed in the dust storms. Most of Mars is very cold, and dry, so though there may be places it could live there, it has to find them. How does it do that from inside a rock?
  • Then, it has to be pre-adapted. This is the same problem as Zubrin's sharks surviving in the Savannah, or is it like a rabbit in Australia? Maybe it could evolve to live on Mars, and microbes can evolve quickly, but it won't have that opportunity unless it can survive right away when it lands there. In the case of Mars this probably means it has to be pre-adapted to tolerate perchlorates (which are pervasive in the dust, and highly oxidizing) as well as as perhaps some amounts of its dissociation products (from UV and ionizing radiation) chlorites, hypochlorites. It probably also has to be pre-adapted to tolerate the hydrogen peroxide. Yet it also has to be able to survive with hardly any free oxygen (must be an anaerobe).

    If it is photosynthetic life, then it also has to tolerate high levels of UV light in the unfiltered sunlight it needs to use to photosynthesize, though it may be able to use a covering layer of dust to help filter out UV. It also has to be able to cope with the near vacuum atmosphere if it is going to survive exposed to the surface air like the DLR lichens and cyanobacteria. Or else, it has to cope with the sulfates, chlorates and perchlorates in soda lake type conditions, if it is able to find its way somehow into a salty surface brine still exposed to sunlight.
  • If there is native Mars life, then the life from Earth has to compete with this too. It has to be either better adapted than Mars life, or at least, it has to be as well adapted, right from the moment it arrives on Mars.

Many microbes that could survive on Mars just couldn't survive the rigours of the journey there, or wouldn't have much chance of finding a suitable habitat once there, or would not be pre-adapted to the habitat once it is found. As for whether they could compete with Mars life right off the bat when they get there, we have no way to know.

It's remarkable that scientists think that there may be microbes that could survive all this. It may well have happened. But if so, it might not have happened for billions of years. The easiest time for it may be in the period soon after formation of our Moon during the "late heavy bombardment". However, was Earth life back then hardy enough to be transferred to Mars on meteorites (or vice versa)?

So, it might never have happened, or it might have happened billions of years ago, or it might have happened more recently. The most recent chance of this happening was 66 million years ago. So, that's one thing we can know pretty much for sure. Any Earth life that got to Mars must have evolved independently on Mars for at least that long. In conditions of greatly increased solar storms and cosmic radiation, with those perchlorates, chlorites, hypochlorites, hydrogen peroxide, almost no ordinary salt, shortages of nitrogen, night time temperatures cold enough for dry ice to precipitate from the atmosphere for 100 days a year in the tropics, an "atmosphere" of almost pure carbon dioxide, and close to a laboratory vacuum. With half the light levels of Earth, those reddish brown skies, and the global dust storms.

Microbes evolve rapidly, most of them with shorter generation times than multicellular life, they make frequent use of horizontal gene transfer, and they have greater tolerance for errors. So, is it not possible, or even likely, that microbes transferred to Mars so long ago evolved in a different way from Earth life over the last 66 million years? But this is for the ones that can get there in a meteorite. What about the ones that can't?

Case study - can photosynthetic life be transferred from Earth to Mars or vice versa?

Surely this must be one of the more dramatic ways that a microbe could transform a planet - to introduce oxygen producing photosynthetic life for the first time. This could transform the composition of the atmosphere and of liquid water throughout the planet, as well as make huge differences in the greenhouse warming effect of a carbon dioxide atmosphere, so it can change global temperatures as well. Oxygen may well have made many species of microbe extinct leading to possibly the first of the great extinctions here on Earth. So, could photosynthetic life be identical on Earth and Mars?

Chroococcidiopsis is our top candidate for photosynthetic life that might be able to survive on Mars. This is a green algae that ticks nearly all the boxes for meteorite transfer. It can withstand high levels of cosmic radiation and solar storms,and is able to repair its own DNA in real time when damaged. It is also UV resistant, and it may even be able to survive on the surface of Mars almost anywhere. To do this it only needs partial shade or a thin covering of dust or dead microbes. It can do this so long as there is liquid water available. In Mars simulation experiments, it can also manage this feat using the night time 100% relative humidity with no liquid water at all.

It's an ancient microbe, probably one of the ones responsible for bringing oxygen to Earth originally, a polyextremophile that can withstand many extreme environments. It also does just fine in ordinary conditions too. You can find varieties of this species anywhere from Antarctic cliff faces through to the tropics, in fresh water, extremely salty water, the most arid conditions where life survives on our planet, in acidic or alkaline conditions, freezing cold, or extremely hot conditions. It can handle just about everything. It's one of the most versatile microbes we know of, with a huge range of different biochemical pathways it can use to deal with different situations it might encounter. Although it's called a species, it reproduces asexually (and can also share gene fragments even with completely unrelated life forms via gene transfer) and there are many varieties of it. For instance desert varieties have especially high resistance to UV light, and lose some of that resistance when brought into the laboratory.

So it could, potentially, have a huge effect on Mars if it is not there already. However, unlike many microbes, it can't use a more hardy spore or dormant resting state to resist the shock of ejection from Earth because it doesn't have that capability as far as is known. It has to survive as an ordinary cell. So, it's not as resistant to the shocks of ejection as some other candidate microbes.The sharing can go both ways, and if chroococcidiopsis evolved on Mars originally, it could survive ejection from Mars, though only at the lower end of the scale. Lichens can survive the shock of ejection rather more easily and so may be better candidates to get from Mars to Earth on a meteorite, and perhaps even from Earth to Mars.

Techy detail: the range of shock experienced by the Martian meteorites we have, when they left Mars, was 5 - 50 GPa (billion Pascals). Chroococcidiopsis can survive up to 10 GPa. The microbe Bacillus subtilis and the lichen Xanthoria elegans survived up to 45 GPa

The shock would probably be greater for an ejection able to send photosynthetic life from Earth to Mars.

So, would chroococcidiopsis get there on meteorites? Is it already there perhaps? Or might it perhaps have come to Earth from Mars? Or must any photosynthetic life on Mars be native to the planet?

The UK astrobiologist Charles Cockell looked into these interesting questions in his paper The Interplanetary Exchange of Photosynthesis. (You can use the Google Scholar button to read the paper in full). He didn't just look at single cell life. He also looked at whether lichens or any other multicellular photosynthetic life could survive transfer from Mars to Earth or vice versa. This section is a summary of a few of his findings in this paper. He found out that it is possible, but difficult and rather unlikely, except perhaps in the early solar system.

As with the other researchers, he found that the easiest direction is from Mars to Earth. He found that the toughest part of the journey from Mars to Earth was actually the entry into our atmosphere, which happens at a minimum re-entry speed of over 11 kilometers per second. Typically 10% - 20% of the radius of a hand sized rock ablates away. The rock also has to be larger than 20 centimeters in diameter to avoid heating up to 100°C all the way through to the center (which would sterilize it of photosynthetic life). Since photosynthetic microbes normally grow at most a few millimeters below the surface of a rock, it's clear already that it's going to be tricky for it to survive entry into our atmosphere.

He did an experiment in which he inculcated some Chroococcidiopsis into an artificial gneiss rock, at a depth where it could be expected to grow naturally. Then he fixed the rock into a heat shield of a re-entry capsule launched by a Soyuz rocket. None of the microbes survived re-entry, nor did any of their biomolecules either.

Charles Cockell's experiment - the circle shows where they attached a sample of gneiss with chroococcidiopsis below the surface at a depth where it could still photosynthesize in natural conditions. None of it survived re-entry, not even biomolecules.

It could survive re-entry to Earth if deep within the rock, for sure. The interior of ALH84001 never got hotter than 40°C during entry into our atmosphere. But how does the photosynthetic life get deep into the rock in the first place? It can flourish in cracks, if light filters in through them - but that also would give cracks that channel hot gases into the interior of the rock. Cracks like that would also be places where the rocks are quite likely to break apart during ejection from Mars or re-entry to Earth.

Also, chroococcidiopsis is rather susceptible to shock of ejection from Mars. It's killed at only 10 GPa. Typically ejection from Mars requires 5 - 55 GPa, based on analysis of the Mars meteorites. So that suggests that it can just survive ejection from Mars at the lower end of impact shock levels. Lichens manage somewhat better here. But that's not much use if they can't survive entry into the Earth's atmosphere when they get here.

In the other direction, from Earth to Mars, with escape velocity 11.2 km / sec, so more than twice the escape velocity of Mars of 5.02 km / sec, the higher shock levels would make it very hard for Chroococcidiopsis to survive ejection.

You can work out scenarios by which photosynthetic life could get from Earth to Mars. For instance, make the original impact into an ocean, and then the photosynthetic life gets forced into rock as a result of the impact and that rock, now impregnated with photosynthetic life to some depth, now gets ejected into space.

You still have the problem of the shock of ejection from Earth. So perhaps you assume that it is some other form of photosynthetic life that is more resistant to shock than chroococcidiopsis, such as lichens perhaps. Also some photosynthetic algae are more resistant to this than Chroococcidiopsis.

If somehow the microbes got into space still surviving, buried deep below the surface of a rock, then they have to get to Mars. Some rocks get there as soon as ten years after ejection from Mars. But most take between a hundred thousand and ten million years to get there. They can survive the low temperatures and vacuum of space and the UV radiation at least for 1.5 years, and probably much longer, tested in experiments flown on the exterior of the ISS. Also, UV light is not so damaging as you might think, as it can be protected by a thin layer of a few mm.

But the cosmic radiation is more serious. This sterilizes the surface to a depth of 2 cm within 100,000 years by breaking up the nucleic acids . That's below the maximum depth you'd expect to find photosynthetic life in normal circumstances, even in fine cracks. So nearly all the rocks that get to Mars from Earth will be totally sterile by the time they get there. Then it has to get out of its rock when it gets to Mars.

Charles Cockell's concludes in this section of his paper that it might not be impossible, but it would need a rather extraordinary combination of events:

"Thus, the planetary exchange of photosynthesis might not be impossible, but quite specific physical situations and/or evolutionary innovations are required to create conditions where a photosynthetic organism happens to be buried deep within a rock during ejection to survive atmospheric transit."

His final conclusion of the paper is that photosynthetic life has the potential to make dramatic changes to a planet, but that this transfer of photosynthetic life is less likely than for heterotrophs (which use organic carbon) or chemotrophs (which use chemical reactions as a source of energy and synthesize all their organics from carbon dioxide, living in places such as hydrothermal vents).

More background on whether photosynthetic life could be transferred from Earth to Mars - other photosynthetic species, shock, and spall zone

I thought I'd look more into the background here, so I've had a look in the literature for more about this question of whether some other form of photosynthetic life could get from Earth to Mars in unusual physical conditions as he described, forced deep into a rock somehow, and with unusual organisms. First, could there be other forms of photosynthetic life that could survive the shock of ejection more easily than chroococcidiopsis? In one paper, samples of a marine photosynthetic algae nannochloropsis oculata frozen in ice were able to survive 6.93 km / sec impacts into water with approximate shock pressure of 40 GPa. (It's not a candidate for present day Mars surface life as far as I know though.)

Also, what is the minimum shock of ejection for the larger impacts needed for the Earth to Mars direction, for a really huge impact like the Chicxulub one? When the Martian meteorites were discovered, researchers were surprised to find that they were so lightly shocked. But this is something that is now well understood, first explained in 1984. The low levels of shock arises from interaction between the shock wave moving away from the forming crater and a reflected shock wave moving backwards. The shock moving back is 180 degrees out of phase so the two shock waves cancel, creating a lightly shocked "spall" zone where the two interact. The spall zone depth is proportional to the radius of the impactor, so a large impactor would have a thicker spall zone (summarized in section 2 of this paper, original paper here). But I can't find any figures for the minimum level of shock expected for a Chicxulub type impact in its spall zone. It seems likely to be a hard to model scenario. Does anyone reading this know of good information on this?

At any rate, whether it can happen in unusual conditions or with especially hardly photosynthetic life, it seems quite likely that photosynthetic life never got from Mars to Earth or vice versa. If it did happen, then it is most likely to have happened in the very early solar system when there were many more impactors and many were also much larger, so that the ejection shock could be more gentle.

If Mars never developed photosynthesis, and never evolved it, then any photosynthetic life introduced on our spacecraft could potentially have a major impact. It could make some species of native Mars life extinct (if they exist) by competing with it for resources or shading it, or by creating chemical byproducts that make the habitats inhospitable to other lifeforms already there. Of course it could also serve as food for the right kinds of organisms there. Either way, it would be a major change in the microbial ecology and the ecology of micro habitats of Mars. If it was was able to survive on the surface just using night time humidity, as in the DLR experiments, and spread over large areas of Mars, perhaps it could also reduce the carbon dioxide levels in the atmosphere slightly too, cooling down the planet more than it is already. Or if not now, it could have that effect in the future when Mars' climate warms up as it does from time to time in chaotic fashion. It could also act to counteract attempts to warm up the planet by artificial means if we ever attempted that, by removing more carbon dioxide from the planet, reducing its greenhouse effect, as the planet gets warmer. You might be interested in my speculations in the section Could oxygen generating photosynthetic life set up an "anti Gaia" feedback on Mars? below.

Photosynthetic life flourished on Earth at just the right time to cool it down as the sun got hotter, but Mars doesn't need cooling down at present. One way or another, it might not be the most brilliant of ideas to introduce photosynthetic life to Mars, if it doesn't have it yet, until we have a clear idea of what the effect may be.

What about other microbes?

General case of transfer of life from Earth to Mars

Most of the papers that study this topic focus on viable life transferred from Mars to Earth. They are generally agreed that this is theoretically possible, though we can't yet prove that it has ever happened. But what about the other direction from Earth to Mars. That direction is central to Zubrin's meteorite argument. This direction is rarely studied, and we get much more cautious statements in the literature in the papers that do mention it.

The big problem with transfer of life from Earth to Mars is the shock of ejection because the material has to leave Earth's surface at very high velocities - not just escape velocity. It has to leave the surface at such a speed that it is still traveling with the escape velocity of 11.2 km / second when it leaves the Earth's atmosphere. The smaller fragments especially have to be traveling much faster than that when they leave the Earth's surface. After all, in the other direction, the debris from meteorites typically hits the atmosphere at many kilometers per second, but slows down to terminal velocity, measured in meters per second, before it hits the ground. Imagine how big a rock has to be, and how fast it has to be traveling, to survive ejection from the Earth's surface and still have some of it left to exit the atmosphere at 11.2 kilometers per second!

So the surface of any rock that gets from Earth to Mars will get boiled away, just as for meteorites re-entering our atmosphere and even more so, at least, so long as it has to punch through our atmosphere on the way there. But even more significantly, it's going to experience high levels of shock of ejection, to achieve an even higher initial velocity than 11.2 km / sec, which will damage cells throughout the rock. The best chance for still viable life to get from Earth to Mars is in the early solar system during impacts that were so violent that they blow away part of the Earth's atmosphere, so clearing a gap for the ejecta to travel through on its way into space, without the resistance of the Earth's dense atmosphere. Large impacts also have a larger "spall zone" where the expanding shock wave from the impact meets its reflection (below the surface of the rock) - for details see

We don't get impacts that large any more, so the last chance for something like this to happen is probably more than 3.75 billion years ago. That leaves us with a big question. Was such early life already robust enough back then to survive interplanetary transfer?

More recent impacts would indeed send material all the way to Mars. The Chicxulub impactor did just that, 66 million years ago. It would surely have transferred organics, and fossils too. The main question is whether any of the life in that debris was still viable after the shock of ejection from Earth and transit through the Earth's atmosphere. There, there is no way to be certain, but the authors of papers that I studied are often skeptical that it happened in such geologically recent times. Here is a quote from the conclusion of a 2007 paper.

“'Lithopanspermia' also includes a potential transfer of microorganisms in the opposite direction, i.e., from Earth to Mars. A direct transfer scenario is severely limited because very high ejection velocities in the solid state are required to escape the Earth's gravity field and to pass its dense atmosphere. Favorable transfer conditions may be only achieved by very large impact events, which blow out at least part of the atmosphere. Such impact events happened frequently during the 'early heavy bombardment phase',”

From Experimental evidence for the potential impact ejection of viable microorganisms from Mars and Mars-like planets (2007)

A planetoid plows into the primordial Earth" - artist's impression by Don Davis for NASA. An asteroid this big would melt the land surface, boil the seas, and sterilize the Earth to some depth. However since it blows away part of the Earth's atmosphere, it gives a gentler ride for the debris that does get into space. Larger impacts can also accelerate the debris to escape velocity over a longer period of time, with less of a shock. Huge impacts like this may have given life it's best opportunity to get from Earth to Mars.

These impacts only happened on early Earth, soon after formation of the Moon. After sterilizing Earth to some depth, they might have reseeded Earth and Mars and maybe other places in the solar system. So if life got off to an early start and was already hardy enough to survive these encounters, Earth and Mars could easily have a common origin for life. Perhaps early Venus, Ceres and other places could also share life with us, or indeed be the source of this life.

We have only limited ground data on the ages of the lunar craters, from Apollo, as they sampled only a small part of its surface, most of it affected by the debris from Mare Imbrium. As a result, most of the earliest cratering history timetable for the Moon, the timescale for the very largest craters, is informed guess work. According to some recent research, such as this paper, perhaps those dates may need to be revised. It's possible that the very largest impacts on the Moon may all date back to as long ago as 4.35 billion years ago or earlier, soon after the formation of the Moon. They think there were no ocean boiling and Earth sterilizing impacts after that. However, there probably were many later large impacts that, though not large enough to boil away Earth's oceans or melt its crust, covered large areas with melts (and could have boiled away the surface of the oceans). See this paper.

There have been no impacts anything like that for nearly four billion years - Jupiter protects us by breaking up larger comets, diverting them to hit the sun or itself, or ejecting them from the solar system. The asteroids in between us and Jupiter are easily large enough, but they are in stable orbits, and have been for billions of years, and should continue in them for at least hundreds of millions of years into the future (there's a very small fraction of a percent per billion years of Vesta hitting Ceres in the distant future). This is confirmed by the impact crater history on Mars, the Moon, Mercury, what we have of the history of Earth, and the recent few hundred million years,, all that's available, since the last global resurfacing volcanic event, on Venus.

This suggests that for microbes generally, the situation may be similar to the situation for the less shock resistant photosynthetic life. There was a brief window of opportunity for a few hundred million years, during which it may have been somewhat easier for life to be transferred from Earth to Mars (if already hardy enough to survive the journey) during the period of bombardment of Earth by huge impactors that blew away great holes in our atmosphere, and also would have had large "spall zone" where the shock wave meets its reflection so reducing the shock of ejection for the microbes (see More background on whether photosynthetic life could be transferred from Earth to Mars - other photosynthetic species, shock, and spall zone (above). Since then, with the smaller Chicxulub type impacts, the situation is much more uncertain and depended on unusual events.

In short, the transfer from Earth to Mars may have happened billions of years ago. Perhaps it could have happened since then. It's also possible that it never happened.

There is another very interesting possibility here. Though the microbes themselves might not be able to survive transfer from Earth to Mars, their genetic information might be able to get there, as fragments of DNA or RNA. If so, perhaps that genetic information could be taken up by Martian microbes. Perhaps we could find indigenous Martian microbes that incorporate genetic sequences that did come from Earth more recently, even as recently as 66 million years ago - even though the microbes themselves did not. This could make us more closely related genetically, through horizontal gene transfer, than you'd expect otherwise, though with our last common ancestor billions of years ago. For more on this, see this paper: Microbial Survival Mechanisms and the Interplanetary Transfer of Life Through Space.

The bottom line here is that so far we have no confirmed case of life transferred to another planet (panspermia) to base all these ideas on. It is just theory. The direction from Earth to Mars is especially challenging. If there is any life transferred to Mars on meteorites, then surely most life on Earth wouldn't be able to do it. It would be a great surprise to find life on Mars that was identical or almost identical in all respects to Earth life. This is a topic of great interest to astrobiologists. There are many papers on the topic discussing it in great detail, exploring many possibilities, but there are no definite conclusions as yet.

Microbe stowaways in a human occupied ship

Meanwhile microbes on a human occupied ship don't have to survive any of these rigours of a journey to Mars after a giant impact.

  • They get a comfortable ride inside a human occupied spacecraft, in a rapid journey, protected from UV light, and extreme cold or heat, with hardly any ionizing radiation (compared to meteorite transfer), and protected from ablative heating from the Earth's atmosphere and the Mars atmosphere by an aeroshell and heat shield.
  • Once they get to Mars then they can slowly adapt to Mars conditions in microhabitats outside the human base. For instance every time humans open an airlock then air from inside, along with moisture, flakes of human skin, shed hair, lint from their clothing, dust from any agricultural activities, and other debris, and numerous spores will be dispersed out into the Mars landscape. This will also happen whenever they use spacesuits, as these are designed to leak small amounts of air constantly through the joints for mobility. All of this will help to create perfect intermediate conditions for microbes to slowly adapt to the surface conditions. If it is possible for them to adapt they have close to optimal conditions in which to do so.
  • The microbes that leak from spacesuits and airlocks can feed on the dead remains of their predecessors, flakes of skin, and other debris.
  • Vast numbers, trillions of these microbes will leak from the habitats, in thousands of species. This makes it easier for rare events to happen. It only needs a few of those to be pre-adapted or to adapt quickly to Mars, and you now have a potentially invasive species on Mars.
  • These microbes are dispersed in the atmosphere right away. They don't have to get out from the interior of rocks or from hidden cracks in the surface of a rover that has been sterilized on Earth and bathed in UV light. Many of them are already suspended in the air inside the habitat, or are on the skin or hair of the astronauts, and they can just float out onto the Mars surface when the airlock is opened - as spores in the air , or on flakes of skin and hair.
  • The UV light is easily shielded, in the shadow of a rock or spacecraft, or by a thin layer of dust, The spores can fall into shadows without getting exposed to sunlight at all, especially at night, and will be protected from the UV light during daytime as well if the airlock entrance is in the shade of the habitat, or if they leak from the shaded side of an astronaut's spacesuit. They can also get caught up in the dust and spread throughout Mars in the dust storms, which can get so dark that any microbes in the dust would be shielded almost completely from the sterilizing UV light of the sun. The iron oxides are great for shielding from UV light. A spore could get into a crack in a grain of dust for yet more shielding.
  • Not just individual microbes, but numerous species, that can work together as communities and biofilms. Often microbes can't do much on their own but really thrive in communities of many species together, especially the "uncultivable microbes". Many microbes are uncultivable because they depend on other species to survive. A microbe like that is not likely to survive on Mars. What's more, Mars has (according to various estimates) 15 to 20 biocidal factors the microbes have to overcome. Also they need to be primary producers too, to have a chance of survival, unless there is other life on Mars for them to eat. Most individual microbes, except in the case of a few very hardy polyextremophile primary producers, maybe chroococcidiopsis, don't have a chance on their own. But in a community, they can survive together, with some microbes making up for the deficiencies of others, forming a biofilm or mat which also creates a microhabitat that they can all survive in. So once you have the possibility of introducing a whole community to Mars in one go, as you do with the tens of thousands of species on a human occupied spacecraft, then the issues become much more serious. Those communities and biofilms can overcome multiple hazardous conditions that they would be unable to deal with individually. This is a point Andrew Schuerger makes in a brief comment 47 minutes into this discussion at AbSciCon 2017 day 1. - he is known for his papers on biocidal factors on Mars.

The stowaways in the human occupied ships have a far easier time than their panspermia cousins hidden inside rocks for a century on the journey to Mars in the cold vacuum and ionizing radiation conditions of interplanetary space. They also have a far easier time than the few hardy microbe spores that may get there on the sterilized robotic explorers.

So, in summary, Zubrin's arguments don't have a lot of force to them, impressive though they may seem when you first encounter them. Even the meteorites argument, which may seem so conclusive at first, has much less force to it, as you work through it in detail.

If you listen to the debates he stages for the Mars society, you may get the impression that he is debating on equal terms with the astrobiologists. However, in actuality, his arguments are not taken seriously by most astrobiologists, and especially, they are not taken seriously by those who are involved in working on planetary protection as their specialist topic. He may seem to have won all of these staged debates, at least to the satisfaction of many of the Mars colonization enthusiasts. However you need to bear in mind that they are, naturally enough, disposed to like his message and cheer him on. This is a case of a confirmation bias I think. He is also a very skilled public debater. Few astrobiologists would agree that he has won these debates.

If you are super keen for someone to "touch Mars", your next obvious question might be

"Is the planet worth protecting in its present state? Does it matter if we contaminate it with Earth life?"

So now, let's look at what many astrobiologists think we might find on Mars. Not fossils, but tiny micro-organisms. Perhaps too small to see with an optical microscope. Why is that so interesting? Do we really need to protect them from contamination by Earth life? Or, as was suggested in the Sky at Night Mars special program, should we just heave a great sigh of relief, when we finally do contaminate Mars irreversibly, that we no longer need to think about such issues? (See Could we get a future news story: "Debate over Moldy Mars is a Tale of Human Missteps"? above)

What if Mars has tiny cells - like the structures in the Mars meteorite ALH84001?

Some of you may remember president Clinton announcing the possible discovery of past martian life in a meteorite. This was the famous ALH84001 (originally found in 1984 but only recognized as a Mars meteorite a decade later). And then perhaps you may remember the anticlimax afterwards when the scientists investigated further, and were not able to prove that it was definitely life?

What you may not realize is that it hasn't been disproved either. We need strong incontrovertible proof to claim discovery of life from Mars. The whole thing ended inconclusively, and the jury is just out on what it is at present, with some scientists arguing in both directions. Maybe in the future as we find out more about Mars, we will discover that these were traces of ancient Mars life all along. Or maybe the abiological explanation will get proven conclusively. So far though, we can't say.

Many astrobiologists think that potential microfossils and traces of organics, like those in ALH84001, give us a much more likely model for what we may find in the search for life on Mars, than fossils you could spot by eye, or with a lens, or even an optical microscope. They also think traces of organics like this are much more likely than massive deposits of organics like the shale oil deposits. The life in this meteorite, if that is what it was, was so small it could only be seen with an electron microscope, and though there were possible biosignatures, they were rather elusive and controversial, mainly because most of the carbon in the meteorite was Earth based contamination.

Here are some of their images from the original press release:

The structures in these photos are between 20 and 100 nm across, well below the resolution of a diffraction limited optical microscope of 200 nm.

If it is life, then the supposed cells seem too small to include all the cell machinery of modern life. This discovery lead to a 1999 workshop to try to figure out if living cells could be so small. And the answer was yes. Although modern DNA based Earth microbes life simply can't be this small and still include all the machinery it needs to function and to reproduce, modern cells must have evolved from earlier simpler forms of life. The relevant section of the workshop concluded that this early life could be as small as tens of nanometers in scale, far beyond the optical resolution limit of 200 nm, and still have all the cell machinery needed to reproduce, especially if it is based on an RNA world type cell, with no DNA and no proteins.

So, to the ordinary person, not an astrobiologist, and especially if you are keen to "touch Mars" or at least if you are keen for someone else to do that - perhaps your thought at this point is something like this:

"Well what's the big deal. Just a few microbes, so small you can't see them in a microscope? This will only interest a few microbiologists.

"Why should anyone else care if we mess up their chances of finding this life. It is so uninteresting that it shouldn't stop humans from doing what we want to do, land boots on Mars and touch Mars."

Well, if you look at it like that, it might not seem that interesting. But if you look at it another way, if Mars life does turn out to be like the structures in ALH84001, there's something much more interesting about this than another obscure microbe that happens to be smaller than any others found to date. Something that would be fascinating and exciting to everyone with an interest in science or biology, I think.

RNA world and the shadow biosphere

To understand how exciting and interesting this discovery would be, first you need to know how similar all modern life is. It might seem that modern life is diverse already - the fish, fungi, trees, birds, animals, starfish, octopuses. Surely most of the variety is in the higher lifeforms like that? Adding a few microbes too small to see, hardly seems likely to add to that diversity.

However underlying all that life on Earth is an almost identical structure. If you look deep inside the cells of every living creature on Earth, seaweeds, plants, amoebae, microbes of all sorts, birds, animals, they all look pretty much the same at that level.

This is not an actual video of the interior of a cell, but scientific art, that depicts it as accurately as possible. Exactly the same amazingly complex process is going on in each and every cell of your body, and what's more, every cell of all Earth life, including microbes - and at roughly the speed of this visualization in every cell too.

What's more, it's not even just a similar process. All Earth life uses the same language here, the same basic components that are used to make proteins, DNA and RNA, and the same translation tables.

It's as if none of us have ever heard anything spoken, or seen anything written except English (substitute your favourite language here, it's just an analogy). We know that it must have evolved from some earlier language or languages, and possibly also as a result of a merge of several previous languages. Yet (in this analogy), all historical records, all inscriptions, everything we have is in modern English, with no evidence even of Shakespearean English or middle English. All we have is the modern language, and only one dialect of it.

In that situation, imagine what an intense interest there would be if someone found an inscription written in another language, or in an earlier form of English. An early form of life on Mars would be easily as revolutionary for biology as that discovery would be for language studies.

What's more, there's even more to this than a new "language of life" discovered by a species that so far is monoglot in its understanding of biology. The interior of every Earth cell is the same or similar in many other ways too, not just the language used in the translation tables and encoding. For instance, consider carotenoids - these are the pigments that make carrots, peppers and poppies red, yolks of eggs yellow, flamingos and shrimps pink, and autumn leaves red or orange. Carrots, poppies, fungi and trees can make the carotene for themselves. This substance is used not just to make them colourful, but to protect chlorophyll and to convert blue and green light (wavelengths in the range 450 to 570 nm) into light at the right frequency for chlorophyll to use. So it's an important part of photosynthesis.

Most animals and insects can't make this substance. Flamingos, birds etc get their carotene by eating plants. But the plants, fungi etc all use the same identical biochemical pathway to produce it. And as it turns out, in a surprising discovery, some red pea aphids can make their own carotene, which they use just to turn themselves red. Astonishingly they got this ability by horizontal gene transfer from a fungus, of all things.

Pea aphids get red coloration by producing their own carotene. The horizontal gene transfer from a fungus didn't transfer carotene, but rather, the instructions for making it. Credit: Zina Deretsky, National Science Foundation

For details of how they made this discovery, see First case of animals making their own carotene and for techy background on carotene and the biochemical pathway by which it is made in cells, see Carotenoid Biosynthesis in Arabidopsis: A Colorful Pathway.

This only transferred the DNA instructions for making carotene, which got incorporated into the DNA in the cells of the aphid. Not only was the aphid's cell machinery able to read these instructions and work with them. It's much more striking than that. The aphid makes carotene using a complex biochemical pathway that is identical in both the fungus and the aphid. This shows how similar the cells are, even though a fungus looks and behaves very differently from an aphid. All life on Earth is fascinatingly similar at a cellular level.

This horizontal gene transfer is an ancient mechanism and works between organisms that had their last common ancestor back in the early solar system. It might even work with modern Mars life, so long as it uses DNA, and we are distant cousins. It could do that, even if our last common ancestor lived billions of years ago. In one experiment 47% of the microbes in a sample of sea water left overnight with a GTA conferring antibiotic resistance had taken it up by the next day. These include microbes in totally unrelated phyla with last common ancestors dating back to early Earth, so they could be as unrelated as Earth microbes, and microbes from Mars that diverged from Earth life billions of years ago. See also, Horizontal gene transfer in microbes much more frequent than previously thought

All present day life is like this. Amazingly complex, yet they all are complex in the same way internally, over nearly all of their cell machinery. Also, the biochemistry in our cells is so complex, there's a limit into how small a cell you can fit it all into. If we found very early cells on Mars, so early that they don't use this same language and don't have the same interior biology inside the cell, this would be a remarkable, groundbreaking discovery.

How small can a living cell be?

This was the question asked in the Size Limits of Very Small Microorganisms (1999). If you are talking about modern life, then even the smallest cells, the ultramicrobacteria as they are so called, have to be quite large. Every cell has DNA for inheritance, which is unzipped and converted to messenger RNA, and then to proteins, always using the same translation table to convert the RNA chain, three bases at a time, into amino acids. But the main limiting factor in all this is not so much the complex DNA to RNA conversion - but rather, the ribosome which does this translation from RNA to proteins. It is a rather huge molecule made up of a mix of proteins and RNA. One well studied ultramicrobacterium, S. alaskensis, manages just fine with only 200 ribosomes though it can contain up to a maximum of 2000 ribosomes. The smallest spherical cell you can fit all the ribosomes into is about 250± 50 nm in diameter.

One of the main questions for this workshop was whether early life could be smaller than this estimated 200 nm in diameter. They identified several ways that fossils of life could be smaller than this:

  • Shrink after death
  • Fragments of larger organisms
  • Pathogens or symbionts which depend on a host - e.g. viruses
  • Live in consortia of smaller cells unable to survive independently on their own
  • Based on biological systems different from the ones we understand (such as the RNA World cells)

Let's look closer at the last of these, in the context of this discussion of RNA life. Early life simply couldn't have started like modern life, no way! The whole thing is far too complex to form spontaneously from non living chemicals. One of the striking things about modern life is how it is so interlocked, that almost everything depends on everything else. How could such a thing spontaneously arise from non living chemicals in one go?

As Cairns-Smith put it in his "Seven Clues to the Origin of Life" (which approaches the problem of the origin of life like a detective puzzle modeled after Sherlock Holmes novels):

"Subsystems are highly INTERLOCKED within the universal system. For example, proteins are needed to make catalysts, yet catalysts are needed to make proteins. Nucleic acids are needed to make proteins, yet proteins are needed to make nucleic acids. Proteins and lipids are needed to make membranes, yet membranes are needed to provide protection for all the chemical processes going on in a cell. It goes on and on. The manufacturing procedures for key small molecules are highly interdependent: again and again this has to be made before that can be made - but that had to be there already. The whole is presupposed by all the parts. The interlocking is tight and critical. At the centre everything depends on everything"

(page 39 of Seven Clues to the Origin of Life)

This also explains why modern life is so conservative at centre, so identical that a GTA from a fungus can be transferred into an aphid and the genetic sequence can continue to produce carotene as easily as it did in the fungus. Maybe there were different ways that life biochemistry could have been organized, but if so, once it is set in place like this, almost nothing can be changed in the overall architecture of modern life. If you made a major change to any part of the architecture, with such strong interdependence, the rest would stop working and you'd just have a non living cell.

So what came before this modern life? First, early life surely didn't have those huge complex ribosomes with their mix of RNA fragments with proteins. They are so large and elaborate that they must be a late development. It probably didn't have DNA either, as that's quite fragile if it isn't held together inside a living cell with a lot of support for it. Also it surely didn't have two biopolymers either, DNA and messenger RNA, as the whole process of separating out the two helixes, translating it to mRNA then to proteins is just too complex to envision that suddenly coming into place in one go from non living chemicals.

Perhaps early life had only RNA (or some other biopolymer). This is the so called "RNA world hypothesis". The huge ribosomes were a mystery until the discovery of the far smaller ribozymes made up of fragments of RNA alone, which gave renewed vigour to the hypothesis. These could do the translations and other cell operations that need enzymes. It may not have needed proteins at all. The interior of the cell may have consisted largely of RNA in different forms, both as strands for the genetic material and reproduction, and also, cut and diced together to make these ribozymes.


This is the key to the RNA world hypothesis - a ribozyme . This particular example is the "hammerhead ribozyme", made up of fragments of RNA, stitched together without any use of protein chains, to make the enzyme. This was a surprising discovery. This reinvigorated the idea of an RNA world with tiny cells and only needing RNA without DNA. The cells would have no need for proteins or amino acids and they would not need all the translation machinery to convert DNA into messenger RNA. Their interior would consist largely of RNA strands as well as these ribozymes. As a result the cells could be far simpler than modern DNA based life. This is one suggestion for an intermediate stage between the earlier organics and modern life.

Early life based on those ideas could have had cells as small as 50 nm across. Stephen Benner and others have suggested that we might be able to find RNA world organisms still here today on Earth, undetected because they have no DNA or proteins and have ribozymes instead of ribosomes. That's the Shadow Biosphere hypothesis. This idea was quite popular a while back. It was one possible way to explain nanobes, structures that visually resemble life, but are far too small to be DNA based life:

from "New life form may be a great find of the century" (1999) The nanobes discovered on Earth are mysterious. Nobody knows if they are life, non life, or something in between.

The idea was that these tiny structures could be a form of life that we miss because all of our tests target DNA based life. What if these were RNA world cells, and we just don't spot them because they only use RNA and don't have proteins or many of the materials that make up the larger cells we are used to? We might have a second "shadow biosphere" living amongst us unrecognized, to this day.

So far nobody has been able to prove that this shadow biosphere exists still on Earth, either now, or in the past. But even if it doesn't exist on Earth, and any traces from the past have long been erased here, what about Mars? Could Mars have had an RNA world shadow biosphere in the past, and if so, could remnants of it still survive to this day? Or indeed, could Mars perhaps have had only RNA world life, right through to the present?

That's actually one of the ideas for the structures in ALH84001, that they might be these RNA world cells. This was originally suggested by the fourth panel in Size Limits of Very Small Microorganisms (1999) - which was convened shortly after the martian life announcements. Now scientists have found alternative ways to explain these structures, including the magnetite, and organics. Instead of being formed by life, they could be the result of rather unusual conditions on the Mars surface. This means we can't use the meteorite to prove that Mars had life in the past. But their research hasn't disproved it either, and the jury is still out on whether the structures in ALH84001 were the result of life or not. In "Towards a Theory of Life" in the book "Frontiers of Astrobiology" (2012, CUP) by Steven A. Benner (notable as the first person to synthesize a gene) and Paul Davies, the authors talk about RNA world cells as a possible explanation of the structures.:

"The most frequently cited arguments against McKay's cell-like structures as the remnants of life compared their size to the size of the ribosome, the molecular machine used by terran life to make proteins. The ribosome is approximately 25 nanometers across. This means that the "cells" in Alan Hills 84001 can hold only about four ribosomes - too few ... for a viable organism.

"Why should proteins be universally necessary components of life? Could it be that Martian life has no proteins?

... Life forms in the putative RNA world (by definition) survived without encoded proteins and the ribosomes needed to assemble them. ... If those structures represent a trace of an ancient RNA world on Mars, they would not need to be large enough to accommodate ribosomes. The shapes in meteorite ALH84001 just might be fossil organisms from a Martian "RNA world".

Though we can't seem to prove the case either way for ALH84001, astrobiologists have learnt a lot from it about ways to search for life on Mars. The structures and organics in this meteorite give us the closest we've ever got to something that could be extraterrestrial life, in a real world situation, and actually accessible to study in our laboratories. The challenges they faced analysing it may well be good training for the challenges we will face interpreting whatever we find on Mars. As Harry McSween put it in an early paper in 1997

"this controversy continues to help define strategies and sharpen tools that will be required for a Mars exploration program focused on the search for life."

So, anyway, surely the life we have on Earth didn't arise all in one go. Perhaps it arose as RNA world cells or perhaps as something else but it must have had a precursor of some sort. It would teach us so much, to find some evidence, anything at all, to help fill in this huge gap in our understanding of the evolution of life.

There is another way to see that we must be missing a huge amount of knowledge about early life.

Half of the pages of the book of evolution have been torn out

This was an idea of the entomologist and ecologist Alexei Sharov, and the mathematician and theoretical biologist Richard Gordon, to plot the increase of complexity of DNA against the time of origin of the lifeform. They found a way to ignore junk and duplicated DNA, so that they could count only what is essential to its genes. They found that as life increases in complexity, it follows a near straight line on this plot, through many different changes of structure of organism, from the prokaryotes, to the eukaryotes with nuclei, worms, fish, and mammals. It's a log plot so the straight line means that it always takes about the same amount of time for the complexity to double.

It's similar to the idea of Moore's law, that the number of transistors doubles roughly every two years. If you draw a line back through the graph you could estimate when the computer was invented.

In this diagram, the number of transistors in a computer chip increased a million fold in 40 years from 1971 to 2011, with a more or less steady growth, doubling every two years, so about a thousand fold every 20 years. From this you'd estimate, extrapolating back, that the computer was invented 22 years before the start of this graph (2*log(2300)/log(2) ) or some time after 1949 (assuming the first computer had more than one transistor or vacuum tube).

That's actually more like the date of the first transistor, invented in 1947. Computers were invented a little earlier, the first electronic computer, the Z3 in 1941 already had 2,000 relays, though of course not nearly as powerful as the same number of transistors.

So - extrapolating back too far can be a bit risky. It's suggestive rather than a proof, but it's still a useful way of looking at things.

So anyway, they traced the timeline back for the complexity of our genes, expecting it to cross the zero line at the time of origin of life, and found to their surprise that the zero line is nearly ten billion years ago. That's over twice the history of the Earth.

This diagram shows the complexity of the DNA as measured using the number of functional non redundant nucleotides. This is a better measure of the genetic complexity than the total length of its DNA. Some microbes have more DNA than a human being - much of that used for other purposes instead of genetic coding. This is the so called C Value Enigma. Measuring the DNA by functional non redundant nucleotides deals with that issue.

Notice that the prokaryotes; the simplest primitive cell structures we know; are well over half way between the amino acids and ourselves. Here, eukaryotes are cells with a nucleus to store the DNA, and prokaryotes don't have a separate nucleus.

So, there are two ways to take this. If you take it on face value, perhaps evolution started before the beginnings of our solar system. The original paper just touches on this briefly and mentions a rather minority view idea that our solar system formed from the remnants of an exploding parent star after a supernova, which might have had life on its planets already. Actually there are many other ways it could happen. One rather likely way it could happen is that it could get transferred from sibling stars in the birth nebula of our solar system, and those stars may in turn have been infected by a passing star and planet born in a much earlier nebula. However, that's a "distant cousins" case so I'll cover this a bit later under Distant cousins with last common ancestor from a planet around another star

The other way to take it is that perhaps evolution was far more rapid in its early stages. The straight line may just show the characteristic slope for DNA based evolution. Perhaps evolution was far more rapid in the RNA world or whatever happened before. For instance, just a thought, perhaps early copying was far more error prone, and also less affected by errors too, leading to faster evolution?

Of course it could also go the other way. Maybe the pace of evolution was been slower in the early stages, like the rate of evolution of computer complexity during the time when computers were made of valves and relays, developing far slower than the later transistor based computers under the processes of technological innovations by their human inventors. There is no way to know for sure as we have no idea how rapidly or slowly pre-DNA based life evolved.

Could we peek into these missing pages of the book of evolution on Mars?

Whether this means that Earth life originated in our solar system or predates it, the graph brings out another interesting point. Let's just duplicate the graph again for reference

With the prokaryotes more than half way across in the graph from left to right, indeed, almost two thirds of the way across, you'd expect at least as many stages of evolution to get from non living chemistry to the most primitive known cells as were needed to get from them to modern mammals. The only difference is that if it got off to a rapid start, it went through those stages more quickly. That might suggest that in the left half of this graph, we are missing steps as radical as the step to cells with a nucleus, multicellular life, creatures with a backbone, warm blooded animals and mammals.

We can look at this another way too. We have a wide variety of things in the modern cell that must have arisen somehow. Astrobiologists have so many questions they don't know the answer to. I'll look at some of this in a little more detail in the next section Life on Mars dancing to a different tune, meanwhile here are a few questions to get you started.

  • How did early life develop the two biopolymers system of RNA and DNA?
  • How did early life replicate? Did it just grow until it got so large it split, or did it have some more sophisticated method like modern life?
  • How did the sophisticated error correcting system for DNA replication evolve?
  • How did the translation table by which RNA is converted into proteins evolve? There is much that seems arbitrary in this translation table by which triplets of RNA base pairs get converted to amino acids to make proteins, including many duplicate redundant entries. Why did it happen like this? Were there experiments with other translation tables that failed?
  • How did life come to use one particular set of amino acids, and no other ones? There are thousands of possible choices.
  • How did cell walls evolve? Has life always had cell walls? Might early cells have had walls that were incomplete or porous or non existent?
  • What about the internal structure if a cell, the cytoskeleton of microtubules and actins, how did that arise?
  • How did cells develop mobility and how did the first motile early cells move?
  • How was the metabolism of early cells powered - what was the source of energy and how was this supplied to the cell?

And in what order did those various things arise? Surely not all in one go.

If we find early precursors to Earth life, it can't possibly work in the same way as modern life because it will be missing most of our cells' complexity, and the internal machinery to make the genetic code work. The genetic instructions to make carotene aren't going to work in some early cell, with only a few thousand distinct chemicals. With so little by way of internal biochemistry, its probably not even able to recognize DNA as an informational biopolymer. Probably it will just ignore it. And surely the first cells didn't have millions of distinct complex chemicals interacting simultaneously as in a modern cell. There is no way that could that arise in one go. So, how many distinct chemicals were there in the first cells, and how did they interact?

You might think, why not just make an RNA world cell in the laboratory, following the "blueprint" that astrobiologists have sketched out, and see if we can get it to work? That wouldn't be proof but it could give us some ideas of how early evolution worked, and answer many questions. Well at present anyway, that idea is a non starter. Astrobiologists don't have anything resembling a complete blueprint for an RNA world cell. They just have a sketch. They don't know how to combine RNA, ribozymes etc to make something that actually works, although by analogy with DNA based life, they think that an RNA based lifeform seems very plausible.

Even with modern life, though we know how it works in great detail, we can't make a modern cell from scratch from non living chemicals. We can tweak modern cells, and tinker with them. We can even make a novel complete gene sequence and insert it into a cell, replacing its existing sequence. We can add an extra base pair to the genetic code that has never occurred in nature. But those are all relatively minor tweaks compared to the complexity of building an entire living cell from scratch. We can't do that. It's just too complex, and interconnected, with all the parts needing to work together right away as soon as it is assembled. The simplest modern living cell is way way beyond anything we could make from scratch with inorganic chemistry. The only way we have to create a living cell at present is to replicate an existing one and tinker with it.

Similarly, astrobiologists can't make a complete blueprint for an RNA world cell, and if someone provided us with one somehow, say an extra terrestrial sends us a complete description of an RNA world cell, how would we put it together to make a living cell? We wouldn't know how to begin, not unless we found a way to tinker with a modern DNA world cell and convert it into an RNA world cell by stripping it down somehow. But we are nowhere near that stage. All they have is a rough sketch of a way that perhaps various components might be able to come together to make an RNA world cell.

Also, our experiments in randomly combining chemicals in conditions to replicate early Earth can only get us a tiny way along the path towards evolution. We can't simulate an entire ocean left to evolve for millions of years. None of our experiments of this nature have come anywhere close to forming living cells. So, there isn't really much we can do to explore these ideas of early life, either by trying to work from the present backwards, or from the early chemicals forward. The only way we can make progress is to actually find it, or find other forms of life that may shed light on what is possible.

I hope that makes it a bit easier to see why it would be the most amazing discovery in biology as well as astrobiology, and a fantastic advance in our understanding, to actually discover such a cell for real, and to find out how it works. So - that is one thing we might be able to find on Mars. If we found something like this, it would be revolutionary. It would be epic.

Also, it's a not unreasonable thing to hope for. We definitely have a possibility of finding out about early evolution of life on Mars because the conditions there should preserve early organics in near to pristine fashion,when the conditions are exactly right. If we are very lucky, then like the shadow biosphere idea, this life might actually still survive there to the present day. If so it could be very vulnerable indeed to whatever made it extinct on Earth.

It would just be so very sad to lose the opportunity to study life that's dancing to a different tune from Earth life. Especially if we lost the opportunity through an accident, through not knowing that it was there for us to find, until it is too late to do anything to protect it. Let's take a slightly closer look at how different it could be from modern Earth life.

Life on Mars dancing to a different tune

Suppose we found early life on Mars, too early to use DNA, or life on Mars that is as evolved as Earth life, but with a separate genesis. We'd have a different dance to compare with the dance followed by all Earth life.

It would have its own internal structures and biology. Here are some examples of things you find in modern Earth microbes, which it might have its own versions of - or maybe not have some of them at all in simpler forms of life.

RNA polymerase used to decode DNA to mRNA, present in all living cells. Does Mars life have DNA? If not, does it have two biopolymers like this, its equivalent of our DNA and RNA, or only one? What are its two biopolymers? What does it use to decode them into each other? Does it do error correction and if so how? Can it repair its DNA or equivalent if it is damaged?

Golgi apparatus - essential organelle in most Eukaryotes (cells with a nucleus) - which acts a bit like a post office. Amongst other things it wraps up items (such as proteins) and sends them out to different parts of the cell. Do any microbes on Mars have a separate nucleus for their genetic material? Do they encapsulate proteins and enzymes like this? Do they have an analogue of the Golgi apparatus, and if so how does it work?

Ribosome translating mRNA into a protein

Does life on Mars use proteins? Early RNA world life might not. If it does, are they encoded in mRNA or some similar molecule like this? Does it use the same 20 or so amino acids to make up the proteins as Earth life does, or different ones? One study found 4,000 biologically plausible amino acids that Earth life could have used. Does it use the same translation table to map nucleotide triples to amino acids?

Amino acids codon table - shows how a triplet of bases gets translated into an amino acid, used to build up proteins. All life uses the same table here, and it seems somewhat ad hoc and arbitrary. It has a lot of redundancy and could code up to 64 different amino acids s that list of twenty amino acids in some way optimal or is it just a historical accident, and other life does it differently? Might life based on a different biochemistry which originated independently from Earth life have a different codon table? Might it use more, or fewer amino acids? Or not use amino acids at all?

Microtubules, strands that stretch through cells. They are a bit like the "skeleton" of a cell, they are used to help cells move, and to keep its structure. They don't do this by themselves but along with actin filaments and intermediate filaments are part of the cytoskeleton.

Do Mars microbes have a cytoskeleton? If so what is it made of, and does it resemble the microtubules and actin filaments? How does cells use it to move around?

Then there are the cell walls, and lysosomes (in eukaryotes) which contain enzymes which break down larger molecules and structures that are no longer useful so they can be built up again into new material. and many other components of the cell.

These processes in Earth life are immensely intricate and complex. One analogy that I've heard is that if you are a cell microbiologist studying the interior of a cell, the whole thing is so complex and unique, it can feel as if you are studying an entire ecosystem. So, let's use that as an analogy to understand why even an ET microbe could be so interesting.

Imagine that you have been brought up in the African savannah - with its grasses, trees, elephants and antelopes. You've never seen a marsh or a forest, or a beach. All your life you've lived in a hut in the African Savannah, and never traveled more than a few miles from your hut, and that's the only thing you've ever known. In this analogy the savannah is like the interior of a cell on Earth, not just one cell, the interior of any cell from any living organism or microbe on our planet.

View of Ngorongoro from Inside the Crater

Then one day someone takes you to the sea shore, with its fish, shellfish, seaweeds, and sea anemones, and perhaps they take you on a dive to see a coral reef.

A Blue Starfish (Linckia laevigata) resting on hard Acropora coral. Great Barrier Reef. Photo by Richard Ling The interior of an ET microbe could differ from the interior of an Earth microbe, by as much as a coral reef differs from the African savannah.

Think how much that would expand your horizons! This gives an idea of what it would be like to find a microbe on Mars with a different biochemistry from Earth life. As boring as it might seem from the outside, just one small microbe looking for all the world like many others, perhaps much smaller - inside it could be as different as this.

Even an early RNA world cell would be so different from modern DNA based life, that it would be like a different ecosystem inside as well.

So hopefully this can help you see why the astrobiologists got so excited by ALH84001, and why it would still be so exciting to find a new form of life on Mars. What they are looking for is not just another boring microbe that happens to be smaller than anything we have on Earth. In the best case, in the case of what I like to call a "super positive outcome" then it could be the most amazing discovery you could imagine, revolutionary for biology, medicine, agriculture, nanotechnology,... There is no way to know how far reaching the implications could be.

Distant cousins with last common ancestor from a planet around another star

The other main possibility is that we find our distant cousins on Mars, DNA based life, following the same tune as Earth life, but evolved separately for tens of millions, or more likely billions of years. One way this could happen is if life originated on Mars, Earth, Venus, Ceres, or somewhere else in our solar system and then spread through the solar system via asteroid impacts. We've already covered the Earth to Mars and Mars to Earth case in the answer to Zubrin's arguments, under What about Zubrin's meteorites argument? and General case of transfer of life from Earth to Mars. It's far easier in the direction of Mars to Earth, but can happen both ways, and is most likely in the early solar system soon after the formation of our Moon. It's the same idea for transfer from Venus, Ceres etc.

Another way we could be distant cousins is if life originated in a a planetary embryo, or protoplanet, one of the hundreds of smaller objects that combined together to make the larger planets. Perhaps it originated in the parent body for some of the carbonaceous chondrites, or perhaps in objects that no longer exist, because they were destroyed when they hit Earth, some time after the formation of the Moon. One of the big issues here is how these small bodies could have held onto enough water for life to form, also the question of where the water came from - planetary embryos that formed at the same distance as Earth should be "dry" with no water, but ones that migrated inwards from further out could be wet, or indeed, even entirely made of ice. For an extensive discussion of this possibility, and some of the ways that it could happen. see From Protoplanets to Protolife: The Emergence and Maintenance of Life

However, there's yet another way it could happen. Both Earth and Mars, and other places in our solar system, could be seeded by life from an earlier star, far older than our one. I've already touched on this in Half of the pages of the book of evolution have been torn out, but let's look at it more closely. The authors of the original paper relied on a very minority view that our solar system formed from remains of a supernova explosion from a star system with life bearing planets. But there's a much easier way this can happen which has been explored in several papers.

First, we could be seeded by life around sibling stars in the Sun's stellar nursery soon after it was "born".

How life could transfer between sibling stars in a stellar nursery by "weak transfer" of meteorites

Normally it's almost impossible for life to get from one star to another as they are so far apart. The problem was that after being ejected from another solar system, the rocks would pass through our solar system with high velocities, higher than the solar system escape velocity. Entering our solar system at perhaps six kilometers per second, due to the relative velocities of the stars, there would be almost no chance of capture.

However, recently Edward Belbruno and Amaya Moro-Martín reexamined this situation using Belbruno's new idea of "weak transfer", and also the idea of transfer while the stars are still in their birth nebula. This is the idea that when you have gravitational tugs from several different bodies pulling in different directions at once, it can sometimes be possible for an object to get into a very weakly bound orbit around another one, in a situation where this would normally be impossible. Belbruno used this to rescue the Japanese lunar probe Hiten in 1991. The probe itself was lost, and its mother ship was stranded in an orbit around Earth without enough fuel to get it into orbit around the Moon by conventional means. He found a way of saving the situation using his new theoretical idea of weak transfer.

He's got an interesting approach. He's an expressionist artist, and he solves these problems using art, to inspire his mind to new approaches, including this idea that saved the Japanese spacecraft in 1991.

His art is here.

Anyway, using his weak transfer ideas, they found that our Sun could have exchanged life bearing rocks at least a hundred trillion times with its nearest neighbour in the cluster. Here they assume that the stars move at relative velocities as low as one kilometer per second as is normal in a young star forming region.

Shows how transfer of life was possible between two stars in the same cluster soon after the stars are born, with relative velocities between the stars of only of order one kilometer per second. It happens through the weak transfer, using the two weak stability regions shown, caused by the two stars themselves, other stars in the cluster and giant planets around both stars.

Our solar system would have had perhaps 700 million years to do this before it got dispersed from the new cluster. It had liquid water already long before the end of that window of opportunity.


Table showing window of opportunity for panspermia in the early solar system - sharing life with its sibling stars. Image by Amaya Moro Martin

Close up of a star forming region in a new version of the famous "Pillars of Creation" photograph by Hubble. Stars are forming right now, especially in the denser "pillars".

It's a tiny part of a vast star forming region. Most of the dust and gas has been blown away as a result of stellar winds from nearby stars, and the whole area is dense with baby stars. This is a zoom in view of a new HD image in visible light from Hubble released in January 2015. Zoomable and HD versions here.

This infrared picture lets us see through the dust and clouds, and you can see how the Pillars of Creation are embedded within a cluster of numerous baby stars, juts forming. High res and zoomable views here.

And this region is just one part of the much larger Eagle nebula.

Photograph of the Eagle Nebula taken with the European Southern Observatory. I've outlined the position of the "Pillars of Creation". This entire region is a star forming nebula, dense with baby stars packed close together.

If life has evolved around just one of those stars in the Eagle nebula, many of them will infect each other by meteorite transfer. It's easy for that to happen right now, because the stars are closely packed together and also not moving fast relative to each other. Typical relative velocities in a young star forming cluster like this are only one kilometer per second.

Within a few million years the dust and gas will be gone. The stars are orbiting the center of our galaxy in independent orbits, with nothing to hold them together as a cluster. Within 700 million years typically they have already spread out so far through our galaxy that it's no longer possible for them to infect each other with life. However, those 700 million years are plenty long enough so that it's possible that nearly all these stars will have life on them, before they disperse, even if just a few or only one of them has life at present.

For more about the Pillars of Creation, see The pillars of creation - a glimpse into how stars are born by Tanya Hill, and this Space.com video about it.

If this is right, then that leads me to a suggestion (my own idea) that one good place to look for life might perhaps be on any planets around HD162826 as it's thought to be a sibling of our Sun, because its composition is similar. We must have thousands of siblings, but most are so scattered through the galaxy that they are probably tens of thousands of light years away. This is the only one identified so far and it is only 110 light years away. If it had the same birth nebula - could it have been seeded by life? Could it have life that's a distant cousin of Earth life and DNA based, with the same amino acids etc as Earth life?

So anyway now, based on that, perhaps we can begin to see how our solar system could also be infected by life from a much older star than our own.

Infecting an entire star cluster with life from planets from an earlier, perhaps billions of years older star

Edward Belbruno and Amaya Moro-Martín's ideas of course just let us exchange life with our siblings, so it wouldn't explain how it could have started ten billion years ago. However, the authors of another paper on this topic take this further, with a suggestion that life could be transferred from one cluster to another more easily than from star to star. With so much exchange of life between stars within a cluster, it would just need a single wandering star from a previous life bearing cluster to pass through a new star forming region, to infect all the stars in the new cluster. The stars from the previous cluster would soon spread throughout our galaxy and it just needs one of them to pass through a new star forming cluster to infect it.

This earlier star of course doesn't need to be a yellow dwarf like our Sun. If this is right, Earth life could have evolved around a different kind of star, maybe an orange dwarf or even a red dwarf.

Artist's impression of the five planet system around the nearby orange dwarf star, Tau Ceti, destination for interstellar travel in so many science fiction stories. Credit: J. Pinfield for the RoPACS network at the University of Hertfordshire

As it turns out, Tau Ceti does have planets, five in all so far discovered. Most seem likely to be uninhabitable, but one of its planets is well within the habitable zone (planet f) and it probably moved into this zone in the last billion years. Tau Ceti is an orange dwarf star. That's a more common type of star than our Sun, and research so far suggests they may be very habitable stars. See this recent survey of the literature: The Habitability of Planets Orbiting M-dwarf Stars

It could also have originated around a red dwarf star. This is by far the most abundant type of star in our galaxy, and it's also a very long lived type of star, lifetimes up to trillions of years and they have habitable regions that the planets could remain in for billions of years.

Artist's impression of the newly discovered planet orbiting our closest stellar neighbour, the red dwarf Proxima Centauri. New ideas suggest that planets huddling close to the smaller red dwarf stars could also be habitable, even though they would be tidally locked with one side always facing to the parent star. This is by far the most abundant type of star in our galaxy. Our nearest star Proxima Centauri has an approximately Earth sized planet, Proxima Centauri b, which is on the edge of its habitable zone and if conditions were right, with a magnetic field to protect its atmosphere, may have been habitable for six billion years. However other research suggests that planets, huddled so close to their parent star, also face more intense UV and X-Ray emissions than our Sun, that may denude the surface of oxygen and water within a few million years.

Whether it was a red dwarf, an orange dwarf, a yellow dwarf like our Sun or some other star, only one of these stars is needed to seed the entire star forming region where our sun was born. Of course a star forming region is a far larger area to target than a single star, and if it originated in a nebula that was itself seeded with life, then there would be thousands of these life bearing stars around, and only one of those needs to pass through the next star forming region, for the life to propagate.

In the same way also, in the future, our Sun, or one of our Sun's siblings, such as HD162826, might infect stars in some future not yet born cluster, and so the process continues. In that way even if evolution of life is very difficult, it could still spread through much of a galaxy from a single origin. It would evolve further and further each time. The authors conclude:

" These results suggest that a young cluster is more likely to capture life from outside than to give rise to life spontaneously. Once seeded, the cluster provides an effective amplification mechanism to infect other members."

So that does seem a distinct possibility. At least, it's perhaps not as implausible as you might think when you first encounter the idea. If that's the situation, nearly all life in our solar system would be related. It would also mean that even the earliest life in our solar system would be hardy and indeed pre-selected as life that can be transferred via meteorites easily. Although many microbes on Earth can't be transferred by meteorite, this would suggest that the first life to arrive in our solar system had this capability already.

However it wouldn't be multicellular life, or even eukaryotes, because those came later on Earth. Our last common ancestor could also predate the prokaryotes, and be somewhat simpler than any form of life we know. Also it might or might not have photosynthesis which seems to have been a later development on Earth.

This scenario doesn't rule out the possibility of places in our solar system where we can study life evolving from scratch. A good candidate for that might be the ocean of Enceladus as according to some ideas, its ocean may be young, only a billion years old. If that's so, it could have started with organics but no viable life left, and any traces long decayed. Or anywhere else where ancient life was sterilized, leaving a situation with organics, and the possibility of life, but no life had reached it yet. I wonder if on this hypothesis, there could even be places on ancient Mars where life tried other experiments in a "shadow biosphere", or pre-biotic chemistry that almost lead to life?

However, it does give another way that Mars life could be a distant cousin of us.

Surprising distant cousins

So, there are many ways we could find distant cousins on Mars. Our common origin could be within our solar system, or perhaps our common ancestors came from another star in our birth nebula, or they might even have arrived on Earth with billions of years of evolution behind them already, transferred from a planet orbiting a star that long predated our sun. If that is what happened, we could find life on Mars that is closely related to Earth life, but yet different. Distant cousins with a common ancestor billions of years ago.

So what might those cousins be like? Well we do have some surprising distant cousins on Earth already which may give us a few clues to get started. As an example, most creatures on Earth with shells, including the microscopic forams, use calcium carbonate. It's the same for creatures with bones, like ourselves, we use calcium carbonate too.

But the tiny microscopic diatoms use silica (found in nature as quartz) to make their shells. Basically their cell walls are made out of a type of glass. Would we have guessed that you could have microscopic creatures with "glass" shells if these creatures did not exist? They are unique - no other Earth lifeform has silicon skeletons or silicon shells. Yet it's also a very successful adaptation for them, as they are numerous with many different species, amongst the most common type of phytoplankton (microscopic single cell "plants").

And then, insects and crustaceans have yet another type of "skeletal material" (as an exoskeleton), to structure their bodies, chitin, which is an organic product, a derivative of glucose.

So could life on Mars use something different yet again? Perhaps they might use the iron oxides in some way, as they are so ubiquitous on Mars and would be useful for protection from UV light?

For another unique ability, most photosynthetic life works by splitting water to make oxygen, taking up carbon dioxide

The basic equation for photosynthesis is 6CO2 + 12 H2O → C6H12O6 + 6O2 + 6 H2O where the oxygen atoms in bold are the same ones on both sides of the equation. Note that photosynthesis doesn't split the oxygen from the carbon dioxide, but rather, from the water. See Plants don't convert CO2into O2, and Notes on lamission.edu

Some photosynthetic life works by splitting sulfides to sulfur. These can even photosynthesize in the dim glow of a hydrothermal vent, a discovery from 2005. In all these cases, the photosynthesis is eventually used to power a "proton pump" to move hydrogen ions across a membrane and this is used with assistance of the enzyme ATP synthase to form ATP which powers the cells (and other bioenergetic processes)

Most remarkably of all, some microbes are able to do bypass all of this chemistry and use light directly to move the hydrogen ions (protons) across a membrane out of the cell, with no byproducts such as sulfur or oxygen. These are the  halobacteria (or haloarchaea) which use Bacteriorhodopsin and Halorhodopsin for photosynthesis. It's similar to the method the rod cells in our eyes use to detect light which make them sensitive to very low levels of light (they use the related pigment rhodopsin). These pigments are most sensitive to green light, and reject blue and red light, and so are purple in colour.

This shows the salt ponds of San Francisco. According to the Earth scientists who maintain the Earth Story, the area of pinkish red (with a somewhat purple hue) here is coloured by the haloarchaea.

The green at top right is from green algae. The orange pool at top left is that colour because of brine shrimp. But the pinkish red (with a purple hue) comes from haloarchaea which use light to generate energy directly without any chemical byproducts, much in the same way our eyes see light.

Many red algae are red for other reasons. For instance the red algae that make the Red Sea red are that colour because of phycoerythrin, a red pigment that is involved in the process of normal chlorophyll based photosynthesis.

These haloarchaea also occur in "Pink Lake" in Western Australia, but that's red partly because of the haloarchaea, and partly because of carotene accumulating in a green algae dunaliella salina, another red pigment that is involved in normal photosynthesis.

Here is how it works:

Shows how halobacterium salinarum gets energy from sunlight using bacteriorhodopsin - similar to pigments we use for vision, as a source of energy, instead of chlorophyll.

The light deforms the molecule which then acts directly as a proton pump which pumps hydrogen ions out of the cell. This proton gradient is then used, for instance, to synthesize ATP. It does this without generating oxygen or capturing carbon dioxide. Ordinary photosynthesis also works as a proton pump, but only indirectly, after carbon fixation and release of oxygen.

Again, if we didn't have examples to show it is possible - who would guess that there could be photosynthetic life that gets its energy in this way? So, even if the life on Mars is a distant cousin of Earth life, it could easily have some unique capability like that which surprises us.

How could our cousins differ? Skins of rust, freezer life, different amino acids translation table, ...

Let's just let our imaginations roam free for a moment, and list a few of the ways that our distant Mars cousins could be surprisingly different. Once again I invite you to a section based on synthesis and speculation. These are just my own ideas, not based on any sources as I don't know of anyone in the literature who has tried listing ways that our Mars DNA based cousins might differ from us. I took the list in Life on Mars dancing to a different tune (above) of the internal structures and biology for Earth life, and asked, "what might it be like if those are varied?". Then I added in a few other ideas that seemed natural extrapolations, such as skins of rust, and freezer life.

Do say if you know of a paper that covers this topic! Also what other possibilities can you think of?

  • Skins of rust - Mars is the "red planet" after all. And iron oxides are great at blocking out UV light. What if there are microbes there that are like our diatoms but they have cell walls that incorporate iron oxides, making them rust red? Walls that are completely opaque to ultraviolet light? I base this on Carl Sagan's observation that the iron oxides on Mars are perfect for keeping out UV light, for cells imbedded in the dust. What if, as a result of evolution, instead of just hiding in cracks in the dust, they actually take up the dust into their own skins?
  • Its own version of microbes with a nucleus. If Mars had multicellular life three billion years ago - what if it developed its own version of eukaryotes which are the basis of all complex Earth multicellular life? If so, it would have developed its own version of the Golgi apparatus, lysosomes etc.
  • Unique cytoskeleton not based on microtubules or actin. Suppose it split off at a very early stage from Earth life, perhaps it has DNA just like Earth life, but doesn't use microtubules or actin at all, but has other ways of structuring its interior and of moving.
  • More or different amino acids in its translation table. The translation table that translates sequences of three base codons to amino acids seems somewhat arbitrary. What if Mars life has found a way to incorporate more amino acids into it, which are never used by Earth life? Or the other way around, what if Earth life uses amino acids that Mars life ignores? Or what if we just have a different set altogether with some overlap, which might perhaps give us clues to the original set of amino acids used by our last common ancestor?
  • Evolves more rapidly and more tolerant of errors in its DNA. Earth life doesn't reproduce DNA exactly, and indeed, perhaps it's adapted to tolerate a level of error to permit evolution to happen. So what if Mars life has a higher tolerance of error than Earth life in its DNA? That could be an advantage on Mars. It would be able to tolerate higher levels of ionizing radiation, and also be able to evolve much more rapidly when faced with a novel situation through its frequent changes of climate.

    That might be worth doing on Mars despite the downside of having more errors, and the higher proportion of cells that just don't work after replication. As we saw, in Tougher conditions for life on Mars - sometimes - and at other times much easier (above), its climate is much more variable than Earth's, with its orbit continually changing between circular and very elliptical, with ice sometimes at the equator instead of the poles, right through to the present. Also, even today, it has temporary short lived habitats after asteroid impacts or volcanic eruptions creating huge lakes that only last for a thousand years. In the early solar system it had frequent huge ocean sterilizing impacts as well as ones that caused massive flooding and created deep basins like Hellas basin filled with water. Its climate probably varying frequently between habitable and ice ball in the early solar system. With all those frequent changes, you can imagine a premium on microbial life that is able to evolve rapidly to face new situations.
  • Ionization resistance way above Earth life. The most resistant Earth life is able to survive in a dormant state for up to hundreds of thousands of years in the ionizing radiation of the Mars surface (and longer if it can wake up briefly to repair its DNA). It seems unlikely that anything could survive in dormant state for billions of years on or close to the surface as that is enough time to break up all its organics into water vapour and other gases - but what if it can survive for millions, or tens of millions of years of dormancy? This might fit in naturally with a higher tolerance of error. After all, Earth life did not evolve in the presence of ionizing radiation, it just has that capability as a byproduct of desiccation resistance. So, how radioresistant would life be that actually evolved in conditions that specifically favoured higher ionizing radiation resistance, evolving for billions of years? If it could remain dormant for millions of years near the surface, then revive, that could be very useful given the way that Mars' climate changes so frequently on the millions of years timescale.
  • Can resist desiccation better than Earth life - the most ionizing resistant Earth life seems to have evolved its capability as a way to resist desiccation Well the ionizing radiation resistance on Mars might have the same effect in reverse. It could be more desiccation resistant on Earth than the most hardy Earth life. Add in iron oxide skins and it may be able to survive UV radiation too in spore form. Also desiccation resistance could be a major advantage and evolution driver in the thin atmosphere on Mars. That might well favour evolution of highly desiccation resistant cells, more resistant than any Earth microbes.
  • Takes in water from dry air- perhaps using perchlorates. Mars life might actually make its own perchlorates. It could run the processes that some Earth life uses to eat perchlorates in reverse, given a source of energy. Perchlorates are great at taking up water from the atmosphere even when it is quite dry. So maybe the Mars life pumps itself full of perchlorates, in dry conditions, and uses that to extract water vapour from the atmosphere? Or expels the perchlorates and creates a little patch of perchlorates salts around itself? Converts normal salts into perchlorates which then take water out of the atmosphere so creating a tiny pool of perchlorate rich brine around it. Perhaps it actually creates Nilton Renno's ice / salt interface "microbe swimming pools" by converting chlorides, hypochlorates, and chlorates into perchlorates in the proximity of ice, so melting the ice. Or uses other ways to extract water in very dry conditions so that it can continue metabolizing and reproducing. Maybe it adjusts the mix of chlorides and perchlorates to make the habitat around it optimal for its metabolism.
  • Freezer life - this seems an obvious adaptation for Mars life -that it can reproduce below the -20 °C that our freezers operate at. This seems a likely adaptation for Mars life. If it is possible at all for life to survive and reproduce way down to temperatures of -80 °C, then surely Mars life has found a way. Our Earth life hasn't yet found a way to adapt to be able to reproduce inside our freezers below -20 °C but we haven't had freezers for that long and there are almost no natural habitats on Earth at temperatures much below that.

I expect you can think of many more ideas. Though as a caution, these attempts to "foresee the future" almost always fail abysmally. Whatever we find there might well surprise us, and not be anything any of us have thought of in advance. Still, it's the best we can do and perhaps it can give a flavour of how much Mars life could differ from Earth life, even if it is a distant cousin of us.

What else could have come before modern life? Alphabet soup of "XNA", Ostwald ripening organic crystals, "naked genes", or almost alive "autopoetic" cells

Let's now return to the idea that what we find on Mars could be an early form of life, whether related to Earth life or independently evolved. Then there are many other options apart from the RNA world cell idea, popular though that idea is. Astrobiologists have often suggested that life might be able to use some other helical structure, neither DNA nor RNA. Their ideas include a  PNA world which has a different backbone from DNA or TNA world, or a molecule that's a hodge-podge mixing different backbones in the same molecule with non heritable variations in backbone structure. They have also suggested a whole "alphabet soup" of other possible precursors such as HNA, PNA, TNA or GNA - Hextose, Peptide, Therose or Glycol NA. They call all of these XNA by convention, though actually many of them are not nucleic acids.


So what were the earliest cells like? What are the simplest possible cells? And what were their precursors? Even early RNA world or TNA world cells or similar may not get us right back to the origins of life. There still seem to be many gaps to fill in, even to get to early cells like that from non living chemicals.

There are various approaches to this. One is the idea of autopoesis, that a minimal cell might be able to reproduce in a less rigorous, approximate fashion, by having a simple structure, a vesicle that takes in material from outside the cell wall.

Diagram of an autopoetic cell, from "Chemical Approaches to Synthetic Biology: From Vesicles Self-Reproduction to Semi-Synthetic Minimal Cells" There, L is the cell boundary, lipids in case of Earth life. P and Q are the basic ingredients of cell growth and W, Z the waste materials.

E is the genetic and metabolic network, which converts the ingredients into the cell wall, as well as the internal components of the cell, creating waste products that leave the cell.

The idea is that , inside the cell, there is a network that turns these precursors into the cell wall itself, as well as using them to regenerate itself, and expels waste products. The vesicle as it gets larger splits to replicate, or alternatively, it creates a daughter cell inside which then leaves the cell. In these primitive protocells, this is regulated, for instance, by the surface area to volume ratio.

This could happen without any DNA or RNA to regulate it. This process happens with some fatty acid vesicles for instance. Some researchers are working with Butschli droplets, a complex mixture of oils and other chemicals such as detergents, that behave rather like cells to explore such ideas.

These are either droplets of oils in water or of water in oil. These are not likely precursors for us, of course, but are examples that work like protocells which let us explore artificial life scenarios in a different medium.

Here is Martin Hanczyc talking about protocells.

Researchers exploring this analogy include Rachel Armstrong and Martin Hanczyc.

This is just one of many ideas. There's a biological survey of some ideas for the earliest cells and their precursors here: The Origins of Cellular life. The next couple of paragraphs summarize some of its ideas.

More generally, there are two general basic approaches for this very early biology and pre-biotic chemistry. Either metabolism first or replication first. The protocells can "reproduce" in a way but imperfectly, just grow and then split. So metabolism comes first for those. We might find "almost alive" protocells that don't replicate exactly and stretch the boundary of our definition of life.

Perhaps protocells used naturally porous cell walls, made of fatty acids. New protocells might form spontaneously as new membranes assemble themselves around genetic material in the solution. Another way it could happen is that clay attracts RNA, and so perhaps a clay particle already covered in RNA also stimulates a cell wall to form around it, leaving the original clay particle, still covered with the RNA crust, trapped inside the newly formed cell wall. Another way protocells could form is inside microscopic channels within the rocks that precipitate around hydrothermal vents. The thermal gradients and the thin channels could concentrate the nucleotides and the larger nucleic acids. Then fatty acids could also be concentrated until they form vesicles at the bottom of the capillaries, with DNA inside, so leading to spontaneous self assembly of protocell like structures with DNA inside at the ends of the capillaries where they meet open sea, and they then could split off.

All in all, making protocells isn't that difficult and they can do that already in experiments. So - surely Mars must have had protocells and probably still does, even if it doesn't have life. The hard bit of the puzzle to fill in is what happens next, and how these cells come to replicate exactly or almost exactly. Getting DNA to form isn't anything like enough as you have to find a way for the two strands to separate and then replicate with the same gene sequence as before, then re-assemble as two identical double helixes. Perhaps the early life DNA just unfolded through variations in temperature. Perhaps in these early cells, DNA separates at higher temperatures, replicates, then reassembles into a double helix at lower temperatures. There are many such ideas. Perhaps we may find the answer on Mars.

Another thing we could find if we can trace back early enough in the origins of life on Mars, is that it formed huge crystals of organics all of one symmetry. When you have a mix of crystals of different sizes, the smaller ones often evaporate and build up into larger ones, because larger crystals are more energetically favourable. Eventually you end up with one big crystal. This is one theory for how modern life got its chirality - the preference for one form of a molecule over its mirror image. The process is known as "Ostwald ripening" and it could take all the amino acids of one chirality out of a solution into one big crystal, amplifying a tiny signal of a slight excess of one version of an amino acid over its mirror image.

Salt crystals - notice how some grow larger at the expense of others. Most of the action happens from about 50 seconds into the video onwards.

If this happened on Mars, who knows, maybe at some point we find traces of these very early pre-biotic Ostwald crystals there.

Or the precursors of life could consist of chemicals that replicate, with no metabolism or cell wall. One possibility, for the RNA world hypothesis, is some kind of a mineral substrate. The chemist Leslie Orgel in his 2004 paper "Prebiotic Chemistry and the Origin of the RNA World" writes:

"A scenario that I personally find attractive is one in which the very first replicators were 'naked genes' adsorbed on the surface of mineral particles, and in which impermeable membrane caps were 'invented' by the genetic system as it became metabolically competent. Escape from the mineral surface, enabled by the development of a closed spherical membrane would occur at a relatively late stage in evolution"

So we might find something like these ideas. Far more complex than any of the things we can make in the experiments in our laboratories, perhaps with thousands of different chemicals interacting, on their way towards becoming life, but not yet what we'd recognize as life as such. Perhaps not replicating exactly for instance. "Almost alive" precursors for life.

And there are many other ideas.

Mars has had such a long period of cold conditions ideal for preserving organics. So, of all the places in our solar system, it may give us one of the best opportunities to trace out some of the steps of evolution of early life. If it is difficult for life to evolve, and Mars never developed life, it would be fascinating to find out what it did develop. Did some of these possible precursors of life develop there? If so I imagine there'd be a lot of interest in trying to find out why they didn't evolve further, all the way to life. It would also give us insight into what happens on the many planets in our galaxy that are Earth or Mars like if they don't evolve as far as life.

If Mars never developed life at all, at any time in the past, does it perhaps have autopoetic cells, or Ostwald crystals, or"naked genes" adsorbed on mineral particles, there today in favoured spots on Mars?

Something amazing to discover - but hard to find

If any of that is waiting for us to discover there, in the past or the present, most astrobiologists don't expect it to be an easy thing to find. Some of the things that make it so hard to know if the ALH84001 meteorite has traces of life or not is that

  • Many of the organics could be produced by non life processes, especially the Polycyclic Aromatic Hydrocarbons (PAH's) in this particular case
  • Non life processes could also produce the magnetite crystals found in the sample, which originally seemed so characteristic of life.
  • The carbonate globules could also be produced by non life processes
  • The possible life structures are so small that most can only be seen with an electron microscope. Could they be artifacts of the process by which the samples were prepared?
  • Nearly all the organic carbon in ALH84001 is known to be terrestrial contamination.

We will have the same, or similar problems on Mars if we study similar samples there. But at least we can deal with the last of these issues. If we can keep Mars samples free of Earth life, we can deal with the contamination problem. But that's not the only problem there.

Organics created on Mars by non life processes

Some of the organics made through natural processes on Mars might mimic biosignatures. They might even be chiral, occur in only one of two mirror image forms. Also any biosignatures we do find are likely to be mixed up with organics created by non life processes making the signal weaker and harder to detect, especially since past life is likely to be damaged and degraded.

Some of the non life organics on Mars could be

If there are any organics from ancient life, they will need to be well preserved for us to detect them. It will be hard enough to disentangle real native Mars life biosignatures from these Mars originated biosignature mimics. Once again, the last thing we should do is to confuse it further by adding Earth microbe biosignatures into the mix as well.

Organics from meteorites on Mars may boost a molecule over its mirror image, mimicking biosignatures (chirality)

Many meteorites, and comets, are rich in organics. So could these mimic biosignatures from Mars life? The Australian Murchison meteorite is a famous example.

Fragment of the Murchison meteorite, and particles extracted from it in the test tube. The meteorite was a witnessed fall, collected soon after it landed in Australia, which means it is fresh with little by way of contamination by terrestrial organics. It has many organics in it. It includes rare amino acids such as Isovaline:

Isovaline, a rare amino acid found in the Murchison meteorite.

This helps confirm that the organics in it are of extraterrestrial origin as this amino acid is not involved in Earth life. Incidentally, it may be of value for treatment of acute and chronic pain.

You might not realize this from the enthusiastic news stories when Curiosity finally discovered organics, but the biggest surprise was that it didn't find them sooner. All those organics "raining in" on Mars from space from comets and carbonaceous chrondite meteorites have to go somewhere. For instance organics found by Curiosity in the Yellowknife bay were consistent with presence of 300 to 1,200 parts per million (ppm) organic carbon from meteorites. They actually expect them to be of meteoritic origin.

A 1990 paper predicted that between 2% and 27% of the Martian soil would be contributed by meteorites, of which 1% to 10% typically is organics. According to calculations, if there was no degradation of the organics, Mars should have 60 ppm of organics from organics deposited into the regolith, averaged over its entire surface to a depth of a hundred meters (see page 10 of this paper)..Also the meteorites from Mars that we have on Earth have lots of organics in them, roughly consistent with that estimate (they come from a few meters below the surface of Mars). So the main puzzle on Mars is actually, what happened to all those meteoritic organics? They seem to be gone from the surface at least (we don't yet have any data about organics centimeters or meters below the surface). The puzzle here is much more a question of how they were destroyed than why some of them are still there. This has been a big puzzle, ever since the time of Viking?

One possibility is that it was under-estimated because the organics get destroyed by perchlorates when the samples are heated up in the ovens typically used to analyse the samples on Mars. The other possibility is that there are processes actively removing the organics from the surface. Probably both of these are factors.

So, when we find non degraded organics on Mars in the future, there's a good chance it is meteorite organics, like the organics already found by Curiosity. So how do we distinguish organics from life, from the organics from meteorites? One way to search for past life is to look for a chiral excess, i.e. the preference of a molecule over its mirror image. But organics in the meteorites we analyse on Earth often have chiral excesses already. Some of this is surely due to contamination from Earth life, but we find these excesses in witnessed falls, where the meteorites are picked up (often in a desert where they are easy to spot) soon after they fell to Earth - or in meteorites from places like Antarctica that have probably not been exposed to much by way of Earth originated organics. Also, sometimes they have an excess in amino acids that are rarely used by Earth life or not at al. Also, sometimes the excess is a chiral imbalance in the opposite direction from Earth life.

In the case of the Murchison meteorite this imbalance is subtle and controversial,. In other meteorites, however, much larger excesses have been detected. In this 2006 analysis of the EET92042 and GRA95229 meteorites from Antarctica, they had chiral excesses ranging from 31.6 to 50.5%.

GRA95229 - another chrondite, collected in Antarctica, had chiral excesses of +31.6‰ for a-AIB to +50.5‰ for the (non terrestrial) amino acid isovaline, while the EET92042 meteorite ranged from +31.8‰ for glycine to +49.9‰ for L-alanine. These excesses seem to be extraterrestrial and not due to contamination by Earth life.

These meteorites are certainly not pristine. They are altered by water, at least. But they come from Antarctica, collected from the ice, so are less likely to be contaminated by organics. Also, the mix of amino acids seems non terrestrial which is another line of evidence to suggest that they may not be a result of contamination. Also GRA95229 has 2.5 times greater levels of organics than are typical in Antarctica. All of this suggests that these excesses may be extraterrestrial. For a more recent review of this, see the Chemistry Society Review article: Understanding prebiotic chemistry through analysis of extraterrestrial amino acids and nucleobases in meteorites.

Also a recent study of sugars in the Murchison meteorite, along with others, found evidence of quite major excesses of sugars in one form rather than the other, excesses of up to 55% for thereonic acid, a four carbon sugar acid. The excess for one of the 5 carbon sugar acids was even more dramatic, it had an excess of up to 82%. It was the same even for biologically rare sugar acids, which the researchers see as evidence that these excesses are not due to contamination from Earth life.

These meteorites have been unchanged since they formed in the early solar nebula. So how did they get this excess (assuming these results are correct)? There are various theories of processes that would do the trick, but nobody is entirely sure yet, and most of the ideas would lead to a rather subtle excess. See this 2000 paper Circular Polarization and the Origin of Biomolecular Homochirality. For a more recent idea which could lead to a large excess, see this 2012 study.

"The researchers propose that, in the solar system’s early days, heating as a result of radioactivity could have melted ice trapped deep inside asteroids. Liquid water then dissolved already present amino acids, which crystallized into mostly left-handed groupings."

Whatever the reason, if this chiral imbalance of organics from meteorites is real, it complicates the search for life on Mars.

Now, it's not a hopeless lost cause, if the meteorite organics do have these huge excesses. If we have a well preserved specimen of billions of years old past Mars life we should be able to figure it out so long as the original organics had only one form of each amino acid. Totally monochiral amino acids would be easy to distinguish from an excess or imbalance in that amino acid.

It's not as easy as you'd think, because if you just leave organics for billions of years even in very cold conditions, occasionally an amino acid will "flip" into its mirror image form. This happens much more quickly in warm conditions so if the sample ever got warmed up, in that long history, for instance if it was sometimes near the surface or if it lay at the bottom of a warm pond for some time, it would lose this chiral signal very quickly, and we would have no way to tell that it ever had a chiral excess.

But if we search long enough and carefully, then we may well find samples there that preserve traces of this chiral signature even after billions of years, in samples that have spent all that time in cold dry conditions. We'd still expect only a small excess, since even in cold dry conditions, much of the chiral signature would get lost. However, different amino acids flip at different rate, so we could look at what the excess is for several different amino acids and then work back to find out when they were originally all the same chirality. If we do that and get a sensible date for the original monochiral sample, and it ends up with the same calculated original date for all the acids in the sample, this would be good evidence that it may have originally had all its amino acids in just one chiral form. That would then be a strong indication that it was once a form of life.

This is part of the detective work astrobiologists would have to do when studying ancient organics on Mars. They could also try to detect life through its preference for lighter isotopes of carbon or sulfur, and there are various other biosignatures they can search for.

Then in addition to all that, the organics could be mixed with a signal from later life, and also of course, with with a later influx of chiral meteorite organics. That would complicate things again of course. But it's possible to deal with that also, by looking at the distribution of the amino acids and at the isotope ratios. It should be solvable, but all that certainly complicates the picture.

So, in short, it's likely to require a lot of careful detective work to work that all out. How much comes from meteorites, how much from degraded organics from life, how much from later life, and how can we accurately figure out if the life originally had a preference for one molecule over its mirror image?

However, it could be even more complex than that. What if some (or all) of our samples on Mars are so early, that they take us back to a time before "homochirality"? The origin of this preference of life for one chemical over its mirror image is still very much a mystery. See a recent paper from Chemistry World, October 2015 on The Origin of Homochirality for some current ideas about this.

The reason many biologists think that the amino acids would all have the same chirality, even if the life is unrelated to Earth life, or very early life, is that if you have some of the mirror image amino acids in a cell, it blocks and poisons the process that joins the other amino acids together to make a longer chain. This is known as "enantiomeric cross inhibition". This leads to a big puzzle though, how then could life evolve in the early pre-biotic "chemical soup". The Urey Miller experiments with lightning and organics produced equal numbers of both types of amino acid. And the excess from meteorites even in the most remarkable cases is nothing like 100% of just one of the two types of amino acid. There are various ideas about how this might have happened.

But what if that wasn't even necessary, at least not at the beginning? We should look briefly at this very challenging possibility for early life. This would be the worst case for exobiologists trying to find out about early Mars life. Though if it started off chirality indifferent and later evolved chirality, then that would make things easier for the astrobiologists. If Mars life somehow stayed chirality indifferent right through to the present, then chirality just would not be a useful biosignature to test for, and we'd need to look into other ways to test for biosignatures.

Joyce's ribozyme which joins together strands of opposite "handedness" - could early life be chirality indifferent - ambidextrous?

This is an intriguing discovery by Gerald Joyce that suggests that perhaps very early RNA world life was ambidextrous. He found a ribozyme using right handed RNA that could join together left handed RNA strands. It could replicate its mirror image also. It couldn't replicate itself, but its mirror image (enantiomer) could in turn replicate its original self. In this way - though it can't directly deal with both handedness of life in one go, it can make a copy of its mirror self, which can deal with the opposite handedness, and its mirror self can copy the original, so together with its mirror image, it can handle everything you throw at it, whatever the handedness, as well as replicate itself in both handedness too. Techy details in their paper in Nature here. Discussion of its implications here.

He did it by a test tube "chemical evolution" gradually building up more and more complex ribozymes, selecting for the ones that can join together the opposite sense of RNA strands. In a few months, after only 16 rounds of this evolution, he finally created an 83 nucleotide ribozyme that works with the opposite sense of RNA. So - though it doesn't fill in all the gaps in the puzzle, it suggests the possibility that early life could have been ambidextrous. So what if early life on Mars is like that, so that there is no chiral imbalance to detect? Equal amounts of all the essential molecules and their mirror images, each happily replicating the other through ribozymes?

Well there are two other main ways to find a possible signature of life from looking at the amino acids. See the section on amino acid target selection in this paper (about sensitive electrophoresis to examine liquid samples). One way to spot it is if there is a large excess of an amino acid that is never formed naturally, only through biotic processes. Another way is to look at the relative abundance of the more complex amino acids compared with the simplest one, glycine. Abiotic samples normally have a much higher abundance of the simpler and easiest to synthesize molecules. So if you saw an abundance of more complex amino acids, that would suggest it is a result of life rather than abiotic chemistry.

In short we don't know what we will find there, and we may have to do a lot of detective work. There are many other kinds of organics we can look for, but amino acids are a likely early starting point. If the biosignatures are very faint and hard to detect, then contamination by Earth microbes, even dead ones, and even in tiny quantities, could be a major nuisance or make the whole thing impossible to sort out, as happened for ALH84001.

Then another thing that makes it even harder is that we don't know where to look for evidence of past life on Mars. It may take a while before we learn which conditions on Mars are best for preserving ancient organics, and then when we do, perhaps they preserve those organics only sometimes. Maybe we find suggestive but ambiguous signals first and then only gradually home in on the best places to find life there. So, let's have a look at this next.

Preservation of past organics

Mars is a great place for preservation of organics in some ways, because it is so cold. One of the things that makes it hard to find past life on Earth is that warm organics gradually either break apart (DNA) or as the molecules jostle around in the warm conditions, they spontaneously swap over into their mirror image forms.

Just as DNA only spirals one way, the other chemicals used by life such as amino acids occur in just one of two possible mirror image forms. But in warm conditions, then molecules can spontaneously flip into their mirror image forms.

So, first, , let's look at the plus points, some of the things that may make it easier to find well preserved billions of years old organics on Mars than on Earth.

  • The Mars surface is very cold. Even just centimeters below the surface, it may be cold enough so that some of the amino acids and other organics haven't yet swapped into their mirror image forms, even billions of years later.
  • Also with no continental drift, much of the Mars surface has remained undisturbed for billions of years.
  • Most of its surface is hardly changed since the formation of the planet, apart of course from the craters from larger meteorite impacts, the regolith gardening by smaller meteorites, and the layers of sediments added during later flooding events.
  • Mars has very little by way of weathering compared to Earth. Lots of dust sediments and some localized flooding, that's all.
  • Either no present day life, or not much present day life to confuse the signal. If it does have life on the surface, it probably can't survive more than a few centimeters below the surface, because it is so cold. That is, except in geological hot spots (if they exist) and deep in the subsurface hydrosphere. Dig a few meters below the surface, the best place to look for past life anyway (protected from cosmic radiation) and there's probably no present day life to confuse the signal.
  • Has large areas of clays and salts - these are ideal for preserving organics
  • Large areas of the surface are thought to have had water in the past, including almost the entire Northern hemisphere. What's more the water seems likely to have been habitable, at times, moderate in temperatures, not too salty, and neither too acid nor too alkali.

However there are other things that make preservation of those ancient organics from any past life harder.

  • Mars was most habitable billions of years ago. This is a long timescale for preservation of organics, during which Mars lost most of its water, had many floods, and changed its inclination, orbital eccentricity, atmospheric density, and climate many times. That's a lot of changes in the Mars climate for the sample to last through. It just needs to warm up for a geologically short period of time to lose its chiral signature, for instance.
  • We don't yet know which parts of Mars had life in the past. For instance, what if the only life occurred around hydrothermal vents, and it never developed as far as photosynthesis? Then we may need to search ancient hydrothermal vents to find it. There are many other ideas about where life might have started. What if it remained in the place where it first evolved and never got any further?
  • The cosmic radiation degrades surface organics. That's because the Mars atmosphere gives little protection from cosmic radiation. Every 650 million years you get a 1,000 fold reduction in the numbers of small organic molecules such as amino acids on the surface because of cosmic radiation. It degrades the organics to water vapour, methane, carbon dioxide etc, leaving nothing but volatiles. Since it's an exponential process, that's a million fold reduction every 1.3 billion years. It becomes a trillion fold reduction after 2.6 billion years and so on.

    This is the main reason that astrobiologists say we may have to dig deep to find life that has escaped this process. Astrobiologists recommend that we search for early life at a depth of ten meters ideally. ExoMars will be able to drill two meters which is enough so that it has a chance of finding evidence of past life. But though that's a necessary condition, it's not by itself enough for the present day sample to be deep down. It must also have been buried quickly in the past and stayed buried well beyond the reach of cosmic radiation ever since. We could also find organics that were unearthed recently as a result of an impact, or weathering by the wind, but it would have to be fresh, recent unearthing, for it to have a chance to preserve the organics.
  • Mixed in with organics from meteorites, volcanoes etc (already covered in Organics created on Mars by non life processes). To make things more confusing, the meteorites often have a chiral signature, which would need to be disentangled.
  • Life is most likely in places that had water in the past. These are the very places where warmth, flooding, consumption of the organics by other lifeforms, and other forms of degradation can happen.
  • Later episodes of flooding

    Artist's impression of Gale crater as it might have looked during one of its flooding episodes (by Kevin Gill). For more on this: Curiosity Rover Data Indicates Gale Crater Mountain Used to be a Lake

    Of course, floods like this may make it briefly habitable, but they can also wash out earlier deposits. Especially as the later floods on Mars were often rapid flash floods, so probably not giving much time for new deposits of life to form.
  • Chemical degradation of near surface materials by the perchlorates, hydrogen peroxide etc. We know that there must be processes actively removing organics because it should have reasonably large quantities of organics from meteorites and comets and instead it only has small amounts.

So, how should we search to try to find a sample that has survived for billions of years?

For a clear signal of past life

For a clear signal, for past life, your sample somehow has to be deposited originally, and then avoid all those things that could destroy or degrade it. It needs to be:

  1. In a place where there was life in the past, for instance in the remains of ancient hydrothermal vents, or salt lake deposits, or wherever it is that life was back then. Perhaps it was almost everywhere in the ancient seas if it had already developed hardy dormant states, and photosynthesis. If not, maybe it is in only a few special locations, so we will need to learn where to look for it.
  2. Form in a place where dead microbes were accumulating, perhaps the bed of a lake. Or the detritus from a flash flood.
  3. Preserved quickly (dried out, caught in clays or salt, or the microbes rapidly entombed in fast forming rocks like chert)
  4. Plunged rapidly into freezing conditions, because in warmer conditions, the organics flip easily into their mirror images, which makes it harder to distinguish life from non life.
  5. Buried quickly, ideally within a few tens of millions of years, to a depth of several meters for protection from the cosmic radiation degradation.

Then once it is safely buried like that, it has to survive for billions of years and then be returned to the surface. So it

  1. Wasn't washed out with later floods, or altered or destroyed by the perchlorates and other chemicals, or returned to the surface temporarily for more than brief time periods.
  2. Wasn't mixed with other sources of organics, or if it was, in a way that is easy to disentangle
  3. Was returned to the surface rapidly (perhaps as a result of a meteorite strike or weathering of the rock by the Martian winds), and did this in the very recent geological past. Or else, your rover needs to be able to drill deep, or search in caves protected from the surface cosmic radiation.

To find out more about all this, see John Grotzinger's Habitability, Taphonomy, and Curiosity's Hunt for Organic Carbon as a starting point:

When you put it like that, it may begin to seem almost a hopeless task. But on the plus side, Mars is a huge and varied planet, with its surface area the same as the land area of the Earth. There are plenty of places to look for this life on Mars. It is also geologically diverse, with many varied geological features and terrains.

Amongst other things, we can search for this life in ancient deltas, shore lines, salt beds, and preserved hydrothermal vents. Also, Mars must have many caves made through the passage of water, and vast layers of sediment (though these caves are hard to spot from orbit, far harder to spot than the lava tube cave entrances we know about already). The whole of Mount Sharp in Gale Crater, which Curiosity is exploring, is a sediment deposit built up over hundreds of millions of years. Surely somewhere amongst all this geology, in all these layers of sediments, we will find the ideal conditions leading to preservation of past life? We also have a great chance to find optimal conditions for present day life somewhere in all this varied terrain too.

Are there any "magic minerals" to preserve microbes from early Mars?

The downside of this vast search area is that we don't know where to look yet. On Earth one key to discoveries of early life was the realization that gunflint chert is a "magic mineral" that preserves traces of early life in exquisite microscopic detail.

Galaxiopsis, one of the fossil microbes found in gunflint chert, which has turned out to be a "magic mineral" for search for evidence of early biology on Earth. These fossils are so exceptionally well preserved that scientists can even detect organics from microbes as old as 1.88 billion years ago. The organics are degraded of course but they could still detect functional groups (which attach to the hydrocarbon chains and play an important role in life processes) such as amides and hydroxyl, and other oxygen based functional groups, and attempt to compare them with modern cyanobacteria and micro algae. See discussion section of this paper.

However, though these newer fossils are uncontroversial, the older Apex Chert alleged microfossils from 3.46 billion years old are much more controversial with the possibility that the traces of organics associated with them may be a later contamination. Papers continue to be published arguing both ways. I'm not sure if it has been resolved yet.

What are the "magic minerals" for the search for life on Mars, in the very different conditions that prevail there? Where are the best places to look? We don't know yet. We are making a great start with Curiosity. We will find out more with future missions like Curiosity's successor and Exomars. But there are likely to be many more steps still to go through before we know where is the best place to look, and for that matter, what to look for. See John Grotzinger's Habitability, Taphonomy, and Curiosity's Hunt for Organic Carbon again for more details.

So in short, we can't expect to just land on Mars, go to a likely spot and find a sample of past life on Mars. We would be extraordinarily lucky if that happened. We may have to search long and hard. And we may have to search for just faint traces of a long degraded signal. That means we may be looking for just a few amino acids in the sample.

Once again, if we get any Earth life on Mars, and it contaminates the samples, it will confuse this search.

Then, we don't know what we are looking for, yet. It may be unknown biology. It could be based on XNA (like DNA but with a different backbone) or it could be something else not DNA at all. What we are searching for is:

  • Likely to be single cell micro-organisms
  • We don't know what it looks like
  • We don't know what chemical signatures to look for
  • It may only form microfossils, which are notoriously hard to identify as life or non life.
  • May easily be so small that you can't see the cells even in a good optical microscope, especially if it is early life

Follow the nitrogen, dig deep and look for biosignatures

So far NASA has been using a "follow the water" strategy. But in some ways it has been too successful. There's abundant evidence of water on Mars past and present. Shannon in his thesis "Elemental analysis as a first step towards "following the nitrogen" on Mars" uses the example of the Leprechaun in a traditional Celtic story.

Leprechaun - by SatyrTN

Here is the story as told on the Discovering Ireland website:

"In one tale, a young farmer captures a Leprechaun and forces him to hand over his gold. The Leprechaun says that the gold is hidden beneath a tree in the woods and shows him which one it is. The farmer ties his red scarf around the tree and after making the Leprechaun promise not to remove the scarf he heads to his farm to get a shovel. But when the farmer returns he finds that the Leprechaun has tied a red scarf around every tree in the woods."

Here is a longer version of the same story.

It's like that with water on Mars. To start with the scientist were excited with every new discovery, and said "Wow this spot had water in the past". Now much of the map of Mars is dotted with the red scarves showing the location of probable past habitable water on Mars. There are so many of these "red scarves" there now, that they don't really give us much focus in the search for life there.

The best way to search for early life, as far as we can tell at present, is to search for organics. However, that also is not nearly specific enough. As we've just seen, the organics are easily confused with organics from non life processes and from space. Eight astrobiologists looked into this in a white paper which they submitted to the most recent decadal review: Seeking Signs Of Life On Mars: In Situ Investigations As Prerequisites To A Sample Return Mission

One of the main conclusions of the white paper was that we should look more specifically for nitrogen rich organics on Mars. These nitrogenous organics are likely to be rare because Mars has few sources of nitrogen. This is important because nitrogen is central to the functioning of biology as we know it. Nitrogen bonds are easily broken and re-attached, which is important in biology, so this may be more than just a prejudice from our experience of Earth life. Even if life on Mars is very different from Earth life, perhaps using different amino acids for instance (see Alien life could use an endless array of building blocks) and perhaps use PNA or some other form of XNA (Xeno nucleic acid) with a different backbone from DNA, still it is likely to use nitrogen if it resembles Earth life. Curiosity recently found evidence of nitrates on Mars, also fatty acids, but it hasn't found these nitrogenous organics which the astrobiologists suggest we look for.

Once we find these compounds, that's not enough, as you also get nitrogenous organics from comets and meteorites and natural processes. We would then need to search for biosignatures to distinguish the ones from life processes from the others. We also need to be able to drill below the surface (as ExoMars will be able to do) to the maximum depth possible to find life less damaged by ionizing radiation.

Their main points are:

  • Need for increasing mobility, and precision landing, supported by orbital observations, to access the many and varied habitable environments including subsurface, layered sediments, gullies and ice sheets.
  • The "follow the water" strategy should now be followed by a "follow the nitrogen" phase combined with a search for biosignatures.
  • The biosignature search can use exquisitely sensitive in situ electrophoresis techniques to identify and characterize and find the chirality of amines, nucleobases, polycyclics and other essential organic molecules.
  • This search should include drilling to the greatest depth possible for the best chance of success for detecting biosignatures of past life on Mars
  • They recommend that we should do a sample return only after we either identify clear biosignatures on Mars, or have exhausted all other possibilities by in situ research

If we follow this program, then our top priority right now should be to "follow the nitrogen" and to send instruments to Mars of exquisite sensitivity to look for traces of past life in situ. For details of instruments we could send there, see In situ instrument capabilities below

The astrobiologists couldn't be clearer in their recommendations for an in situ search first,. Several other groups of astrobiologist have published papers arguing this point very strongly in exactly the same way. In situ is the way to go right now, not sample return, not until we find unambiguous biosignatures, or have exhausted our tools for searching for life in situ. But for some reason, NASA continues to see a sample return as a top priority for the search for life. They continue to take this as their main near future goal, even though hardly any astrobiologists think this is a priority, and most think it is actually a distraction and diverting of funds away from what should be our top astrobiological priorities on Mars.

When commenting on ideas to return a sample from Mars at an early stage, the astrobiologist tend to say (paraphrasing their papers)

"Right, that would be nice, and it will be good to have that for later on. But right now it is more of a technology demo, and a geology mission. If we do it now, it is likely to be little more than an expensive way to add extra samples to our collection of controversial Martian meteorites. Without a way to intelligently select samples with evidence of past life, on Mars itself, it's just not worth the price tag of millions of dollars per gram for a sample return. It would be much better value to use the funding for in situ instruments searching for biosignatures directly on Mars".

Hopefully what I said here is enough to give some background to help understand this point of view. I go into this in much more detail, discussing several papers on the subject, in my section Astrobiologists arguing strongly for an in situ search on Mars first (below) .

NASA's plan for safe zones - based on finding Mars life easily

If we knew where to look, and what to look for, then we could just land on Mars, dig up a well preserved sample of ancient life, and then that answers the question of whether there was life on Mars. Then we find a present day habitat, and find present day life and that answers the question of whether there is present day life there. Enthusiasts for humans on Mars seem to imagine it happening like that, pretty quickly.

If you find life as quickly as that, and supposing you are content in just making the discovery, and you are not so much worried about what happens later as Earth life spreads to Mars habitats - then it's a matter of landing somewhere, making sure the humans don't contaminate too much of Mars too quickly, and then sending out robotic scouts to bring back materials for the astronauts to analyse, first on Mars, and then, back on Earth.

That's NASA's current plan - an exploration zone, with the human occupied field station in the center, and teleoperated rovers heading off for in situ study around the perimeter, and returning samples to the center. Meanwhile the astronauts do their own survey missions within the area that they have set aside as "cleared for human exposure".

To them, this seems like a good compromise. It lets humans "touch Mars" but they do their best to limit the effects of the contamination by Earth microbes and organics by restricting human movement geographically on Mars. Here is one example from a paper in 2010, with the human exploration zone shown close to an area of special interest which humans can't visit directly.

Illustration from Mission to Mars: The Integration of Planetary Protection Requirements and Medical Support . This paper is from 2010, so before the discovery of the RSL's, first reported in Science in 2011, and at that time the gullies shown were thought to be amongst the most likely to have present day life.

These particular features are still a matter for much discussion, but are no longer top candidates for present day habitats on Mars. One suggestion is that they formed from water originally, perhaps just from a small amount of water mixed in with dust. They are still active today with occasional new features forming, and this seems to be the result of subliming carbon dioxide mobilizing the dust, rather like a pyroclastic flow. It's a complex situation, see this discussion by Tanya Harrison in Astronomy magazine. See also this announcement from NASA which she comments on.

A modern version of this diagram would probably show the Recurring Slope Lineae (warm seasonal flows (see below) ) in place of the dry gullies, but it's much the same idea. See also Mars colony will have to wait, says NASA scientists

The "Safe Zone - cleared for human exposure" shown on this sketch is a region set aside for the astronauts to roam, where you don't mind Earth contamination. The idea is that the human exploration zone is contaminated with Earth microbes and this is just accepted as a necessary part of human exploration of Mars, but only clean rovers are permitted to travel to the habitats that potentially could host surface life on Mars. They bring samples back to the human base for analysis, or the rovers are used to study the regions beyond the zone remotely.

That could work fine on the Moon. Indeed, perhaps that could be a good place to test it out first? If humans don't travel too far from their base, they will preserve pristine lunar surfaces just a few kilometers away, untouched by human footprints, wastes or garbage. So long as the rovers can also be sterilized sufficiently in a human base, which may be quite a big "if", they could be used in just this way to do clean studies of, say, the volatiles at the lunar poles. The rovers could travel from a human base into the pristine craters just kilometers away and collect samples and return them to the base for analysis, then be sterilized, and go out again to repeat the process.

Even on the Moon there will be some issues of contamination immediately around the base. For instance the Apollo astronauts left a fair bit of organics behind when they left the Moon. It is enough to confuse analysis of samples if you are looking for just traces of organics from the cosmic wind impacting on the Moon. The Apollo samples were recently re-analysed and the composition of amino acids does suggest some extraterrestrial sources, However, that analysis was a tricky one due to contaminants from Earth in the form of rocket fuel, organics taken to the Moon by the astronauts, and organics introduced while handling them on Earth.

This suggests we need to take care to avoid this sort of thing in the future as we explore the Moon. Then there is some transport of the contamination further afield, even on the Moon, by electrostatic levitation of dust, but it's a rather minimal effect. There are no winds and there is no way for water to flow either. Also, how easy would it be to sterilize the rovers at the base? It would probably depend on the design. For instance if the exterior can withstand high temperatures, you could just heat sterilize them and then send them out again.

There are many details to be filled in, however, it seems reasonable to be optimistic that with a bit of care we should be able to keep the Moon surface relatively free of contamination except for the region immediately around a human base. Even this is quite challenging and the Moon may be a good place to learn about how effective such methods can be in practice.

But when it comes to Mars, it begins to seem far more of a formidable challenge. First, it's not just a risk of contaminating ice, but of liquid water where microbes could reproduce (not a risk for the Moon). Could the astronauts really sterilize the rover to Viking levels of cleanliness or better every time it returns to the base with a sample, and then send it out again to study an RSL or similar habitat? And could they keep the returned samples clean while handling them in their base as they analyse them to try to find out what is in them? Especially when they need to do very sensitive analyses to detect even single molecule biosignatures.

Or would they sterilize the rover only once, on Earth and then use it on Mars only for a single mission to the RSL where it would collect a large number of samples in one go and return it to the base, then use it for missions that don't require such cleanliness later on? Or how would it actually work out?

It may seem easy to do on paper, but when it gets to actual practice, are they going to be able to keep the Earth microbes away from an RSL even for the duration of their visit, with rovers driving back and forth between the human base and the possible habitats for present day life? Perhaps this is something we can find out on the Moon first, where it will be equally important to keep the organics from the base away from the study region, especially when searching for organics in the polar ices. It will be far easier to do on the Moon, so we can get preliminary ideas there first about whether it is possible in an easier situation.

However, there is an additional major complication with Mars of course

How could this work on Mars with dust storms and a globally connected environment?

If it does work on the Moon, how can this work at all on Mars with the Martian dust storms? The main problem here is that microbes can form hardy spores, and on Earth these can survive for long periods of time, hundreds of thousands of year. In rare cases, they can survive for millions of years of dormancy. Though the Mars grains of dust are so fine, they are plenty large enough for a microbe to hide in a crack in the dust, and so be protected from the UV radiation. And any microbes that get into a shadow are sheltered completely from the UV. Even in equatorial regions, some areas under rocks will be permanently shadowed from UV light.

And then you get these:

This is a Martian dust devil - they race across the surface of Mars picking up fine dust and would also pick up any microbes imbedded in the dust. The microbes would be protected from UV radiation by the iron oxides in the dust.

HiRISE image from Mars Reconnaissance orbiter, of a dust devil in a late-spring afternoon in the Amazonis Planitia region of northern Mars. The image spans a width of about 644 meters.

The strongest winds on Mars, though fast, would barely move an autumn leaf, because the air is so thin. But the dust is also so fine on Mars, as fine as cigarette ash, and easily lifted by these feeble winds. Also, it's made of iron oxides, which would help to shelter any spores imbedded in cracks in the dust, from UV light.

Then from time to time dust storms will cover the entire planet. A microbe imbedded in a typical dust particle transported around Mars during one of these thick dust storms would be much more shielded from UV than it would be in normal conditions.

Global Mars dust storm from 2001 Mars has local storms every two years, and from time to time it has larger global storms. The first global storm recorded is from 1873: the other ones reported were in 1909, 1924, 1956, 1971, 1973, 1975, 1977 (2 storms), 1982, and more recently in 1994, 2001 and 2007. So we get a global dust storm roughly every decade or so, though sometimes several per decade (five storms in the 1970s)..

This relates to an observation Carl Sagan made in an old paper "Contamination of Mars", back in 1967.

"The prominent dust storms and high wind velocities previously referred to imply that aerial transport of contaminants will occur on Mars. While it is probably true that a single unshielded terrestrial microorganism on the Martian surface ... would rapidly be enervated and killed by the ultraviolet flux, ... The Martian surface material certainly contains a substantial fraction of ferric oxides, which are extremely strongly absorbing in the near ultraviolet. ... A terrestrial microorganism imbedded in such a particle can be shielded from ultraviolet light and still be transported about the planet."

He continues:

"A single terrestrial microorganism reproducing as slowly as once a month on Mars would, in the absence of other ecological limitations, result in less than a decade in a microbial population of the Martian soil comparable to that of the Earth's. This is an example of heuristic interest only, but it does indicate that the errors in problems of planetary contamination may be extremely serious."

Of course we know much more about Mars than they did back then. But the situation is still the same, the dusts do indeed contain large amounts of iron oxides. We have also found out that some microbes are far more UV hardy than realized in the 1960s. Some especially hardy strains of bacillus can survive many hours of Mars surface conditions unshielded from UV by dust or other shields. In one experiment in 2010, one of their strains survived four hours of Martian conditions and in one case 28 hours of Mars surface conditions, in both cases, unshielded from UV light, in simulated Mars winter conditions. This is far longer than the few seconds to minutes that researchers used to think was the maximum for Mars.

This shows survival of spores in Mars daytime summer conditions (left) and winter conditions (right) exposed to the full UV flux of the Mars sunlight. As you see a significant percentage of the most resistant strain B pumilus DSMZ 27 survived for the entire 90 minutes shown on the table. In later experiments they found that one of the strains could survive for at least 4 hours, and in one case 28 hours of simulated Mars surface UV flux in winter conditions

Their paper is summarized in this article in Universe Today: Bacteria Could Survive in Martian Soil.

The dust storms and high wind velocities are the same as in the 1960s. The dust does contain perchlorates, which they didn't know back then, but microbes can survive exposure to perchlorates at the low temperatures on Mars.

The dust particles in dust storms range from less than a micron to 50 microns in diameter. Endospores are from 0.25 microns upwards. Some experiments suggest, that Earth microbe spores could survive at least twelve hours of being blown over the surface within a Martian dust storm. See also Survivability of Microbes in Mars Wind Blown Dust Environment. They could also be transported at night during a dust storm, when there's no UV light, yet still dust suspended in the atmosphere.

The largest global dust storms occur only every 30 years or so. With wind speeds of 10 to 30 meters per second (22 to 67 miles per hour) average for the faster winds during a dust storm, the dust could travel 240 to 720 miles every twelve hours, and some of the dust rises to many kilometers in the atmosphere, and it takes months before all the dust settles. If the dust particles, and any spores embedded within them, end up in a shadow at the end of that, they will then be protected from UV radiation until the next time they get transported by the winds. The NASA save zones idea suggest that the human habitat may be positioned close to a special region - one that could potentially have habitats for present day life. If so, these figures suggest that they might get to a vulnerable region near the base, in a dust storm in much less than twelve hours. That would seem to suggest that the microbes could get to nearby habitats perhaps quite early on, perhaps even during the first human mission to the Mars surface, if there is a dust storm to carry them in that direction. After all the contamination zone around the human base would have large numbers of spores in it, also additional protection for the spores in the form of flakes of skin, hair etc.

I've tried to find experiments simulating transport of microbial spores in Martian dust storms. But I can't find much. There are plenty of experiments to show that microbes can survive covering by a thin layer of dust on the surface. I suppose it is understandably hard to simulate conditions in a dust storm accurately.

The best I can find is a recent experiment from 2016, "Assessment of the Forward Contamination Risk of Mars by Clean Room Isolates from Space-Craft Assembly Facilities through Aeolian Transport - a Model Study" which uses Staphylococcus xylosus, a microbe that is commonly found on the skin of humans. It's an aerobe so not likely to survive on Mars, but it could introduce contamination by biomass and genetic material. Their experiment simulated Mars conditions, but with quartz dust instead of the iron oxides, and only for a few minutes. They found that the vacuum conditions had an effect on survival, but the sub zero conditions and transport in the Mars winds had little effect, One of their main focuses was to simulate electrical charge effects in the suspended dust, but they only simulated that for twenty minutes. The experiment itself wasn't very conclusive, but in their conclusion they combined their data with previous results on survival of microbes when shielded by dust on Mars, and said that they thought that microbes removed from a spacecraft surface by impact of dust would not be killed so long as they were not exposed to UV radiation for long periods of time.

There isn't any other more complete experiment mentioned in their list of citations at the end, so assuming they did a reasonably thorough literature survey, perhaps that means that a more complete experiment has just never been done. Their paper has no citations yet in Google Scholar (which would be a way to find more recent experiments of that nature). The only other paper I've found so far is this one from 1970 which found that UV light shining on simulated Martian dust storms did not sterilize the spores, but it's just an abstract ,and it doesn't go into much detail, and of course we knew much less about Mars conditions back then:

"A chamber was constructed to create simulated Martian dust storms and thereby study the survival of airborne micro-organisms while exposed to the rigors of the Martian environment, including ultraviolet irradiation. Representative types of sporeforming and non-sporeforming bacteria present in spacecraft assembly areas and indigenous to humans were studied. It was found that daily ultraviolet irradiation of 2 to 9 X 10(7) erg cm-2 was not sufficient to sterilize the dust clouds. The soil particles protected the organisms from ultraviolet irradiation since the numbers of survivors from irradiated environments were similar to those from unirradiated environments. Pending further data of the Martian environment, the contamination and dissemination of Mars with terrestrial micro-organisms is still a distinct possibility."

Do say if you know of any other experiments to test survival of microbe spores in Mars dust storms, thanks!

Given the challenge of keeping the samples clean of Earth life, and the difficulty of finding nanoscale fossils and traces of degraded organics mixed in with the organics from meteorites, comets and non life processes on Mars, how can this approach keep Mars pristine for long enough to complete the search for past life? Never mind the search for present day life which I'll cover later. Also, do we not have some responsibility to keep Mars free of Earth life for rather more than the duration of our own first missions there? Do we not have a responsibility for future generations, or indeed even ourselves in future decades when we come back again, to learn more about Mars and any life there may be on Mars, long after the first human landings on Mars? We may have much more sophisticated ways to study Mars in the future. There may be, experiments we will want to do, to answer questions that we don't even have the understanding to ask yet, but it will be too late if it is already irreversibly contaminated by Earth life introduced there in the early twenty first century.

So far we have only considered microbes that escape from air locks, and from spacesuit joints and such like - and any wastes intentionally released onto the surface. Those are all ways we could contaminate the surface in the course of a "nominal" human mission to the surface where all goes as planned. That's hard enough to cope with.

But all those issues pale into insignificance when you consider what happens if a human occupied spacecraft crashes on Mars.

Why do spacecraft crash so easily on Mars?

The basic problem is that Mars gravity is twice lunar gravity. To get a first rough idea of the issue, the Mars escape velocity is 5.03 km / sec, and for the Moon it is only 2.38 km / sec. The delta v to low lunar orbit is under 2 km / sec for the Moon, for example Apollo 14 ascent stage trajectory had a total delta v of 1.845 km / sec (6053.4 fps, close to optimal). By comparison, for an optimal ascent trajectory, it's around 4.2 km / sec to get to a low Mars orbit from Mars.

So, very roughly speaking, the delta v is around double for Mars compared to the Moon, to get down to the surface, if you use rocket propulsion all the way, or to ascend back to orbit. The difference in the amount of fuel needed isn't that huge, since even a 4.2 km / sec delta v is still fairly small, for decent thrust rocket motors, with an ISP of say 302 seconds (exhaust velocity 2.96 km / sec).

How ISP relates to exhaust velocity - techy aside. The fuel efficiency of a rocket is measured using the "specific impulse". This is often abbreviated as Isp or just written as ISP.

This is the ratio of the change in momentum to the amount of fuel used up to achieve that change. If it's a conventional rocket and you measure the fuel used in units of mass such as the kilogram, and the rocket is flying in a vacuum, then the ISP is just the exhaust velocity. When flying in an atmosphere, it's the effective exhaust velocity.

But often it's given in units of seconds. That's for convenience as you then have the same number for the specific impulse no matter what units you use for the length measurements (meters or feet). The specific impulse in seconds gives the ratio of the change of momentum to the weight of the fuel changed. Here the weight is measured as the force acting downwards on the fuel, in pounds force, or newtons. When you divide a momentum (mass times velocity) by a force (mass times acceleration), it turns out that the result is a number measured in seconds.

Anyway the main thing you need to know is that you get from the specific impulse in seconds to the exhaust velocity in meters per second by multiplying by the standard gravity of 9.807. That's because one kilogram exerts a force (weight) of 9.807 newtons in standard gravity (the Earth's gravity varies slightly depending where you are, reduced in equatorial regions because of the Earth's spin, counteracting it, also less as you get higher and varying depending on whether or not you are above a gravity anomaly, so they use a standardized gravity). Similarly, if you want the exhaust velocity in feet per second, multiply the ISP in seconds by 32.175.

You can try this rocket equation calculator here to get an idea of how much difference it makes to the fuel needed to increase the delta v from 1.845 to 4.2 with a specific impulse of (say) 302 seconds. However, you also need to take account of the need to discard the first stage for the ascent to orbit as well, and the maximum fuel fraction for a first stage. When you do that, it turns out that the ascent from the Moon, and return to Earth, is far easier than it is from Mars. This paper makes some comparisons between a Mars sample return to orbit around Mars, with a Moon sample return all the way back to Earth in terms of payload ratios. Although the Mars sample return only has to get to orbit, and then is picked up by another spacecraft, in their plan, it still is much harder to do than the sample return from the Moon all the way to Earth.

Anyway our focus here is on the descent to the Mars surface. A spacecraft landing astronauts on Mars would need to have a capacity of many tons. All the spacecraft landing on Mars so far, have used aerobraking, instead of relying on rocket propulsion all the way. It's likely to continue like this for the foreseeable near future,, as otherwise they would have to carry a lot of fuel, especially if the plan is to be able to abort back to orbit during a failed descent. This is what makes it so hazardous to land on Mars, especially since the atmosphere also contributes to make it more of a one way process. The Apollo astronauts could fly as close as they liked to the surface. Apollo 11 had enough fuel to delay landing on the Moon while Neil Armstrong looked for a better place to land. They also had the ability to abort back to the lunar orbit and return to Earth at any stage, if a problem arose during the landing sequence. The only external danger was from the lunar mountains. Also the landing sequence is slow enough so that a human can pilot a spacecraft to a landing on the Moon by hand, as Neil Armstrong did with Apollo 11. There would be no chance of doing any of that on Mars with present day technology.

On Mars, once you start the landing sequence, and you hit the atmosphere, you are committed. There is now no way to abort back to orbit again, unless you carry huge amounts of fuel with you. You are streaking through the atmosphere at kilometers per second. Everything after that has to work in a perfect sequence with timings accurate to fractions of a second, with critical steps in the timeline passing by faster than a human being could assess the situation and react. Also, a landing on Mars is far more complex than a landing on the Moon or indeed anywhere else in the inner solar system. It should be no surprise when spaceships crash on Mars. Indeed, it's rather remarkable that we've had as many successful landings there as we have. Only the Americans have achieved totally successful landings on Mars to date, and they also have had their share of failures too.

All landings on Mars so far started with an aeroshell and aerobraking to slow down in the upper atmosphere. Next comes the parachute, because it would just take so much fuel to do all the rest of the slowing down using rockets. But a parachute can't slow you down enough for a landing, because the atmosphere is so thin. So then you have to find a way to slow your spacecraft down even further, from those hundreds of miles an hour to a slow enough speed for a soft landing. This shows the sequence for the Schiaperelli entry, as it was supposed to happen.

See Schiaperelli: the ExoMars Entry, Descent and Landing Demonstrator Module

So that’s why you have the retro propulsion stage for many landers on Mars. But you have to take care because if you do retropropulsion when the parachute is still attached you will get the lander tangled up in the parachute. So you have to release the parachute first before you fire the rockets. The moment of parachute release is very important, to get that right. Schiaperelli released its parachute too early, because of bad data from its inertial measurement unit, which was the start of its problems. The other main method used to date is the one used by Opportunity, and Spirit, to use air bags, followed by bounces on the surface. But nobody has suggested we use air bags for a human landing.

Now even after that, you still are not quite home and dry. The problem is that unlike a landing on the Moon you have no precise control over where you land. Instead you have a landing ellipse. This is the one for Schiaperelli, 100 km by 15 km

There is almost no chance of steering your landing craft during the landing, except possibly in the last few meters. Up to then, it is dependent on whatever the atmospheric conditions are as you land. The Mars atmosphere is very thin, a near vacuum, but it also varies hugely in density between day and night and there are lots of variations depending on altitude, temperature etc, also dependent on the dust content and dust storms, and it is hard to predict exactly. There’s also always some small amount of uncertainty in the speed and position of the spacecraft as it enters the atmosphere, and all of that adds up to contribute to the overall uncertainty shown by the landing ellipse.

The size of the ellipse depends on the spacecraft and the technology used. Curiosity had a smaller landing ellipse of 20 kilometers by 7 kilometers. That's why it could land in Gale Crater - which would be too small for ExoMars to land safely. ExoMars couldn't land there, as it would risk hitting the central mountain or the crater walls. But even Curiosity had nothing like the precision we had for lunar landings back in the 1960s.

Neil Armstrong could decide exactly where to set down the lunar rover, and if necessary just fly a bit further to find a good spot. With modern technology we should soon be able to do pinpoint landings on the Moon. It is not likely to be a major problem for the ESA village, to land the astronauts near to their habitats. On Mars you have to be able to land safely wherever you happen to be in that huge landing ellipse. After that, you have to find your way to your habitat somehow, if you have a previously landed habitat on Mars.

Either that, or you take a risk that if you hit a boulder, that’s the end of the mission. Viking 1 landed not far from a boulder which would have certainly ended its mission if it had landed on it

Photo by Viking 1 showing "Big Joe", a boulder two meters in diameter close to the landing site in western Chryse Planitia. It could not have survived a landing on this boulder.

The landing site was chosen, after much deliberation, by a combination of radar and photographic observations from orbit. They couldn't see surface features this small from orbit, and the radar observations were confusing. We can take photographs from orbit now down to 30 cms resolution (with HiRISE), but we still can't control where exactly our spacecraft land on Mars.

Mission planners deal with the issue of boulders as best they can by choosing regions on Mars that are very flat. Ideally you want to have hundreds of square kilometers with no boulders or steep slopes in all directions. That’s why Curiosity had to drive for so long before it got to Mount Sharp. It wasn’t safe to land it any closer because it would risk landing on a big boulder or on a steep slope. Curiosity could never have survived landing on a huge boulder like the one in the Viking 1 photograph. Similarly, unless we achieve pinpoint landings first, of course, astronauts would have to drive several kilometers from the landing site, to reach any previously emplaced habitats on Mars (you don't want to risk the spaceship landing on a habitat).

Of course, it's possible that we develop the ability to do pinpoint landings on Mars, but we don't have that quite yet. It would require some way to steer the spacecraft as it descends through the atmosphere.

Supersonic retropropulsion - or huge parachutes

If we ever land humans on Mars we'd need to be able to land much larger payloads than any attempted to date. There are two ideas of ways to simplify this process. The first is supersonic retropropulsion. That’s what Elon Musk plans to do for SpaceX. It's safer in some ways, and it does permit a much heavier payload, but in other ways it is riskier.

Conceptually it is about as simple as you can get, and the simpler, the less there is to go wrong. The rocket doesn’t need to have an aeroshell or parachute or anything. It just decelerates. Though in practice it may well have a heat shield as well.

Early artist’s impression of supersonic retropropulsion

It slows down by coming in very very close to the surface in the thicker atmosphere at huge speeds. Its rockets switch on when it is still traveling at supersonic speeds. It skims across the surface below the height of the higher mountains on Mars. Indeed it has to come in so low, that if landing in the Valles Marineres, a big rift valley in the Martian highlands, it would have to skim down between the walls of the canyon. All this time the rocket is firing and it is also affected by the air resistance of the atmosphere, so it slows down for both reasons at once, the retropropulsion and the air resistance. Finally, it comes to a vertical landing on the surface. It may have a heatshield and backshell but no parachute.

Entry, descent, and landing sequence, figure 4 from recent 2016 paper by Humphrey Price, Robert Manning, and Evgeniy Sklyanskiy

SpaceX has actually done something very like this on Earth. Their barge landings of the first stage used supersonic retropropulsion, in the very early stages when the first stage slowed down from 70 km down to 40 km, at just the right altitude to stand in for the tenuous Mars atmosphere.

What’s more, they can achieve a pinpoint landing as well, as they have demonstrated several times - when it works. Perhaps that means pinpoint landings on Mars will be possible after all, once this technology matures. So it can certainly be done, but it is rather risky and tricky to do on Mars with the very thin atmosphere and its atmosphere far more variable in density than Earth’s.

The other way to do it is to use an absolutely enormous parachute. If the parachute is big enough, you can have a conventional landing just as for Earth. Simply use an aeroshell, and then parachute down, and it will slow you down enough so you get a soft landing.

The problem is deploying those parachutes and making sure they work. You can work it out with computer models, test tiny parachutes etc. But at some point you have to test it with real parachutes. The parachutes they use so far were tested by firing rockets in suborbital trajectories in the upper atmosphere, because the Earth's upper atmosphere is similar in density to Mars'. This required many expensive tests. To make even larger supersonic parachutes will require a new set of these very expensive rocket tests. NASA are working on this with their Low-Density Supersonic Decelerator.

What I've presented here is just a first rough idea of how it all works. For more details of these ways of landing on Mars with supersonic retropropulsion or large supersonic parachutes etc, hear Robert Manning talk about it here Mon, 03/28/2016 - 14:00 Elon Musk's idea is to use supersonic retropropulsion, and to use data from his experiences returning first stages and later second stages of his rockets on Earth to help develop the design.

Artist's impression of red dragon doing supersonic retropropulsion over Mars, image SpaceX

Elon Musk recently announced that he is canceling the Red Dragon - he says referring to the idea of a retropropulsion landing on Earth

"The reason we decided not to pursue that heavily is that it would have taken a tremendous amount of effort to qualify that for safety for crew transport. There was a time when I thought the Dragon approach to landing on Mars, where you've got a base heat shield and side mounted thrusters, would be the right way to land on Mars. But now I'm pretty confident that is not the right way."

His Dragon 2 will now land with parachutes on water. Then he said in a tweet:

"Plan is to do powered landings on Mars for sure, but with a vastly bigger ship"

So he still has the idea of supersonic retropropulsion eventually, but on a much larger scale. However it would be slightly smaller than his ambitious Interplanetary Transport System with its ability to carry 100 people in one go.

Elon Musk's fun but dangerous trip to Mars

With this background, it's no wonder that Elon Musk said in his talk to the International Astronautical Congress that the mission to Mars carries a high chance of death for the first would be colonists. See Elon Musk envisions 'fun' but dangerous trips to Mars

"I think the first trips to Mars are going to be really, very dangerous. The risk of fatality will be high. There is just no way around it," Musk said. "It would basically be, 'Are you prepared to die?' Then if that's ok, then you are a candidate for going." (emphasis mine)

He isn't talking about dangerous as in a scary haunted house or a fairground ride, where it's scary but you know that you are in safe hands. It's not at all the idea that the equipment is inspected and though it seems dangerous, you won't actually be hurt by it. He is talking about dangerous as in something that is far more dangerous than base jumping. You could easily be killed by it for real.

So, yes, for sure, he may find many people willing to sign on for such a ride. But what would the consequences be for Mars?

It will surely take a while to perfect this technology. Even if, say, he has four successful previous unmanned missions, this doesn't prove it is safe. With a 50/50 chance of success for each mission, you can get four successes in a row with a 6.35% probability. So four successes would not show at all conclusively even that it is 50% reliable. Other ideas such as enormous parachutes far larger than any tested to date also have similar issues.

So, if we accept that there is a high risk of a crash, how can you be sure you won't get this sort of thing happening?

Debris from Columbia - broken into tiny pieces by the crash. If something like this happened on Mars, with the debris spread over the surface and dust and small debris and organic materials from the crash carried throughout Mars eventually in the global dust storms - that would be the end of any chance of planetary protection of Mars from Earth life.

The debris field for Space Shuttle Columbia, with a debris track around 350 miles long, and about fifty to a hundred miles wide (depending on whether you measure to the most distant debris). An accident, especially if it happened early during the supersonic retropropulsion entry to the Mars atmosphere, could scatter debris over a large area of Mars.

With this background, how can we land humans there, without a significant risk of a crash? As for the space shuttle, this would mean dead bodies, broken up into minute fragments, food, air, and water spread over the surface of Mars and mixed in with the dust. It could then spread anywhere on the planet. This would have an immediate impact on science studies throughout the region of the debris field. Your first assumption, if you found biosignatures anywhere near the crash site would be that they came from Earth. That could be immediately devastating for science, especially if the humans crash happens close to somewhere biologically interesting on Mars, which they might well if the plan is to situate the human base close to a special region such as the RSLs. Or any other place where they hope to search for life on Mars, past, or present.

However, it gets worse than that. Because Mars is a connected system through its dust storms, as we saw, the crash site would be a source for life itself to spread throughout the planet. I think most would agree that if there are Earth microbes able to adapt to live on Mars, and any habitats there for it to inhabit, a crash of a human occupied mission on Mars would mean essentially the end of all planetary protection of the planet.

This section includes material from my articles:

Elon Musk, though he is so in favour of sending humans to Mars as quickly as possible, does care about the science impact of introducing Earth microbes to Mars. Here he answers a question on this topic, in the 2015 AGU conference in San Francisco, 30 minutes into this video:

Q. "I am Jim Cole from Arizona State University. I was listening to Chris McKay, another advocate of humans to Mars, and he was talking about how if we do go to Mars and we find life either there or extinct, we should consider removing human presence so that we can allow the other life to thrive. I was wondering what your thoughts on that were. "

A. "Well it really doesn't seem that there is any life on Mars, on the surface at least, no sign of that. If we do find sign of it, for sure we need to understand what it is and try to make sure that we don't extinguish it, that's important. But I think the reality is that there isn't any life on the surface of Mars. There may be microbial life deep underground, where it is shielded from radiation and the cold. So that's a possibility but in that case I think anything we do on the surface is not going to have a big impact on the subterranean life.".

So, it's clear (as I'd expect actually), he does think it is important we don't extinguish any native Mars life. But he thinks there isn't any present day life on the surface. Many of you reading this may be of the same view.

But is that right? Up until around 2008, and possibly for a year or two later,, many scientists would argue that the surface of Mars is sterile, and that if there is any life on Mars it is deep underground and not connected to the surface in any way. With that background, it seemed reasonable to suppose that anything humans did on the surface wouldn't matter, as Elon Musk suggests in that interview. But that's no longer the situation. Indeed even Robert Zubrin now says that he thinks life on Mars is likely. So, let's take a look at this, what has changed?

I'll come to the surface habitats in a bit - if you want to jump ahead, it's the section Habitats for life on the surface of Mars - warm seasonal flows. (see also Habitability of the Mars surface (top few centimeters). But let's first start with the life below the surface. As Elon Musk said, scientists have thought for a long time that the subsurface may be one of the best places to look for life. Is it as cut off from the surface as scientists used to think?

Methane plumes on Mars and the possibility of water deep below the surface in its hydrosphere

The Mars crust, like Earth's, gets warmer as you go deeper down. It might have a hydrosphere, a layer of liquid water rapped below thick layers of rock and ice. There's probably ice at great depths, even in equatorial regions, and below it, water kilometers below the surface even in the equatorial regions. This water would be trapped beneath many layers of rock and ice, and so could stay liquid, warmed up by the geothermal heat of the interior of Mars. So, even before the Phoenix observations in 2008, which lead astrobiologists to re-evaluate the possibility of surface habitats, astrobiologists thought that there could be life deep below the surface.So then, a natural question, "What about hot spots closer to the surface?" Despite many searches, we haven't yet found any sign of current volcanic action or hot spots.

However , there are signs of geologically recent volcanic eruptions in the Olympus Mons caldera, and other volcanic features that formed as recently as a few million years ago. The phoenix lander also found evidence from isotope ratios that some of the carbon dioxide in the atmosphere came from volcanoes in the recent past. So there could easily be geothermal hot spots still there, closer to the surface. So far, our searches have turned up some slightly puzzling infrared anomalies but no clear signs of hot spots. However, localized hot spots would be easy to miss. Our orbiters can only measure the temperature of the top few millimeters of the surface, so we have to work out the subsurface temperatures through modeling. Perhaps there is liquid water in caves not that far below the surface, or just damp rock, the moisture trapped in liquid form by overlying layers of rock and ice, and kept warm by geological heating, yet sufficiently insulated from the surface that we haven't detected them yet from orbit?

All this was rather abstract theory until the methane plume observations in 2009 by astronomers using NASA's infrared telescope facility, and the Keck telescope, both at Mauna Kea, Hawaii. They were puzzling, as the methane seemed to disappear from the atmosphere so rapidly that it was hard to work out a physical process that could fit the observations. Also these were delicate measurements and needed to be confirmed.

Now, Curiosity seems to have confirmed these observations, though its results continue to be puzzling because they appear and disappear over such short timescales. Perhaps that means they form somewhere close to Curiosity's location. It's also possible that the methane is contamination from Curiosity itself, but so far, that seems unlikely. Hopefully ESA's Trace Gas Orbiter will help clear up some of these mysteries once it starts its science mission. It has to circularize its orbit first, and the current plan is to start its science mission in early 2018. It is by far the most sensitive instrument of its type ever sent to Mars. It's sensitive to up to an amazing 10 parts per trillion to many different chemicals in the Mars atmosphere, including methane. It may help answer many of our questions about the methane, and may detect other interesting trace gases as well.

So where does the methane come from, if these signals are genuine? Well there are various ideas but most suggest a connection between the surface and the subsurface. The methane plumes on Mars could be results of

  • Past inorganic processes such as serpentization (reaction of olivine with water at high temperatures - olivine is an ingredient of many rocks including basalt). Other sources include inorganic processes in the atmosphere, volcanoes, or it could be that it was already present on Mars when it formed. In all these cases, it would be locked in clathrates for maybe up to billions of years, and then released
  • Products of past life, again locked in clathrates
  • Present day deep subsurface life using the hydrogen from serpentization as an energy source
  • Present day deep subsurface inorganic processes

We may have spotted methane on Mars. If so this figure from NASA / JPL shows some possible sources. One possibility is that early Mars had large amounts of methane in its atmosphere which helped keep it warm, and its been trapped in the methane clathrates for billions of years, now released.

Whether it is the product of present day life or not, these plumes may show a connection between the surface and a habitable region below. But a connection can go both ways, especially if, for instance, the methane plumes are accompanied by seeps of liquid water that reach to the surface or to caves or cracks or other features open to the surface. What happens if Earth life gets into this habitable region after a human crash or landing on Mars? It could be contaminated by methanogens that generate more methane, or methanotrophs that eat them, confusing the scientific study of what caused the plumes. And if there is Mars life down there, the Earth life could confuse the search for the Martian microbes, or compete with them, maybe even make them extinct (especially if it is some vulnerable early form of life)..

This is not the only way that the surface could be connected to the deep subsurface. One of the theories for the warm seasonal flows (see below), or Recurring Slope Lineae is that they might be the result of water from deep below getting to the surface in regions of geological hot spots. Again this means it could be possible to contaminate the subsurface, and maybe even the entire hydrosphere, if it is connected to the surface via the RSL's.

Cassie Conley, astrobiologist and NASA's planetary protection officer, made an interesting observation, that this could also contaminate subsurface aquifers with microbes that are known to create calcite when exposed to water with CO2 dissolved in it. Later explorers might find subsurface aquifers converted to cement as a result of contamination by Earth microbes. See Going to Mars Could Mess Up the Hunt for Alien Life (National Geographic).

Porous basalt

If Mars does have a deep hydrosphere and it's inhabited with Mars life, it may live in porous (vesicular) basalt, a rock that's ideal for life to live in. 90% of the rocks in the vicinity of Viking 2's landing consisted of porous rocks like this, riddled with holes. If there's some source of hydrogen, for instance, it could be very habitable. Basalt has has the chemical elements needed to support a million cells per gram (limiting factor is phosphorus) and there are likely to be perchlorates, nitrates and sulfates from the surface to increase its fertility for life. If it is brought to the surface, then this might be a good place to look for traces of life on Mars, past or present.

See this paper.

A - sample of vesicular basalt from Earth (Columbia River Basalt).
B - from Mars, false colour photo by Spirit in Gusev crater.
C - lunar basalt collected by Apollo 16.
D - Ordovician basalts with the holes (vesicles) filled with carbonates.

Two ways Curiosity's methane spikes could be generated in the shallow subsurface (centimeters deep at most)

Curiosity's methane spikes could also come from the shallow subsurface, rather than the deep subsurface. There are two ways it could happen, as described in a paper from 2016. Both rely on the low temperature brines that Curiosity detected indirectly, in the sand dunes it drives over. See Liquid brines beneath the surface of sand dunes at night (below). These brines are just centimeters below the surface. Both theories require this water to be very abundant.

First, the regolith could take up methane when dry - and release it when wet. That would work, because methane can't dissolve in water, so wetting the rock, somewhat paradoxically, forces the methane into the atmosphere.

Techy detail. The reason methane can't dissolve in water is because unlike ammonia, it can't do hydrogen bonding with water. Also it's able to force its way between the hydrogen bonds that join water molecules to each other in liquid water).

The main problem with this theory is that the regolith might not able to take up enough methane quickly enough. Its adsorption energy is around 18 kJ/mol for a Mars soil analogue JSC-Mars-1. That's just an analogue, of course and not the real soil, which might have rather different properties. But for this theory to work, it needs an adsorption energy of 36 kJ/mol. That's rather high, higher than activated carbons and similar to that of synthetic nano-porous titanium silicate, an artificial material which can separate methane from nitrogen.

Another way it can happen is that methanogens living in the water would make it themselves by taking in carbon dioxide from the atmosphere. The main difficulty with this model is that it doesn't explain how the methane suddenly vanishes from the atmosphere with a residence time of a few days. But it could be that Curiosity was no longer downwind of the plume, so that it wasn't dissipated quickly. It just changed position, or Curiosity moved out of it.

So, they concluded that neither of these theories can be ruled out quite yet. The third theory they looked at is methane from the deep subsurface, the one we already mentioned, and there were no major issues with it.

How else could Mars have deep subsurface water with some connection to the surface?

Subsurface ice in equatorial regions

Mars could also have subsurface ice from past more habitable times. There have been several papers suggesting ice in the equatorial regions below the surface. But they are none of them definitive, though interesting. But let's just take a look at them. First here is a map of some of the features on Mars that may be associated with subsurface ice.:

Shows the position of Medusae Fossae, Utopia Planitia, the mountains Arsia Mons, and Olympus Mons and the rift valley Valles Marineres on Google Mars. All of those have evidence of ice deposits beneath the surface, but not confirmed.

One of the most promising places to find subsurface ice in equatorial regions is Hellas Basin, the deepest point on Mars, which has many features that suggest ice. It may even have liquid water occasionally in the depths of the basin - as the lowest point on the Mars surface, the atmospheric pressure is higher there, and even fresh liquid water could form though it would be close to its boiling point and evaporate quickly. Salty brines would be more stable there and longer lasting than anywhere else on Mars. It's often obscured by dust and clouds.

The right hand image shows a photo of three craters in Hellas Basin. The left hand image shows what ground penetrating radar suggests maybe lies just below the surface, covered in an insulating layer of debris. This is a discovery from 2008.

In another more controversial measurement, ground penetrating radar showed what may perhaps be ice below the surface, in the Medusae Fossae Formation - it's either miles deep layers of equatorial dirty ice or volcanic ash all the way down. If it's ice then it was probably deposited there when Mars' axis was tilted so far that it had equatorial ice sheets, as happens occasionally - unlike Earth, it's axial tilt varies chaotically on long time periods (as discussed in Oceans that are only liquid part time as Mars' tilt and orbital eccentricity change (above) ) .

The vertical distance below the surface here shows the radar time delay, roughly corresponding to depth below the surface. Do you see how there is a faint extra line of white in the middle of the picture showing a secondary reflection layer in the radar image? That's the region with probable ice deposits in equatorial Mars.

See also Water on Mars May Have Piled Up as Ice Near Equator and for the technical paper Radar Sounding of the Medusae Fossae Formation Mars: Equatorial Ice or Dry, Low-Density Deposits? (abstract). Later research confirms these observations, strongly suggesting that this indicates ice in some form.

This is not at all conclusive however. It might be volcanic ash as this paper suggests.

Another set of observations suggested ice at the base of craters in the Sinus Sabeus region. It's strong evidence that this area had ice in the past, when Mars' axis was tilted more than it is now, but they thought there was a possibility that some ice might still be there today, buried and hidden from view, concluding (from the paper):

"It is unclear from available data whether any relict ice is currently present at these locations, although estimates for fill thickness are noteworthy. The equatorial setting suggests that if present, this ice is likely buried by a thick, insulating debris layer or a near-surface layer of reduced permeability."

Another paper suggested there may be still be large amounts of ice buried beneath the surface in the Valles Marineres region. Then there could be ice on the flanks of Arsia Mons area and of Olympus Mons. More recently, the Mars Reconnaissance Orbiter found a massive sheet of ice in the Utopia Planitia area but that's a rather higher latitude, not in the equatorial regions.

So, in short, there is a fair bit of indirect evidence that now suggests that the subsurface ice may be more widespread than just in the higher latitudes. If that is so, it is hard to tell how much there is. The idea that there is ice and possibly even occasionally liquid water in Hellas basin is perhaps the surest bet of all these ideas. After all it's one of the best locations for stable ice covered in debris in equatorial regions, because of the higher atmospheric pressure, and even water could survive for a while there, though close to its boiling point.

Ice covered lakes habitable for thousands of years after large impacts

When comet Siding Spring was discovered in 2013, before they knew its trajectory well, there was a small chance that it could hit Mars. Calculations showed it could create a crater of many kilometers in diameter and perhaps a couple of kilometers deep. If a comet like that hit the martian polar regions or higher latitudes, away from the equator, it would create a temporary lake, which life could survive in.

Artist's impression of Mars as seen from comet Siding Spring approaching the planet on 9th October 2014. It missed, by less than half the distance to our Moon. But sometimes comets will hit the Mars ice caps or higher latitudes. If that happens, it will create lakes and hydrothermal systems that last for thousands of years.

These lakes can last for a surprisingly long time, insulated by the ice and heated from below by the rock. The models suggest that large craters of 100 - 200 km in diameter in the early solar system would have made lakes that stayed liquid for as long as one to ten million years. This happens even in cold conditions, so it is not limited to early Mars. A present day comet a few kilometers in diameter could form a crater 30 - 50 km in diameter and an underground hydrothermal system that remains liquid for thousands of years. The lake is kept heated by the melted rock from the initial impact in hydrothermal systems fed by water from deep underground.

Also, there's another way to keep water liquid. Any ice deep enough below the surface, only 100 meters deep, can actually stay liquid indefinitely if covered by an insulating layer of gravel. There'd be enough heat from below, just from the heat of Mars itself and enough insulation above from the gravel, to keep the water permanently liquid. See section 2.2.3 of Niton Renno's article. This is also one theory for the Martian "dry gullies" that they formed through liquid water suddenly flowing out of a subsurface aquifer like this. This was the most popular theory for them at one point, though there are other explanations for them now.

It's much harder to keep water liquid below ice, since rock is much more insulating than ice. It's especially hard for water to form below an ice sheet. If the ice cap was four to six kilometers deep, then you'd expect the base of it to be liquid water, melted from below just through the heat of Mars itself. Though Mars does have ice at both poles, its ice sheets aren't quite as deep as that. But it could still have liquid water at the base of its ice sheets, if there's localized geothermal heating from below.

Also, if a lake formed, originally by geothermal melting or a meteorite impact, it's much easier to keep the lake liquid than it was to melt the water in the first place. In one model, then if a lake forms at a depth of over 600 meters below the ice (originally open to the surface) then it can remain liquid indefinitely from the heat flux from below, even without local geothermal heating.

We'd be able to detect this water using ground penetrating radar because of the high radar contrast between water and ice or rock. MARSIS, the ground penetrating radar on ESA's Mars Express is our best instrument for the job. After several searches, it hasn't found anything yet. See page 191 of this paper. Their resolution isn't that great, however, around a kilometer.

From the searches done to date, we can say with reasonable certainty that Mars doesn't seem to have an equivalent of our Lake Vostok (250 km by 50 km by 0.43 km deep) beneath its ice caps at present. It could however still have small subglacial lakes of up to a kilometer or so in diameter. They were looking for water liquid through geothermal heating, but their search would surely have found impact lakes too.

So, Mars doesn't seem to have any large lakes created from impacts just now. Nor does it have any major lakes formed through geothermal activity below glaciers or ice caps, though it could have smaller lakes.

However, we do have good evidence of past subglacial lakes on Mars, caused by volcanic activity. One of these is rather recent on the geological timescale, backing up the research suggesting that such lakes could form even today.

Ice covered lakes from volcanic activity

There is evidence that volcanism formed several lakes 210 million years ago on one of the flanks of Arsia Mons. That's relatively recent in geological terms, so recent that there seems no reason why it couldn't happen again today. It probably formed two lakes with around 40 cubic kilometers of water each, and a third one of 20 cubic kilometers of water. They probably stayed liquid for hundreds, or even thousands of years.

By comparison, Loch Ness, the largest fresh water lake in the UK has a volume of 7.5 cubic kilometers. The smallest of these three lakes would be similar in volume to the combined volume of the seven largest lakes in the UK: Loch Ness, Lough Neagh, Loch Lomond Loch Morar, Loch Tay, Loewr Lough Erne, and Low Awe. So, though it's nothing compared to the largest fresh water lakes in the world, which have volumes of tens of thousands of cubic kilometers, you are talking about a seriously large volume of habitable water here all the same.

This image shows scratch lines on the flanks of Arsia Mons which must have been made by a glacier. The lead author Kathleen Scanlon, at the time a doctoral student at Brown university, also found evidence of "pillow-like lava". That means, rounded globules of rock which are signs of lava melting under water, or beneath a glacier at high pressure. This type of lava is quite common on the sea floor on Earth.

If Mars has present day life capable of living in such habitats, then there must be a good chance that it proliferated when these lakes formed. I wonder if perhaps there might still be signs of it on the flanks of Arsia Mons today? There might even be ice still preserved from those times as remnants of glaciers long buried as this paper suggests.

Possibility of geological hot spots in present day Mars

There is clear evidence that Mars is not yet geologically inactive

Mars might even have present day volcanism, releasing gases to the atmosphere if not lava. NOMAD on ExoMars' trace gas orbiter will be looking for geological as well as biologically relevant molecules in the atmosphere. It's targeting carbon, oxygen, sulfur and nitrogen atoms and carbon-hydrogen bonds.

Probably Mars also has magma plumes deep underground, at least, to explain this recent volcanism. Also, given that there has been activity on Olympus Mons as recently as two million years ago, it seems very unlikely that all activity has stopped permanently. We should see more again in the future, millions of years into the future at least. The main question is, is there any activity right now.

The Mars Global Surveyor scanned most of the surface in infrared with its TES instrument searching for hot spots. The Mars Odyssey's THEMIS, also scanned the surface in wavelengths that measure temperature. So far they haven't found any hot spots. If Mars does have these geological hot spots, they could melt the subsurface ice. The water would need to trapped under overlying deposits to keep it liquid. Perhaps the water could then come to the surface from time to time. If so that could explain why some of the Martian hillsides have warm seasonal flows (see below), and others, apparently identical, don't. Perhaps the ones with RSLs have geological hot spots beneath them - though that's just one hypothesis for the warm seasonal flows. We will look into this in more detail below.

It's not too surprising that these hot spots are hard to spot from orbit as we can't measure the heat below the surface directly. We can only measure the heat radiated from the surface. That also leads to a rather intriguing possibility that the hot spots could be hidden from view by ice.

Life in ice towers hiding volcanic vents

So, this is another suggestion, that we could find habitats on Mars inside ice fumaroles. It's a nice idea, and perhaps ice fumaroles do form on Mars from time to time. So far we haven't found any on present day Mars. But it may well be worth keeping a look out for them, as it would be a very interesting habitat if we find one, or one of them starts to form, around a volcanic vent on Mars. If Mars does have any volcanic vents which vent water rich gases through a fumarole, they are likely to form ice towers like this, as happens in Antarctica.

Let's look at the idea in some more detail. This photo shows an ice fumarole - an ice tower that forms around a vent of volcanic gases in the extremely cold conditions right near the top of Mount Erebus in Antarctica.

One of the numerous Ice Fumaroles near the summit of Mount Erebus in Antarctica. If these also occur on Mars, they could provide a habitat for life, and would be extremely hard to spot from orbit due to the low external temperatures. Image credit Mount Erebus Volcano Observatory

For more photos of ice fumaroles see "Ice Towers and Caves of Mount Erebus",

They were originally discovered by the Antarctic explorer Shackleton during his 1908 Nimrod expedition, when he and a few others set out to climb Mount Erebus.

Photograph from Shackleton's Mount Erebus expedition with a fumarole in the background

He described them like this.

"The ice fumaroles are specially remarkable. About fifty of these were visible to us on the track which we followed to and from the crater, and doubtless there were numbers that we did not see. These unique ice-mounds have resulted from the condensation of vapour around the orifices of the fumaroles. It is only under conditions of very low temperature that such structures could exist. No structures like them are known in any other part of the world."

Ice caves form below the fumaroles, and these are especially interesting as a habitat for life.

Entrance to Warren Cave on Mount Erebus. Credit Brian Hasebe. Volcanically heated, the temperatures inside their three study sites were 32, 52 and 64 degrees Fahrenheit (2,11 and 18 degrees Celsius), far warmer than the surroundings.

These ice caves on Erebus are of especial interest for astrobiology, as analogues for habitats outside of Earth, because they are so biologically isolated. Most surface caves are influenced by human activities, or by organics from the surface brought in by animals (e.g. bats) or ground water. These caves at Erebus. are high altitude, yet accessible for study. There is almost no chance of them being affected by photosynthetic based organics, or of animals in a food chain based on photosynthetic life. Also there is no overlying soil to wash down into them.

As described in this paper, these ice towers eventually collapse and then rebuild themselves, but though temporary features, they persist for decades. The air inside has 80% to 100% humidity, and up to 3% CO2, and some CO and H2, but almost no CH4 or H2S. Many of the caves are completely dark, so can't support photosynthesis. Organics can only come from the atmosphere, or from ice algae that grow on the surface in summer, which may eventually find their way into the caves through burial and melting. As a result most micro-organisms there are chemolithoautotrophic i.e. microbes that get all of their energy from chemical reactions with the rocks. They don't depend on any other lifeforms to survive. They survive using CO2 fixation and some may use CO oxidization for their metabolism. The main types of microbe found there are Chloroflexi and Acidobacteria.

This makes them very interesting as an analogue for Mars habitats. If Mars is currently geologically active, then in such cold conditions, it may well have ice fumaroles around its vents, and if so they would be only a few degrees higher in temperature than the surrounding landscape and hard to spot from orbit. We haven't found these yet. The closest we have got so far is that the silica deposits in Home Plate which Spirit found, might have been formed by ancient fumaroles on Mars, (not necessarily ice fumaroles) though they could also have been formed by hot springs or geysers.

This article Martian Hot Spots in NASA's Astrobiology magazine presents Hoffman's ideas. He explains that ice fumaroles on Mars could be up to 30 meters tall in its lower gravity and 10 to 30 meters in diameter, circular or oval in shape. So, potentially these things could grow to be huge on Mars, as high as a nine story high skyscraper, and potentially some of them could be as wide as they are high.

He suggests searching for them on Mars from orbit, and he wondered if some temperature anomalies in Hellas Basin could be ice fumaroles. They wouldn't need to be in polar regions because the fumaroles themselves would bring large quantities of water vapour to the surface to keep replenishing the ice towers as they sublime away in the thing Mars atmosphere. They might be quite easy to spot as white circles or ovals, probably in permanently shadowed regions, and they would be slightly warmer than their surroundings. This shows one of his candidates.

Daytime infrared from Odyssey IR

Anomalous warmth in infrared at night as well on all nine infrared bands, so not a chemical signature.

That candidate is in Hellas Planitia and is from 2003. Despite a search of high resolution visual images they were unable to find anything visual corresponding to them, they were only visible in infrared. But it shows the sort of thing they would be looking for. Lots of small dots around 10-30 meters in diameter each, clustered around a potential fracture. For details see their paper.

The idea is that just as on Earth, volcanic action could bring water vapour and other gases from below. The water vapour, as in Antarctica, would freeze out to form these ice towers. If these environments do occur on Mars, they would provide a warm environment, high water vapor saturation, and some UV shielding. The ones we have on Earth don't have significant amounts of liquid water. However, as they have close to 100% humidity inside, that doesn't matter. They sustain microbial communities of oligotrophs, i.e. micro-organisms that survive in environments that are very poor in nutrients. The same could be true of Mars.

Though we haven't found ice fumaroles on Mars yet, we have found recently formed rootless cones, which are the results of explosive contact of lava with water or ice. This shows that ice (or water) and lava were in close proximity as recently as around ten million years ago.

This shows rootless cones on Mars (to the left) and in Iceland. They are the locations of small explosions of steam, when lava surges over the surface over water or ice. These rootless cones on Mars formed around ten million years ago which shows that Mars has had ice and lava in close proximity very recently. They range in diameter from 20 meters to 300 meters.

So, could there be other ways that volcanic processes on Mars produce habitats by interacting with ice, such as the ice fumaroles? From this 2007 paper:

Hoffman and Kyle suggested the ice towers of Mt. Erebus as analogues of biological refuges on Mars. They combined the idea of still existing near surface ice deposits with the assumption that there is still some localized volcanic activity on Mars today.

There are several examples from Mars that show a direct interaction between lava and ice in the geological history of Mars. The most obvious cases are the rootless cones seen in the northern lowlands. HRSC images show direct and violent interaction in the relatively recent geological history, for example at the scarps of Olympus Mons. Mars today is in relatively dormant phase, and any interactions which might be occurring today are presumably on a much less dynamic scale. Nevertheless, they may be driving local hydrothermal systems. Studying the geothermal processes in the first few tens to hundreds of meters below the surface of Mars today might thus uncover a wide variety of new habitats where biological activity may survive on this cold and dry planet.

For more about this topic see Volcano-Ice Interaction as a Microbial Habitat on Earth and Mars. These ice fumaroles would be of great interest, but of course, being open to the surface, would easily be contaminated by Earth life from surface explorers or brought in to them through dust from the Martian storms.

So far we've been looking at habitats deep below the surface of Mars, though perhaps connected to the surface. But what about habitats on the surface itself? They would make planetary protection even more of an issue, so it's important to look at the possibility. First we need to look at the question, is surface life possible there at all. Just a decade ago, most scientists (with the exception of Gilbert Levin) would have answered with a resounding "No". But that's all changed.

Habitability of the Mars surface (top few centimeters)

First, a short summary of the latest conclusions about the habitability of the Mars surface and near surface.

?The first thing Earth life needs is liquid water, and water that's not too cold or too salty. It can't survive in very acidic or very alkaline conditions either - but those limits are quite extreme and not likely on Mars. The UV radiation is shielded by a thin layer of dust or a shadow. The ionizing radiation is well within the tolerance limits for life as we will see. Perchlorates are not a problem, indeed are a food source for many microbes and are now considered to be a habitability factor for Mars.

It doesn't need organics for photosynthetic life, as photosynthetic microbes can create them from the atmosphere and water. It's short of organics for other types of life (heterotrophs) but there are sources for instance from organics. It does need nitrogen, which is likely to be the element most in short supply on Mars. But there are some nitrates on the surface.

Anywhere where Earth life can survive is a potential habitat for Mars life. In the other direction though, Mars life may have adapted to tolerate colder or saltier conditions than Earth life.

So, let's look at all of this a bit more closely and see which factors limit habitability of the Mars surface, and which factors we can ignore as not limiting in any way.

Temperature limits for Earth life

I'll summarize the 2014 paper A New Analysis of Mars ‘‘Special Regions’’ The "special regions" by definition are places where present day life may be possible on Mars. Spacecraft have to be especially thoroughly sterilized to enter them, so it's important to study them carefully. They did a survey of the literature to date, and these are their conclusions:

  • The usually stated limit for cell division is - 20 °C with it confirmed down to - 18 °C but genome replication (which probably means cell division) occurs down to -20 °C, which is the temperature for our freezers

    However, it is hard to study this because cell division proceeds so slowly at such low temperatures, with a doubling time as long as 50 days already at - 18 °C, and it gets longer, the colder the temperatures. It's especially hard to study this because it's so cold, making the working conditions tricky. Very slow cell division may be possible down to - 32 °C or lower, but there are no experiments to prove this yet.

In more detail: experiments with yeast show doubling at - 18 °C, so it's confirmed at that temperature. One study showed genome replication in permafrost at - 20 °C which is "highly suggestive of cell division". Another experiment show ammonia oxidation at - 32 °C sustained for 300 days, the duration of the experiment. Since cell division would be so very slow at those temperatures, then, so far, it's impossible to be sure whether this is just maintenance metabolism, or whether it actually did support very slow cell division.

Some substances can help microbes be active at lower temperatures, which could reduce the limits even further. These are known as chaotropic agents, and include ethanol, urea, butanol etc. They work by disrupting the hydrogen bonding of water molecules with each other. There are many chemicals on Mars which could act as chaotropic agents and so reduce the minimum temperatures for cell division, including MgCl2, CaCl2, FeCl3, FeCl2, FeCl, LiCl, chlorate, and perchlorate salts

Typically, these chaotropic agents reduce the lowest temperatures for cell division by 10 °C or 20 °C for many microbe species. However the authors couldn't find any experiments testing these agents at the very low temperatures that would be needed to reduce the lowest temperature limit for cell division for life.

Salinity limits, and more generally, water activity limits for Earth life

Salinity is measured as "water activity" which is a measure of how available the water is to Earth life, which can also be used to measure other solutions (e.g. honey). Salty water has a lower partial vapor pressure of water which means that there is less water in the atmosphere above salt water than above fresh water in equilibrium conditions. Water activity is defined simply as the ratio of the partial water vapour pressure of the solution divided by the partial water pressure of pure water. So, fresh water has water activity 1.

Here the limit is much clearer. There is no evidence at all of cell division or metabolic activity in solutions with a water activity level below 0.6.

In more detail: Honey has a low water activity level of 0.6. That's why honey doesn't spoil - you don't need to keep honey in a fridge, because its water activity level is so low that though microbes would find plenty to eat, and though there is plenty of water there in the honey, the water is not available to the microbes because of the low water activity level.

The record at the moment is 0.605 is for a fungus Xeromyces bisporus which was discovered in 1968 in a study of spoilage in prunes. It can divide at these low water activity levels, so can germinate, but needs higher levels of water to create fungal spores through asexual sporulation and even more for sexual sporulation. Most microbes can't handle a water activity level below 0.755. However, there have been a number of other reports of microbes that can manage lower levels similar to the bisporus fungus. In a recent 2014 survey paper of the literature on the subject "Multiplication of microbes below 0.690 water activity: implications for terrestrial and extraterrestrial life", the authors came to the conclusion that the best consensus at present is that the lowest level of water activity needed for cell division is about 0.605, and that some halophiles (salt living microbes) are able to tolerate such low levels.

They remark on the difference between the situation for water activity and the situation for temperatures, that there's this sharp cut off for water activity, but much better evidence of microbes able to tolerate temperatures below the usually cited -20°C.

Nilton Renno's droplets that form where salt touches ice. Why did he call a salty droplet on Mars "a swimming pool for a bacteria"?

This is perhaps one of the most striking discoveries in recent years because of its implications for habitability of Mars. Nilton Renno found that liquid water can form very quickly on salt / ice interfaces. Within minutes in Mars simulation experiments.

Erik Fischer, doctoral student at University of Michigan, sets up a Mars Atmospheric Chamber on June 18, 2014. These experiments showed that tiny "swimming pools for bacteria" can form readily on Mars wherever there is ice and salt in contact.

This is striking as it could open large areas of Mars up as potential sites for microhabitats that life could exploit. The professor says

""Based on the results of our experiment, we expect this soft ice that can liquefy perhaps a few days per year, perhaps a few hours a day, almost anywhere on Mars. So going from mid latitudes all the way to the polar regions. This is a small amount of liquid water. But for a bacteria, that would be a huge swimming pool - a little droplet of water is a huge amount of water for a bacteria. So, a small amount of water is enough for you to be able to create conditions for Mars to be habitable today'. And we believe this is possible in the shallow subsurface, and even the surface of the Mars polar region for a few hours per day during the spring." (transcript from 2 minutes into the video onwards)"

That's Nilton Renno, who lead the team of researchers. See also Martian salts must touch ice to make liquid water, study shows . He is a mainstream researcher in the field - a distinguished professor of atmospheric, oceanic and space sciences at Michigan University. For instance, amongst many honours, he received the 2013 NASA Group Achievement Award as member of the Curiosity Rover " for exceptional achievement defining the REMS scientific goals and requirements, developing the instrument suite and investigation, and operating REMS successfully on Mars" and has written many papers on topics such as possible habitats on the present day Mars surface.

His announcement sparked headlines in many papers such as:

This helps to explain how the droplets of liquid formed on the legs of Phoenix lander. At least, that's the leading hypothesis, that these are droplets. They look and behave like droplets, including coalescing and falling off. But sadly, Phoenix had no way to analyse them to prove what they were.

Possible droplets on the legs of the Phoenix lander - they appeared to merge and sometimes fall off. In this example the rightmost of the two droplets - coloured green in this black and white image just to pick them out - grows apparently taking up the water from its companion to the left.

If you remember news stories and press releases from a decade ago, you might wonder what has changed. Until around 2008, when Phoenix took those photos, most scientists believed that the surface of Mars was uninhabitable. But now you can hear them talk enthusiastically about the possibilities for life there. Hear for instance Nilton Renno talking about a discovery of a new way of creating habitats with liquid water on Mars on the interface between salts and ice

So, you might wonder what has changed, if you have read the many articles on this subject from about six years ago which seemed to show that present day life on Mars is impossible. For instance, Encyclopedia Britannica now says

"It could be argued that the best strategy is to look for fossil remains from the early period in Mars’s history when conditions were more Earth-like. But the Martian meteorite debate and disagreements about early terrestrial life point to the difficulty of finding compelling evidence of microbial fossil life. Alternatively, it could be argued that the best strategy is to look for present-day life in niches, such as warm volcanic regions or the intermittent flows of what may be briny water, in the hope that life, if it ever started on Mars, would survive where conditions were hospitable.""

From The question of life on Mars (Encyclopedia Britannica)

So it might help to explain why we no longer think that this rules out indigenous life on the Mars surface. The planetary protection report A New Analysis of Mars ‘‘Special Regions’’ covers UV radiation, and ionizing radiation. Their findings in brief are that ionizing radiation from solar storms and galactic cosmic radiation have negligible effect on habitability. UV is severely limiting but easily blocked by a thin layer of dust. But let's look at this in more detail.

Ionizing radiation

Before 2008 - the idea was that if there are any spores on or near the surface, they must have been dormant for a long time, ever since the last time Mars had a slightly thicker atmosphere. They would have to have been dormant for millions of years. This seemed to make perfect sense since the Mars atmosphere is so thin that any water on the surface would either be above boiling point already or close to it as soon as it melts. Even salty water would be close to boiling point. So then they worked out that the levels of ionizing radiation from cosmic radiation in the surface layers could easily destroy even the most radioresistant microbes in a million years. So (they deduced), any remaining viable life would have to be well below the surface.

It is not easy to simulate Mars condition in experiments on Earth, so it was quite a surprise to researchers when Phoenix found evidence for liquid drops forming in Mars surface conditions. They then did the experiments to try to duplicate the Phoenix observations, and found that liquid water is possible in those conditions after all, and what's more, if you balance the conditions carefully, the liquid doesn't have to be too salty for life to use it. The researchers now had to redo all their calculations.

There are some places in our solar system that are made totally uninhabitable because of radiation. The surface of Europa is laced with sterilizing radiation. Jupiter has such strong ionizing radiation that humans would only last hours in vicinity of Europa before they died of radiation poisoning. Even highly radioresistant microbes wouldn't last long on the surface of Europa

But the surface of Mars - though it gets far more radiation than the surface of Earth - gets roughly the same amount of radiation every year as the interior of the ISS.

The surface of Mars, with its thin atmosphere to shield from radiation.

Receives about the same amount of cosmic radiation per year as the interior of the ISS (above the Earth's atmosphere but shielded by our ionosphere )

Curiosity measured radiation equivalent to an estimated 76 mGy per year at the surface - or 0.076 Grays per year. Humans would find such hazardous long term - increases your risk of getting cancer. Also, there is not much to do to protect from it except stay buried under meters of radiation shielding. Spacesuits and rovers would be no protection.

But microbes don't get cancer. Their DNA gets damaged, yes, but the most radioresistant microbes have remarkable abilities to heal the damage to their own DNA, in real time, with no need even to reproduce. They have structures that keep the DNA fragments in proximity to each other when they are damaged. When they are able to metabolize again, their cell machinery can join those fragments together to repair the DNA. They can do this even in semi dormancy. They can wake up for a few hours, repair their DNA damage for the last few thousand years, and then go back to "sleep" again.

Take Chroococcidiopsis for example, one of the microbes we have on Earth best able to survive in Mars surface conditions. Experimenters have found that it can repair, 2.5 kGy of damage within 3 hours given the opportunity to wake up for a few hours and metabolize. Here a kGy is a thousand Grays, and a mGy is a thousandth of a gray. So 2.5 kGy corresponds to 2500/0.076 or over 32,000 years of radiation on the surface of Mars. So if it wakes up for a few hours every year, it will have no trouble at all keeping its DNA repaired. And - that's nowhere near a lethal dose. If able to wake up for 24 hours, it can repair 64,000 years worth of damage.

The most remarkable thing about it is that these are microbes from Earth that have never encountered such high levels of cosmic radiation - at least as far as we know. Perhaps this ability is a side effect of its evolution of mechanisms to stop the DNA from getting broken up when it dries out (resistance to desiccation)? At least that's the most usual explanation. Perhaps Mars microbes, evolved in conditions of high levels of radiation may be far more radioresistant even than this.

The most radioresistant microbe currently known is Thermococcus gammatolerans, which lives in conditions just about as as sheltered from ionizing radiation as you can imagine, in hydrothermal vents.

Thermococcus gammatolerans - an obligate aerobe from hydrothermal vents, the most radioresistant organism known, able to withstand 30 kGy of gamma radiation, and still reproduce. That's about 400 thousand years (30,000/0.076) worth of surface radiation on Mars at the radiation levels detected by Curiosity during the current solar maximum - possibly it could survive surface radiation for longer than that when you include periods of solar minimum.

It's not a likely candidate for the Mars surface as it lives at the bottom of the sea and requires high temperatures. But - its radioresistance seems to be a side effect, it certainly doesn't encounter cosmic radiation down there. So a Mars microbe, adapted through billions of years of evolution to the ionizing conditions on the surface of Mars may well be as radioresistant as this, or more so.

The findings on ionizing radiation in the 2014 report: A New Analysis of Mars ‘‘Special Regions’’ were (Finding 3-8):

  • From MSL RAD measurements, ionizing radiation from GCRs at Mars is so low as to be negligible. Intermittent SPEs can increase the atmospheric ionization down to ground level and increase the total dose, but these events are sporadic and last at most a few (2–5) days. These facts are not used to distinguish Special Regions on Mars.

Here a SPE is a Solar Proton Event (solar storm), and a GCR is a Galactic Cosmic Ray. So why then was it sterilizing for organisms dormant for millions of years, when it has negligible effect on active life?

Ionizing radiation is exponential in its effects. So though it has negligible effect on timescales of decades, it is devastating on timescales of hundreds of thousands of years.

Example, suppose that a dormant population is halved in ten thousand years by ionizing radiation (within the range of possibility for hardy ionization resistant life). That would have negligible effect on life able to reproduce every year.

Yet if a population of such microbes is kept dormant for 500,000 years you would have only 1 in 250 or around 1 in 1015 left, or one cell remaining out of a quadrillion.

After a million years only one viable cell will remain out of an original population of a nonillion (1030).

So ionizing radiation on Mars is devastating for life that remains dormant for millions of years but has no noticeable effect on life that can either replicate or "wake up" for a few hours to repair its own DNA as many ionization resistant microbes can do.

So that's how it works with ionizing radiation. Dormant populations can be completely sterilized over millions of years. Yet the radiation causes no problems at all, so long as they can revive enough to repair their DNA at least once every few millennia, which only takes a few hours. Life in these Mars habitats, if they exist, could revive every year. So ionizing radiation is not a limiting factor any more, for these new ideas about possible present day surface habitats on Mars.

Ultraviolet radiation

The levels of UV on the surface of Mars are also very high and would destroy most microbes within seconds. So scientists used to think that there was almost no chance of any life able to live exposed to the UV on Mars. However, first of all UV light is easily shielded. It differs in that way from cosmic radiation, which goes through meters of rock without noticing it. UV light is like ordinary light - it can be blocked by just about anything that casts a shadow. A mm or so of soil will block it. Also if a microbe is in the shadow of a rock, or pebble, it is shielded. Even if it is in a tiny microscopic crevice in a grain of Martian dust, it is shielded, especially since the Martian dust contains iron oxide, which is rather effective at shielding out UV light. Also in a microbial mat, microbes can be shielded by the cells of their dead siblings.

Then, even a cell can shield itself from UV light using pigments. It turns out that some microbes can last a lot longer than a few seconds exposed to UV light. Indeed, it also turns out that our pal Chroococcidiopsis has adapted to shield itself from UV. Unlike radiation, we get significant amounts of UV light in cold deserts and high mountains. Though they are nowhere near the Mars levels, there is enough UV so that cyanobacteria have evolved some protection from it This UV resistance is so good, that when the German aerospace company DLR (sort of their equivalent of NASA) researched into this, they found that our friend Chroococcidiopsis, could survive partial shade in conditions on the surface of Mars. And to their surprise, it doesn't just survive - in their Mars simulation chambers, they found that these microbes could metabolize and photosynthesize, slowly, on the simulated Mars surface. And it wasn't just microbes. - they found that some lichens, also have developed UV resistance, with various specialized pigments to block it out.

These are still early stage experiments - but it looks promising that you might get photosynthesizing lifeforms actually on the surface of Mars using the sun's light for energy. Either in partial shade, or shaded by a thin layer of dust - or protected by transparent rock such as quartz or gypsum (which has been found on Mars). In the Atacama desert, in regions of high UV light, enough to sterilize the surfaces of rocks, then 1 mm of gypsum is enough to protect from UV light. So, UV light also is a challenge, but some life will have no problems with it.

The findings in the 2014 report: A New Analysis of Mars ‘‘Special Regions’’ were (Finding 3-7):

  • The martian UV radiation environment is rapidly lethal to unshielded microbes but can be attenuated by global dust storms and shielded completely by < 1 mm of regolith or by other organisms.

After ionizing radiation and UV light, the next thing you are likely to say is "What about the perchlorates, don't they make the Mars surface uninhabitable even to microbes?"

Perchlorates on Mars

This is another thing you often hear, that Mars surface life is impossible because of the perchlorates. Now, for humans, the perchlorates are quite nasty. The Mars dust has these chemicals in it, at levels 10,000 times higher than on Earth. This is harmful to humans as perchlorates prevent us from absorbing iodine. We need iodine for our thyroid glands, which regulate our metabolism. They may also have other effects on us, see this chemical might make Mars more dangerous. Also, to make things worse, the ionizing radiation may decompose these perchlorates into the reactive chlorites (ClO2) and hypochlorites (ClO) which have more serious and immediate effects on us

"Ionizing radiation can decompose small quantities ofClO4-into other Cl-oxyanions, such as ClO2-and ClO-(Quinnet al. 2013), which are much more reactive and can be the cause of other health concerns such as respiratory difficulties, headaches, skin burns, loss of consciousness and vomiting "

(quote from page 3 of this paper). For more on this, see Perchlorate on Mars: A chemical hazard and a resource for humans.

So, it's nasty stuff for humans for sure. But microbes don't have thyroid glands and don't get headaches or lose consciousness or vomit. Also perchlorates are more potent at the warmer temperatures of a human body. As it turns out some microbes actually eat perchlorates. It is nutritious food for them. So it doesn't by any means rule out microbial life on Mars. Indeed it figures on both sides of the ledger as it were, as a habitability limiting factor for some microbes, and a source of food for others.

Even when Phoenix first discovered perchlorates on Mars, scientists were clear that they are not a problem for native Mars life, for instance in this article in Scientific American: NASA Says Perchlorate Does Not Rule Out Life on Mars - Unexpected chemical in Martian soil is a food source for some Earthly microbes. Before the announcement there were rumours that they had proven that the Mars surface was uninhabitable forcing them to make an early statement to quell those rumours.

Cassie Conley, current planetary protection officer for Mars, put it like this (as reported in the NASA Astrobiology magazine)

"The salts known as perchlorates that lower the freezing temperature of water at the RSL's, keeping it liquid, can be consumed by some Earth microbes. “The environment on Mars potentially is basically one giant dinner plate for Earth organisms,” Dr. Conley said."

Perchlorates irradiated with UV light to create sterilizing chlorites and hypochlorites

This is an experiment reported in Nature in 2017 as "Perchlorates on Mars enhance the bactericidal effects of UV light" by Jennifer Wadsworth and Charles Cockell (her supervisor for her research). It got widely reported with headlines such as Mars covered in toxic chemicals that can wipe out living organisms, tests reveal . As is often the case, the experiment wasn't as dramatic as those headlines suggest, but it was interesting, well worth mentioning in a section here. It suggested that the UV light irradiating brines on Mars could produce toxic products of perchlorates - the same chlorites and hypochlorites mentioned in the previous section (Perchlorates on Mars), and that these can be bactericidal.

The experimenters studied the effect of UV light (not cosmic radiation as discussed in the last section) on the perchlorates. Also they looked at its effects on perchlorates in solution - as they might be, for instance, in the RSLs and other possible liquid brine layers. They weren't testing the effects of UV light on dry dust.

What they found is that though the perchlorates are not normally bactericidal, this changes when they are irradiated with UV light simulating the UV flux on Mars. Of course UV light by itself is also bactericidal.

Some details of the experiments: They tested Bacillus Subtilis, a microbe which is a common spacecraft contaminant. They mixed the cells in with a nutrient solution they call M9 which contained magnesium sulfate, glucose and calcium chloride dissolved in distilled water. They then added perchlorates, or hydrogen peroxide, or iron oxides, and mixtures of all three, and irradiated it with UVC monochromatic light, and in other experiments, with polychromatic light for a more realistic UV flux similar to that for Mars. They also repeated the experiments with silica disks soaked in the medium (after addition of the perchlorates) which they used as an analogue for microbes beneath the surface of rocks on Mars.

They found that vegetative cells (not spores) of Bacillus Subtilis were completely sterilized (no viable cells left) within 60 seconds when irradiated by UV at 25 °C. When they added perchlorates, they were sterilized much more quickly, within 30 seconds. When they used the silica disks (which provide some protection from the UV radiation), then 60 seconds of irradiation lead to a 9.1 fold drop in viability after 60 seconds compared with a 2 fold drop without the perchlorates. Those experiments were all done in room temperature conditions, of 25 °C. They then tried colder temperatures of 4 °C and found that the samples were completely sterilized after three minutes, with or without the perchlorates, but the sample with the perchlorates had many fewer viable cells after two minutes (11.4 times fewer).

All of this demonstrated that the perchlorates enhanced the bactericidal effects of the UV light. So how did it do it? They found evidence of hypochlorite and chlorite in the absorption spectrum of the UV irradiated sample, and conjectured that this might be what caused the bactericidal properties. They found that adding hematite (a form of iron oxide) on its own increased viability (as you'd expect, as it shields from UV) but that hydrogen peroxide (which is also present on Mars) decreased viability further - and adding all three decreased viability most of all.

Many of the news story write-ups of this research said that this meant the surface of present day Mars os uninhabitable, and that we will have to search at least three feet below the surface to find life. I'm not sure where that three feet figure came from as it isn't mentioned in the paper as far as I can see. Three millimeters would be plenty to block out UV light. The primary author of the paper, Jennifer Wadsworth, was more circumspect, when interviewed, as reported in the Smithsonian magazine:

"It's also possible that hypothetical Martian bacteria could be much tougher than the common Bacillus subtilis. On Earth, researchers have found all types of extremophile organisms with the ability to survive under intense heat and pressure, in the presence of acid, without water and even inside rocks. “Life can survive very extreme environments,” Wadsworth tells Fecth. “The bacterial model we tested wasn’t an extremophile so it’s not out of the question that hardier life forms would find a way to survive.”

If I can make a few more observations

  • This is for vegetative cells in a liquid water media. So it wouldn't necessarily impact on spores embedded in the dust - spores are protected by a coat and are far more resistant to oxidizing agents, bactericidal agents chlorites, hypochlorites etc than vegetative cells. They are also more UV resistant. Some spores can withstand many hours of surface UV radiation on Mars, including one strain still viable after 28 hours of direct UV radiation in Mars simulation surface conditions. See How could this work on Mars with dust storms and a globally connected environment? (above).
  • It's also for liquid media irradiated with UV light. Many of the Mars brine habitats would be a few millimeters below the surface, inhabited by chemolithoautotrophic microbes. These get energy from the rocks and chemicals in the dust and regolith, and don't need sunlight at all. Some of them would use the perchlorates as a source of food, and they would not need to be irradiated by the UV light.

    Even if the UV light did create reactive chemicals, say, at the head of the RSLs, would these reactive chlorites and hypochlorites persist, as the thin film of brine flow down the forty five degree slopes beneath the surface of the Martian soil? Or would they react with other constituents of the brine at an early stage? A few millimeters of dust would be enough to shield out most or all of the UV light.
  • Indeed, since UV light is sterilizing even without the perchlorates, then photosynthetic life would need to either filter it out by huddling in semi shade or sheltered by the iron oxide dust - or incorporate specialist UV absorbing pigments such as are used by many microbes and lichens on Earth adapted to cope with high levels of UV
  • Also UV light of course can be blocked almost completely by a shadow of a boulder, hill, in an overhang, crevice or cave, or indeed in the shadow of a spacecraft (leaving only a small amount of ambient light that reflects into the shadow).

    In the DLR experiments, then the lichens survived in semi shade. In Antarctica, lichens huddle in cracks and photosynthetic microbes live beneath the surface of the rocks for protection from UV light.
  • The UV light could also be blocked by gypsum, as happens in the Atacama desert. As we saw in the Ultraviolet radiation (above), 1 mm of gypsum is enough to protect from UV light in the Atacama desert. Somewhat more would be needed on Mars of course.
  • Also - what effect would this UV radiation of perchlorate solutions have, if any, on the lichens and cyanobacteria that take their water directly from the humidity of the air? They don't need to be in contact with perchlorates as they don't depend on perchlorates for water.
  • Also even the 4 °C experiment involves temperatures far higher than are normally considered for Mars habitats for microbes. Often the discussion centers over whether the habitats would be warmer than the -20 °C that's usually thought to be the minimum temperature for cell division. Such high temperatures as 4 °C or higher could only be attained in the very surface layers, and by the time a brine reaches those temperatures in the thin atmosphere of Mars, then, depending on its composition, most often, the liquid in the brines has dried out completely.
  • In their experiment they were careful not to add enough of the B. Subtilis to form a biofilm - as this could protect against the UV. Microbes on Mars could form biofilms which could protect the rest of the colony from both the UV and also the reactive chemicals formed from splitting the perchlorates, much as the coat of a spore protects microbes in spore form. The microbes on the surface, exposed directly to UV, would be killed but the dead microbes protect the ones beneath them.
  • Then, also, as Wadsworth said, there are many microbes that are much more UV hardy than B. Subtilis - for instance the ones that withstand hours of UV radiation in semi shade in the Mars simulation conditions, taking in moisture from the atmosphere and photosynthesizing (see Lichens and cyanobacteria able to take in water vapour directly from the 100% night time humidity of the Mars atmosphere (below) ).
  • Microbes form communities in biofilms - the biofilms are more hardy than any individual species can be. Andrew Schuerger makes this point in a brief comment 47 minutes into this discussion at AbSciCon 2017 day 1. - he is noted for his research on biocidal factors on Mars, and he remarks that with 15 - 20 recognized biocidal factors on Mars, it's easier for a community of microbes to overcome them than individual microbes.
  • Also there are many other salts on Mars such as the sulfates, that can create brine habitats when in contact with ice, or deliquesce, and take water out of the atmosphere.
  • Some of the suggested microhabitats, as we will see later, can potentially have fresh liquid water at 0 °C, with no need for salts at all to keep it liquid, trapped beneath a solid state greenhouse of transparent blue ice (see Southern hemisphere flow-like features - these may involve fresh water! (below) . Also, the concentrations of perchlorates and the mix of other salts will vary across the surface of Mars.
  • One possibility for Mars is life based on hydrogen peroxide and perchlorates within the cell in place of the chlorides used by all Earth life. Life like that based on a hydrogen peroxide biochemistry is not going to be affected by hydrogen peroxide clearly - and may well be able to cope with other reactive chemicals that modern Earth life can't tolerate. See. Life that uses hydrogen peroxide, or perchlorates, or both, INSIDE the cells (below).

So, those are a few things to think about.

It's interesting research for sure. It suggests that there are more challenges for life on Mars than we thought before the experiment. But I think it's going too far to say it means the surface of Mars has been proven to be inhospitable to Earth or native Mars life, or that any life has to be at least several feet below the surface. I think it's just another example of interesting research that got rather overhyped by the media, as happens so often in this topic area.

Let's now look at how perchlorates can help with the formation of liquid brines on Mars.

Perchlorates as a way to scavenge water from the atmosphere

The perchlorates are not just useful as an energy source for food. The salts lying on the Mars surface also extract water from the atmosphere and could be the basis for microhabitats for life. Many salts will take in water from a humid atmosphere, with no need for rain or surface water flow. But perchlorates are amongst the best at doing this.

(One of the slides for the NASA press conference Water flowing on present day Mars)

The Mars atmosphere reaches 100% relative humidity at night because it gets so very cold at night. Warm air holds more moisture than cold air (which is why clothes on a clothes line can dry) .So, though Mars has hardly any water vapour in its atmosphere, these huge night time drops in temperature raise the humidity so high that the thin atmosphere gets saturated at night, and Mars often has frosts even in the extremely dry equatorial regions of Mars, as the Viking landers found out.

Ice on Mars- Utopia Planitia, photo taken by Viking Lander 2 at its Utopia Planitia landing site on May 18, 1979,

These frosts formed every morning for about 100 days a year at the Viking location. Scientists believe dust particles in the atmosphere pick up bits of solid water. That combination is not heavy enough to settle to the ground on its own. However, carbon dioxide, which makes up 95 percent of the Martian atmosphere, then freezes and adheres to the particles and they become heavy enough to sink. Warmed by the Sun, the surface evaporates the carbon dioxide and returns it to the atmosphere, leaving behind the water and dust.

The ice seen in this picture, is extremely thin, perhaps no more than one-thousandth of an inch thick. These frosts form due to the 100% night time humidity, which may also make it possible for perchlorate salt mixtures to capture humidity from the atmosphere at night, through to the early morning. This process could occur almost anywhere on Mars where suitable mixtures of salts exist.

Perchlorate salts on the surface would take this humidity out of the atmosphere and form damp patches of liquid water at certain times of day. The best times to find this water depend on the site but typically it's around midnight and early morning.

So could this water be habitable?

(One of the slides for the NASA press conference Water flowing on present day Mars)

Well the temperature range is fine. Perchlorates actually can permit liquid brines on Mars at higher temperatures than for fresh water, though they also permit water at temperatures far too low for Earth life. A pool of perchlorate brine below 24 °C would be below boiling point even in the thin Martian atmosphere, but it would dry out pretty quickly, like a puddle on a hot sunny day on Earth. So don't expect puddles of hot salty water on Mars, but it's a little easier for warmer perchlorate rich water to be present there in daytime than fresh water (which is already boiling when it melts over much of Mars, and in low lying places like Hellas Basin is close to boiling point in the thin atmosphere)..

The problem is that in practice, perchlorate based brines on Mars are likely to be very cold, or very salty, or quite likely, both. The next couple of sections are a little techy, so if you want to skip them, just jump to "So deliquescing water is liquid in a wider range of conditions than you'd expect".

Mixing perchlorates with other salts lets them take up water from drier air than either separately

The ability of the salts to form liquid brines on Mars is improved hugely by the process of eutectic mixtures. The name comes from the Greek "ευ" (eu = easy) and "Τήξις" (tecsis = melting). If you have a mixture of two salts, for example, a mixture of chloride with perchlorate, then the mixture stays liquid at a lower temperature than each of the salts separately. The melting temperature is the "eutectic point". This phenomenon is related to the way antifreeze works, and the reason why salt keeps roads free from ice, even though the melting point of salt is far higher than that of water. See also Freezing-point depression.

The same thing happens with humidity too, in which case it is called a eutonic mixture, or a eutonic solution (when it has taken up enough water vapour to become liquid), and the relative humidity at which this happens is the eutonic point. A mix of salts is able to take up water from drier air (lower relative humidity) than either of the salts separately. Interestingly, it doesn't matter much what the actual percentages of the two salts are, so long as there is some of both in the mixture.

This diagram shows how it works - for a fictitious mixture A and B.

DRH = Deliquescing Relative Humidity, ERH = Eutonic Relative Humidity

Here the graph shows the mixture of A and B along the bottom with 100% A to left and 100% B to right. The ERH or Deliquescing Relative Humidity is the humidity at which some of the mixture of the salts starts to take up water from the atmosphere. Below that, everything is solid. Above that point, if you have the optimal mix of A and B then the whole thing goes liquid. If not, you get a mix of liquid (L) with the solid, and then eventually as you increase humidity then the whole thing goes liquid. As you see, the perfect mix of salts means that it can all go liquid at a much lower humidity than for either salt separately.

You can also see from the diagram that even if you don't have the perfect mix, so long as you have some of each salt, then it will start to take up water as soon as it reaches the much lower Eutonic Relative Humidity (ERH), or "eutonic point". Depending on the mix of salts, then at the ERH you get either a mixture of liquid and salt as A + L or L+ B (L = Liquid). Either way you will get some liquid right away at the ERH.

The upper curved line shows the Deliquescing Relative Humidity (DRH) at which all of the mixture goes liquid. The point at which this happens depends on the mix of A and B.

So as the humidity is increased, for a given A / B mixture, first the lower horizontal line is reached, at which point some of the mixture of salts becomes liquid. This is known as the "eutonic relative humidity" - the point at which any mixture will start to take up some water vapour.

As humidity is raised further, more and more of the mixture becomes liquid. Eventually the upper, curved line is reached - and at that point, the entire mixture will be in its liquid phase.

Because of this eutonic mixture effect, then even if you only add a tiny amount of perchlorates to the less deliquescent chlorides, this is enough to reduce the minimum relative humidity needed to deliquesce hugely, right down to the eutonic relative humidity for the mixture. This is not only lower than the deliquescence relative humidity of the chlorides, it is also lower than the deliquescence relative humidity for the perchlorates as well.

You can also get similar eutonic mixtures of three or more different types of salts, which typically have even lower ERH than any of the mixtures of two salts. Salts on Mars could have a mixture of perchlorates, chlorates, sulfates, and chlorides and perhaps nitrates also if present, along with cations of sodium, potassium, calcium, and magnesium. So there are many possibilities to consider here.

Similarly if the axis is temperature - then as the temperature is raised, first part of the mixture will go liquid, at a temperature corresponding to the optimal mixture of the salts, and then when the upper curved line is reached, the entire mixture will be liquid.

After taking up water, perchlorates retain it for a long time even when very dry, or very cold

There's another effect that makes things even better for microbes on Mars. It's much harder for salt mixtures to lose water, than it is for them to take it up. So, as the air it gets more humid, the salt mixtures start to form liquid solutions at the eutonic point. But as it gets drier, stay liquid even when the humidity is reduced well below the eutonic point (this is known as delayed efflorescence). Similarly for eutectic freezing, the salty brine solutions can be supercooled well below the temperature at which they would normally freeze, staying liquid for a fair while below the eutectic point.

You get a eutectic also for freezing of a single salt in solution. If you have a mixture of salt and water then different mixtures will freeze at different temperatures. The eutectic is the optimal mix of water and salt with the lowest freezing temperature. As you freeze a mixture, then no matter what the original concentration, some of it will remain liquid down to the freezing point of the eutectic mixture. Then, as you freeze further below that temperature, you may find that the salt continues to remain liquid. The reason for this is that for a salt to come out of solution through nucleation, it has to form a new interface between the crystal surface and the liquid, which requires energy. Once the nucleation starts, then crystallization is rapid, but the nucleation can be delayed often for many hours.

Here is a table of some salts likely to be found on Mars, showing the eutectic temperature for each one (with the molar concentration for the optimal eutectic concentration in brackets) and the amount of supercooling below that temperature that they found with experiments (adapted from table 2 of The formation of supercooled brines, viscous liquids, and low-temperature perchlorate glasses in aqueous solutions relevant to Mars- omitted some of the columns).

Salt system Eutectic (°C) Amount of supercooling below eutectic (°C)
MgSO4 -3.6°C (1.72 m) 15.5
MgCl2 -33°C (2.84 m) 13.8
NaCl-21.3°C (5.17 m) 6.3
NaClO4 -34.3°C (9.2 m) 11.5

As the salt / liquid solution cools in Mars simulation conditions, then the results can be complicated, because for instance MgSO4 has a eutectic of -3.6 °C but it releases heat in an exothermic reaction when it crystallizes. This keeps it liquid for longer than you'd expect. In their experiments, it remained liquid for twelve hours as it gradually cooled below the eutectic temperature before eventually it froze at 15.5 degrees below the eutectic temperature. In simulated Mars conditions you also have to take account of the effect of soil mixed in with the salts. Surprisingly, when you work with the Mars analogue soil, instead of the solution, this does not reduce the supercooling and can in some cases permit more supercooling. (see The formation of supercooled brines, viscous liquids, and low-temperature perchlorate glasses in aqueous solutions relevant to Mars and "Formation of aqueous solutions on Mars via deliquescence of chloride–perchlorate binary mixtures).

With some of the salt solutions, depending on chemical composition, then the supercooling produces a glassy state instead of crystallization, and this could help to protect supercooled microbes from damage.

So deliquescing water is liquid in a wider range of conditions than you'd expect

The combination of all these effects means that mixtures of salts, including perchlorates in the mixture, can be liquid at lower temperatures than any of the salts separately, and also take up water from the atmosphere at lower relative humidity, and once liquid, can remain liquid for longer than you would predict if you didn't take account of these effects. Then in addition to all that, if there are micropores in the salt deposits, any life within them could also take advantage of an internal relative humidity higher than the external humidity of the atmosphere, and so have access to water for even longer.

On Mars the relative humidity of the atmosphere goes through extremes. It reaches 100% humidity every night in the extreme cold, even in equatorial regions. In the daytime the relative humidity becomes much less, and becomes very dry indeed, approaching 0%, and any exposed salts would lose their liquid. Then at night, the surface temperatures of Mars, even in the equatorial regions, drop to tens of degrees below freezing every night. So, the surface temperatures of the top few also change enormously from day to night (more stable but lower temperatures are encountered deeper below the surface). But because of these other effects these liquid layers, may resist efflorescence and remain liquid longer than you'd expect as the air dries out in the daytime, and also stay liquid longer than you'd expect through supercooling as the temperatures plummet at night.

The result is that you could have layers of liquid, on Mars, and especially if they are some way below the surface 1 or 2 cms, then liquid brines can form and be stable for hours every day. So this discovery of perchlorates on Mars has major implications for presence of liquid, and so habitability.

Challenges for life in these liquid layers of deliquescing salts

Mars has so many salts and perchlorates that the optimal mixes are bound to be present in places, so these deliquescing liquid layers must form. They are probably very thin layers but these would be plenty thick enough for a microbe to live in. The main focus of much of the research is on whether there are mixtures of salts able to deliquesce on Mars, that at the same time are also warm enough and not too salty for life. Many of these potential habitats would be far too cold or too salty, but it seems that in optimal conditions, with the right mixture of salts, at the right depth below the surface, these might just possibly form habitats for salt tolerant microbes - the haloarchaea. They would have to be perchlorate tolerant, and ideally, able to use it as a source of energy The perchlorates could act as a substitute for oxygen, as an "electron acceptor" on Mars in the anaerobic (low oxygen) environment.

One interesting thing about these deliquescing salts is that since they take up water vapour directly from the atmosphere, they can give a way to form liquid water where there is no ice present on the surface such as the arid equatorial regions of Mars. They could also could form in very dry conditions inside salt pillars, perhaps one of the most interesting possibilities of all.

Microbes could find tiny oases of water inside micropores in salt pillars

This is a way for life to access water when the atmosphere is far too dry for normal deliquescence, so at relative humidities way below those in the laboratory experiments. This section is based on  several papers that studied life in salt pillars in the Atacama desert, and suggested that the way the life gets water there may be of interest to Mars. Here are some of them:

In experimental studies of salt pillars in the Atacama desert, microbes are able to access liquid through spontaneous capillary condensation, at relative humidifies far lower than the deliquescence point of salt (NaCl) of 75%.

'The Atacama desert hosts the closest analogue of what a real, live Martian might be like', in its salt rock formations in the hyper-arid core of the Atacama desert, where microbes were found, living inside the salt, and getting their water entirely by deliquescence. Perhaps the same process might work on Mars. See Paul Davies' Blog The key to life on Mars may well be found in Chile. Cassie Conley also talks about these as a possible habitat for life on Mars.

These micro-organisms can survive because of numerous micro-pores in the salt, less than 0.1 micrometers in diameter in the salt. Theoretically, this reduces the limit for trapping water from the atmosphere down to relative humidities as low as 50-55% instead of the usual limit of 75%. Also once the water is trapped, it is retained for a long time, as the air gets even drier, right down to an extremely low relative humidity of 20%.. The authors suggest this seems a likely niche for Martian life to exploit. See Novel water source for endolithic life in the hyperarid core of the Atacama Desert

In year round studies of the pillars, they found that the external atmosphere reached a minimum relative humidity of 2.90%, maximum 74.2% and average of 34.75% (see table 2 of their paper). So the external atmosphere never quite reached the 75% level where salt deliquesces naturally, but inside the salt pillars it was able to capture the water in these micropores, easily, and retain it as well. The values for the interior of the salt pillars were: minimum 2.20%, maximum 86.1, average 54.74%.

The researchers, Wierzchos et al, did detailed studies with scanning electron microscopes. At 75% relative humidity then brine was abundant inside the salt pillars. As the humidity was reduced, even at 30% relative humidity for the atmosphere outside of the pillar, the clumps of cyanobacteria in the micropores shrunk due to water loss, but still there were small pockets of brine in the salt pillars. In "Novel water source for endolithic life in the hyperarid core of the Atacama Desert" the authors write:

"Endolithic communities inside halite pinnacles in the Atacama Desert take advantage of the moist conditions that are created by the halite substrate in the absence of rain, fog or dew. The tendency of the halite to condense and retain liquid water is enhanced by the presence of a nano-porous phase with a smooth surface skin, which covers large crystals and fills the larger pore spaces inside the pinnacles... Endolithic microbial communities were observed as intimately associated with this hypothetical nano-porous phase. While halite endoliths must still be adapted to stress conditions inside the pinnacles (i.e. low water activity due to high salinity), these observations show that hygroscopic salts such as halite become oasis for life in extremely dry environments, when all other survival strategies fail.

Our findings have implications for the habitability of extremely dry environments, as they suggest that salts with properties similar to halite could be the preferred habitat for life close to the dry limit on Earth and elsewhere. It is particularly tempting to speculate that the chloride-bearing evaporites recently identified on Mars may have been the last, and therefore most recently inhabited, substrate as this planet transitioned from relatively wet to extremely dry conditions"

Microbes also inhabit Gypsum deposits (CaSO4.2H2O), however Gypsum doesn't deliquesce. The micropores can still enhance the humidity of the atmosphere. Researchers found that the regions of the desert that had microbial colonies within the gypsum correlated with regions with over 60% relative humidity for a significant part of the year. They also found that the microbes were able to imbibe water whenever the humidity of the atmosphere increased above 60% and gradually became desiccated when it was below that figure.

For more recent work, see for instance Microbial colonization of halite from the hyper-arid Atacama Desert studied by Raman spectroscopy

This shows images of salt crystals from the hyper-arid core of the Atacama desert. The dots show regions that were analysed with Raman spectroscopy and found to contain life.

In short, it's a major challenge for life to find water on Mars, but it may be possible, and there are many potential methods that life could exploit to find it. We will come across several other possibilities later on. We won't know for sure whether life on Mars actually can use these various sources of water in practice until we have a much better understanding of the Mars surface conditions.

There is one other major thing that could make Mars, or parts of Mars, uninhabitable. That's the very low level of nitrogen in its atmosphere. It does have some, but the partial pressure is only 0.2 mbar. Earth's atmosphere has 781 mbar.

Sources of nitrogen - essential for life on Mars

Nitrogen is important for life, on Earth at least. It's what lets DNA zip up and unzip and it's also what holds together the helical structures of proteins. In more detail, when hydrogen is attached to nitrogen, it forms weak bonds with other elements like oxygen,

What about life on Mars though? Well, these bonds are central to biology as we know it because the bonds are so easily broken, which is so important, for instance, to let the DNA zip and unzip. Hydrogen can't make these special bonds when it is attached to carbon, instead of nitrogen. So even if life on Mars is very different from Earth life, perhaps using different amino acids, or with a different backbone from DNA, still it is likely to use nitrogen as a way to create these weak hydrogen bonds. Indeed, the astrobiologists think that anything that even remotely resembles Earth life would be likely to use nitrogen. Here is how it is used in DNA

How Nitrogen mediated hydrogen bonds are used in DNA

And here is how it is used to hold proteins together

Diagram of helical structure of a protein, showing how the nitrogen mediated hydrogen bonds hold it together

For more, see Searching for Organics in a Nibble of Soil. Diagrams credit J. Bada.

So, if this is right that nitrogen would be essential to life on Mars, how can it find the nitrogen it needs to survive?

First, there is that 0.2 mbar of nitrogen in the atmosphere. This may be too little for life to fix the nitrogen. Or is it? Let's look at this first, because if it was possible, you could have nitrogen fixing microbes anywhere on Mars. Some Earth microbes can actually make do with astonishingly low levels of nitrogen. I can't find much recent research about this for some reason. The main paper is from 1989, Biological nitrogen fixation under primordial Martian partial pressures of dinitrogen. The authors grew a couple of species of microbes often used for nitrogen fixation experiments, Azotobacter vinelandii and Azomonas agilis under conditions of low nitrogen levels, though at full Earth atmospheric pressure. They found that they could continue to fixate nitrogen down to 5 mbar. They found no evidence of fixation below 1 mbar.

However that's just a study of two species. A natural follow up would be to look at some of the extremophiles that might be able to survive on Mars. Could any of these, chroococcidiopsis (say) do nitrogen fixation at these very low pressures? The authors of a more recent paper in 2006 propose doing this experiment with Antarctic cyanobacteria, but as far as I can see, though they did some preliminary research, they haven't actually tried it yet in Mars simulation chambers. It does seem a bit of a stretch. Perhaps the answer would indeed be "No". Maybe it was only possible in the past when nitrogen concentrations were a bit higher. It's quite a long way to go, from a limit of somewhere between 1 and 5 mbar all the way down to 0.2 mbar. Yet, it would make present day Mars far more easily habitable if the answer was "Yes". If anyone here knows of any more research on the limits of low pressure nitrogen fixation for extremophiles, do say.

Luckily, there are many other ways to fix nitrogen on Mars, especially in the early solar system, including

  • By volcanic processes and lightning.
  • By chemical reactions. A recent (2012) paper suggests reaction of FeS with NO might have fixed nitrogen in the primordial Martian atmosphere.
  • By meteorite impacts. The very early Mars had far more nitrogen than now, between 200 and 600 mbars of nitrogen (present day Earth has 780 mbars). That's enough so to make it likely that impacts on Mars fixed large quantities of nitrogen in the past.

The impacts there are especially promising. One estimate suggests that 80 to 150 mbars of Nitrogen was fixed by giant meteorite impacts on early Mars. If so, then much of those nitrates would get washed into the northern seas. Assuming about 100 mbar was fixed in this way globally, the authors suggest that Mars could still have a layer of nitrates a hundred meters deep, with 10% sodium nitrate by mass. The likely place to find this is in the northern lowlands. See Nitrates on Mars: Evidence from the 15/14N isotopic ratio

We haven't drilled deep below the surface yet. So the situation on Mars could easily be like the Atacama desert, that it does indeed have rich deposits of nitrates buried just a few meters below the surface. That estimate of a layer a hundred meter deep covering the Northern lowlands of Mars below the surface would give life on Mars vast amounts of nitrates to use. The main problem would be how to get it to the surface,as it would be below the permanently frozen permafrost layers. But perhaps it could get churned up by the wind in moving dust dunes. It could also get unearthed by meteorite impacts ("meteorite gardening"), and so become useful to surface life.

Anyway that's just one of several solutions. There are other ways for Mars to get fixed nitrogen. It can be delivered on meteorites, since some carbonaceous meteorites are rich in nitrogen. Another possible way for the life to get nitrates is through nitrogen fixation in interstatial thin films of fresh water. These films can form at well below the point where ice would normally freeze. They are really thin, less than a micron in thickness. For details see this paper:An active nitrogen cycle on Mars sufficient to support a subsurface biosphere. The author finds that these films can fix between 5 and 54 trillion molecules per square centimeter per second. That's 1.577 * 10^24 molecules per square meter per year, or about 2.6 moles per square meter per year, which is plenty enough to be biologically useful.

Life on Mars doesn't need much by way of nitrates. For instance in Antarctica, the main source of nitrates is from nitric acid created partly in the stratosphere as well as in photochemical reactions in the lower atmosphere. This is a slow process but it's enough to support small populations of microbes. The amounts available are tiny, around  1 to 2 micro moles per square meter per year (measured using snow pits). That's around 0.6 to 1.2 milligrams per square meter per year. That's far less than, for instance, the estimated quantity from the nitrogen fixation in interstatial thin films.

Though much here is still unknown, we do know that there are at least some biologically useful nitrates on Mars, as Curiosity discovered them in 2015. More details. Also the Martian meteorites have fixed nitrogen in them. But it's probably patchy and in short supply on the surface. For an overview with more about this issue and possible ways that life on Mars could access nitrates, see page 189 of Charles Cockell's "Trajectories of habitability". He also looks into many other requirements for habitability of past and present day Mars life.

This issue of the distribution of fixed nitrogen on Mars remains one of the most important unknowns for both past and present day habitability. Astrobiologists have recommended that NASA follow up its current "Follow the water" strategy with a "Follow the nitrogen". I cover this in Follow the nitrogen, dig deep and look for biosignatures (below) .

So, in short, there mightn't be much by way of nitrates on Mars, especially on its surface, but we know there is some, and there may well be enough to support small populations of microbes, as for the McMurdo dry valleys in Antarctica. There are a few ways it could be fixed even today, it could also get there on meteorites and comets. Also Mars may well have thick subsurface deposits of nitrates, and if so, perhaps other processes bring them to the surface from time to time.

Other requirements for life on Mars: major and trace elements, organics, and pH (alkaline or acid)

Charles Cockell looks at the other elements that life needs. First, most of the elements needed for life are no problem on Mars. The igneous rocks such as basalt are a rich source for trace elements as well major elements used by life (such as calcium). These are abundant on Mars, and have all the same major and trace elements as they do on Earth. There are no obvious omissions. There's plenty of sulfur there. Phosphates are also common. Hydrogen and oxygen are no problem. Hydrogen also is not a problem, as it can come from water, through chemical reactions like serpentization or can be produced by interactions with radioactive elements common in the crust.

What about organics? Many microbes need organics as food, because they can't make their own (the heterotrophs). Carbon dioxide fixing photosynthetic life can make organics, just from water, and air, but there can't be much by way of photosynthesis of that sort in the surface conditions, because we'd notice the effect of the oxygen it produces on the atmosphere if there was (I look at this in How much oxygen would surface photosynthetic life produce on Mars? below).

However even if Mars has no photosynthesis, organics shouldn't be much of a problem for life, as organic compounds continually rain down in meteorites. The reactive chemistry removes them quickly, but there should still be some organics available for life to use. Also, there's carbon in the basalt, which could have reacted with hydrogen to make organic compounds for the heterotrophs.

Most of the Mars surface is slightly alkaline. It may have acidic environments due to weathering of sulfates but these would be well within the tolerance limits of Earth life. See the section on pH, page 191 of Cockell's Trajectories of Martian Habitability. Finally, there are various chemical gradients that life can exploit as a source of energy (electron acceptors and electron donors). For details see his paper "Trajectories of Martian Habitability"

So in short, there is nothing else that should be a problem for life, on Mars, apart from the need for water, and nitrogen, which we've already covered. So, anyway, all that's just theory so far. Is there anywhere on the Mars surface where we can hope to find life on Mars?

Well as it turns out, yes there are quite a few suggestions for places to look, and some of them seem rather promising. We've already mentioned Nilton Renno's droplets that form where salt touches ice. (above) which may form rapidly, anywhere where there is ice on the surface in contact with salts. Now let's look at a few more of these suggestions.

Habitats for life on the surface of Mars - warm seasonal flows - now known as Recurring Slope Lineae or RSLs

When we got our first look at Mars in detail, it hardly seemed to change at all, apart from the dust storms. There were many signs of activity in the past -evidence of river beds, even seas, huge craters, and the volcanoes and the Valles Marineres rift valley. But since then, nothing.

However, when we started to put high resolution cameras in orbit, looking down on the surface, scientists started to notice many small scale seasonal changes. Most of these are caused by the wind blowing the dust, or by dry ice. The scientists found a few candidates for features that might be caused by liquid water, especially the dry gullies, which were interesting for a while, but they all had alternative explanations and didn't prove the case.

However, back in 2011, NASA announced discovery of the Warm Seasonal Flows. This discovery was a lucky break, made by a young Nepalese American student Lujendra Ojha (Luju for short), who rather remarkably made this startling discovery while working on an undergraduate thesis, under the guidance of Alfred McEwan. He first spotted them in 2010, with the results published in a paper by McEwan in Science.

“When I first saw them, I had no idea what it was. I just thought it was a streak made by dust or something similar. It was a lucky accident”, - Lujendra Ojha

They appear in spring, gradually extend during the summer, and broaden out and fade in autumn. And they always appear on sun facing slopes when the temperatures rise above 0 °C. Right from the beginning, the scientists were only able to come up with hypotheses that involved liquid water in some form. The temperatures are far too high for dry ice (which also would be a surprising thing to find in equatorial regions). The other likely explanation is that they could be wind formed features. There are many temporary wind formed changes on Mars, and you can get dark streaks rather like this from avalanches of dust after a strong wind. However, the RSL's are not associated with winds and the seasonal changes, with the streaks gradually extending, broadening and fading through the season, again don't match what you'd expect from winds.

This made them hard to explain as anything else except features caused by liquid brines in some form. We haven't detected water yet, but it's now pretty much confirmed that they are caused by liquid brines, indirectly through detection of changes of hydration levels in salts, in a paper published on 28th September 2015 along with a press conference.

Unusually, these can form even close to the equator, These locations have no surface ice at all, at any time of the year (except for the ephemeral morning frosts photographed first by Viking). The RSLs extend downwards from bedrock outcrops, with many streaks extending from the same outcrop, as you can see in the video above. They only form on very steep slopes. Typically the slope is 33 degrees (about 1 in 1.54), on a concave slope, with the runoff a slightly gentler 27 degrees, or about 1 in 2 (online angle to slope calculator). For details see section 5.2 of this paper.

For some reason they only form on some steep slopes and not on others. Nobody knows why yet. These examples are from the slopes in Newton crater. High resolution version and techy details here.

These dark streaks are not damp patches. Rather, it's some other effect on the surface due to the brines flowing beneath. They are very hard to study from orbit because they can only take the highest resolution photos during local afternoon, the worst time to detect the water. That's due to the orbit of the spacecraft, which is optimized to let it take close up photographs always at the same local time of day - more on that in a moment.

Warm seasonal flows on Mars. This is in Palikir crater, which is 41 degrees North. They have also been spotted in regions close to the equator.

There is some chemical process going on here as the streaks show traces of ferric and ferrous iron. So far nobody has detected water in them, but that might be because they are so thin, well beyond the resolution of their spectroscopic observations, and any water may be in small quantities.

They may also be caused by water which then dries up, or the water may be easier to spot in the morning, perhaps the flows happen in the morning depending on melting frost - the spectroscopic observations were made in the afternoon. See Are These Water Flows On Mars? Quite Possibly, New Observations Reveal

Even before the spectroscopic observations, our only explanations for these streaks involved liquid water in some form. But the explanations involving water aren't easy either. It's easy enough to explain warm seasonal flows for one year, but they keep coming back every year in the same spot. This means that something must replenish all the water that flows down the slope over geological timescales.

(for details, see Seasonal Flows on Warm Martian Slopes).

So, first things first, why they are so confident this means flowing water?

Details of what they found

Although Mars is currently surely the most studied planet, outside of Earth, even more thoroughly scrutinized than our Moon, still it has nothing like the constellations of satellites continually observing Earth. They did the measurements using a spectroscopic mapping instrument called CRISM.

CRISM is one of the instruments on Mars Reconnaissance Orbiter. It lets us do spectroscopic mapping of Mars.

We can't expect too much of CRISM. Our best optical telescope in orbit around Mars. HiRISE has wonderfully fine resolution. It is able to spots the streaks at widths of 5 meters widest, right down to 30 centimeters - its optical resolution limit. It benefits from Mars's thin atmosphere. This let's us spot fine details on the surface rather more easily than on Earth, and do it with comparatively small satellites also.

CRISM however has a best resolution of 18 meters per pixel. So there is no way you can use it to distinguish the composition of the streaks from the composition of the slopes around them, and they didn't try. Also, it can only look at the streaks at around 3 pm Mars time. That is, unfortunately, the time of day when they are likely to be at their driest.

We'd dearly like to be able to observe them in the mornings, which is probably when they are actually flowing. Perhaps we might even get the spectral signature of water in that case. But sadly it's not possible. MRO is in a slowly precessing sun synchronous orbit inclined at 93 degrees (orbital period 1 hr 52 minutes). Each time it crosses the Mars equator on the sunny side, South to North, the time is 3.00 pm, in the local solar time on the surface, and that is true all the year round. (See page 8 of Mars Reconnaissance Orbiter Communications).

What they did was to focus it on the streaks at times of the year when the streaks were very broad. At those times - it saw hydrated salts. At other times of the year it didn't see them.

Why its convincing even though they didn't spot flowing water

They did not actually see the water itself. But the only models for these RSLs involve liquid water flowing down the streaks. It is hard to see how else the streaks could grow in spring, and then fade away in autumn. The RSLs form at temperatures from -23 to 0 °C - dry ice evaporates at -78.5 °. So there is no way it can be anything to do with dry ice. Also there's no association with winds or dust storms.

So, we are in this situation where the only models that make much sense involve liquid water flowing. Flowing water would hydrate the salts. And we spot hydrated salts. It seems very convincing evidence, even though they haven't spotted flowing water directly.

How did they form?

The hydrated perchlorate salts don't need to be hydrated by flowing water. On Mars they can also take up water from the atmosphere, the process of deliquescence. Like this, another of the slides in the presentation:

This is one of the three main hypotheses for the formation of the streaks. However they were careful to say, they haven't proved that the perchlorates cause the streaks.

So, the perchlorates could be already hydrated, picked up from the soil by the flowing water as it flows down the slope. From the shape of the streaks, and the way they spread, they seem to come from a source at the top of the slopes. That source could be liquid from deliquescing salts. However, there are two other possibilities. The three main hypotheses are:

  • The deliquescing salts. It's a bit of a challenge to get huge quantities of water forming in this way enough to feed the slopes, but it seems to be possible.
  • Ancient reservoirs at the top of the streaks - this is hard to explain though as some of the streaks start from near the peaks of mountains, and the reservoirs would soon be exhausted. Still, it hasn't been ruled out, the reservoirs could be filled when the Mars atmosphere was thicker, so more precipitation - and it's orbit is continually changing in eccentricity and its axis changing and there is evidence of flowing water in gullies as recently as 500,000 years ago.
  • They come from deep aquifers, perhaps even connecting to the subsurface hydrosphere kilometers below. To be liquid below the surface, you would have to have geothermal heating. Some of the models here have flows of water feeding those aquifers in turn from very deep down, the kilometers deep hydrosphere where Mars is warm enough, and pressures are high enough for liquid water - there may be a liquid layer on Mars at great depth (this depends on how much water is left from Early Mars - if a lot is left there should be an extensive deep hydrosphere, which could in turn feed aquifers closer to the surface, if not, then perhaps there is no water down there or very little).

So - they all have their good points, and all are also challenging. This discovery doesn't settle the question. The discovery of hydrated perchlorates makes the deliquescing salts more plausible, but it hasn't disproved any of the other ideas, not yet. Also another possibility is that different RSLs have different sources of water.

Indeed, another point is that though it would seem reasonable to suppose that the hydrated salts occur in the visibly dark patches, since they observed them only when the dark patches were very broad, this isn't proven yet. They can't actually prove that, since the resolution of CRISM isn't good enough to prove that the salts are in the dark streaks rather than the ground around them.

What we have so far is a correlation rather than an established causal connection. Surely there is some connection between the hydrated salts and the RSLs. Maybe one causes the other or both have a common cause.

The total picture isn't at all clear yet, with nothing conclusively proven about how the RSLs form. However when you put these new observations of hydrated salts together with everything else, it is a convincing case for flowing water.

Perhaps a hundred thousand metric tons of flowing water per year in the Valles Marineres

The amounts of water are quite large. Alfred McEwan, project leader for HiRISE, said at the end of the press conference that they made a rough estimate, for the amount of water in the streaks only in the Valles Marineres region (see 54:44 into this video).

They came up with an estimate of at least a hundred thousand metric tons of water flowing throughout the Valles Marineres region. They assumed only 5% water in the solution, and a thickness of only 10 mm which is around what you need for the material to flow at all.

That may sound a lot, and it is for Mars, but it's not much by Earth standards. Suppose you have a stream, flowing at one meter per second, a slowish walking speed. Make it a tiny stream, say average 25 cm deep and 2 meters wide - deep enough so you need to get your wellies on to cross it. That would make the cross sectional area around half a square meter, so even a small stream like that would have 100,000 cubic meters of water flow past in just over two days (200,000 seconds). As much water as flows through all the RSLs in the entire Valles Marineres region for a year will flow through your little stream in two days.

So it's not a lot by terrestrial standards. But for a microbe, or small colonies of microbes, it's a lot of water. So that leads to the question, could it be habitable?

Are the RSLs (Recurring Slope Lineae) habitable?

This of course is the big question on everyone's minds - could the RSLs be habitable?. The salts can be liquid right down to -40 °C or depending on the mixture, down to -60 °C or lower. The RSLs start to form when the surface temperature is between -23 °C to 0 °C. But that doesn't mean the water is at those temperatures. It could easily be the case that the surface heats up, and triggers release or deliquescence of waters deeper down, just centimeters below the surface, which flows at far lower temperatures. Mars has a very steep temperature gradient in the top couple of cms of soil. Even when it is 20 °C at midday on the surface, then a couple of centimeters down, you have reached the permafrost layer, and you'd hardly notice anything has happened. Or the water could be warm enough, but too salty for life.

You can read the paper in Nature if you follow up the link from the BBC story here, as Nature have an arrangement by which they make their papers available to anyone to read for major news stories like this, so long as you get to their site from one of the big name journalist news outlets.

In the conclusion they say

"These results strongly support the hypothesis that seasonal warm slopes are forming liquid water on contemporary Mars. The spectral identification of perchlorate in association with RSL, also suggests that the water is briny rather than pure. Terrestrially, in the hyper-arid core of the Atacama Desert, deliquescence of hygroscopic salts offers the only known refuge for active microbial communities and halophylic prokaryotes. If RSL are indeed formed as a result of deliquescence of perchlorate salts, they might provide transiently wet conditions near surface on Mars, although the water activity in perchlorate solutions may be too low to support known terrestrial life. The detection described here warrants further astrobiological characterization and exploration of these unique regions on Mars. This enhanced evidence for water flow also provides new clues as to the nature of the current Martian hydrological cycle"

So - these observations don't settle anything about the long standing question of whether the RSLs are habitable. If the water comes from deliquescing salts, it depends on the salt mixture and its temperature, and how salty it is. When they refer to "water activity" there, they mean, how salty it is. If very salty the water is not available even to microbes adapted to salty conditions - there is just too little water for them to be able to take it up even though the solution is liquid.

On the other hand, it could easily be habitable. We just don't know at this stage. Though the RSLs get most of the publicity at present, researchers have found some other seasonal features on Mars that behave rather like the RSLs, growing in the spring, then fading away in autumn. As we'll see in the next section, some of them may actually involve fresh water!

Southern hemisphere flow-like features - these may involve fresh water!

There are two types of these flow-like features. For a technical overview of them, see the Dune Dark Spots section in Nilton Renno's survey paper. These ones in the southern hemisphere which form in Richardson crater are particularly promising because all the current models involve liquid water in some form and what's more, in the models, these features start off as fresh water trapped under ice.

These often get confused with the rather similar looking Northern hemisphere flow-like features (see below) - which are far too cold to be habitable in the models,at least to Earth life, and may not involve water at all. The two Martian ice caps are rather different. The northern cap is low lying, mainly ice, with a thin layer of dry ice that disappears in summer. The flow like features in the northern hemisphere form at 12.5 degrees from the pole at surface temperatures of about -90°C, which is low enough for dry ice to be stable on the surface. Their models involve either extremely cold salty brines or dry ice and sand. These features are far too cold to be habitable to Earth life and may not even involve liquid water It's important not to confuse these two very different phenomena. They are easily confused because they are so similar in appearance, and because both are referred to as "flow like features".

The more interesting ones, for habitability, are in the south. The southern ice cap consists mainly of dry ice. It is colder, and higher up (at a higher altitude). It stretches as far as forty degrees from the pole in winter (so spanning over 4,700 km), but it reduces to just 300 km across in summer, Richardson's crater is 17.4 degrees from the south pole (that's over 1,000 km).

So though the features resemble each other in appearance, the conditions in which they form are very different and not directly comparable. The southern hemisphere features from at much higher surface temperatures than the northern hemisphere features, and they appear late in spring, after the rapid disappearance of a vast and thick layer of dry ice that covered the entire southern polar region, and beyond. In the summer then surface temperatures at Richardson crater can actually get above the melting point of ice at times in daytime, as measured by the Thermal Emission Spectrometer on Mars Global Surveyor. (See figure 3 of this paper)..

This map shows where the crater is. It is close to the south pole - this is an elevation map showing the location of Richardson crater in Google Mars, and I’ve trimmed it down to the southern hemisphere. You can see Olympus Mons as the obvious large mountain just right of middle, and Hellas Basin as the big depression middle left. Richardson crater is about half way between them and much further south.

Here is a close up - see all those ripples of sand dunes on the crater floor?

Link to this location on Google Mars

Well it’s not the ripples themselves that are of special interest, Mars is covered in many sand dune fields like that planet wide. What interests us are some tiny dark spots that form on them which you can see if you look really closely from orbit.

And, would you ever guess? Although it's one of the colder places on Mars, there's a possible habitat for life there in late spring? It is due to the "solid state greenhouse effect" which causes fresh water at 0°C to form below clear ice in Antarctica at a depth of up to a meter, even when surface conditions are bitterly cold.

The Warm Seasonal Flows often hit the news (probable salty brines on sun facing slopes). But for some reason, the flow-like features in Richardson crater are only ever mentioned in papers by researchers who specialize in the study of possible habitats for life on Mars. I first learnt about them in the survey of potential habitats on Mars by Nilton Renno, who is an expert in surface conditions on Mars (amongst other things, he now runs the Curiosity weather station on Mars). You can read his survey paper here, Water and Brines on Mars: Current Evidence and Implications for MSL. The models I want to summarize here are described in his section 3.1.2 Dune Dark Spots and Flow-like Features under the sub heading "South Polar Region". But it's in techy language so let's unpack it and explain what it means. I will also go back to the papers he cited, and some later papers on the topic.

In the case of Richardson's crater, both models involve liquid water in some form, and also potentially habitable liquid water. One of the two main models involves relatively thick layers of fresh water below optically clear water ice, up to tens of centimeters thick, and so is very promising for microhabitats. The other model involves microscopically thin layers of fresh water that join together to make a larger stream and pick up salts on the way out. That's very promising too. So let's now look at these two ideas in detail.

First, early in the year, you get dry ice geysers - which we can’t image directly, but see the dark patches that form as a result and are pretty sure this is what happens:

Geysers which erupt through thick sheets of dry ice on Mars. Clear dry ice acts as a solid version of the greenhouse effect, to warm layers at the bottom of the sheet. It is also insulating so helps keep the layers warm overnight. Dry ice of course at those pressures can't form a liquid, so it turns to a gas and then explosively erupts as a geyser. At least that's the generally accepted model to explain why dark spots suddenly form on the surface of sheets of dry ice near the poles in early spring on Mars.

So that would be cool enough, to be able to observe them, video them and study them close up. I hope the rover would be equipped with the capability to take real time video. These geysers are widely known and many scientists would tell you how great it would be to look at them up close, and see them actually erupt.

But most exciting is what happens later in the year, when it is getting too warm for the thick layers of dry ice needed for geysers. These layers of dry ice vanish rather quickly in spring. You would think that the dark spots that you get in the aftermath of the geysers would just sit there on the surface and gradually fade away ready to repeat the cycle next year. But no. Something very strange happens. Dark fingers being to form and creep down the surface as in this animation. Very quickly too (for Mars). I haven't been able to find a video for this, as the papers just use a sequence of stills, so I combined together some of the images myself into an animation to show the idea:

Flow-like features on Dunes in Richardson Crater, Mars. - detail. This flow moves approximately 39 meters in 26 days between the last two frames in the sequence

I made this animated gif using HiView: the image viewer for the HiRISE database and the images

ESP_011640_1080 : 19 January 2009, 4:14 PM Mars local (sol 396)
ESP_011706_1080 : 24 January 2009, 4.22 pm Mars local (sol 401)
ESP_011772_1080 : (29 January 2009, 4:28 PM Mars local(sol 406)
ESP_011917_1080 : 10 February 2009, 4:21 PM Mars local (sol 418)
ESP_012273_1080 : 09 March 2009, 4:10 PM Mars local (sol 444).

You can use the viewer to explore the terrain yourself - this is just one detail. The dunescape here is covered in numerous dark spots like this, most of which have flow like features that extend during the spring in this way. For more details see the wikimedia commons description.

BTW I found it hard to align these images exactly. I cut them out from the raw data, and aligned them by eye - unlike the RSL's there aren’t any widely shared images of them, and the figures in published papers weren't really suitable for this sort of thing. I’ve done my best to register them with each other but I couldn’t figure out a way to do it automatically.The problem seemed to be that the images are centered on different spots on the terrain. This means that there is no correct registration that puts each frame entirely in sync with the next one. So that’s why you may see some alignment shifts from one image to the next. It’s the best I can do - perhaps there is some way of remapping them to get a more exact alignment. However this was enough to give a general idea of what they are like.

All the likely models for these features, to date, involve some form of water. Alternatives that one might try to use to model them might include a second ejection of material by the dry ice geyser, or dust deposition, but researchers think these are unlikely to produce the observed effects.

So, these southern hemisphere flow like features seem very promising. That’s not as surprising as you might think. The same thing happens in Antarctica - if you have clear ice, then you get a layer of pure water half a meter below the ice. The thing is any water on Mars exposed to the surface would evaporate quickly, so quickly that there would be none left. If ice melts there, it turns directly to water vapour because the atmosphere is a laboratory vacuum, it’s so thin. But - water beneath a layer of transparent ice - that’s a different matter. The water is trapped by the ice so stays liquid. And what’s more, if they model it assuming clear ice like the ice in Antarctica they find that the ice there gets enough heat from the sun in the day to keep it liquid through the night to the next day so the layer can actually grow from one day to the next (ice is an excellent insulator). Also the Mars atmosphere is so thin that it doesn't matter at all that the air above the ice is very cold in these regions. The atmosphere is a near vacuum and works as a great insulator. Better in some ways than Antarctica.

Möhlmann's model is pretty clear (abstract here). If Mars has transparent ice like the ice in Antarctica, then it should have layers of liquid fresh water 5 - 10 cm below the surface and a couple of cm in vertical thickness in late spring to summer in this region. His model doesn't involve salt at all, so the water would be fresh water. The only question here is whether clear ice forms on Mars in Mars conditions and whether the ice is sufficiently insulating. We can’t tell that really from models, the only way is to go there and find out for ourselves.

Blue wall of an Iceberg on Jökulsárlón, Iceland. On the Earth, Blue ice like this forms as a result of air bubbles squeezed out of glacier ice. This has the right optical and thermal properties to act as a solid state greenhouse, trapping a layer of liquid water that forms 0.1 to 1 meters below the surface. In Möhlmann's model, if ice with similar optical and thermal properties forms on Mars, it could form a layer of liquid water centimeters to decimeters thick, which would form 5 - 10 cm below the surface.

In his model, first the ice forms a translucent layer - then as summer approaches, the solid state greenhouse effect raises the temperature of a layer below the surface to 0°C, so melting it. This is a process familiar on the Earth for instance in Antarctica. On Earth, in similar conditions, the surface ice remains frozen, but a layer of liquid water forms from 0.1 to 1 meters below the surface. It forms preferentially in "blue ice".

On Mars, in his model, the melting layer is 5 to 10 cm below the surface. The liquid water layer starts off millimeters thick in their model, and can develop to be centimeters thick as the season progresses. The effect of the warming is cumulative over successive sols. Once formed, the liquid layer can persist overnight. Subsurface liquid water layers like this can form with surface temperatures as low as -56°C.

Creates potential for flowing fresh liquid water on Mars!

That's for fresh water. The liquid layer below the surface is warmed by the solid state greenhouse effect to 0°C even when the surface temperature is as low as -56°C. The same thing happens in Antarctica, that you get fresh liquid water forming below the surface when the surface temperatures are far too low for liquid water. It's because ice traps heat in much the same way that the CO2 on our atmosphere does, and then the ice and snow is also is very insulating (the reason the Inuit build igloos), so keeps the heat in. That's why the layer forms up to a meter below the surface in Antarctica and why it would form 5 to 10 cm below the surface on Mars, so that the solid state greenhouse effect can warm the subsurface to a much higher temperature than the surface and so that there is enough ice to insulate it to keep it warm.

Inuit village, Ecoengineering, near Frobisher Bay on Baffin Island in the mid-19th century - ice and snow are very insulating.

In the model, then the ice below the surface is first warmed up in the daytime sunshine, due to a greenhouse effect, the infrared radiation is trapped in the ice in much the same way that carbon dioxide traps heat to keep Earth warm. Then because the ice is so insulating, the heat is retained overnight, and the water remains liquid to the next day. To start with it would be only millimeters thick but over several days, gets to thicknesses of centimeters.

This should happen on Mars so long as it has ice with similar properties to Antarctic clear ice.

If there is a layer of gravel or stone at just the right depth, the rock absorbs the infrared heat and that can speed up the process. In that case, a liquid layer can form within a single sol, and can evolve over several sols to be as much as several tens of centimeters in thickness. That is a huge amount of liquid water for the Mars surface.

In their model it starts as fresh water, insulated from the surface conditions by the overlaying ice layers. This fresh water of course can't flow across the surface of Mars in the near vacuum conditions, as it would either freeze back to ice, or evaporate into the atmosphere. But the idea is that as it spreads out, it then mixes with any salts also brought up by the geyser to produce salty brines which would then remain liquid at the much lower temperatures on the surface and flow beyond the edges to form the extending dark edges of the flow-like features.

Later in the year, pressure can build up and cause formation of mini water geysers which may possibly explain the "white collars" that form around the flow-like features towards the end of the season - in their model this is the result of liquid water erupting in mini water geysers and then freezing as white pure water ice

This provides:

  • A way for fresh water to be present on Mars at 0 °C, and to stay liquid under pressure, insulated from the surface conditions.
  • 5 to 10 cm below the surface, trapped by the ice above it
  • Depending on conditions, the liquid layer is at least centimeters in thickness, and could be tens of centimeters in thickness.
  • Initially of fresh water, at around 0°C.

If salt grains are present in the ice, then this gives conditions for brines to form, which would increase the melt volume and the duration of the melting. The brines then flow down the slope and extend the dark patch formed by the debris from the Geyser, so creating the extensions of the flow-like features.

They mention a couple of caveats for their model, because the surface conditions on Mars at these locations is unknown. First it requires conditions for bare and optically transparent ice fields on Mars translucent to depths of several centimeters, and it's an open question whether this can happen, but there is nothing to rule it out either. Then, the other open question is whether their assumption of low thermal conductivity of the ice, preventing escape of the heat to the surface, is valid on Mars.

The process works with blue ice on Earth - but we can't say yet what forms the ice actually takes in these Martian conditions. The authors don't go into any detail about this, but ordinary ice can take different forms even in near vacuum conditions. As an example of this, the ice at the poles of the Moon could be "fluffy ice"

"We do not know the physical characteristics of this ice—solid, dense ice, or “fairy castle”—snow-like ice would have similar radar properties. [then they give evidence that suggests fluffy ice is a possibility there] "
(page 13 of Evidence for water ice on the moon: Results for anomalous polar)

That's the main unknown in their model, whether the ice is blue ice like Antarctic ice, or takes some other form. The ice should at least be in the same hexagonal structure crystalline phase as ice is on Earth - Mars is close to the triple point in this ice phase diagram

Phase diagram by Cmglee, wikipedia. Ice outside of Earth can be in many different phases. For instance in the outer solar system it is often so cold that it is in the very hard orthorhombic phase, where it behaves more like rock than what we think of as ice. However ice on Mars is likely to be in the Ih phase similar to Earth life. The Mars surface is close to the triple point of solid / liquid / vapour in this diagram.

So, the ice is likely to be of the same type as the blue ice in Antarctica. Not likely to have bubbles of air in it. But it could still take a different forms. The model shows that Mars should have layers of liquid water ten to twenty centimeters below the surface if there are any areas of clear blue ice as in Antarctica.

This solid state greenhouse effect process favours sun facing slopes (equator facing). Also, somewhat paradoxically, it favours higher latitudes, close to the poles, over lower latitudes, because it needs conditions where surface ice can form on Mars to thicknesses of tens of centimeters. (The examples at Richardson crater are at latitude -72°, longitude 179.4°, so only 18° from the south pole. There is no in situ data yet for these locations, of course, to test the hypothesis. Though some of the predictions for their model could be confirmed by satellite observations.

Interfacial liquid layers model

Another model for these southern hemisphere features involves ULI water (Undercooled Liquid Interfacial water) which forms as a thin layer over surfaces and can melt at well below the usual melting point of ice. In Möhlmann's sandwich model, then the interfacial water layer forms on the surfaces of solar heated grains in the ice, which then flows together down the slope. Calculations of downward flow of water shows that several litres a day of water could be supplied to the seepage flows in this way.

The idea then is that this ULI water would be the water source for liquid brines which then flow down the surface, mixing with dust, to form the features. That would still be interesting as you end up having flowing liquid water on Mars, several litres a day what’s more. Here is a paper from 2016 describing the idea.

See also Möhlmann's paper The three types of liquid water in the surface of present Mars

Those are the only two models so far. So it does seem very likely that there is liquid water here, and even with the interfacial liquid layers, the water starts off as fresh water beneath the ice, or possibly salty (in either model) if there are salt grains in the ice for the water to pick up. Either way the features start out as a flow of fresh water trapped beneath a layer of ice. This is one of the least publicized types of habitat on Mars, seldom mentioned outside the specialist literature. Yet in some ways it's one of the most interesting, if it exists, because of the potential for fresh water at 0 °C.

This liquid water is hard to observe because the features are so small, beyond the resolution of CRISM. However, analysis of the larger spots, at around the spring equinox, produced a signal that just possibly could be liquid water, where the ice is in contact with the dark material of the dune spots.

" In the gray ring area the water ice 631 surrounds darker surface, where liquid interfacial water layer or brine (Möhlmann 2004, 632 2009, 2010) may form. We found no firm evidence for the presence of liquid water in near-IR 633 spectra, although linear unmixing results show that the data are not inconsistent with a 634 possible slight contribution (a few %) of liquid water in the dark core unit." page 26 of this paper.

Could Mars have patches of fresh water in summer, throughout the polar regions, at a depth of six centimeters below optically pure ice?

Möhlmann has also suggested that this could be a more widespread phenomenon in the Mars ice caps, as for Antarctica. Liquid water could form at a depth of around 6.3 cm wherever there is optically clear ice on Mars in snow / ice packs, just as it does in Antarctica. In summer, it could form layers from centimeters to tens of centimeters in thickness.

Results of Mohmann's modeling of the solid state greenhouse effect in clear ice on Mars. The plateaus show temperatures that get above the melting point of water regularly every Martian sol, at depths of about 6.3 cms. L here is 11.4 cm. Ice at this level will melt periodically, and especially in summer can stay liquid overnight, leading to subsurface liquid water in layers of from cms to tens of cms in thickness. This should happen on Mars not just in the flow-like Features of Richardson crater, but also, anywhere where there is optically clear ice.

In another paper he writes "This liquid water can form in sufficient amounts to be relevant for macroscopic physical (rheology, erosion), for chemical, and eventually also for biological processes. "

His models seem clear enough. The air temperature hardly matters, because the Mars air is so thing it's a near vacuum, insulating the ice, like a thermos flask. The only unknown here is whether Mars does have optically clear ice like this, which is common on Earth in cold conditions like this in Antarctica.

Dark streaks in Russell Crater, 55 degrees South

These streaks form in the southern hemisphere, but in a different crater, Russell crater, 55 degrees from the south pole (compared to 17.4 degrees for Richardson crater). They are visually rather similar to the flow like features. Again one needs to be careful not to confuse them with the Richardson features as they seem to behave rather differently when studied in detail. These are also different features from the better known constant width linear grooves on Russell crater which formed some time between a century and a few thousand years ago (two suggestions are that they may have been formed originally by slurry flows involving liquid water, or they may be features cut out by blocks of dry ice rolling down the slope). They occur higher up the slope than the grooves.

This shows the location of the dark flows they studied for the paper relative to the better known grooves

Detail of HiRISE image PSP_002548_1225. Location of the the zoom in for the next image shown in blue.

Zoom in on previous detail of the HiRISE image PSP_002548_1225 which you can explore with HiView (drag and drop the J2 link into the viewer) - showing a late stage in formation of the dark flow features in Russell crater.

Notice how the features flow in between the wind formed ridges in the dunes, and also cross each other's tracks, and split and braid (unlike the Richardson crater features), all suggestive of avalanche type features. They also found evidence that these features extend episodically and very rapidly, at speeds of 2-4 meters per second. They could work out the speed, because they were able to pass up and over features in the terrain up to 1 to 2 meters in height, which is only possible for very rapidly moving flows. The evidence is reasonably conclusive that these are dust avalanche features. This image is a zoom in on the location of their figure 4d. For the complete sequence, see figure 4 in this paper.

These are braided, divide, recombine and cross each other's tracks. They flow down the slopes channeled by wind formed ridges in the dunes, and most distinctive of all, they are able to rush up over small features of up to two meters high and down the other side.

These seem to be dry features associated with defrosting and small dust avalanches as they are episodic, moving rapidly at speeds of 2-4 meters per second like an avalanche. The authors call them "dark flows". For details see this paper.

Northern hemisphere flow-like features - not likely to be habitable

Note that there are rather similar looking flow-like features in the Northern hemisphere, but these typically form at much colder temperatures for some reason, around -90°C - the two hemispheres on Mars have a very different climate. These are sometimes confused with the Richardson features which may be partly why those features get so little attention?

Flow-like features in the Northern polar dunes . These are thought to form at much lower temperatures. Some of the models for these also involve liquid water but there are other hypotheses as well, some of them involving dust and ice slipping down the cliff faces. This is another animation I made by hand cutting out the images from the raw data, and I was unable to do exact alignment throughout the image, due to the changing angles at which the photos were taken from orbit.

I created this animation myself by combining the following HiRise images from NASA/JPL/University of Arizona, all taken in 2008 - PSP_007468_2575 - 29 February (2.03 PM), sol 80, PSP_007758_2575 - 22 March (2:06 PM) , sol 101 and PSP_007903_2575 - 3rd April (2:07 PM), sol 113. For more details see the wikimedia commons description.

The northern hemisphere has shorter warmer winters (due to Mars’s eccentric orbit), and a lower elevation, but the flow-like features there form at times when the surface temperatures are lower than in Richardson crater. There are several different mechanisms for the northern hemisphere flow-like features, not all the models for those involve liquid water, and the ones that do involve very cold water. So the Richardson crater ones are the surest bet, seems to me, for a habitable flow-like feature.

Note that some authors have come to the conclusion that all these features may have more in common than they seem to at first sight, and may be formed in a similar way. This is a paper from 2012 which studies the Northern hemisphere features. They acknowledge the papers requiring liquid water in their models for the southern hemisphere features, but point to the similarities in appearance, and the sequence of events in all the cases, although the timing is different in detail.

They think that perhaps the features they studied in their work, the features in Russell crater, and the features in Richardson crater may all form in a similar way, in which case they might all be the result of avalanche like dry flows, which is a likely explanation for the northern hemisphere features, and is pretty much conclusively the explanation of the features in Russell crater. (To read the paper in its entirety you can use a google scholar search for "Observations of the northern seasonal polar cap on Mars III: CRISM/HiRISE observations of spring sublimation" to download the pdf from academia.edu, or use the google scholar button for chrome).

Sun warmed dust grains embedded in ice

This is another suggested habitat for life in the Mars higher latitudes based on processes that happen in the Antarctic ice. Dust grains in the ice often produce tiny melt ponds around them in the heat of the summer sunshine. The dust grains absorb the heat (preferentially over the ice), and so heat up and melt the surrounding ice. Then this heat gets trapped because of the insulating effect of the solid state greenhouse effect, because ice traps heat radiation, so forming tiny melt ponds of a few millimeters thickness or more. This could happen on Mars too, so is another possible habitat with fresh water.

It's just a few millimeters of fresh water, but that could be significant on Mars. Another example of this process, then meteorites in Antarctica are often found associated with gypsum and other evaporates - minerals that can only form in the presence of liquid water and must have formed after they fell in Antarctica. Sometimes the researchers find capillary water, or thin films of water, and sometimes they even find evidence of a rather large meltwater pond which formed around the meteorite, or find the meteorites in depressions filled with refrozen ice.

A similar process could be at work in the Martian icecaps too. This process could melt the ice for a few hours per day in the warmest days of summer, and melt a few mms of ice around each grain. Indeed, if I can venture a speculation of my own, perhaps just as in Antarctica, there could be larger melt ponds around meteorites embedded in the ice too - as Mars must have many meteorites embedded in the polar ice sheets.

This could explain another puzzle. Particles of gypsum (the same material that is used to make plaster of paris) have been detected, first in the Olympia Undae dune fields that circle the northern polar ice cap of Mars, See this paper for details. Later on, they were detected in all areas where hydrated minerals have been detected, including sedimentary veneers over the North polar cap, dune fields within the polar ice cap, and the entire Circumpolar Dune Field. There's strong evidence that the gypsum originates from the interior of the ice cap. See this paper for details. Gypsum is a soft mineral that must have been formed close to where it has been discovered (or it would get eroded away by the winds) and as an evaporite mineral, it needs liquid water to form. Opportunity later found veins of gypsum in the equatorial regions, in 2011, a clear sign of flowing water on ancient Mars. But these polar deposits are more of a mystery because they are found in the dust dunes on Mars, so must be produced locally, but where?.

Losiak, et al, modeled tiny micron scale dust grains of basalt (2-2 microns in diameter) exposed to full sunlight on the surface of the ice on the warmest days in summer, on the Northern polar ice cap. They found that these tiny dust grains were large enough to provide for five hours of melting which could melt six millimeters of ice below the grain. They say that with pressures close to the triple point, on windless days, you should get a significant amount of melting. They speculate that this might possibly explain the deposits of gypsum in the polar regions. Could it have formed in a similar way to the gypsum that sometimes forms around Antarctic meteorites?

Möhlmann did a similar calculation. This time he was looking at the possibility of liquid water forming inside snow on Mars. The snow would be exposed to the vacuum, but as the ice melted it would plug all the pores in the snow and eventually form a solid crust of ice on the snow, and so protect it from further evaporation. It would trap the heat as well and so encourage melting. This could happen anywhere between a few centimeters depth down to ten meters below the surface.

Could Mars life in the polar ice exploit thin films of ULI water in the ice, wrapped around individual microbes?

This is an interesting suggestion by Möhlmann in an article in Cryobiology magazine, that life may be able to make use of thin film monolayers of the " ULI water" (Undercooled Liquid Interfacial water) wrapped around a microbe, even in tiny nanometer scale layers of liquid water only two monolayers thick.

"In view of Mars it should be mentioned, that there is water ice in the permanent polar caps. At mid- and low-latitudes, ice can form, at least temporarily, via adsorption and freezing in the soil. There, the adsorbed and frozen water overtakes the role of ice, as described above. So, ULI-water can be expected to, at least temporarily, exist also in martian mid- and low-latitudinal subsurface soil. A similar environment can be expected to exist in isolation heated parts of icy bodies in the asteroidal belt, and analogously in the internally heated icy moons of Jupiter and Saturn. It is thus a current and challenging question if ULI-water can act as supporting life in environments with temperatures clearly below 0 C by delivering that water, which is necessary for metabolic processes, and by permitting transport processes of nutrients and waste. It is the aim of this paper to demonstrate the potential importance of ULI water in view of the possible biological relevance of nanometric undercooled liquid interfacial water."

He cites research suggesting life can remain active in the presence of just two monolayers of water wrapped around a microbe.If there is just a small thermal gradient in the ice, of one degree C pre meter, then enough liquid water will form to fill a micrometer sized microbe once a month. Enough will form to fill it once a day if there is a locally steeper gradient of one degree C per 10 cm. This can lead to a constant transport of fresh water to bring fresh nutrients to the microbe, and to remove wastes. The main question is whether this is a sufficient flow of water to sustain life. For more details of this intriguing idea, see his article.

Other surface and near subsurface habitats for life on Mars

Nilton Renno's "swimming pool for a bacteria" on salt / ice interfaces, the RSLs, the Richardson flow-like features and these potential microhabitats within the polar ice, covered so far are just a few of many possible microhabitats suggested on the Mars surface. I cover some other possibilities below, see

However, let's take a break now, and try to speculate about what forms of life could live in these microhabitats, if they exist. Also are any of them accessible to Earth life?

Plausible microbial metabolisms for present day Mars - what else apart from photosynthesis?

One way to examine the possibility for life on Mars is to look at the Redox pathways that the life could use as a source of energy. This involves a pairing of an electron donor and an electron acceptor. For details see Electron transport chain, and Microbial metabolism.

Here is a table of some of the available donors and acceptors in Mars conditions, table from this paper: Plausible Microbial Metabolisms for Mars (added CO2, also added available to NO3- and removed "not shown to support life" from ClO4- because we now know that there are microbes that metabolize perchlorates).

electron donors, any of: electron acceptors, any of:
FeSO2+: available in Fe-rich silicates Fe3+: available in numerous alteration


S : suggested at Gusev Crater SO42- available in salts
H2: available in subsurface? O2: partial pressure too low
CO: available in atmosphere NO3-: available
H2O available for oxygen photosynthesis ClO4-: available and abundant
organics: meteoritic likely to be present at surface CO2: in the atmosphere
organics: endogenous available in subsurface CO2: in the atmosphere

A candidate metabolism would use one of the electron donors in the first column paired with one of the electron acceptors on the right as a source of energy. (The final dash on left hand side is there just because the list of electron donors is shorter than the list of electron acceptors).

See also the presentations in: Redox Potentials for Martian Life. Cockell gives a long list of examples of potential redox couples on page 190, table I of his Trajectories of Martian Habitability.

Note, you might think that there can't possibly be any photosynthesis on Mars right now, because there isn't much oxygen in the Mars atmosphere, only 0.13%. If Mars was an exoplanet, we'd look at it from a distance, analyse its atmospheric composition by looking at the spectrum of a single pixel, and might well say "No oxygen, so probably no photosynthesis". However in conditions on Mars similar to the Antarctic dry valleys and if that corresponds to a few localized optimal places for life on Mars, it wouldn't produce much oxygen, in a tiny seasonal and long term signal easily masked by 0.13% of the atmosphere. See How much oxygen would surface photosynthetic life produce on Mars? (below) . Also we have forms of photosynthetic life on Earth that don't produce oxygen, such as the haloarchaea, so the same could be true on Mars.

Earth life able to contaminate Mars habitats

None of this would matter if there was something that made Mars so different from Earth that no Earth life could survive there. For instance, temperatures on Titan are well below the temperatures for Earth life and the only water is thought to be in the form of solid rock, while the fluid is ethane or methane. There are no issues contaminating Titan unless it has cryovolcanism - volcanoes with liquid water as lava.

And it's true, no humans, animals, birds, or insects could survive on Mars, and most plants couldn't either. But some lichens and microbes from Earth have been shown to be able to survive in Mars simulation chambers and other experiments to simulate Mars-like conditions on Earth. So these could potentially survive there, contaminate them and make it difficult or impossible to study them to find out what was there originally.

This all depends on whether or not the habitats exist, which is not known yet. They could all be too cold or too salty for Earth life. But if they do exist, here is a list of lifeforms that might potentially be able to "emigrate" from Earth to Mars. For the cites for this section, see my Candidate lifeforms for Mars in my Places on Mars to Look for Microbes, Lichens, ...:

  • Chroococcidiopsis - UV and radioresistant, and can form a single species ecosystem. It needs no other forms of life, and only requires CO2, sunlight and trace elements to survive.
  • Halobacteria - UV and radioresistant, photosynthetic (using hydrogen directly - proton pumps, doesn't generate oxygen or sulfur), can form single species ecosystems, and highly salt tolerant. Some are tolerant of perchlorates and even use them as an energy source, examples include Haloferax mediterranei, Haloferax denitrificans, Haloferax gibbonsii, Haloarcula marismortui, and Haloarcula vallismortis
  • Some species of Carnobacterium extracted from permafrost layers on Earth which are able to grow in Mars simulation chambers in conditions of low atmospheric pressure, low temperature and CO2 dominated atmosphere as for Mars.
  • Geobacter metallireducens - it uses Fe(III) as the sole electron acceptor, and can use organic compounds, molecular hydrogen, or elemental sulfur as the electron donor.
  • Alkalilimnicola ehrlichii MLHE-1 (Euryarchaeota) - able to use CO in Mars simulation conditions, in salty brine with low water potentials (−19 MPa), in temperature within range for the RSL, oxygen free with nitrate, and unaffected by magnesium perchlorate and low atmospheric pressure (10 mbar). Another candidate, Halorubrum str. BV (Proteobacteria) could use the CO with a water potential of −39.6 MPa
  • black molds The microcolonial fungi, Cryomyces antarcticus (an extremophile fungi, one of several from Antarctic dry deserts) and Knufia perforans, adapted and recovered metabolic activity during exposure to a simulated Mars environment for 7 days using only night time humidity of the air; no chemical signs of stress.
  • black yeast Exophiala jeanselmei, also adapted and recovered metabolic activity during exposure to a simulated Mars environment for 7 days using only night time humidity of the air; no chemical signs of stress.
  • Methanogens such as Methanosarcina barkeri - only require CO2, hydrogen and trace elements. The hydrogen could come from geothermal sources, volcanic action or action of water on basalt.
  • Lichens such as Xanthoria elegans, Pleopsidium chlorophanum, and Circinaria gyrosa - some of these are able to metabolize and photosynthesize slowly in Mars simulation chambers using just the night time humidity, and have been shown to be able to survive Mars surface conditions such as the UV in Mars simulation experiments.

Most of these candidates, apart from the lichens, are single cell microbes (or microbial films). The closest Mars analogue habitats on Earth such as the hyper arid core of the Atacama desert are inhabited by microbes, with no multicellular life. So even if multicellular life evolved on Mars, it seems that most life on Mars is likely to be microbial, especially on the surface

For more about the value of Mars for biology and implications of sending humans there, see

For more about the flow-like features habitat, and many other possible habitats on Mars, the lifeforms that could live there, Mars analogue habitats and other topics, see my:

Places on Mars to look for Microbes, Lichens, ... Salty Seeps, Melt Water Under Clear Polar Ice, Ice Fumaroles, Dune Bioreactors, ...: Where early Mars lifeforms could survive to the present day,

It’s also available to read online for free at Places on Mars to Look for Microbes, Lichens, ... and the section on the Richardson flow-like features is here: Flow-like features (notice I put the Richardson flow-like features on the cover - for me, this is the most exciting feature of all on Mars for exobiology)

I also recently got the material there accepted on wikipedia (they rejected it when I first submitted it in 2011, saying that, though well written, it was more suitable for an essay or a journal article than wikipedia. However, they accepted it when I tried again earlier this year. I added it as two separate articles this time, as it was a bit long for a single article there. See Modern Mars habitability and Present day Mars habitability analogue environments on Earth

Vulnerable to photosynthetic life

In the section above: Case study - can photosynthetic life be transferred from Earth to Mars or vice versa? I looked at Charles Cockell's study where he showed that it's quite possible that photosynthetic life has never got to Mars via meteorites.

Many of the Earth lifeforms that could survive on Mars are photosynthetic Chroococcidiopsis , Halobacteria the black molds such as Cryomyces antarcticus and Knufia perforans, and lichens such as Xanthoria elegans, Pleopsidium chlorophanum, and Circinaria gyrosa. So what would those do to native Mars life if they got there somehow as a result of planetary contamination, and Mars never had any photosynthetic life before? Chroococcidiopsis particularly is a polyextremophile, seems pre-adapted to survive in the Mars conditions, is even able to survive in Mars simulation chambers without any liquid water, taking up the humidity from the atmosphere, has efficient protection from UV radiation, can repair damage from desiccation and ionizing radiation rapidly within hours, and is widespread in many different habitats on Earth.

Analogy of ET microbes on Mars with microbes from a planet orbiting another star

Imagine if you could learn about life on a planet or in the ocean of an icy moon around another star? Not just investigate the planet as a faint single pixel speck of light. But actually send a rover there to study it in situ?

Even if all you found was extraterrestrial microbes or lichens, imagine how exciting that discovery would be? Well Mars, Europa and Enceladus may be like exoplanets and exomoons in our own solar system, they may be as interesting as that. They could easily be as interesting to biology as an exoplanet or exomoon is to geology. When, or if, astrobiologists discover the first biology from another planet, this will be just as revolutionary, indeed more so, as the first ever discovery of an exoplanet. We could easily find something that overturns understanding of biology in a similar way to the way the discovery of hot Jupiters overturned our understanding of the formation of planets. Indeed, it would be in some ways as revolutionary as the first discovery that the planets in the night sky are other worlds like our Earth. We don't know what we will find out until we study other forms of biology close up.

Microbes on Mars, in the more interesting case, would be so different from us, they'd be more like a microbial version of ET than like a tiger. See Will We Meet ET Microbes On Mars? Why We Should Care Deeply About Them - Like Tigers

What we could learn - some examples - Early life or proto life - Life originated from Mars - Distantly related life - life not based on DNA style double helixes - life with capabilities Earth life hasn't developed yet

What sort of things might we hope to discover on Mars? This is another "synthesis" section. What might we find on Mars? Surprisingly, I can find hardly anything by way of detailed speculation about this - as usual do say if you know of a good paper to cite here. So I've based this section on various ideas for early life and alternative forms of biochemistry, and especially on any ideas that seem particularly relevant to Mars. I've also added a couple of speculative ideas of my own, clearly labeled (the idea of life based on replicating sheets instead of replicating strands is one of them).

Here are a few ideas.

  • Early life or protolife, to fill in the huge gap between the organics and cell-like structures that turn up in laboratory experiments, and the immense complexity of modern life. We might find the much simpler RNA world cell with no proteins, or ribosomes, or DNA. Instead of the huge ribosome it could use RNA sliced into pieces and recombined to make a ribozyme, that tinier distant cousin of the ribosome. For more on this see RNA world and the shadow biosphere (above). Or, we might find the so called autopoetic cells that replicate just by producing daughter cells with a similar mix of chemicals when they get too large. They might have no genetic code yet to regulate the process, or they might be able to regulate it, but only achieve very approximate, imperfect reproduction, leading to questions about whether they count as life or not. For many other ideas of what we could find, see What else could have come before modern life? Alphabet soup of "XNA", Ostwald ripening organic crystals, "naked genes", or almost alive "autopoetic" cells (above)
  • Evidence that life originated on Mars. DNA and RNA are both particularly fragile, and DNA especially is rather hard to form naturally. It needs the environment of the cell or special conditions to keep it stable. RNA is not quite so fragile and is more stable when it is very cold. Also, the ribose in its backbone is stabilized by the presence of special chemicals, borates. As it happens, these are found on Mars in useful quantities.

    The white crystals here are Ulexite, (the yellow ones are calcite). Ulexite is also known as "TV rock" because it is a natural optical fibre.

    This is one of the minerals that include borates - compounds involving complex chains of boron and oxygen atoms terminated by hydrogen atoms. These minerals can help stabilize ribose in the backbone of RNA, and so may have helped with early stages of evolution of life. Boron is very rare in extraterrestrial materials, and quite rare on Earth, and dissolves in hot water. However it is found in in the Martian meteorite MIZ 09030 in similar concentrations to Earth clays, and Curiosity found it in Gale Crater.
    Steve Benner presented an interesting theory in 2013. He thinks that the borates prevented early organics from turning into tars, instead forming carbohydrates. He thinks that molybdenum in the form of molybdate played a role too, as it helps catalyse the formation of ribose, the "R" in "RNA". Both of these are common on Earth now, but being soluble in water, would have dissolved in our early oceans. Early Earth may have been an almost entirely ocean world, with far less dry land than today. Meanwhile Mars being drier could have created better conditions for this process and so might be favoured for the origins of life.

    If that's right, then maybe we can get evidence of this process of evolution of RNA based life on Mars with borates and molybdate at at some point, as we trace the evolution there back and back. For more on this idea, with many cites to the literature, see also Romulus Scorei's Is Boron a Prebiotic Element? A Mini-review of the Essentiality of Boron for the Appearance of Life on Earth

    For other approaches suggesting that life originated on Mars:
    • Mars, Panspermia and the origin of Life: Where did it all being? by Joseph. Kirschvink and Benjamin Weiss. They argue that though both Earth and Mars had strongly reducing conditions in the early solar system, only Mars had strongly oxidizing conditions early on. Indeed oxygen arose on Earth at around the time that it disappeared on Mars (see their figure 2). Early Mars also probably had a more reducing crust than Earth. This gave early Mars more opportunity for niches where life could exploit large chemical gradients between oxidizing and reducing conditions. Also, the early Mars atmosphere would have had ozone to shelter life from UV, which was not present on early Earth.

      Finally, they argue from the inferred biochemistry of our last common ancestor suggests it arose in conditions with large redox chemical gradients, already able to exploit oxygen, nitrate, sulfate and sulfur. They say that later massive horizontal gene transfer can be ruled out, at least for oxygen, because most of the genes responsible are found in rRNA (ribosomal RNA) and the sequences in rRNA tend to be conserved within species and not transferred horizontally so much (see page 9). So, it looks as if our last common ancestor of all modern Earth life may have arisen in an environment where it could exploit oxygen. If that's true, it seems to rather strongly favour Mars as the most likely place for it to evolve.

      Figure 1 from their paper. Here, the black dashed lines in the bottom figure show various possibilities for oxygen levels for Earth's atmosphere, and the red dashed line shows a possible scenario for Mars. The top figure shows the origin of various microbes - see especially the origin of the cyanobacteria, not long before the first oxygen spike in the Earth's atmosphere. (The process of oxygenation of Earth's atmosphere may have begun as early as three billion years ago).

      Mars had more oxygen in its atmosphere early on, and so a greater redox potential. It was also a more likely place for an organism to evolve able to exploit oxygen, which our last common ancestor may have been able to do. They conclude:

      "At face value, all of these lines of evidence suggest that, compared to early Earth, early Mars might have had a greater supply of biologically useable energy and was perhaps, by implication, a better place for the origin of life. And so we salute you, all you descendants of space-traveling microbes from the Red Planet!"

    • Paul Davies, see Was Mars the cradle of life?,

      "Mars is the most Earth-like of our neighbouring planets and enjoyed a number of advantages during the early history of the solar system. Though a freeze-dried desert today, Mars was warm and wet before about 3.6 billion years ago . Being a smaller planet, it cooled more quickly, making it suitable for life sooner than Earth. Gene sequencing indicates that the oldest and deepest branches of the tree of life are occupied by hyperthermophilic archaea and bacteria , hinting that the earliest life forms dwelt deep beneath the oceans near volcanic vents, or even kilometres underground in the crust itself. "

      "The deep subsurface zone remains populated on Earth today, and probably offers the most promising location on Mars for finding any extant life. It would have become cool enough for hyperthermophilic microbial life on Mars perhaps as long ago as 4.5 billion years, when the Earth’s crust was still sizzling. Ensconced in this Hadean niche, shielded by a kilometre of two or rock, Martian life could have withstood the ferocious early bombardment that afflicted Mars just as it did Earth."

      He has an interesting calculation based on this. He assumes that Mars was favourable for the emergence of life before Earth and had a longer window of opportunity - as he suggests there. He also assumes that life was an unlikely event (if life starts very easily, then it would develop quickly on both Earth and Mars, so is likely to have made a fresh second genesis on Earth). Finally he assumes that the length of time needed for life to transfer from Mars to Earth in the early solar system via meteorite transfer after a large impact is a million years, and the window of opportunity during which both Mars and Earth had conditions ideal for life, so that transfer was easy, was a hundred million years.

      Based on that, he finds that Mars is hugely favoured over Earth as an origin for life. This is especially so if the origin of life required a succession of several different improbable steps to happen. It is also especially likely to originate on Mars if early life is fragile and goes extinct easily. The calculation is in Does Life’s Rapid Appearance Imply a Martian Origin?

      Techy note: He works out that pM/pE = 100.8nm(NE/PM)mn

      pM is the probability of life originating on Mars, pM/
      pE is the probability of it originating on Earth,
      n is the number of successive improbable steps needed for life to evolve,
      m is the number of genesis attempts before a robust form of life is evolved
      τM is the average length of time you'd expect it to take for each of those improbable steps of evolution necessary for life in early Mars surface conditions.
      τE is the average length of time you'd expect it to take for each of those improbable steps of evolution necessary for life in early Earth surface conditions.
      100.8 = 6.3 is his estimate for how much longer life had available to evolve on Mars than on Earth. For this, he assumes that life was present on Earth 100 million years after it became habitable, and that, by then, Mars had been habitable for around 600 million years.

      For those who want to read his calculation in detail, note, that for the deduction from equation (3) to (4) he is using a mathematical trick that some readers may not be familiar with. By the McLauren series expansion of the exponential function - you can express ex = 1 + x + x/2! + ... If x is very small (positive or negative, then this simplifies to 1 + x.

      He calculates that the chance of an interstellar origin is very small. However that part of his calculation doesn't take into account of the possibility of life around another star seeding another star in our sun's stellar birth cloud, see Distant cousins with last common ancestor from a planet around another star (above)
  • Unrelated or very distantly related life, perhaps based on some form of XNA (Xeno Nucleic Acid) instead of DNA. This would be one of the most amazing discoveries we could make. It would lift biology into a new dimension, by showing that life can exist based on completely different biochemical principles from DNA based life.

    It's a reasonable hypothesis, because DNA is very fragile indeed, and RNA is fragile also. Perhaps what that's telling us is that life started as one of the many alternatives to DNA? Some think we may have started with a PNA world for instance, as it is far more robust than RNA and forms more easily.

    Other ideas for a more robust early life include TNA world, or a molecule that's a hodgepodge mixing different backbones in the same molecule with non heritable variations in backbone structure (or a whole alphabet soup" of other possible precursors such as HNA, PNA, TNA or GNA - Hextose, Peptide, Therose or Glycol NA). XNA based life can also be used to create enzymes - one of the necessary requirements for life as we know it.

    Perhaps we find one of these forms of life on Mars and assume that it is unrelated life at first - but later we find it is an ancient predecessor to Earth life that still remains on Mars barely changed since those early times. Or perhaps it has no connection to Earth at all and we discover that life originated separately on Mars. Or perhaps Earth life originated as an early form of life on Mars, yes, but it hasn't just stayed "as is" but has evolved and elaborated hugely in some different direction on Mars, into a different modern form of life, just as complex as our DNA and mRNA based life, but organized on different principles altogether. Perhaps life on Earth and Mars both began as PNA, for instance, with a common ancestor but on Mars it evolved in a different direction, for instance a more elaborate form of PNA based life, or to something else. Maybe we are such distant cousins that the Mars life is not DNA or RNA based at all.
  • Life with mirror helixes, triple helixes quadruple helixes, or side by side molecules that don't wind round each other. The first idea here is life based on an exact mirror of DNA. To get that to work, all the molecules in the cells would be reflected as if the whole thing was just reflected in a mirror. As far as we know aa mirror image cell like that would function perfectly well in its mirror image world. That's mirror image life, a trope in countless science fiction stories - but it is one of the more sensible ones, as the idea is perfectly feasible for extra terrestrial life, as far as we know.

    The idea that we could flip someone or even a single cell into their mirror image is pure science fiction at present, and way beyond anything we know about. But idea of life that evolves independently using mirror image chemistry seems to be eminently sensible and perfectly possible as far as we know.

    However there is also a form of ordinary DNA here on Earth that spirals in the opposite direction in a rather zig-zaggy way. It's called "Z DNA. It is the right most of these three models.

    A, B and Z DNA - image by Richard Wheeler . The one in the middle, B type is "normal DNA". The A type, on the left, is wider, with a shallower pitch on the outside, and occurs in dehydrated samples but may also occur in cells in pairings of RNA with DNA. The Z DNA, on the right, spirals in the opposite direction from ordinary DNA in a rather zig-zaggy way - it does this when it is methylated (has the methyl group CH3 added) and can be stabilized in this form by using special Z-DNA-binding proteins and may play a role in transcription.
    Could life be based on Z DNA?

    Then what about a helical structure with three strands instead of two? Well, Earth life does that as well in some situations

    Triple helix - which forms in certain circumstances with Earth microbes and is very stable. The third strand rests in the large gap of the 2 strand DNA. It may be part of a larger molecule which is entangled with the DNA as shown - the dashed lines are meant to show connections to the rest of the molecule.
    Before the double helix structure of DNA was discovered, triple DNA was one of the hypotheses they were looking for. They found it was difficult to get it to fit the theory and observations. But now we do have an actual form of triple DNA that we know is stable. Could ET life use a triple helix for some reason?

    Or could ET life even use a quadruple helix? Earth life uses those sometimes, it sometimes, forms four strand helixes, a recent discovery!

    Cross section of a DNA quadruplex which can create four strand DNA. Forms very slowly but once formed is reasonably stable.
    If life on Earth can use DNA in those different forms in special situations - is it possible that extra terrestrial life could use something like one of these as its "normal" form of DNA?

    Or what about Rodley's "Side by side DNA"? This is just a hypothesis. There is no evidence at all that Earth life ever uses it, but if it doesn't exist here, perhaps ET life could use it? It spirals one way for a while, then changes direction and spirals the other way then back again in such a way that the two strands aren't actually twisted around each other. This could help them to separate more easily, which may perhaps be an advantage for replication.

  • Life that is based on completely novel principles, for instance replicating sheets instead of replicating strands. Are we being blinkered in some way because all we know of is life with double helix DNA and single helix RNA (along with a few variants like the triple helix DNA mentioned above)?

    Suppose it is much more radically different than that? To help stimulate a few ideas here, suppose for instance that ET life uses a sheet-like two dimensional structure, planar rather than linear, and replication happens by a second layer forming on top of the original sheet? This is one of my own "fun speculations" to stimulate the imagination and get you thinking a bit "out of the box" perhaps.

    This is related to Graham Cairns-Smith's Clay Hypothesis for the origins of life. He outlines it in his controversial 1985 book "Seven Clues to the Origin of Life". Clays consist of alternating layers of alumina and silicate. His example is the common clay montmorillonite Like all clays it is made up of alternating sheets of alumina and silica. The alumina sheets consist of edge to edge connected octahedra and the silica layers fill in the gaps with tetrahedra.

    It's formula is (Na, Ca)0.33 (Al, Mg)2 (Si4O10) (OH)2·nH2O. There the comma in (Na,Ca) means sodium or calcium can substitute for each other - they are not shown in this diagram but are attached to the outside layers of oxygen - the layers have a net negative charge which is balanced by the positively charged sodium or calcium ions attached to them. Similarly the (Al, Mg) shows that either Aluminium, or Magnesium can substitute for each other in the Alumina layer. Other atoms not shown in this formula can also substitute in the Alumina layer, such as iron and phosphorus.

    The layers are separated by water molecules and when you add a lot of water they can slip relative to each other which is what makes clay slippy.

    Anyway the idea is that defects, variations in the choices of ions, and irregularities in one layer somehow propagate through from the layers below to the weakly bonded layers on top of them. Some might be better at replicating than others, breed more true by this stacking method - and the ions in the layers could speed up catalysis of various reactions including polymerization of RNA.

    The main problem with this model is that nobody has ever got anything like this to work in the laboratory - breeding true from one layer to another in a consistent way and doing something useful as a result. So, you just have to wave your hands and say "perhaps after millions of years of evolution, Nature will find a way". So though it does seem very promising as an idea, with a lot about it that is in common with the way that DNA works, it is hard to test it as we have nothing that works like this at present. Sadly, we can't leave oceans to evolve for millions of years to see what happens.

    Graham Cairns-Smith's idea is that it would be a very early stage of life, with no cell walls, just these replicating sheets of clay, which would transition to RNA and DNA chemistry and eventually lead to us. As just said, we don't have good evidence for it at all.

    But as a purely speculative idea, for fun, just to stimulate ideas, what if we run with it? What if ET life similarly started off as essentially two dimensional sheets of clay just a few atoms thick, separated by water only loosely bound together and they replicated in such a way that later forming layers duplicated the structure of the layer below? And then suppose they went a step further than Graham Cairns-Smith suggested, and incorporated those sheets inside their cells and used them to replicate? Eventually, could they perhaps build up a whole complex biochemistry, as complex as our helix based life, but based on using 2D sheets for replication instead of linear or helical strands? As with the "side by side DNA" it would have the advantage that the 2D biopolymer doesn't have to uncoil to replicate - and what's more, it could hold a high density of information in a small piece of material.

    Techy aside for sci. fi. geeks, this is not quite the same as Greg Egan's fun but very speculative science fiction story "Wang's Carpets" based on the mathematical idea of "wang tiles" - in his short story the lifeforms themselves are entirely two dimensional, evolving patterns of tiles within larger two dimensional sheets, exploiting the complexity of the patterns you can make with a few simple shapes. These can form in a "Wang carpets", with patterns so complex to define that questions about them can be mathematically undecidable. Here though, it's just a 2D sheet of "instructions" standing in for the one dimensional sheet of instructions we know as a strand of DNA, within a larger 3D microbe otherwise similar to Earth life.

    Or could the genetic instructions even be preserved somehow in a 3D informational polymer? Is there any approach that avoids the need to uncoil to read it? We can do this mechanically through laser scanning, in prototypes for future memory devices, so the idea is not so far fetched as to be totally impossible.

    This is just fun speculation at present. But suppose that you are an ET biologist and your life uses 2D sheets to replicate or even, 3D informational polymers scanned in some way without altering its structure. Would you not find the idea of a helical structure that has to uncoil and unzip to replicate implausible and unlikely too?

    Could anything like that be possible? What do you think?
  • Life that has evolved further than Earth life. Mars has had such very harsh conditions in the early solar system, alternating between a frozen "snowball Earth" type ice phase and more habitable phases. It's also been subject to strong ionizing radiation, extremes of cold, and near vacuum atmosphere. Some think that we have multicellular life on Earth as a result of a snowball Earth phase. If that's true, you could make a case for Mars life to be more highly evolved than Earth life - more complex, with more robust cells, extra non redundant nucleotides, additional amino acids, anything that it can exploit to add to the complexity and make it more capable than Earth life, maybe even with totally novel capabilities never explored here.

    Present day Mars probably only has microbes, or perhaps lichens, at least if it is fair to make a comparison with similarly harsh environments on Earth. Microbes and lichens seem to be better adapted to such conditions than complex multicellular life. But the harsh environment may mean it evolved further on Mars than on Earth. Microbes yes, but microbes with the equivalent of several billion years of evolution advantage over us. Could it be vastly more sophisticated than Earth life, even if it is just a single cell lifeform?

    The harsh conditions on early Mars could mean it didn't get as far and is an early form of life. It's hard to say in advance which way this would go. See also See Life that has evolved further on Mars (below) and What are the effects of these frequent ups and downs in habitability? (above)
  • Life with a capability Earth life doesn't have at all, e.g. a new form of photosynthesis

    We have three ways of doing photosynthesis on Earth - broadly speaking:
    • Green sulfur bacteria, which use light to convert sulfides to sulfur, which is then often oxidized to sulfur dioxide
    • Normal photosynthesis which splits water to make oxygen, also taking up carbon dioxide in the process. (basic equation 6CO2 + 12 H2O → C6H12O6 + 6O2 + 6 H2O where the oxygen atoms in bold are the same ones on both sides of the equation - see Plants don't convert CO2into O2, and Notes on lamission.edu)
    • The photosynthesis of the haloarchaea which works similarly to the receptors at the back of our eyes, based on a "proton pump" which moves hydrogen ions across a membrane out of the cell using bacteriorhodopsin similar to the rhodopsin in our eyes, with no byproducts such as sulfur or oxygen, just creates energy directly from the proton gradient. For more on this see Surprising distant cousins.
    ET microbes might well use some fourth form of photosynthesis that has never been explored on Earth.
  • Life that is better adapted to some conditions than Earth life e.g. better able to resist ionizing radiation, or perchlorates, able to use hydrogen peroxide, better more efficient metabolism, more efficient photosynthesis.

    As an example, C4 photosynthesis as used by maize is more efficient than C3 in conditions of drought, high temperatures and limitations of nitrogen, and is a later development over C3 photosynthesis, though it requires more energy so not so good in cold conditions. But it's not the most efficient possible. It still relies on the enzyme RuBisCO which helps catalyse conversion of CO2 into glucose which is not the fastest of enzymes.

    Scientists have found a way to increase the efficiency of this carbon dioxide fixation step 20 fold, - though only in a test-tube, not in an actual microbe. They combined enzymes from the human body and gut bacteria, plants, microbes that live in the sea and microbes from the surface of plants, in total they used 17 different enzymes from 9 different organisms, which they combined into a new 11 step cycle. They worked out and synthesized the gene sequence to reconstitute this pathway in a test-tube. More details here.

    Nature never discovered this cycle on Earth. What about on Mars? Might Mars life have found this pathway, or if not this one, some other novel CO2 fixation cycle? The scientists didn't invent it from scratch, but rather obtained it by serendipity, combining enzymes that already exist in nature. What else might nature be able to do, given hundreds of millions of years of evolution in a planet spanning sea punctuated by numerous impacts and changes of climate?
  • Life similar to Earth life in most respects - this is Zubrin's idea - and it would raise many questions. How has it evolved in such a different environment, since last transfer from Earth, surely at least tens of millions of years ago. How did it get there? We can test the theory of panspermia, find out in practice how easy it is for life to be transferred to another planet. Or test ideas of convergent evolution on microbes that have evolved on different planets.
  • Some think that all life has to be essentially similar to DNA in almost all respects, that DNA life is optimal. If so, we might find something that closely resembles Earth life, but independently evolved. Based on DNA and RNA. With a similar number of amino acids. With many of the amino acids the same as with Earth life. It's hard to imagine it could be identical in all respects, but it might be so similar that the differences require specialist studies to discern them. There's a short survey with cites to various authors presenting these views in section 4 of this paper. This incidentally gives another way that life from Mars could look so similar to Earth life that it's not easy to tell it apart. It might even use DNA, mRNA, and similar or even the same nucleotides, and proteins, again similar or even most of the same amino acids, if the ones used by Earth life are optimal in some sense. Depending how optimal Earth life is, we might discover the result of a convergent evolution of many characteristics of Earth life, yet be independently evolved.
  • Uninhabited habitats - no life but with organics, and all the ingredients for life. This may seem boring at first sight. But it could also be wide reaching in the consequences. It would tell us a lot about how hard it is for it to evolve on a planet, and about the paths it follows on the way to life. If not life itself, there has to be some complex organic chemistry going on, and cell like structures surely form, as that happens even in short term laboratory experiments. So how far did it get and what exactly happens on a world similar to Earth in many ways (especially in the early solar system), but without life?

    Also, on Earth it's impossible to study uninhabited habitats, except for a very short time after a volcanic eruption. Life appears rapidly on any uninhabited habitat here. On Mars, we might have the opportunity to study uninhabited habitats on a planet that hasn't been inhabited for billions of years. This could help us to understand exoplanets and the origin of life and maybe find out that life is harder to evolve than we thought. It can also help to disentangle effects of life and non life processes on Earth.

    Indeed, if no life ever evolved on Mars, that would suggest that evolution of life is hard and often doesn't happen. If so, Mars could be an example of one of the most common types of planet in our galaxy, and the only one of its type close at hand for us to study.
  • Some major unexpected discovery that nobody currently is likely to predict. We need a catch all wild card here as there's no hope of being exhaustive surely :). Not when all our experience is based on trying to generalize from a single example of biochemistry based on a single origin for life.

If you think of anything I've left out here, do say!

All the possibilities here are of exceptional interest for biology. If there are habitats for life at all on Mars, whether inhabited or uninhabited, then biologists world wide will want to study them as they are now, and the results in the best case could be revolutionary for biology. Let's look a bit closer at some of these ideas.

Life that has evolved further on Mars

This is another of my speculative synthesis sections.

If all that survives are microbes, perhaps Mars life evolved further than Earth life. Its life may have novel capabilities our life doesn't have. What will Earth microbes develop in the future after another several billion years of evolution? The answer to that question could tell us about possibilities for present day Mars life.

  • More efficient photosynthesis.
  • More efficient metabolism
  • Better adapted to sudden changes of temperature - any life on the surface has to cope with huge day / night swings
  • More resistant to drying out, UV radiation, ionizing radiation.
  • Able to repair itself more quickly,
  • More robust and longer lasting spores and resting states.
  • More resistant to oxidizing chemicals like hydrogen peroxide.

If Mars ever developed life as robust and varied as DNA based life on Earth, or even more so, then it is probably still there. Yet the conditions are so harsh that some of the habitats could be uninhabited. If life is rare on Mars and there are few spores in the dust, then it might take quite a while to find it. Indeed you could imagine a situation where some of the RSL's on Mars are inhabited, some uninhabited, and different RSL's are even inhabited by different lifeforms.

So a search even for life on Mars that has evolved further than on Earth could be elusive. And of course it could be more advanced in some ways and less advanced in others. E.g. it could be better at photosynthesis than Earth life, but less efficient metabolism, or vice versa.

See also What are the effects of these frequent ups and downs in habitability? (above)

Uninhabited habitats

This is something that Charles Cockell has explored in a series of articles. His latest is Trajectories of Martian Habitability. One thing that greatly complicates the search for life on Mars is the possibility of uninhabited habitats. On Earth, if you find a habitat with all the conditions that life needs to survive, you expect to find life also. The only uninhabited habitats are new ones, such as recently cooled lava flows, or artificially created habitats such as petri dishes, or occasionally in very extreme conditions such as patches in the McMurdo valleys (as mentioned in the quote above)..

On Mars though some or all of the present day habitats may well be uninhabited. That is to say, habitats with all the conditions for life to arise, even with organics, but there is no life there. Perhaps life never evolved, or it evolved but became extinct.

So then there are three states for Mars at any given time in its history (the abbreviations U, H and L are my own):

  1. Uninhabitable - doesn't have the conditions for life (U)
  2. Habitable but uninhabited. Has habitats but they are all uninhabited (H)
  3. Has at least some habitats with life (L)

As Mars evolved, initially when it first formed in the early solar system, it was too hot for life, and so was uninhabitable. Then there are various trajectories it could follow after that, starting from the early Mars, during which it passes through one or more of those three states. In his paper "Trajectories of Martian Habitability" he identifies six main possible trajectories.

Let's use the abbreviation U for uninhabitable, H for habitable but uninhabited (either permanently or transiently) and L if there are at least some habitats with life. Then his possibilities are:

  • "Trajectory 1. Mars is and was always uninhabitable."
  • "Trajectory 2. Uninhabited Mars, has hosted uninhabited habitats transiently or continuously during its history."
  • "Trajectory 3. Uninhabited Mars, was habitable and possessed uninhabited habitats but is now uninhabitable."
    (H → U)
  • "Trajectory 4. Mars is and was inhabited."
  • "Trajectory 5. Mars was inhabited, life became extinct, but uninhabited habitats remain on Mars."
    (L → H)
  • "Trajectory 6. Mars was inhabited, life became extinct, and the planet became uninhabitable."
    (L → U)

He also suggests other more complex trajectories. For instance that it starts with uninhabited habitats and the life evolves there at a much later date (H → L) . Or perhaps, it is seeded from Earth at a later date. He also suggests trajectories where life on Mars becomes extinct, and then reoriginates on Mars or is transferred to Mars from Earth (L → H → L) . Or even, a logical possibility but seems unlikely, that it was for some reason uninhabitable in the early Noachian and became habitable later (U→ H).

In his paper he discusses ways that this could be tested with observations. For instance, if you find that promising environments with water in present day and past Mars lacked some fundamental requirement for all known life, or if we found that the conditions have always outside the range of physical and chemical tolerances of all known organisms, then that could be evidence for trajectory 1 (U). If you find it was habitable for life in the past, and also in the present, but find no evidence of life, past or present, that's evidence for trajectory 2 (H), and so on.

In his Uninhabited habitats on Mars he points out that if Mars does have uninhabited habitats, these would be a useful control to investigate the role of biology in planetary scale biological processes on Earth.

Uninhabited habitats on an inhabited Mars - potential habitats that life can't reach or hasn't reached yet

Cockell also mentions the possibility that a large habitat may form on an inhabited Mars, but no life from Mars gets into it. He gives the example of a habitat created by a meteorite impact surrounded by an inhabited surface. In his example the life can't get to it from above because of UV. Here is his example:

Cockell's Figure 6

We now know about some very UV hardy life able to survive hours of exposure to Martian UV, and given that the iron rich Mars dust could protect spores from UV, and the dust-storms, I wonder if a better example might be an impact into an ice sheet?

For this idea, see Ice covered lakes habitable for thousands of years after large impacts and Ice covered lakes from volcanic activity (above) See also Ice covered lakes habitable for thousands of years after large impacts and Ice covered lakes from volcanic activity (above).

If liquid water forms but rapidly freezes over, it could trap a system of liquid water and the hot rocks and hydrothermal system below the ice before any spores get a chance to get into it from the surface. If so, and if there are no spores in the ice already and if the newly formed hydrothermal system does not connect to the deep hydrosphere, perhaps the lake could stay liquid for decades or a thousand years, or as long as it lasts, with no life in it. Perhaps that might happen most easily for a small impact creating a smaller shorter lived lake - so that it freezes over more quickly and is not so deep.

Another suggestion - even when the habitat is open to the surface and can be colonized - it might just take a very long time for life to colonize a new habitat in the harsh conditions on the surface of Mars. Not just years or decades. Perhaps it takes hundreds of thousands of years or millions of years for life to colonize a newly formed habitat on Mars?

If that's the case, then some of the RSLs for instance, could be like that in habitable, but uninhabited lake or crater. Some may be uninhabited, some may be habited and some may be in the process of being gradually colonized perhaps from one side to the other right now.

In Antarctica then layers of microbes on Antarctic rocks take from a thousand to ten thousand years to completely colonize a thin layer on the surface of a rock before the rock flakes off and the process starts again. So we have something not unlike that in Antarctica.

Processes like that could be even slower on Mars. So - how long would it take for a microbe to colonize a new habitat - also given that the habitats may be hundreds of kilometers apart and quite small scale and only viable seasonally - and given that the original population is similarly slow growing and may not produce many spores per year to be carried in the dust storms? (Individual RSLs are far closer together, but many of the outcrops and steep slopes that host RSLs are far away from each other). It might go more easily if some of the regions are colonized from below, e.g. RSLs colonized from the deep hydrosphere.

Also - if Mars life never developed photosynthesis, that would make it much harder for life to colonize it. In that case, many habitats that we might consider habitable for Earth life might be uninhabitable for Mars life, because they require photosynthesis for life to thrive there. See Case study - can photosynthetic life be transferred from Earth to Mars or vice versa? above.

Uninhabitable liquid water on Mars

There's another complication for water on Mars that we rarely encounter on Earth. Some of the water might not be habitable at all. You find life on Earth almost everywhere where you find water, or even water vapour from the atmosphere. That includes salt lakes, concentrated sulfuric acid, permafrost, inside the ice of glaciers, and places like the Atacama deserts and the McMurdo dry valleys. But you could get liquid water on Mars which is even more inhospitable for life than any of these.

We do have some natural uninhabited habitats on Earth. A nice example is honey. Though it's got plenty of moisture, the water activity level is too low and it also has anti-microbial properties. No life can colonize it; though spores can survive there in dormant form.

About the only other place where we have uninhabitable permanent liquid water on Earth may be the extremely salty Don Juan pond in Antarctica. This small pond, 100 meters by 300 meters, and 10 cm deep, is of great interest for studying the limits of habitability for present day life on Mars. Research using a time lapse camera shows that it is partly fed by deliquescing salts revealing dark tracks that resemble the Recurring Slope Lineae on Mars. The salts absorb water by deliquescence only, at times of high humidity. This then flows down the slope as salty brines. These then mix with snow melt, which then in turn feeds the lake. The deliquescing salts which start this process may be related to the processes that form the RSLs on Mars.

Though microbes have been cultivated from this pond, they have not been shown to be able to reproduce in the salty conditions present in the lake, and it is possible that they only got there through being washed in by the rare occasions of snow melt feeding the lake.

The tiny Don Juan pond in Antarctica, 100 meters by 300 meters, and 10 cm deep. This pond is about as salty as it could possibly be, with the CaCl2 levels approaching saturation at 60% w/v. It's so salty it stays liquid all the year round, at temperatures ranging from 0°C to -40°C. It has a eutectic of -51.8°C so is believed to be liquid all the year round. The water activity level measured is an exceptionally low 0.3 - 0.6. Though the temperature range is fine for life, it may be too salty for life to reproduce there. Microbes have been found, but they could only grow in less salty conditions. It might be that microbes sometimes can grow there when the water activity level is occasionally raised through influxes of water, and then die. Or they might be washed in from the surroundings.

So far there is no evidence that microbes can actually grow there. It's of great interest to scientists studying the water activity limits of habitability for astrobiology.] There is some doubt about whether it is completely uninhabited. But if it is, it might be the only natural body of water on the Earth of any size which doesn't have any form of indigenous life.

The interesting thing here is that though an uninhabited pond like this is so rare here on Earth that we have only one possible natural example, water like this could be the norm on Mars. Perhaps much of its liquid water is just not available for life to use. Reasons could include, too much by way of salts like Don Juan Pond (including too many chlorates, and sulfates), too much acid, or lacking essential trace elements and nitrogen. Mars also gets so cold that the potential habitats there could also just be too cold for life.

Conditions were better in the past, even in recent times when the Mars atmosphere was a bit thicker on occasion. But as we saw above, on present day Mars, even when water is not already at boiling point, it is so close to it that only salty brines could be stable in habitats exposed to the surface,- and these may be too salty for life to use. There's a narrow habitability zone between water that is salty enough to remain liquid, and water that is so salty that life can make no use of it. Since 2008 scientists have been saying that it may be possible for Mars to have habitable water. But we haven't been able to study any of these potential habitats close up yet, and we just don't know if any of them are habitable. If it does have habitable water, it may have many uninhabitable patches of liquid brines on Mars for each habitable patch.

Also habitats that seem similar may actually form in different ways. What if all three of the main hypotheses for RSL's describe different ways they form? Some due to hot spots leading to liquid water from the deep hydrosphere reaching the surface by repeated sublimation and refreezing. Some due to ancient ice from times when Mars was so tilted in its axis that it had equatorial ice sheets. Some due to salts that deliquesce in the night time humidity of the very cold though thin air on Mars.

Then some of them could be inhabited by different forms of life, and others uninhabited. Maybe even some of the streaks in a particular group of RSL's are inhabitable and others are uninhabitable, depending on the mixtures of the salts that feed them. So, on a Mars that has surface life, you could get

  • Uninhabitable RSLs, water is just too cold or too salty for life
  • Otherwise habitable RSLs, but they are lacking in some essential ingredient for life. Not enough nitrogen, or organics, or life is prevented by some other limiting factor
  • Otherwise habitable, apart from presence of a biocidal agent (such as the chlorites and hypochlorites in the section Perchlorates irradiated with UV light to create sterilizing chlorites and hypochlorites above)
  • Habitable but uninhabited RSLs which life hasn't got to yet.
  • Inhabited RSLs

We could get all that variety even in what superficially seems to be the same kind of habitat from orbital photos.

There might also be a difference in habitability for Mars and Earth life. Any of these habitats could be habitable to Earth life but uninhabitable to Mars life, or vice versa. For instance if Mars life hasn't developed photosynthesis, there may be many habitats that are only habitable for Earth life. Or, as we'll see in the next section, depending on the biochemistry of the cells, there may be many "habitats" that are only habitable for Mars life.

Life that uses hydrogen peroxide, or perchlorates, or both, INSIDE the cells - habitable to Mars life and not to Earth life

So far, the idea is to look for habitats on Mars that would be suitable for at least some forms of Earth life, on the basis that if Earth life could live there, potentially Mars life could as well. As we saw already in the section Perchlorates on Mars (above), perchlorates were first found on Mars they seemed a major issue for life. But now they seem more like an advantage. They take in water from the atmosphere so could help create local habitats and as well as that, many microbes are able to metabolize perchlorate. So nowadays they are thought to make Mars more habitable for microbial life, not less so.

However what if, more radically, the microbes used perchlorates mixed with water as a solvent in their cells? I.e. they don't just eat it, or tolerate an environment of perchlorates, they actually have perchlorates inside their cells? They would take the place of the sodium. potassium and magnesium chloride salts that we have inside our cells. That's Dirk Schulze-Makuch and J. M. Houtkooper's suggestion in A Perchlorate Strategy for Extreme Xerophilic Life on Mars?

Also what if it has hydrogen peroxide inside its cells as well? Just as life on Earth evolved in conditions of salty water and the interior of our cells resemble those ancient seas, perhaps life on Mars could have evolved in conditions of perchlorates and hydrogen peroxide mixed in the water, and developed cells with those chemicals inside them.

If life can cope with perchlorates internally like that, then it has many advantages.

  • It's a great antifreeze so they can stay active at lower temperatures, right down to -70 °C for magnesium perchlorate. That would be very useful on Mars.
  • It deliquesces, takes in moisture from the atmosphere. If the perchlorates are inside the cells body it may be able to take up water directly from the atmosphere into its body at night
  • Being adapted to highly oxidizing compounds could help it also deal with DNA damage from radiation

For more on this: A Perchlorate Strategy for Extreme Xerophilic Life on Mars?

Also, if Mars life can use perchlorates, what then about the hydrogen peroxide idea? The same authors presented this interesting hypothesis that some life on Mars could use a mixture of water and hydrogen peroxide as a solvent inside their cells instead of water. This is not as strange as it seems even Earth life uses hydrogen peroxide.

The Bombardier beetle uses a 25% mixture of hydrogen peroxide as one of the two fluids it mixes to make a steam explosion to fire at a predator. Mammalian cells produce it to catalyse many processes. There doesn't seem to be a fundamental issue with using hydrogen peroxide inside every cell, it's just that Earth life doesn't need it. Selection on Mars might well have favoured it strongly. They say:

"There does not appear to exist a basic reason why H2O2 could not be used by biology. On Earth, utilizing H2O2 in the intracellular fluid has little advantage with regard to temperature and availability of oxygen and water, thus the majority of Earth organisms never developed extensive adaptation mechanisms. On Mars, on the other hand, directional selection may have favored organisms which developed on an early warm and wet Mars to adapt to the progressive cooling and desiccation of Mars"

A Possible Biogenic Origin for Hydrogen Peroxide on Mars: The Viking Results Reinterpreted

Earth life almost never needs to be able to reproduce below -20 °C, at least, not before the invention of the modern freezer. For life on Mars, this would be a major advantage.

Hydrogen peroxide has many advantages for Mars life too:

  • Lower freezing point - for a mixture of water and hydrogen peroxide, it's -56.5 °C. But they can also supercool sometimes forming glasses, right down to liquid air temperatures.
  • Source for oxygen
  • A mixture of water and hydrogen peroxide can also take in water from the atmosphere

It does have its downsides. Hydrogen peroxide is easily decomposed by UV light, so it would need UV protection. Hydrogen peroxide also decomposes spontaneously, so the cells would need some way to stabilize it. The cells would die if exposed to too much water or water vapour, so attempts to culture them for testing might easily kill them. Also if you were to heat them to analyse them in an oven, the cells would auto-oxidize because of the reactive hydrogen peroxide, leaving only gases and very little by way of solid residue and almost no organic content.

This idea of hydrogen peroxide based life was originally suggested as a possible explanation of some of the Viking observations. See: A Possible Biogenic Origin for Hydrogen Peroxide on Mars: The Viking Results Reinterpreted

Life on Mars could combine both those strategies of the hydrogen peroxide and the perchlorates. Life based on these principles would find colder and drier conditions optimal for it, so we might need to look for it in places that would be thought uninhabitable for Earth life. For instance we never get perchlorate rich liquid water at temperatures of -70 °C on Earth, so there is no reason for Earth life to be adapted to use it. Also perchlorates and hydrogen peroxides are rare on Earth, and are common on Mars. The combination of very cold water laced with perchlorates and hydrogen peroxide may be a form of liquid water that is very common on Mars. If so, and if life can adapt to life in it at all - well this may even be the most common "habitat" for Mars life. Perhaps it has both forms of life, with ordinary salt inside like Earth life, and with the perchlorates and hydrogen peroxide, the first adapted to warmer conditions (evolved first in the early oceans on Mars) and the second adapted to the colder conditions of the very cold perchlorate brines. Or perhaps - what if Mars only has the perchlorate based life, and the few patches of fresh or somewhat fresher fresher water are as uninhabitable for Mars life as Don Juan pond is for Earth life?

If this is possible, it might even be that most habitats on Mars are habitable to Mars life and uninhabitable to Earth life.

Since the perchlorate life might be very fragile in warmer moister environments - then we might also have habitats on Mars that are habitable for Earth life but uninhabitable for Mars life, perhaps with an overlap that both can occupy.

There's one other habitat for life on Mars that may exist there, though it is hard to be sure, and where we'd never think to look if we base our strategy on looking for places for known Earth life. That's in liquid ("supercritical") CO2 - which probably exists on Mars at depths. of 100 meters if geothermal processes make the local temperatures above 31.1°C at that depth (with pressures above 73.8 atmospheres). Interestingly, that habitat might exist on Earth as well below the ocean floors.

This habitat would indeed be like Robert Zubrin's Sharks in the savannah. Again it would be uninhabitable for ordinary Earth life - though we might have lifeforms we haven't discovered yet here on Earth living in these habitats deep below the sea floor. At any rate if we do have them - you'd expect they probably wouldn't be able to get to Mars on our spaceships. More on that below.

There are several other ways that we could have habitats on Mars that Earth life could colonize but Mars life can't.

Early life not so versatile as present day life - habitable to modern Earth life and not to Mars life

This is most likely for early Mars. You might think that uninhabited habitats would be rare in the early Noachian, so long as life evolved on Mars at all. It had oceans covering much of the planet, and organics delivered from comets and meteorites. Unless its water was extraordinarily acid, alkaline, or salty, then once life arose, surely it would have spread almost anywhere?

However, if you start thinking in terms of early life, even before the evolution of the first archaea on Earth, the early Noachian may not seem so hospitable after all. For one thing, it might have taken a while before life developed hardy resting states and microbial spores. Without that, it would be confined only to habitable regions and couldn't spread from one to another easily.

So then - what would those habitable regions be, before it has hardy spores and resting states? Well, first, once again, nobody knows when photosynthesis first evolved on the Earth. Perhaps it was present almost from the beginning, but maybe it developed rather later. In an early Mars without photosynthesis, life would be confined to places where it could take advantage of chemical energy.

Perhaps it lived in hydrothermal vents, but there are many other ideas for abiogenesis (origins of life). Some think that life could have evolved in icy conditions, where melting and refreezing ice concentrates organics (eutectic freezing). Or it might have evolved on a clay substrate in a hydrogel which experimenters found can be used as a "cell free" medium for protein production from DNA, amino acids, enzymes and some components of cellular machinery. Or perhaps it evolved on pumice rafts.

One theory of the origin of life (amongst many) is that it might have started in pumice rafts like these. If this is what actually happened on Mars, and if it took it some millions of years to evolve to the stage where it could colonize harsher conditions, then we might have to search for pumice rafts to find evidence of the earliest life on Mars.

This is one out of dozens of suggestions for the origins of life. The hydrothermal vent hypothesis is perhaps the most popular but there are many others.

Another theory is that it originally evolved kilometers deep underground, rather than on the surface.

Wherever life started on Mars, the big question then is - how long did it take to spread to other habitats of the same type, and how long did it take to diversify to other habitats? Or did it ever?

What if it is still localized to its original habitats? That could lead to a planet that is very habitable to Earth life, even a planet with seas like early Mars - but with almost no life there. Only small colonies around hydrothermal vents say. Or in pumice rafts. Or wherever they are able to survive. If this is right, it would of course have huge implications on our search for past life on Mars.

Early pre-archaea type life on Mars could be extremely localised

This is another speculative section, where I invite you to think over possibilities, and present ideas of my own here to stimulate thought. I don't know of any paper that looks into this in depth. Do say if you know of anything.

So, if we search for early Noachian and pre-Noachian period life, we may find the primitive pre-archaean cells only in the hydrothermal vents. Other apparently equally habitable areas could be devoid of life. Also, even the hydrothermal vents of the Noachian period might not all have life in them. Assuming early life hasn't yet developed hardy resting states or spores ,then - it might just fall apart when it is washed away from the habitat where it originally formed. It might die because it is too cold, or too warm, or doesn't have the right chemicals in the water around it to use as "food". It would be hard for such primitive life to transfer from one vent to another. Perhaps we find that the earliest life on Mars is localized to a single hydrothermal vent. You could also envision a scenario where life evolves around a single vent there, survives maybe for as long as that vent lasts, but becomes extinct when the vent ceases to function, because it hasn't yet developed a way to transfer from one vent to another. Then, if that's possible, then different vents might have life, or protobionts, that developed independently through different pathways. There are exposed remains of hydrothermal vents on Mars, so is that where we need to go? And which one should we explore?

The first cells also might not have reproduced like modern cells with their complex transcription methods and error correction. Or, if they could reproduce in the modern sense, yet it might be with many errors and changes, and not reproduced as exactly as present day cells. Early life might have got going in fits and starts, with the first cells easily going extinct.

So another possibility there is that the remains of one attempt at life provided the raw materials for the next attempt until finally it succeeded long term. We might find long gaps with no life, then a build up of organics from life, but then conditions turn unfavourable, upset the delicate balance that let that fragile early life thrive, and it goes extinct. But not before it leaves behind layers of dead organics. So then, that life is then the basis for the next lifeform to evolve and so on. These early protobionts might not have had any informational coding molecules at all. Just split apart and the daughter cells inherit similar chemistry and chemistry of the cell wall particularly to their parent.

So, then, to take this a step further, what if Mars never developed life as robust as modern Earth life? Then life may have evolved, for instance in the hydrothermal vents, then gone extinct when the vent was no longer active. Then perhaps it evolved again from scratch around another vent. Perhaps evolution happened in slow motion until eventually, millions of years later, more robust forms developed that could survive the end of activity of the birth place hydrothermal vent. If Mars life never developed photosynthesis or hardy resting states, then the search for past life there may be elusive.

The same could be true for present day life as well. We might find that it is only present in a few locations even if there are many places that modern Earth microbes could survive. Imagine if life on Mars is like that, not photosynthetic life, only occurs in particular habitats, perhaps only in particular RSL's, and is an early life less developed than DNA life, perhaps even only recently re-evolved. It could be so vulnerable to introduced Earth microbes, and also, hard to find. Then the events of Arthur C. Clarke's "Venus story" of the introduction could happen so easily. (See Why don't explorers in science fiction have these problems when exploring other worlds? )

Indeed, it could be worse than that - though we might never know what happened in the very worst case. It could easily be extinct before we can find it. We might never know it was ever there. Perhaps eventually someone might suspect what happened and then you'd probably get endless debates about whether there was early life on Mars just before the humans got there, that just never gets resolved, like speculations about the behaviour and life habits of Dodos.

The main problem is we have no timescale for this. Of course life must start somewhere, or perhaps several places at around the same time; but how long does it take for it to develop a robust reproduction system, to develop the ability to colonize many different habitats, and to spread from these starting points to cover a planet? It might have needed millions or hundreds of millions of years in stable conditions such as hydrothermal vents for primitive pre-archaea to evolve to the complexity of a modern cell. Or maybe all this is possible within a million years or less, even just a few millennia? Or very fast - at some lucky moment, everything came together in some situation we can't yet duplicate in a laboratory and it rapidly evolved to a recognizable lifeform with exact replication in years, days, even hours??

Nobody knows. We can't yet create these conditions in a laboratory, have no near future prospect of doing so either, and have no evidence at all yet from early Earth.

Life evolving from scratch many times - multiple genesis - and could Mars have multiple "shadow biospheres", perhaps some extremely localized?

This is another of these fun speculative sections. We are used to the idea of a single genesis of life, over four billion years ago, with all present life derived from it. But is that typical of a planet with life on it?

After all with the shadow biosphere hypothesis, if that turns out to be true, Earth will have two distinct forms of life living here at the same time. Distinct in the sense that they don't use the same genetic code or use the same structures at a cellular level. If the life on Mars was less robust, no hardy spores, no photosynthesis, evolving around multiple vents (say) then it could have evolved in many different ways. There are at least some things it could have done differently. Different amino acids choice, different translation table, some subtle or some very obvious differences.

So - what if Mars has a "shadow biosphere" - and not just one, but multiple simultaneous different biochemistries? Maybe including Earth life as well, the few microbes that got there on a meteorite maybe are able to co-exist with whatever is there already. For instance, Mars could have a form of life that uses perchlorates and hydrogen peroxide inside to live at very low temperatures, and that could be the dominant life there. Then it could have Earth life like a shadow biosphere living in habitats that are warmer and wetter. Then it could have other shadow biospheres as well.

ere on Earth though many microbes are found widespread throughout the world, fresh water lakes have distinct populations of diatoms, see Invasive diatoms in Earth inland seas, lakes and rivers (below) . Mars though globally connected through its dust storms and in another way possibly through geothermal hot spots and the deep biosphere, may easily have many biologically very isolated regions. It just depends how easily the local life is able to spread in the dust storms.

Some of the life there may be able to do that while others may not. Microbes on Earth can spread anywhere through the wind. But we still have diatoms in fresh water lakes that can only be transferred from place to place on materials damp with fresh water, such as the invasive diatom that got to New Zealand on wet diving gear. So in the same way,there could be species as localized to regions of Mars as the diatoms in our freshwater lakes.

For instance perhaps there is a small shadow biosphere of relic lifeforms from billions of years ago that only lives in Hellas basin - and another one only on Olympus Mons where there may be ice, and possibly geothermal heating below the surface, and another only in Richardson crater, and another only in the RSLs along the Valles Marineres.

Hellas Basin as an example one of the many unique features on Mars, the deepest point on Mars on the Mars surface. If Mars life never developed photosynthesis and some of it is unable to spread easily in the dust storms - or for some reason requires conditions only available in particular places - then Mars could have life that is there and nowhere else. Life that is able to use the moisture in the atmosphere at night would find this one of the easiest places to live on Mars because the atmospheric pressure is highest.

Valles Marineres has the densest population of RSLs on the planet. Each blue dot here marks a site with many Recurring Slope Lineae (RSLs). The RSLs here are also associated with downward slumping of the slopes which is adding to a picture in which perhaps significant amounts of water are involved in the RSLs. Nobody knows why the Valles Marineres has so many of these. It might be due to the clouds and low level haze often seen in this area - or perhaps deliquescing salts, but if so why do they form preferentially here? Or could it be due to subsurface aquifers, but if so,how did it form? It's easy to see a newly formed crater puncturing an aquifer, but again, why here?

As we know so little about what causes the RSLs, except that they involve liquid brines in some way, different areas of RSLs might have different mechanisms. Perhaps the RSLs in Valles Marineres have unique conditions that favour a particular relic form of life. Even different RSLs in the same area could have different formation mechanisms or be biologically different.

Mars has many permanent features that could provide locally distinct conditions like this.

Or maybe they co-exist in the same habitat. There doesn't seem any reason, on the face of it, that all Earth life has to be related through a common ancestor. It's just the way it is here, and we've got used to it. But for all we know, this may be rare. Maybe most planets have a variety of biochemistries, with no common ancestors, or only very early past common ancestors?

After all, we have the fungi, plants, animals, and other branches of the tree of life, that did have a last common ancestor, but long ago. Is there anything to stop us having a similar "tree of life" but with multiple trunks and no common ancestor to them all? Is Earth perhaps even unusual in this respect?

So, we could have a very complex situation. To expand on Charles Cockell's three divisions of uninhabitable, uninhabited, and inhabited - at any stage in the history of Mars, it might be

  • Uninhabitable
  • Has habitats but they are all uninhabited.
  • Inhabited but only in special locations, such as hydrothermal vents, or rock pools on ocean margins, or deep underground. Perhaps even each of these could be locations for a different genesis for life or a multiple genesis in the same place, different forms each time.
  • Inhabited almost everywhere, so that you can find traces of early Martian life in any habitat with conditions suitable for modern life, so long as it was buried and then rediscovered in a way suitable to preserve it.
  • Inhabited by a mix of forms of life with a different genesis. May have multiple "shadow biospheres". If there is Earth life, the life that has got there so far could even be a shadow biosphere compared to the dominant Mars life. Other shadow biospheres could have RNA world cells, PNA life, maybe even mirror life, etc. Some biospheres could be localized to particular types of habitat or geographical regions. Perhaps some of these could be localized to a single geographical region on Mars like some of the relic life we have on Earth.

Then any of those separate shadow biospheres could go extinct, and maybe from time to time new ones evolve too, perhaps in separate uninhabited habitats such as the isolated lakes and craters (depending how rapidly life can evolve)?

There is no way to decide between these various scenarios on theoretical grounds. The only thing we can do is to search, everywhere we can think of, and see if we can find it. We might finally find the first traces of early life on Mars in some unexpected place nobody predicted.

Mars could also have extremely localized "relic species" . As an analogy, look what happened with the "dawn redwood":

Fossil leaf of Metasequoia

Metasequoia, or the "Dawn Redwood" was originally only known from fossil leaves like this, described in 1941, and scientists thought it was extinct. That was until Zhan Wang found a small stand of a new species of tree in Moudao in China in 1944 (then called Modaoxi). He didn't know what it was.

After the end of the World War II, in 1946. the eminent Chinese botanist Wan Chun Cheng and Hu Hsen Hsu went back to this location, and described it as a new living species of metasequoia. If it weren't for that one small relic stand of metasequoia in China, we wouldn't have any living species of metasequoia left in the world today.

Now it is often grown as an ornamental tree - here is one of them in San Jose State University campus.

So, perhaps Mars similarly might have some precious relic species of some unusual form of life, microbial or even multicellular, that is still hanging on there as a relic species but is extremely localized. Perhaps to a single patch of RSLs, or it's only in Richardson crater, or only in the Hellas basin etc. Again we have no way to know about such a thing until we explore it thoroughly. It's also the sort of thing that could easily go extinct before we find it, especially if it is a relic species of microbe, of unusual biochemistry.

But will we get the opportunity? Perhaps it is already too late to find present day life on Mars? Maybe the signal from present day life is also already confused beyond recognition by life we have already introduced?

Idea that we have contaminated Mars too much already, so there is no point in protecting it

This is another argument that enthusiasts for humans on the Mars surface often bring up. The idea is that Mars is already contaminated with Earth life. It's a sad loss for astrobiology, and a total loss. We have lost all opportunity to study a pristine Mars already. There may have been life there in the 1960s but if so it is now extinct or has merged with Earth life to the extent that we can't tell it apart. But it's too late to do anything about it, and we might as well press ahead and send humans there and study what we have now, such as it is.

That would be an overwhelmingly sad message for astrobiologists. But a message of encouragement and hope to some space colonization enthusiasts.

Well, there is some truth in the background to this. It's surely true that there is Earth life there already from our spaceships. But our planetary protection measures take this into account and they did take care. Carl Sagan's aim has often been misunderstood I think. He and the other early planetary protection scientists never aimed for a certainty of keeping Mars free of Earth life as there was no way they could have achieved that, at least, not with the technology of their day. It was either not to go there at all, or to set an acceptable probability of contaminating Mars. So his idea never was to be 100% sure we can't contaminate it.

He actually proposed a 1 in 10,000 chance of contaminating Mars per mission and a 1 in 1000 chance of contaminating it during the exploration period. Those were the actual numbers used for the Viking missions.

Of course ideally we would like a 100% sure chance that we won't contaminate it. But we can't do that at present. I think personally that we should aim for 100% or as close as makes no difference, for Europa and Enceladus by sampling the plumes rather than landing, and eventually finding a way to 100% sterilization. That's not impossible for near future technology. I think we should also aim for 100% sterilization for anything that contacts a liquid water habitat on Mars. The Viking spacecraft were not sterilized sufficiently for direct contact with a liquid water on Mars, but this is something we could achieve in the near future. See Can we achieve 100% sterile electronics for a Europa, Enceladus, Ceres or Mars lander?

But for Mars the die is cast. However, if we have managed to achieve Carl Sagan's goal, we can be at least 99.9% certain that it is not yet contaminated.

The decision to stop sterilizing to Viking level also is sometimes presented as if the scientists just threw in the towel and gave up on protecting the planet properly. But far from it. This was a carefully thought out decision based on the Viking discovery that the conditions on Mars are so harsh that in their opinion they roughly corresponded to the heat sterilization stage of preparation for the Viking lander. So after Viking they did all the sterilization for the Viking lander except for the final heat sterilization phase on the understanding that the Mars surface would do an equivalent level of sterilization for us.

Critics say that they stopped protecting Mars after Viking, but that's not true or was not the intention at least. We still have planetary protection officers and regular biannual meetings of COSPAR to protect Mars and the rest of the solar system. They continue to protect Mars with the same idea as Carl Sagan had originally, that the chance of contaminating Mars per mission is tiny, somewhere around 1 in 10,000 (probably less).

The Viking lander being prepared for dry heat sterilization for purposes of planetary protection. Since then this stage of planetary protection has been omitted, based on the reasoning that the harsh Viking surface roughly corresponds to the heat sterilization stage.

The reason for the change in sterilization policy is that before Viking they didn't know quite how hostile conditions were there. After Viking, these measures are still needed if the spacecraft contacts regions in Mars that could be habitable for life- the"Special regions".

They did give up on the use of probabilities pioneered by Carl Sagan et al, because of the impossibility of doing a calculation to find an approximate probability to life contaminating Mars when conditions on Mars are so hostile to life. However the basic objective hasn't changed. The aim is still to have a tiny chance of contamination, of the order of 1 in 1000 during the "exploration phase" of perhaps 57 ground missions an 30 orbiters (Carl Sagan's figures). Yes, we've had crashes on Mars, notably of the