If Humans Touch Mars

Like the Lascaux Story - Another Tale of Human Missteps?

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

You can also read this book on kindle. First published online and on kindle in January 2017. For my other kindle booklets, see my author page on Amazon.com

Cover picture shows an astronaut searching for fossils on Mars. (Higher resolution version of cover). It's called "20/20 vision" and is by Pat Rawlings, courtesy of NASA .

Higher resolution version of the cover here:

The main sections in this book are

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Picture this - an astronaut has just found life on Mars. How do you think it happened? I expect most of us will answer "Found a fossil" as in the painting by Pat Rawlings used for the cover. However, Curiosity is exploring a lake bed that has been dry for three billion years, and that is typical for most of Mars. That's long before there were any easily recognizable fossils on Earth. Astrobiologists focus their hopes on ancient microbes instead. That may seem boring, but in the best case, they hope to find microbial ET's, possibly so ancient that they predate DNA, or maybe microbes based on different principles from Earth life. This could be the biggest discovery in biology of our century, revolutionizing our understanding of evolution, of how life works, and perhaps medicine, agriculture, and who knows what else.

These astrobiologists have designed exquisitely sensitive instruments, including "labs on a chip", miniaturizing equipment that a few years back would fill an entire room. They can detect the faintest of ancient degraded biosignatures, even a single molecule in a sample. Hopefully some of these instruments will fly one day. Perhaps life still exists on Mars, right through to the present, perhaps even early life hardly changed for billions of years, long extinct on Earth. If we aren't careful, something as vulnerable as that could be made extinct by whatever made it extinct here. Sadly, humans can't be sterilized of our trillions of microbe companions. As we try to touch Mars we may lose the most precious thing we could find there. As our microbial spores spread in the global dust storms, native Mars life might be gone before we knew it was there.

It would be so wonderful if we could just reassure everyone that Earth microbes on human crewed spaceships will cause no problems on Mars. If only real life were like the movies, built on the imaginations of authors and using evolving dramatic tropes. However, this time, we don't get to write the script for the sequel. Star Trek doesn't give us any real experience in exploring planets. We often make mistakes when we try something new;, sometimes huge ones, It's time to look at this carefully. At last we are getting a few journalists and TV presenters who touch on the subject, but we need more awareness as it is so often treated as a minor matter, soon dismissed. It's time this discussion moved from the specialist papers and workshops on planetary protection to the general public.

If we decide to keep Earth microbes away from Mars, at least for now, what happens to all our plans to explore the red planet? Well, we can continue to use our robotic eyes and hands on Mars, with increasing autonomy and mobility, and eventually, broadband streaming of video back to Earth. That would be a dramatic change. Meanwhile our astronauts can start extraterrestrial fossil hunting on the Moon. We should find meteorites there from early Earth, Mars and perhaps even Venus, wonderfully preserved deep in the extremely cold ice deposits at the lunar poles. Later, as we learn to send humans safely further afield, to Mars orbit, and its moons, they can explore it using "telepresence" in immersive 3D virtual reality. All of us back on Earth can join in, exploring a landscape built up from the hours of binocular HD video streamed from the surface. I will also cover searches for life in the oceans of Europa and Enceladus, and more exotic places, ranging from the molten sulfur lakes of Io through to the liquid nitrogen geysers of Triton.

The book starts by comparing Mars with the Lascaux cave paintings, damaged by microbes from human visitors. I hope this book will bring these issues to the attention of a wider audience, now that it seems that we may have technology to send humans to Mars within a decade or two. This is a decision for all of us, not just scientists and space enthusiasts. This book has hyperlinks which take you to the scientific literature. Many of the papers are open for anyone to read , so it's easy to follow through to find out more for yourself about anything here that interests you..

In more detail

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 - credit NASA / Pat Rawlings

However there is another side to this picture. As these brave astronauts explore Mars, their bases and rovers leak Earth microbes into the dust, every time they open an airlock. Their spacesuits also leak air constantly (they have to be able to bend their arms and legs at the joints and the spacesuit designers achieve that by leaving tiny gaps which the air leaks through). They leak a trail of microbes, wherever they go.

Then, the ground they walk on is covered with dust as fine as cigarette ash, light and easily moved, even in the near vacuum winds of the Mars atmosphere. These particles can travel hundreds of kilometers in a few hours during the fast winds of the seasonal dust storms. Every decade or so, these storms combine and spread to cover the entire planet and last for weeks. The dust blocks the sun and turns day into night, and it takes months for all the thick clouds of dust to fall back out of the atmosphere. 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 dust grain, could eventually fall to the surface undamaged, thousands of kilometers from its point of origin. Trillions of hardy microbial spores will stream out from a human base in the winds, and if there are any Mars habitats for them to find, they would surely get there eventually. For more on this see How could this work on Mars with dust storms and a globally connected environment? (below).

That's especially so if humans crash on Mars. After all the space shuttles Columbia and Challenger crashed. Minute fragments of the astronauts bodies, food, air, water and the spacecraft itself would spread in the dust and could irreversibly contaminate Mars with Earth life. If that happens, then it will impact on all nations on Earth with an interest in exploring Mars, and also our descendants, and all future civilizations in our solar system. For the entire billions of years future of Mars, nobody would ever again have the opportunity we have now to study the present day pristine planet. Why don't explorers of other planets in Star Trek and the many movies, books and TV series have these same problems? Perhaps it is because they are the result of the author's imaginations. They aim is to entertain, after all, and over many films and movies, through collaborations of script writers, directors and sometimes ideas from the actors, they build up movie tropes that reinforce each other. Eventually the audience come to expect the films to be done this way. These are often things which help move the plots forward, and make the stories more dramatic. However, none of this is based on any actual experiences at all of exploring other worlds.

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. Anyone who writes or says that is likely to be more popular, and their articles and videos will get more widely shared. But our actions will have real world consequences, not just lead to 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.

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 with 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"? 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 Sky at Night program in the UK (hosted for many years by Patrick Moore until his death). In a recent episode,Life on Mars the presenters 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 rather minor matter. The discussion starts about sixteen minutes into the program. The presenter 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, but it is available to buy and watch online, I think probably for UK residents only.)

In other words, the idea is that our present situation is frustrating, and 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, if you haven't looked into it in detail. 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. 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 the word "Touch" in the title of this book after listening to their video, and this book is in a way a response to it. It covers some of the same issues that they cover (starting nine minutes into that video), but there is so much more to be said.

Raising awareness - fossil optimists and early life enthusiasts

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

We have a long way to go by way of raising awareness of these issues, and I hope to help with this book. Before we can make the right decisions for the future, we need a clear understanding of what the issues are.

Many of us, without thinking about it, are "fossil optimists" as I characterize it in this book. The cover photo shows this fossil optimism in artwork done for NASA by Pat Rawlings. After all, that's how it works on Earth. We are used to learning about past life from fossils, so it's not too surprising that we expect the same to happen on Mars. Enthusiasts, including scientists, even search the Opportunity and Curiosity photos for what they think may be fossils of past Martian life.They are usually searching rock formations from dried up lake beds that are unlikely to have seen any life for more than three billion years. Nearly all Earth macro fossils date from the last half billion years of our geological history, apart from some hard to recognize stromatolites and other fossils that are ambiguous and took a lot of proof before they were accepted as life. To look for clear unambiguous macrofossils in Gale crater is to show optimism that life on Mars had at least a two and a half billion year head start compared to Earth. Such fossil optimism is not absurd, indeed you can come up with some interesting reasoning in favour of it, but the case for it isn't very strong either, and you can argue the case both ways.

Many professional astrobiologists are "early life enthusiasts". Their professional focus and instrument design is mainly orientated towards life similar to whatever existed on Earth over three billion years ago. Such early life may very well even be so early that it predates DNA based life. It may consist of single cell organisms, 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 in these deposits. Instead they pin their hopes on the ability of the Mars conditions to preserve organics for billions of years. But they expect this signal too,to be weak, degraded, mixed in with organics from other sources, and only present in a few rare locations. They also expect that they will need to drill to depths of several meters to find it. For this reason they think that in situ searches with sensitive biosignature detectors are the way ahead for the search for past life. And even then it may also require a fair bit of detective work. Present day life is also likely to be 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 subcrtiical water as a solvent for non destructive extraction of organics, but 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, so sensitive that it could detect a single amino acid in the sample. The target goal is a mass of 2.5 kilograms, a quarter of the mass of UREY.

Asrtobionibbler's predecessor, UREY was approved for ExoMars but was descoped when NASA pulled out of the partnership. Then the Life Marker Chip, mass of 4.7 Kg, which uses polyclonal antibidoes to detect biosignatures was approved for ExoMars but later descoped. Another version of it, LDChip300 was tested in the Atacama desert and was able to detect a layer of previously undiscovered life at a depth of 2 meters below the surface in the hyper-arid core of the desert from analysis of less than half a gram of material.

To find out about the exquisitely sensitive instruments designed for in situ searches by exobiologists, 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. Curiosity would not be able to detect biosignatures for past or present life in the low concentrations most astrobiologists expect.

With that background, the tiny microbes are the very thing you are looking for. Introducing Earth microbes and organics could be disastrous for ones hopes of finding out about life on Mars. So what then is the role of humans in this vision?

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. It's also a place of great interest in its own right, and with little by way of planetary protection issues to deal with.

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, fossil diatoms are still recognizable after a simulated impact on the Moon, indeed the smallest ones are intact, complete fossils. There must be a lot of material from the Chicxulub impact on the Moon. 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 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 too, from before its atmosphere became as thick as it is now. Early Venus might have had oceans and might have been as habitable as early Earth and Mars. For more on this see Search for early life on Ceres, our Moon, or the moons of Mars below.

The Moon is also 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. This time they take the entire crew back to Earth within a couple of days, and are 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. We don't have any practical lifeboats for such a journey. Here are my two online and kindle books where I go into detail about these ideas.

"MOON FIRST Why Humans on Mars Right Now Are Bad for Science", available on kindle, and also to read for free online.

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

In those books I also argue that with our lunar adventures, we will learn about 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, before 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, just outside its dangerously intense radiation belt, is less than two years journey time on a fast Hohmann transfer orbit.

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 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 thought it's best to say 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 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 more ordinary conditions. They can 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, we can be there in person without these possibly devastating consequences of touching Mars.

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. I argue in my Moon First books that this has potential to be hugely positive or hugely negative. It depends very much how it is done, and it may be a good thing that we are likely to have few humans in space to start with. Though I'm keen on humans in space, I'm no advocate for sending large numbers as fast as possible. After all think what the consequences would be if we had the likes of ISIS and North Korea in space colonies, with space technology far advanced over ICBMs. Once there are tens of thousands, and millions of people in space, we can't restrict this to the "good guys or gals" whoever we think those would be. I cover this in my Case for Moon First in these sections:

I argue 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's a similar situation for human exploration without settlement, which is the main focus of this book. That also can be either hugely positive or hugely harmful. 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, Triton even, but the main focus is on Mars as there are no plans to send humans to those other places in the near future.

Humans can probably help a lot with 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. 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?


Touching Mars

We love to touch things. If you put a sculpture in an art gallery and say "please touch" you can guarantee that both children and adults will touch it. So it's natural that we want to touch other planets. 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 instance,

Photograph of the Lascaux paintings by Prof Saxx.

Many of us would love touch these paintings, as the original painters dido, and feel the texture of the rock they were painted over. But not only are we not permitted to touch them - we have to take care even about going into the caves at all. The warmth, humidity and carbon dioxide from our breath have all taken their toll. Fungi and black mold are attacking the 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 Lascaux cave with these marvelous paintings found in the 1940s by four children with their dog. They were opened to the public immediately after WWII by the owners who enlarged the entrance, added steps and replaced the sediment that covered the cave floor with concrete. The humidity, carbon dioxide and warmth of all the visitors since then have taken their toll leading to microbes, fungus and black mold. Even though the cave has been closed to all except occasional specialists, for some time now, it is too late to restore it back completely to its original condition.

Our attempts to fix the many issues have lead to one more misstep after another. For instance, after a white fungus spread over the floor and up the walls, scientists took care to photograph every single painting in detail, to keep track of the damage. What they didn't realize is that the bright lights they needed for the photographs were damaging the cave paintings, by encouraging the growth of 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 say it is possible to restore the original environmental conditions of the cave. But the microbiologists say that it is not possible to restore the pre 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 (trying to destroy them will only make things worse), and try to find a new equilibrium.

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".

You ask them, what about planetary protection from Earth life, and they say

"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. They just have to find a way to make it work for us, 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 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.

And there are some places humans can't go at all. Even if you desperately want to visit the lake Vostok in Antarctica, kilometers below the surface of the ice, even if that was the one thing you most want to visit, even if you had 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 fund the expedition entirely yourself, you 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, so confusing scientific study of a body of water that has been cut off from the surface possibly for millions of years. Scientists would dearly love to explore this lake, but they haven't yet found a way to do it that preserves its science interest in the way they would like to.

Microbial ethics

So, could we harm Mars as much as we did with the paintings in the Lascaux cave, or perhaps more so? The debate about this often centers around ideas of "microbial rights" and microbial ethics. Of  course, these are not rights for individual microbes, but if we discover life on Mars, in whatever form, does it 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?

Some argue that microbial life on another planet deserves a "biorespect" from us independently of whether we can actually make use of it or 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"

Whatever ones views on that, our present reasons for protecting planets and moons in our solar system from Earth life are much more practical. We do it to protect their science value for us. 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. I'll look at issues with looking for present day life on Mars later on, but first, let's look at how microbes from Earth could confuse the search for past life 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,

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 belemnites and sell them in her fossil shop at Lyme Regis. A nd indeed, if we found something like this, the search would probably be over, could anything like 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.

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 its favour. Mars may well have had an oxygen rich atmosphere early on, over three 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 dots 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 which developed photosynthesis, as that's how similar manganese deposits formed on Earth. But Mars had another way to make oxygen. With no magnetic field, the solar storms could split water vapour in its upper atmosphere. The lighter hydrogen then would escape 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

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 second sea briefly, and the Amazonian which continues to the present with localized flooding, and the atmosphere getting thinner and thinner, cold and dry.:

  • The early Noachian and pre-Noachian periods, which had an extensive sea (though possibly often ice covered) over the Northern hemisphere of Mars. That entire hemisphere is lower in elevation than the southern hemisphere.
  • The Hesperian period of volcanic eruptions and extensive flooding and a second sea three billion years ago.
  • The Amazonian period of more localized flooding, though with a second sea at one point. This is followed by billions of years of cold dry conditions with occasional small flows of water and some flooding. The climate is quite variable depending on the tilt of Mars, the eccentricity of its orbit, and local effects of impacts. At times the atmosphere is thick enough for pure water to be liquid though at present ice everywhere is at or close to boiling point as soon as it melts. Mars is still in the late Amazonian period.

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?

Fossil optimists and early life enthusiasts

There are quite a few obstacles in the way of fossil forming life.

  • 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 on Earth date back to the last half billion years, out of over four billion years of evolution.

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?

Tougher conditions for life on Mars - sometimes - and 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. But it's more complex than that. Its orbit is much more variable than Earth's, varying hugely through the influence of the other planets.When its orbit was at its most eccentric, it would get 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, though there is plenty of evidence that it did. Perhaps it had strong greenhouse gases such as methane in the atmosphere. However it did it, maybe it had oceans that were frozen over every time it was furthest from the Sun, then melted a year later when closest to the Sun. Since the early seas formed in the northern hemisphere, which is consistently lower than the mountainous southern hemisphere - then the best times for liquid water seas might be when the northern summer coincided with times when it was closest to the sun. This would depend on the direction of its axis which precesses just like Earth's axis. So at times it would have warm habitable summers in the northern ocean and at other times, thousands of years later, it would be frozen for the entire Mars year.

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

So what would all these changes in habitability do to evolution? Perhaps with long periods of time with permanently frozen oceans and then times when the oceans melt every two years? That's very different from anything that happened on Earth. 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 had many more large impacts than Earth in the very early solar system at the times of its oceans. These impacts would boil the oceans and melt the rock, 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. Perhaps this accelerated evolution, with the life continually faced with new challenges to overcome. Or on the other hand it 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. If you are optimistic about macro fossils on Mars, however, you could go with that hypothesis 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 possible hypotheses 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.

There was a huge diversity of strange creatures back then. Only a few survived and continued to evolve into present day life. If we find something like that in three billion years old deposits on Mars, it will mean that evolution on Mars had a head start over us of two and a half billion years. Which 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?

Or perhaps life on Mars evolved more or less in pace with Earth life (though it may have got off to a slightly earlier start as the Earth - Moon system formed quite late). If so then we might, just possibly, find stromatolites and acritarch's in the Hesperian deposits. Stromatolites are large 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 actually 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, these 1-2 cm high putative stromatolites found in Greenland. However ancient stromatolites are hard to identify conclusively and there might be much debate 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 too. That's often a matter of debate for early Earth putative microfossils, whether they were life or not. It would be even more so on Mars.

So, in short, if there are easily recognizable macrofossils in Hesperian deposits on Mars, like Gale crater, then evolution there has to be at least two and a half billion years ahead of Earth life. 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, with many setbacks. then we might find very early life there, so early that even the stromatolites and acritarch fossils might be unlikely. Though 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 which would put present day evolution there three to four billion years behind Earth.

I don't think we can distinguish between those 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. 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. We can speculate endlessly but without at least a bit more data it's hard to draw any definite conclusions.

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 which is made up entirely of shells? Well it did have enough time for this to happen. It probably had hundreds of millions of years of relatively stable conditions in the early solar system, and continued to have seas and lakes for over a billion years. That would be plenty of time to build a thick deposit of oil shale in ideal conditions.

If we found something like this, even without the multicellular life fossils, just the remains of single cell life but in deep meters thick beds of organics, our task would be easy:

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 life based organics. Or, it could be that they were washed out by the later floods, and what's left was destroyed by surface conditions. Maybe Mars still has deposits like this, many meters below the surface beyond the reach of the cosmic radiation? Any surface deposits, even meters thick, would soon be degraded to just water vapour and other gases by the cosmic radiation over the billions of years timescale. 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 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 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.

How would we recognize fossils on Mars?

The other problem is that we don't know what to look for on Mars. If we found a fossil archaeopteryx or a fish it would be obvious. Even a fossil multicellular plant. But for billions of years, the only macro fossils on Earth were microbial mats and 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. But 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 - 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 very different geological conditions from Earth. As an example, it doesn't have chlorides, but it has abundant chlorates, sulfates and even hydrogen peroxide. There are only small amounts of oxygen in the atmosphere, but the surface is far more highly oxidized than Earth's surface. It also has some geological processes that we know only happen on Mars such as the processes that involve dry ice (e.g. the dry ice geysers and dry ice blocks sliding down slopes). Dry ice is a significant causation factor in many Mars geological formations and it is never a factor on Earth at all. Even the sand formations are created by winds that blow the dust in ways that they wouldn't on Earth because of the low gravity and the near vacuum atmosphere. The low gravity also lets geological structures form as a result of wind erosion that would be unstable on Earth. And there is no water. And because of the low pressure, the fastest winds on Mars would be just strong enough to gently move an autumn leaf. The dust storms only are able to lift up the dust because it is so fine, as fine as cigarette ash. It also has much larger temperature variations, able to change between the freezing point of dry ice and melting point of water and above in the same day.

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.

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

Opportunities blueberries, to take another example, are made of iron oxides, so far not very life like. But on Earth similar nodules form in the presence of life.

Blueberries - photograph 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 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. The Mars "blueberries" seem to have formed in the same way.

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

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, based 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 just say that they must have been formed in the same way because they look so similar - Mars is so different from Earth.

Practical science reasons - why small quantities of present day life can confuse the search

So, if there is life on Mars, how will we find it and recognize it? Well perhaps we identify it through the organics. After all, that's how the ancient stromatolites on Earth were eventually proven to be fossils rather than geological formations. We need to look for organic biosignatures. Once we prove that they are the results of life in that way then we may be able to identify them by their shape too, as fossils. For instance if the blueberries turn out to be the result of life processes, they will be easy to find after that. But it's unlikely that we recognize them first through their macrostructures.

So how easy or hard will it be to search for life on Mars?

It would be great to find macrofossils, shale deposits or similar, but, it takes a lot of optimism to pin your hopes on that possibility. First, present day life is likely to consist mainly of microbial life, or at most lichens, even if multicellular life evolved in the past. That's because similarly inhospitable locations on Earth such as the hyperarid core of the Atacama desert and the Mc Murdo dry valleys in Antarctica have only microbes, and sometimes lichens in them. It's also likely to have small populations and be slowly metabolizing, again because the conditions there are so harsh.

What's more, we have yet to send any landers to look at the possible habitats for present day life. Curiosity is just a few kilometers away from a possible habitat but it can't go up to it to look at it because it isn't sufficiently sterilized to do this. As for microbial spores mixed in the dust, we haven't sent anything to Mars yet that could spot something like that, not since Viking, unless Viking itself did discover life on Mars.

So, both past and present day Mars life is likely to be very hard to detect and also hard to distinguish from Earth life. The first problem is that life may be there only in minute traces. Modern life may be scarce and hard to find, because it is so inhospitable. It would be life at the edge, only just surviving. Past life may have been destroyed long ago except in a few favoured patches which may have only a few trace amounts of organics, and not only might it be microbial, it might be early life, that hasn't yet evolved to be as large as a modern microbe on Earth.

I'll talk about present day life later. But first, let's look at why past life is likely to be hard to find and how introduced Earth organics can interfere with the search. Before we do this, though, let's look at Zubrin's arguments because if you've heard those you may think there is no need to read any further, because he argues that Earth life can't harm the scientific exploration in any way, either the search for past or present day life. It would be great if that was true. Would be no need to do anything special to protect Mars. But is that really the situation?

What are Zubrin's arguments?

The enthusiasts who want to send humans to Mars 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 even in the same talk:
"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 perhaps rather unlikely ones at that.

He also talks about the advantages of human astronauts over robotic rovers on the surface, citing an example of a fossil discovery he and a team of others made in Arizona in a Mars exploration simulation. From his log book:

"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.

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 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.

Demolishing Zubrin's arguments

I go into this in some more detail in my Moon First books. But let's look at them briefly here, because if you've been convinced by these arguments you may well think there is no need to read any further. So we do have to address them early on.

  • 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 rabbits, rats, cats, etc. The wallaby is perfectly adapted to Australia, and the rabbit isn't; it's a generalist. However, he rabbit happens to be better at living in Australia than the marsupials that evolved there. The same could happen for Earth microbes competing with whatever might live on Mars.
  • Yes anthrax on Mars would be easy to detect. But how likely is it that we find a well known species like that on Mars after a human landing there? Most Earth microbes have not been sequenced. 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 (new techniques make it possible to sequence a microbe from a single cell even if you can't cultivate it). That makes it only 0.00001% of all microbial species on Earth that have been sequenced to date. Nine tenths of those can't be cultivated in the lab and have been sequenced from a single cell. See Largest ever analysis of microbial data (May 2016).

    So, if a wide range of species of Earth life was introduced accidentally to a Mars habitat then typically only the tiniest fraction of a percent of the species in that habitat could be confirmed as definitely coming from Earth. For the rest of the lifeforms that actually did came from Earth, you'd just have to say you don't know where they came from.
    Photomicrograph of the anthrax microbe, Bacillus anthracis using Gram-stain technique. This is one of 100,000 microbes that have been genetically sequenced. Robert Zubrin is fond of using it 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, 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.
  • Advantages of human astronauts for fossil hunting on Mars - for one thing, this all depends on Mars having features that the astronauts can recognize by eye. Even early stromatolites are likely to be tiny (one or two cm in size), hard to recognize, and controversial once found. They could spot things like the blueberries easily- but how much use is that if they can't tell if they are life or not? It would work for dinosaur bones and other macrofossils, which they can recognize to be life easily - but how likely are those?

    For an example of how the search could be done robotically, let's look at 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 very sensitive tests for organic biosignatures. They tested it in the "hyper arid" core of the Atacama desert, drilling into the extremely salty "hypersaline" subsurface. From in situ analysis of just half a gram of sample in situ, 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 SOLID by researchers from the Center of Astrobiology in Spain and the Catholic University of the North in Chile. SOLID is one of several extremely sensitive instruments, low mass, which astrobiologists hope one day can be sent to Mars for in situ search for life there. Image credit Parro et al./CAB/SINC

    For many more such instruments, see In situ instrument capabilities below

    Also, before we send humans to Mars, whether to orbit or on the surface, we would need broadband communications back to Earth. 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 HD video every day, where they could 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.

    Future rovers would also be more robust and capable than Sojournor, Opportunity or even Curiosity. They could be far more mobile, as even in the 1970s the lunakhod traveled as far as Opportunity did in a decade, in a few months. With better autonomy, or driven from orbit, they could travel tens of kilometers per day or even in an hour, like the lunar rover. And we'd have 2D binocular vision, haptic feedback and many other technological improvement, with existing technology given an increased level of funding and commitment. Or this may happen anyway as technology develops. For instance the successor to ExoMars.

    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, white balanced to make the landscape easier to read, and as before, stream everything they see back to Earth, where we can explore everything they found at our leisure and find things that they missed.

    More on all this later in the section Objective for humans to Mars

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 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.

What about Zubrin's meteorites argument?

This may seem one of the most convincing arguments 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 there is no need to read any further if I don't answer this.


This one takes a bit longer to answer properly. In short, it is really hard for a microbe to get from Earth to Mars, and it's remarkable that it now seems possible at all. Most microbes that could get to Mars on a human occupied spaceship, and perhaps survive on Mars once there, would probably never get there in a meteorite. Also it's not easy for life that gets there in a meteorite to find a suitable habitat once there. 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, and it is challenging even then. By far the easier direction is from Mars to Earth, but that direction also is still a huge challenge for most lifeforms, and of course we don't yet have any proof that it has happened in either direction.

Also, if we did find that Mars life is nearly identical to Earth life, that by itself would be a surprising discovery to many, of some considerable importance . We would want to study this, to learn about panspermia and to find out how much of a difference there is,between Mars and Earth life as surely there would be some differences. The last thing we'd want to do in that situation is to introduce modern Earth life to Mars when we have this wonderful opportunity to learn about the effectiveness of panspermia for introducing Earth life to Mars and about what happens to life transferred to another planet in this way, whether it evolves in a different direction or in parallel.

As Cassie Conley, said, 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 present planetary protection officer for NASA.

Alberto Fairén and Dirk Schulze-Makuch cover this meteorite argument in detail, in "The Over Protection of Mars". it was rebutted in a follow up article "Appropriate Protection of Mars", in Nature 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.

Let's look at it in more detail.

Microbe stowaways on meteorites traveling from Earth to Mars

So, yes, it's true, meteorites get to Mars from Earth, on average tons of them every century. But that's an average over timescales of billions of years. The numbers fluctuate hugely. We get meteorites from the Moon, from Mars, possibly from Mercury but so far we haven't found any confirmed "Earth meteorites" in modern times - not any that actually left Earth, and then came back again some time later. So probably there are no meteorites from Earth arriving on Mars right now.

Even meteor crater in Arizona wasn't large enough to send ejecta all the way through the atmosphere with escape velocity.You need a rather huge impact to do that, like the Chicxulub meteorite impact 66 million years ago. Earth clears its orbit over a period of twenty million years. So all that 66 million years old material is probably gone by now. The last meteorite to get from Earth to Mars may have got there over forty million years ago. Anyway solar storms and cosmic radiation would sterilize it thoroughly unless buried deep within rocks many meters in size. 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 gets there 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. Remarkably, experiment suggest that some very hardy extremophiles could survive the century in the vacuum, cold, and solar radiation of space as well as the shock of ejection and impact on Mars. However we don't yet have any examples yet of panspermia - transfer of microbes on a meteorite between planets. It is currently a theoretical idea not confirmed by any observations. 

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 and acceleration of ejection from Earth and impact on Mars.
  • Survive inside a rock - because anything on the surface would be destroyed - from the heat, and from the UV radiation in space.
  • Withstand the hard vacuum of space.
  • Withstand the extreme cold of space - so after the heating up and high gravity during the ejection, it then has to survive freezing well below the freezing point of water.
  • Survive the cosmic radiation and the solar storms of the journey to Mars.

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 chlorates and chlorites, and the hydrogen peroxide. Yet it also has to be able to survive without 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 also has to be able to cope with the near vacuum atmosphere. 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 there.
  • If there is native Mars life, it 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 away 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.
  • 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 this to happen may be during the first few hundred million years during the late heavy bombardment. But was Earth life back then hardy enough to be transferred to Mars on meteorites (or vice versa)? Since the most recent chance of this happening was 66 million years ago, we know at least that any Earth life that survived there has evolved independently for at least that long

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

One of the most interesting cases to look at here is photosynthetic life, particularly, oxygen producing photosynthesis. This is particularly important because by making oxygen, it could make a huge difference to a planet, transforming the atmosphere and liquid water habitats throughout the planet. It might have a major effect on any Mars life already there - either positive (as a food source for instance or a source of oxygen locally) or negative (competing with it or changing a habitat so it can't live in it any more). It could have had an even larger effect on early Mars, when it had oceans and lakes, when it could have transformed the atmosphere just as it did on Earth. We need to look at it both ways because if the life came to Earth from Mars it would also be another way to have the same species on both planets.

However it turns out that these microbes are less able to survive transit from Earth to Mars and vice versa than some of the other species studied. So as well as being of great interest in their own right, these are a good examples of life that would find it very challenging to get from Earth to Mars via meteorite transfer. It's also very challenging for it to get from Mars to Earth. Yet it might well be able to survive once there and could get to Mars easily in a human occupied spaceship.

So let's look at it in more detail. Chroococcidiopsis is our top candidate here, a green algae that can withstand high levels of cosmic radiation and solar storms, able to repair its own DNA in real time when damaged. It is also UV resistant, and sophisticated Mars simulation experiments suggest it may be able to survive on the surface of Mars almost anywhere, photosynthesizing in partial shade or if there's a thin covering of dust or dead microbes. It can do this so long as there is liquid water available, or in the 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, which may have been one of the ones responsible for bringing oxygen to Earth originally, a polyextremophile that can withstand many extreme environments, but also does just fine in ordinary conditions too. You can find varieties of this species anywhere from Antarctic cliff faces through to the tropics, fresh water, extremely salty water, the most arid conditions where life survives on our planet, acidic or alkaline, freezing cold, or extremely hot. 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 would it get there on meteorites? Is it already there? 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. He also looked at whether lichens or any other photosynthetic life could survive transfer from Mars to Earth or vice versa. This section is a summary of few of his findings in that paper. He found that it was possible, but difficult. The toughest part of the journey from Mars to Earth was actually the re-entry into the atmosphere as typically 10 - 20% of the radius of a hand sized rock ablates away during re-entry. The rock also has to be larger than 20 centimeters in diameter to avoid heating up to 100°C through to the center. Since they 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 re-entry, or indeed ejection from Mars through its atmosphere at escape velocity, which would also heat up the rocks.

He did an experiment in which he grew 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 them 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 the biomolecules.

It could survive this if deep within the rock, as Martian meteorites never got hotter than 40°C during re-entry. But how does the photosynthetic life get deep into the rock in the first place? It can survive in cracks if light filters in through them - but that also would give cracks that channel hot gases into the interior of the rock as well as being places where the rocks are quite likely to break apart during ejection from Mars or re-entry to Earth.

Chroococcidiopsis also is rather susceptible to shock of ejection from Mars, killed at only 10 GPa. Typically ejection from Mars requires 5 - 55 GPa so it can only just survive at the lower end of impact shock levels. Lichens manage somewhat better here. In the other direction, from Earth to Mars, with escape velocity 11.2 km / sec more than twice the escape velocity of Mars of 5.02 km / sec, leading to higher shock levels, then it would be very hard for Chroococcidiopsis to survive ejection.

You can work out scenarios by which photosynthetic life could get from Earth to Mars. For instance, 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 depth, now gets ejected into space. You still have the problem of the shock of ejection from Earth. Then it has to get out of its rock when it gets to Mars.

If somehow the microbes got into space still surviving near 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. UV light is not so damaging 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 the maximum depth you'd expect to find photosynthetic life. So nearly all the rocks that get to Mars from Earth will be totally sterile by the time they get there.

They can however survive the low temperatures and vacuum of space and the UV radiation at least for 1.5 years, tested in experiments flown on the exterior of the ISS.

Charles Cockell's concludes in this section of his paper:

"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 transfer of photosynthetic life has the potential to make dramatic changes to a planet, but that it is less likely to happen than for heterotrophs (which use organic carbon) or chemotrophs (which basically use chemical reactions as a source of energy and synthesize all their organics from carbon dioxide, for instance living in hydrothermal vents).

General case of transfer of life from Earth to Mars

The big problem with transfer of life from Earth to Mars is the shock of ejection because the material has to leave the Earth's surface at very high velocities - not just escape velocity. It has to leave the Earth's 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 Earth's surface at only terminal velocity of meters per second rather than kilometers per second, after it gets slowed down by the atmosphere.

So the surface of the rock will get boiled away, just as for meteorites re-entering our atmosphere -, but even more significantly, it's going to experience high levels of shock as it leaves Earth, 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 are 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. For that reason, the best time for life to get from Earth to Mars is in the early solar system, soon after formation of the Moon. Impacts large enough to do this would probably also sterilize the Earth to some depth. So we are talking about very early stages in the evolution of life here. Probably more than 3.75 billion years ago. Was such early life already robust enough to survive interplanetary transfer?

More recent impacts would send material all the way to Mars. The Chicxulub impactor did 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. It's definitely possible in the other direction to get viable life ejected from Mars and arriving on Earth. There I'm summarizing the conclusion of this paper from 2007. Experimental evidence for the potential impact ejection of viable microorganisms from Mars and Mars-like planets

There have been no impacts this large 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.

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 - even though the microbes themselves did not. For more on this, see 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. If there is any life transferred to Mars on meteorites, then surely most life on Earth wouldn't be able to do it. This is a topic of great interest to astrobiologists and there are many papers by them published on the topic.

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.

  • They get a comfortable ride inside a human occupied spacecraft, protected from UV light, and extreme cold or heat, in a rapid journey to Mars,
  • Once they get to Mars then they can survive in marginal habitats outside the habitats as they adapt to Mars . For instance every time humans open an airlock then air from inside, along with moisture, flakes of human skin, hair and other debris, and numerous spores will be dispersed out into the Mars landscape. This will also happen whenever they use spacesuits as spacesuits are designed for mobility which at least with spacesuit designs so far also means that they are also designed to leak air constantly through the joints.
  • The microbes that leak from spacesuits and airlocks can feed on the dead remains of their predecessors.
  • You have trillions of them, and it only needs a few of those to be pre-adapted or to adapt quickly to Mars.
  • They 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 spaceship, but just float out onto the Mars surface as spores in the air, or on flakes of skin and hair.
  • They may fall into shadows, so protected from UV light, can get caught up in the dust and spread throughout Mars in the dust storms.

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.

ALH84001 as an example of what we may find in the search for past life on mars

Some of you may remember when president Clinton announced the possible discovery of past life on Mars in a meteorite, the famous ALH84001. And then perhaps the anticlimax afterwards when later investigations were not able to prove that it was definitely life? It hasn't been disproved either. The scientific jury is just out on what it is at present with some scientists arguing in both directions.

Many astrobiologist think that ALH84001 is 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, or 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. 

If it is life, then the supposed cells seem to be too small to include all the cell machinery of modern life. The discovery of the possible life in this meteorite lead to a 1999 workshop to try to figure out if such small things could be alive. And the answer was yes, though present day life simply can't be so small and include all the machinery to reproduce, early life could be as small as tens of nanometers in scale, far beyond the optical resolution limit of 200 nm. 

So, well to the ordinary person, not an astrobiologist, and especially if you are keen to "touch Mars" or at least for someone to do that, if not you - perhaps your thought at this point is

 "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, and apart from them, who cares 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 there's something much more interesting about this than another obscure microbe that happens to be smaller than any others found to date (if Mars life does turn out to be like this).

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 widely diverse - the fish, fungi, trees, birds, animals, starfish, octopuses. Adding a few microbes too small to see hardly seems likely to add to that diversity. However underlying all that life 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, bird, animals, they all look pretty much the same at level.  This amazingly complex process is going on in each and every one - and at roughly the speed of this visualization. This is not an actual video of the interior of a cell, but scientific art that depicts it as accurately as possible, a scientific visualization of the cell processes.

All Earth life uses the same language here. To find a new form of life would be like discovering your first new language if all you have ever known before is English (say) and you knew in principle that the words must come from other languages but you have never heard any other language or seen any other language written.

What's more the interior of the cell is the same or similar in many other ways too.  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 as a pigment but to protect chlorophyll and to convert blue and green light in the range 450 to 570 nm in the visible spectrum 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.

Credit: Zina Deretsky, National Science Foundation

They got this ability through horizontal gene transfer from a fungi. This didn't transfer the actual carotene. Rather, it transferred just the instructions for making carotene, which when incorporated in the cells of the aphid lead to it making carotene also. What's more, it does it through a complex biochemical pathway that is identical in both the fungus and the aphid. 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 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 if it uses DNA too, and we are related, even if our last common ancestor is billions of years ago. In one experiment 47% of the microbes (in many phyla) in a sample of sea water left overnight with a GTA conferring antibiotic resistance had taken it up by the next day See also, Horizontal gene transfer in microbes much more frequent than previously thought

All present day life is like this. It always has DNA for inheritance, which is unzipped and converted to messenger RNA, and all life uses the same translation table to translate particular triplets of nucleotides in the RNA into amino acids which then combine together to make the emerging protein chain. The ribosome which does this translation is made up of a mixture of RNA and protein, and has to be a rather huge molecule. This makes the minimum size for a modern cell, if spherical, about 200 nm. Anything smaller and there just isn't enough room to fit in everything the cell needs to function.

Early life just couldn't have started like this as the whole thing is far too complex to form spontaneously from non living chemicals. It probably didn't have DNA as that's quite fragile if it isn't held together inside a living cell. It may have had only RNA (or some other biopolymer). Instead of the huge ribosomes made of protein and RNA, it may have used the far smaller ribozymes made up of fragments of RNA alone, to do the translation and other cell operations.  It may not have needed proteins at all. The interior of the cell may have consisted largely of RNA in different forms - this is the so called "RNA world hypothesis". Early life based on those ideas could have had cells as small as 50 nm across. This leads to the idea of a shadow biosphere on Earth. This idea was quite popular a while back. It was an idea of a 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 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.

The hammerhead ribozyme made up of fragments of RNA, stitched together with no use of protein chains to make the enzyme - a surprising discovery. This reinvigorated the idea of an RNA world with tiny cells and only needing RNA with no need to translate from DNA. The cells would only need nucleotides with no need for proteins or amino acids and would not need all the translation machinery to convert DNA into messenger RNA. As a result the cells could be far simpler than modern DNA life. This is one suggestion for an intermediate stage between the earlier organics and modern life, and is the basis for the RNA world hypothesis.

Stephen Benner and others have suggested that there could be RNA world organisms still here today, undetected because they have ribozymes instead of ribosomes. That's the Shadow Biosphere hypothesis. The theory has not yet been confirmed on Earth. However the RNA world hypothesis is also an alternative theory for the small cell like structures in ALH84001.

Well 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, it may exist on Mars, or it may have been on Mars in the past, and remnants of it still survive there.

The idea that the structures in ALH84001 might be these RNA world cells was suggested originally by the fourth panel in Size Limits of Very Small Microorganisms: Proceedings of a Workshop (1999), convened after the announcements of ALH84001. Now that scientists have found alternative ways the structures, magnetite, and organics could form without using life, based on unusual conditions on the Mars surface, this meteorite is no longer thought of as proving the existence of past life on Mars. But it hasn't disproved it either, and the  jury is still out on whether the structures in ALH84001 are life. 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 amongst many other accomplishments) and Paul Davies.

"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 (Benner 1999). The shapes in meteorite ALH84001 just might be fossil organisms from a Martian "RNA world".

There is another way also to see that we must be missing a huge amount of knowledge about early life.

Half of the pages of book of evolution have been torn out

This was an idea some researchers had to plotted the increase of complexity of DNA. They found a way to ignore junk and duplicated DNA so that they count only what is essential to the genes of the organism. 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 this straight line means that it always takes about the same amount of time for the complexity to double.

They traced the timeline back, expecting it to cross the zero line at the time of origin of life, and found 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 of the organism than the total length of its DNA. Some microbes have more DNA than a human being - much of that used for other purposes rather than for genetic coding, the so called C Value Enigma. Measuring it this way 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. Eukaryotes are cells with a nucleus to store the DNA, and prokaryotes don't have a separate nucleus.

So, either evolution started before the beginnings of our solar system (perhaps brought here by impacts on another planet around another star that passed through the collapsing nebula as our solar system was forming) - or else - evolution was far more rapid in its early stages. Both are plausible. The straight line may just show the characteristic slope for DNA based evolution so earlier life could have evolved far more rapidly.

Either way, you'd expect that as many stages of evolution were needed to get from non living chemistry to the most primitive known cells without a nucleus (prokaryotes), as were needed to get from them to modern mammals. We are missing steps there as radical as the step to cells with a nucleus, multicellular life, creatures with a backbone, warm blooded animals and mammals.

How did early cells work? How did they evolve all the complexity of modern life? How did they get to the two biopolymers RNA and DNA? How did the translation system by which RNA is converted to proteins evolve? There is much that is arbitrary, such as the translation table by which triplets of RNA base pairs get converted to amino acids to make proteins. What about the cell walls and internal structures of cells? Astrobiologists have lots of ideas but have no idea how it actually happened. Nor can they create any novel lifeforms however primitive to test out the ideas. They just don't know how to combine RNA, ribozymes etc to make something that actually works.

If we find early life, precursors to Earth life, then it can't possibly work in the same way as modern life because it's not complex enough. Transfer the genes for carotene into an RNA world cell, and it won't be able to make carotene because the cells won't be complex enough, and won't even be able to cope with DNA. So how did they work?

You might think, why not just make an RNA world cell, and see if we can get it to work. Well, we can tweak modern cells,tinker with it, we can even make a novel complete gene sequence and insert it into a cell. We can add an extra base pair. But we can't make a living cell cell from scratch. It's just too complex, and interconnected, with all the parts needing to work together. The simplest modern living cell is way way beyond anything we could make from scratch from inorganic chemistry, if we didn't have it already.

We'd never have invented DNA based life if we didn't have it already. So they can't make a blueprint for an RNA world cell either. All they have is a sketch of a way that various components might be able to come together to make an RNA world cell. Our experiments in randomly combining chemicals in conditions to replicate early Earth can only get us a tiny way. We can't simulate an entire ocean left to evolve for millions of years.  So, there isn't really much we can do to explore these ideas of early life, except actually find it, or find other forms of life that may shed light on what is possible. So it would be the most amazing discovery 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, the biggest discovery in biology of this century most likely. We definitely have a possibility of finding out about early evolution of life on Mars.

Life on Mars dancing to a different tune

We'd have a different dance of life from Mars to compare with the dance followed by all Earth life.

If independent in origin, it would have its own versions of DNA, mRNA, ribosomes, RNA polymerase, mitochondria, cell walls, lipids, proteins, gogli apparatus, lysosomes, microtubules, and all the other things that make up the complexity of modern living cells.

RNA polymerase used to decode DNA to mRNA, present in all living cells.

Golgi apparatus - essential organelle in most Eukaryotes

Ribosome translating mRNA into a protein

Microtubules, strands that stretch through cells, a bit like the corals in a coral reef.
ET microbes, if independent in origin, would have a completely different "ecosystem" of these structures.

 One analogy that I've heard is that if you are a cell microbiologist studying the interior of a cell, it is so complex and unique it's like studying an entire ecosystem. So, imagine that you have been brought up in the African savannah - with its grasses and trees and 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, never traveled more than a few miles from your hut, and that's the only thing you've ever known. In this analogy this is like the interior of a cell on Earth, any cell from any living organism or microbe here.

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. Lighthouse, Ribbon Reefs, Great Barrier Reef. Photo by Richard Ling

Here I'm using the analogy, that the interior of a cell is so complex it resembles an entire ecosystem.

The "ecosystem" of the interior of an ET microbe could differ from the "ecosystem" of Earth life, as much as the ecosystem of this Australian coral reef differs from that of 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 more small microbe like many others but perhaps much smaller - inside it is as different as a coral reef is from the African Savannah.

So hopefully this can help you see how what the astrobiologists are looking for is not just another boring microbe that happens to be smaller than anything we have on life. In the best case, 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.

If so it could be vulnerable to whatever made it extinct on Earth.

Something amazing to discover - but hard to find

But if that is something waiting for us to discover there, 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) 
  • 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?

We will have those same problems on Mars if we study similar samples there. But at least, if we can keep them free of Earth life, we will know for sure that it is not contaminated by life from Earth. Nearly all the organic carbon in ALH84001 is known to be terrestrial contamination.

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. 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.

Organics on Mars could be

How do we distinguish between those different forms of organics? If there are any organics from ancient life on Mars, they will need to be well preserved for us to detect them. The very last thing we want to do is to add in an extra spurious signal from modern Earth life to make our task harder than it is already.

Preservation of past organics

Mars is a great place for preservation of organics in some ways. 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.

The Mars surface is very cold, just centimeters below the surface, perhaps 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 is billions of years old, hardly changed since the formation of the planet.

However there are other things that make preservation of ancient organics harder. The main problems here are that the organics can be degraded by many processes on Mars, they may have been present only in a few favoured spots originally, and the search is confused by a constant influx of organics from meteorites and comets which may have chemical signatures that mimic some life processes.

  • Later episodes of flooding

    Artist's impression of Gale crater as it might have looked during one of its flooding episodes (by Kevin Gill). 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.
  • It gets supplied by organics from meteorites, comets and created in volcanic processes. These would get mixed with the organics from life which we are looking for. And to make things more confusing, the meteorites often have a chiral signature.
  • n this 2006 analysis the EET92042 and GRA95229 meteorites 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 isovaline, while the EET92042 meteorite ranged from +31.8‰ for glycine to +49.9‰ for L-alanine. It's thought that these excesses are extraterrestrial and not due to contamination by Earth life.

    They are certainly not pristine, are altered by water, but they come from Antarctica so less likely to be contaminated, and the mix of amino acids is non terrestrial so they don't seem to be a result of contamination. Also it has 2.5 times greater than typical levels of organics in Antarctica. So these excesses may be extraterrestrial and not due to contamination by Earth life. For a more recent review of this, see the Chemistry Society Review article: Understanding prebiotic chemistry through analysis of extraterrestrial amino acids and nucleobaess in meteorites.

    There are various theories about how meteorites may have got this excess originally, see Circular Polarization and the Origin of Biomolecular Homochirality Whatever the reason, it complicates the search for life on Mars.

  • 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 of organics. All the organics found so far probably came from meteorites and comets.

  • 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.
  • High levels of cosmic radiation - either originally when the deposit is formed, if it is not buried rapidly enough - or later when it is unearthed again on or near the surface. Every 650 million years you get a 1000 fold reduction in the concentrations of small organic molecules such as amino acids on the surface because of cosmic radiation. So that's a million fold reduction every 1.3 billion years. We may have to dig deep to find life that has escaped this process. Probably at least meters deep. The usual recommendation of astrobiologists is 10 meters deep ideally. ExoMars will be able to drill 2 meters which is enough so that it has a chance of finding evidence of past life.
  • 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.
  • 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 events to find it. There are many other ideas about where life might have started, so what if it is only in the place where it first evolved and never got any further, wherever that is?

So, this research suggests it is likely to be far more difficult to find past life than you might expect. It's no surprise that Curiosity hasn't found it yet - it is just not looking in the right way in the right place to find it. It's just searching for past habitability, a brief that it has fulfilled rather well. But the organics it has found already are thought to have come from meteorites or comets. 

Any organics in samples the team for Curiosity 2020 selects to return to Earth are almost certainly going to come from those as well. The Mars surface has a continuous influx of organics ,and meanwhile the surface perchlorates, hydrogen peroxide and the cosmic radiation and solar storms damage and remove whatever organics are there already. It's unlikely that we will find traces of past organics unless we do the search for life and unambiguous biosignatures on Mars itself in situ.

For a clear signal of past life

So, for a clear signal, for past life, your sample somehow has to 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. No use looking in a place where they were being washed out.
  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. That's because the Mars atmosphere gives little protection from cosmic radiation. This radiation has little effect over thousands of years, but over millions to billions of years timescales even large quantities of organics get broken up into water vapour, methane, carbon dioxide etc. This could easily degrade it beyond recognition through cosmic radiation.

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
  3. Was returned to the surface rapidly (perhaps as a result of a meteorite strike), 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.

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 surface area the same as the land area of the Earth. There are plenty of places to look for this life on Mars, many varied geological features and terrains. Including ancient deltas, shore lines, salt beds, preserved hydrothermal vents. It must have caves made through the passage of water, and vast layers of sediment. 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 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.

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. 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.

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.

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 many more steps still to go through. See Habitability, Taphonomy, and Curiosity's Hunt for Organic Carbon 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 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.

If we get any Earth life on Mars 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.

  • 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

The best way to search for early life, as far as we can tell at present, is to search for organics. However 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 for organics with nitrogen on Mars. Nitrogenous organics are likely to be rare because there are few sources of nitrogen on Mars. This is important because nitrogen bonds are easily broken and are central to biology as we know it. So 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 that wasn't a detection of these nitrogenous organics.

Once we find these compounds, that's not enough as you also get nitrogenous organics from comets and meteorites and natural processes. We then need to search for biosignatures. We also need to be able to drill below the surface (as ExoMars will be able to do) to the maximum depth possible. That's because our best chance of finding evidence of past life is to drill down below the surface layers damaged by ionizing radiation, ideally to ten meters depth or more (though the two meters depth of ExoMars is a good start here).

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 biosignatures on Mars, or have exhausted all other possibilities by in situ research

If we follow this program we need to send instruments to Mars of exquisite sensitivity to look for traces of past life in situ. Astrobiologists have designed instruments for Mars so sensitive they can detect a single amino acid in a sample. For details see In situ instrument capabilities below

Nasa's plan for safe zones - based on finding Mars life easily

If we knew where to look, 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 seem to imagine it happening like that, pretty quickly. If you find life as quickly as that, and supposing you are content so long as  you discover it first and 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 them to analyse.

That's NASA's current plan - an exploration zone, with the human occupied field station in the center, and robotic spacecraft heading off for in situ study around the perimeter, and returning samples to the center. To them this seems like a good compromise, with humans on the surface, lets humans "touch Mars" but they do their best to limit the effects of the microbes by restricting human movement geographically on Mars.

Here is one example, with the human exploration zone shown close to an area of special interest - the recursive slope lineae or warm seasonal slopes, which may have liquid salty brine seasonally, one of the suggested habitats for present day life on Mars:

See Mission to Mars: The Integration of Planetary Protection Requirements and Medical Support and Mars colony will have to wait, says NASA scientists

The "Safe Zone - cleared for human exposure" is a zone without any present day Mars habitats in it, and a region where you don't mind if there is Earth life introduced to Mars. So, 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 are used to study the regions beyond the zone remotely.

That could work just fine on the Moon. 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 or wastes or debris from the habitat. So long as the rovers can also be sterilized sufficiently in a human base, they could be used in just this way to do clean studies of, say, the volatiles at the poles just a few kilometers from the human base. There is some transport even on the Moon by electrostatic levitation of dust, but it's a rather minimal effect.

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 which can form as a result of the cosmic wind impacting on the Moon. The Apollo samples were recently re-analysed and the composition of amino acids suggests some extraterrestrial sources, The 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, which suggests we need to take care to avoid this sort of thing as we explore the Moon.

So even this is quite challenging. 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 it clean while handling it in their base as they analyse it to try to find out what is in it? Or would they sterilize the rover only once, on Earth and then use it on Mars for a single mission only to the RSL?

It may seem easy to do on paper, but when it gets to actual practice, are they going to keep the Earth microbes away from an RSL with rovers driving back and forth between the human base and the possible habitat 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, e.g. when searching for organics in the polar ices. It will be far easier to do on the Moon so can get preliminary ideas there about whether it is possible in an easier situation than on Mars.

How could this work on Mars with dust storms and a globally connected environment?

But 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 years, and in rare cases, 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 would barely move an autumn leaf. 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 too, 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.

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 Carl Sagan raised 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 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 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.

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 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 they 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. If the human habitat is positioned close to a special region as in the suggestion by Jim Rummel above, these figures suggest that they might get to a vulnerable region in a dust storm in much less than twelve hours. So, the microbes could get to nearby habitats perhaps quite early on. 

I've tried to find experiments simulating transport of likely microbial spores in Martian dust storms. But can't find much. Plenty of experiments to show that microbes can survive covering by a thin layer of dust on the surface. I suppose it is hard to simulate.

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, but the sub zero conditions and transport in the Mars winds had little effect - including simulating electrical charge effects in the suspended dust, but they only simulated that for twenty minutes. So the experiment itself wasn't very conclusive, but combining it with previous results on survival of microbes when shielded by dust on Mars, they 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, and their paper has no citations yet in Google Scholar. The only other paper I've found so far is this one from 1970 which found that simulated Martian dust storms did not sterilize the spores:

"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 more detailed 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 amongst 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 future generations or indeed even ourselves in future decades after the first human landings on Mars? It's hard enough if you only need to worry about microbes that escape from air locks, and from spacesuit joints and such like - and any wastes intentionally released onto the surface.

But all those pale into insignificance when you consider what happens if a human occupied spaceship crashes on Mars.

Crashes of spacecraft on Mars - robotic or human occupied

Mars is probably the hardest place to land in the inner solar system. If you imagine humans landing much as they did on the Moon - well no, it can't happen like that on Mars. basic problem is that Mars has double the gravitational field of the Moon. To fight against double the gravitational field requires a lot more than double the amount of fuel by the rocket equation (fuel has to carry more fuel), and the lunar module would have no chance at all landing there.

Also as well as that, on the Moon you can orbit as close as you like to the surface and the only problem is that you have to avoid hitting the mountains. You can adjust your orbit, wait for as many orbits as you like until ready, if you have enough fuel you can delay your landing looking for a good place to land (as Apollo 11 did), abort back to a higher orbit if it fails, and with enough fuel you can try again if needed, and take your time about it. A human can pilot a spacecraft to a landing on the Moon by hand, as Neil Armstrong did with Apollo 11. That's impossible on Mars.

On Mars, once you start the landing sequence, and you hit the atmosphere, you are committed. You are streaking through the atmosphere at kilometers per second. Everything after that has to work in a perfect sequence with timings accurate to seconds,, far faster than a human being could react. The result is that 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 if spaceships to Mars crash.

First the aeroshell and aerobraking. Then you need the parachute, because it would just take so much fuel to do all the slowing down using rockets.

See Schiaperelli: the ExoMars Entry, Descent and Landing Demonstrator Module

So then you have to find a way to slow it down from those hundreds of miles an hour to a slow enough speed for a soft landing.

So that’s why you then have the retro propulsion stage for most 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. So the moment of parachute release is very very important, to get that right. It seems that Schiaperelli for some reason released the parachute a bit too early, which was the start of its problems.

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 control over where exactly you land. Instead you have a landing ellipse. This is the one for Schiaperelli, 100 km by 15 km

There is no chance at all 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 so 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 and it is hard to predict exactly. There’s also the uncertainty of the speed and position of the spacecraft as it enters the atmosphere.

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. On Mars you have to be able to land safely wherever you happen to be in that huge landing ellipse. 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 been the end of the mission if it had landed on it

They deal with that as best they can by choosing regions on Mars that are very flat, ideally you want to have hundreds of square kilometers that are pretty much completely flat with no boulders or steep slopes. 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 to Mount Sharp because it would then risk landing on a big boulder or on a steep slope.

Now 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, it permits a much heavier payload also, but in other ways it is riskier.

Conceptually it is about as simple as you can get. The rocket doesn’t have an aeroshell or parachute or anything. It just decelerates.

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. Indeed if landing in the Valles Marineres, big rift valley, rift in the Martian highlands, it would need to skim down between the walls of the canyon. All this time the rocket is firing and it is also affected by the friction of the atmosphere. Finally, it comes to a vertical landing.

SpaceX has actually done this on Earth. Their barge landings of the first stage actually have to use supersonic retropropulsion and what’s more, they can achieve a pinpoint landing as well - when it works. So it can certainly be done, but it is rather risky and tricky to do on Mars with the very thin atmosphere there and the atmosphere far more variable in density than Earth’s atmosphere too.

The other way to do it is to use absolutely enormous parachutes. If the parachute is big enough, you can have a conventional landing just as for Earth. Simply use aeroshell, and then parachute, and parachute down and the parachute 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 on Mars so far were tested by firing rockets in suborbital trajectories and then releasing parachutes and required many tests.

To make even larger supersonic parachutes will require many expensive rocket tests. NASA are working on this with their Low-Density Supersonic Decelerator - Wikipedia

This is just a rough idea of how it works. For more on 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. The rocket lands on the Mars surface in reverse. It has to use the atmosphere for aerobraking, and simultaneously fires its rockets to bring it to a standstill on the surface. The atmosphere is only thick enough for this close to the surface, so it skims down to a landing within a few kilometers to the surface - so close that it can't land on mountainous areas of Mars because the air is so thin. 

Artist's impression of red dragon doing supersonic retropropulsion over Mars, image SpaceX

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."

He isn't talking about dangerous as in a scary haunted house or fairground ride, where it's scary but you know that you are in safe hands. It's not 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 much more dangerous than base jumping. You could easily be killed by it for real. And 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.

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 study near the crash site. Your first assumption, if you found biosignatures near the crash site would be that they came from Earth. That could be devastating for science, especially if the humans crash happens close to somewhere biologically interesting on Mars. And that in turn is likely if the human base is situated in a 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, the crash site would be a source for life itself to spread throughout the planet. If there is any life able to adapt to live on Mars, and any habitats there for it, a crash of a human occupied mission on Mars would mean the end of all planetary protection of the planet.

See also my 

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, 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. 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?

Methane plumes on Mars and deep hydrosphere

Mars, like Earth, gets warmer as you get further below the surface. It might have a hydrosphere, a layer of liquid water perhaps a few hundred meters thick, trapped below thick layers of rock and ice. There's probably ice and then water kilometers below the surface even in the equatorial regions. So, even before 2008, astrobiologists thought that there could be deep down habitats for life on Mars. Then there could be geological hot spots near the surface too. Mars is still geologically active, though not nearly as much so as Earth. Despite many searches, there's no sign of any current volcanic action or hot spots. But there are signs of volcanic eruptions in the Olympus Mons caldera and other volcanic effects as recently as a few million years ago. So there could be hot spots not far below the surface, masked by the surface ice and extreme cold from our orbital instruments. There could even be fumaroles, with gas and vapour escaping to the surface but hidden from our sensors by an ice tower.

All this was just theory until we had observations of methane plumes from Earth. They were puzzling though, as the methane seemed to disappear from the atmosphere so rapidly that it was hard to work out a physical process that could do this. Also these were delicate measurements and needed to be confirmed. But 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. They could also be 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 in 2017. It is sensitive to up to 10 parts per trillion to many different chemicals in the Mars atmosphere, including methane.

So where does the methane come from, if these signals are genuine? Well there are various ideas but all 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) inorganic processes in the atmosphere, volcanoes, or it could be that it was already present on Mars when it formed, locked in clathrates and released
  • Products of past life, again locked in clathrates
  • Present day life using serpentization as an energy source
  •  Present day inorganic processes

We may have spotted methane on Mars. If so this figure from NASA / JPL shows possible sources. One possibility is methane clathrate storage. It's possible that early Mars had large amounts of methane in its atmosphere which helped keep it warm. 

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. So, 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 or compete with them. 

This is not the only way the surface could be connected to the deep subsurface. One of the theories for the warm seasonal flows, or Recurrent Slope Lineae is that they might be the result of water from deep below the surface getting to the surface in regions of geological hot spots. Again this means it could be possible to contaminate the subsurface, maybe even the entire deep subsurface hydrosphere, if it is connected, via the RSL's. 

As Cassie Conley pointed out 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. See Going to Mars Could Mess Up the Hunt for Alien Life (National Geographic).

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? If there are surface habitats, this makes planetary protection even more of an issue. First though, is surface life even possible there.

UV light, cosmic radiation and perchlorates on Mars - why they aren't lethal for surface life - and how can the atmosphere be in equilibrium if there is life there?

Until around 2008, 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 

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

Nilton Renno 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:

So, you might wonder what has changed, if you have read the many articles on this subject from about six years ago.

Go to the Life on Mars article on wikipedia, and you read:

"Although Mars soils are likely not to be overtly toxic to terrestrial microorganisms, life on the surface of Mars is extremely unlikely because it is bathed in radiation and it is completely frozen..."

Hasn't the professor read wikipedia? Why does he think there could be life on the surface of Mars?

Read Encyclopedia Britannica, and it is a bit more up-beat about life on the surface

 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. I'll look at each in turn.

Ionizing radiation

Before 2008 - the idea was that if there are any spores on or near the surface, they have been dormant all the time, since the last time Mars had a slightly thicker atmosphere, 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 Ionizing radiation could easily destroy even the most radioresistant microbes in a million years. So any remaining viable life would have to be well below the surface.

It is not easy to simulate Mars condition in experiments and it was quite a surprise to researchers when Phoenix found evidence for liquid drops forming in Mars surface conditions. They then did the experiments 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.

Mars is not laced with lethal radiation for living organisms. The surface of Europa is. Jupiter has such strong ionizing radiation that humans would only last hours in vicinity of Europa before they die of radiation poisoning. And even highly radioresistant microbes wouldn't last long.

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 it 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, no need 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, the cell joins those fragments together to repair the DNA. They can do this even in semi dormancy, wake up a bit, and repair their DNA then go back to "sleep".

As an example, take Chroococcidiopsis, 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 microbes have never encountered such high levels of cosmic radiation - at least as far as we know. Perhaps it 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. So Mars microbes, evolved in conditions of high levels of radiation may well 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 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.

So why then was it sterilizing for organisms dormant for millions of years? Well suppose for instance that the radiation was strong enough to kill half the organisms in a hundred thousand years. Then after two hundred thousand years there'd be only a quarter left, and it halves every hundred thousand years. After a million years there'd be only a thousandth of the original organisms still viable. After two million years, only a millionth, and I think you can see that after a few million years, there'd soon be nothing viable left at all. So that's how it works with radiation. That lifeforms can be completely sterilized over millions of years, yet the radiation causes no problems so long as they can revive enough to repair their DNA at least once every few millennia. Life in these Mars habitats, if they exist, could revive every year.

Ultra violet 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. So, UV light also is a challenge, but some life will have no problems with it.


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 chlorates (ClO3) and chlorites (ClO2) which have more serious and immediate effects on us "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.

James Lovelock's argument for a lifeless Mars before the Viking Landers

This idea goes back in 1967. James Lovelock, originator of the Gaia hypothesis, found a way to use a planetary atmosphere to detect life. He suggested that we look for simultaneous presence of pairs of gases like oxygen and methane that react together. We can also search for gases such as oxygen above  levels expected from abiotic processes.

As  far as we can see, Mars atmosphere seems to be close to equilibrium in this way. So when Viking I and II landed there in 1976, and found a barren desert-like surface, it seemed natural to conclude that there is no life on Mars.

However, just the following year, 1977, scientists trawling the ocean depths found life which could not be detected by his methods. Since then we've found many other ways life can get hidden from these observations of the atmosphere:

  •  Deep underground
  • In caves
  • Past life that goes extinct
  • Life that flourishes every few million years like poppies in a desert
  • Sparse populations on the surface right now.

The last of these is the newest, sparking many researches into habitability of present day Mars over the last few years, since Phoenix. Life on a planetary surface could be hidden from these atmospheric measurements simply because of low populations, and a slow metabolism. For details, see  "How much oxygen would present day life produce on the Mars surface" below.

(You can get this article as a kindle ebook - 32 printed pages equivalent)

Original argument

For his original paper see Life detection by atmospheric analysis (1967). 

When he wrote this, we had no observations from the Mars surface. The only close up information he had was from the Mariner 4 flyby.

Our best image of Mars when James Lovelock's paper was published - this was taken by Mariner 4 in 1964.

But he didn't need any observations from close up. Its atmospheric composition is something we can measure from Earth, by clever disentangling of doppler shifts - a technique pioneered in the Lick Observatory observatory in 1926,

Lick observatory in 1900. First conclusive proof that there is almost no oxygen on Mars was a paper from this observatory in 1926

Atmosphere out of equilibrium without life

James Lovelock's argument is not about oxygen alone, because it can be created abiotically.

For instance, on the early Mars, its low gravity makes it easy for the early oceans to evaporate - and then the hydrogen and oxygen dissociate - because it has almost no protection from solar radiation - and then the hydrogen could escape, leaving an oxygen rich atmosphere.

You wouldn't need much, not the 20% levels of Earth, to turn Mars red.

This is the suggestion of Bernard Wood in a paper published in Nature last year.

This shows Gusev crater. He came to his conclusions after studying rocks from Gusev crater, amongst other evidence.

Jupiter's moon Europa has a similar story going on - though for different reasons.

Jupiter's intense radiation splits hydrogen directly from the ice in its icy surface, leaving it oxygen rich. Eventually enough of this may find its way to the under ice ocean keeping it oxygen rich.

See Europa's churn leads to oxygen burn

James Lovelock's life detection method

His life detection method is based on search for methane and oxygen simultaneously, and other similar trace gases.

"The biological significance of an atmospheric mixture lies in the relative concentrations of a variety of constituents and not wholly in the presence or absence of any single one of them. Such a mixture is biologically significant if it represents a departure from a predictable abiological steady state. The strongest evidence is the simultaneous presence of two gases, like methane and oxygen, capable of undergoing . irreversible reaction; even with gases which can be expected to occur under abiological conditions, departures from the abundances to be anticipated will be of biological significance."

He also suggested that we search for life by looking at isotope ratios. 

Viking results

Most of the experiments on Viking, didn't detect any life on Mars. 

One experiment gave ambiguous results - for more about this, see Rhythms From Martian Sands - What Did Our Viking Landers Find in 1976? Astonishingly, We Don't Know

There is no doubt that his paper was an important contribution to the field - and it went on to become the foundation of his influential and important "Gaia" theory.

However almost immediately, scientists began to find ways that life could evade detection by James Lovelock's method when they discovered hydrothermal vents in the ocean floor, with some lifeforms there that didn't depend on the surface at all. They put the ocean out of equilibrium locally but would not be detected as life by his method, in an ocean world with no other life on it.

Mars hydrosphere

Mars might also have life in its hydrosphere. Unlike Earth, Mars has  a cryosphere over its entire surface - a deep layer which remains at temperatures below freezing point of water. But deep down, temperature rises, and a few kilometers below the surface - it gets warm enough - and also pressures high enough - so that there may well be a layer of liquid water. Possibly a hundred meters thick, possibly deeper. This could be an ideal habitat for microbial life - almost completely disconnected from the surface.

Pole to pole cross section of Mars showing the cryosphere - permanently cold enough for ice - and below it, the saline groundwater. This may well be the most habitable region of Mars - on Earth similar habitats deep below the surface have abundant life - but may not be much exchange with the surface.

Mars probably also has hot spots - as it has been geologically active in the recent geological past (though no confirmation yet of any present day hot spots and geological activity). If so - if the hot spot is beneath a "trapping layer" much like an oil field - it could provide another habitat with liquid water and almost no communication with the surface.

Snowball or slushball Earth

Then a bit later we discovered - that life doesn't need to cover a planet continuously.

Earth has gone through phases in the past when it was covered in ice, or almost completely so. This is the Snowball - or Slushball Earth

Slushball Earth

Perhaps Earth had patches of open ocean in the tropical regions, or perhaps it was completely covered in ice. 

Possible Snowball Earth models

For millions of years - the entire surface of Earth -except possibly a small region of pack ice near the equator - was completely covered in ice. Was as barren as Antarctica - but without the penguins of course. There was life there - but so little - that if you had landed a Viking probe on a random spot on Earth, chances are you would not have spotted a thing.

Once a planet is covered in ice - then it becomes  highly reflective, it doesn't absorb as much heat from the sun - so it is pretty hard to get out of it. Earth got out of this phase because of continental drift, then eventually limestone that deposited in the oceans during its warm phase gets subducted beneath the continents, and then returns to the atmosphere in volcanoes. With no life to turn that back into organics, and no oceans for the CO2 to dissolve into - then the atmosphere got richer and richer in CO2 Finally the ice over the oceans began to melt, and life started to flourish again.

Snowball Mars

This then raises the interesting idea - what if the same thing happened on Mars? Scientists began to wonder if Mars might have had life in the past - now vanished. Or perhaps life there - but only occasionally depending on the tilt of its axis. Mars is dry right now, but might have shallow ponds occasionally as the atmosphere gets thicker. At other times, it might survive as dormant spores, perhaps buried deep below the surface in caves or underground, protected from cosmic radiation.

We now have pretty conclusive evidence that Mars had a shallow ocean covering much of its northern hemisphere in the first few hundred million years of the solar system (and briefly again a billion years later). At the very beginning, it's thought that Mars, Earth and Venus were almost identical with dense CO2 rich atmospheres of tens of atmospheres of pressure - which raised the boiling point of water permitting all three planets to have global oceans of water well above boiling point - planetary pressure cookers. These atmospheres were also nitrogen rich - and the seas were rich in nitrates formed during asteroid impacts in the "Late heavy bombardment" - and probably also organics delivered by comets.

Mars is further out from the sun of course, and had no Moon impact. So life may have begun there earlier than on Earth. It cooled down more quickly however - and entered the snowball phase far sooner than Earth. And - Mars has no continental drift. So unlike Earth which went through many snowball phases and survived - Mars just got colder and colder. There were temporary respites - floods  perhaps due to giant impacts - also its axial tilt and orbit changes far more than the Earth's so that also led it's oceans to melt from time to time. But before long it went into a permanent snowball phase. And then eventually - the ice disappeared as well - either lost to space - or underground - we don't know. At any rate it is currently as cold or colder than Snowball Earth ever was - but without any ice so dry as well.

What would happen to life on present day Mars

So - let's suppose for purposes of discussion that Mars did have life in the early solar system? What would happen to it as a result of this snowball phase? Well - to start with for sure, would be like Earth. There is life there - but it's hard to detect and mainly underground. What little there was left on the surface - would have no detectable effect at all on the atmosphere.

If we were to spot an exoplanet like that around another star - then again - we would have no way of knowing that there is life there. And if you landed Curiosity on it - you would not expect to find anything. Eventually - you might think - with the surface completely frozen, and no liquid water - it would be totally underground - so - this seems, on the face of it - to feed into the idea that Mars must be totally lifeless on the surface - and only have life deep underground.

However, various discoveries in Antarctica and in the permafrost layer have changed this picture

Mars can't get out of its current "dry snowball" phase through biology.

There is no way that life on Mars could turn it back into a planet like Earth. It's too cold, too far from the sun, too dry etc. Earth was only able to escape its snowball phase because of continental drift, which Mars doesn't have But - it could have continued to survive, by "going slow" as the planet became less and less habitable. Or by going deep below the surface.

Microbes that are almost dormant - but not quite

Some deep sea microbes have such a slow metabolic rate that they take 1000 years to divide, They have a metabolic rate of a millionth, or a ten millionth of a grams of carbon per  hour for every gram of carbon in the cell (typical rates are between a tenth and a thousandth of a gram of carbon per hour).  This obviously would have minimal effect on the atmosphere. 

And some metabolize even more slowly.

Mars polar regions - one of the places where slowly metabolizing microbes may be able to survive, similar to P. cryohalolentis, an ordinary microbe but able to repair 10 DNA base pairs a year.

P. cryohalolentis can't survive 600,000 year's worth of damage on the Earth, but was found still alive in 600,000 year old deposits in permafrost and Antarctic sea ice. The researchers believe it did this by metabolizing extremely slowly throughout the time period, using its ability to slowly repair DNA damage at about 10 base pairs of DNA a year. 

This makes it a good candidate for a Mars microbe. The researchers said that it might also have reproduced occasionally - they couldn't tell - but it might have survived simply by using a slow metabolism + DNA repair. See Even Ordinary Microbes May Survive Radiation on Mars

Underground caves

Also underground caves that have almost no communication with the surface. Such as  microbes in the Lechugulla cave, isolated from the surface for millions of years. 

The caves were carved out by sulfuric acid - which recently was drained exposing the giant crystals.

Life in the rocks themselves

Also life kilometers underground within the rocks themselves.

the "Worm from Hell" from 1.3 kms deep, about half a mm long, lives on microbes, need minimal oxygen.

So - perhaps similarly Mars could have life - but only in caves or deep underground, in some way not connected to the surface.

Hydrothermal vents

First of all, just a few years later, scientists discovered the hydrothermal vent communities around the black and white "smokers" on the sea beds. They form at great depths, where the Earth's plates are pulled apart along the mid ocean ridges.

This was in 1977, just a year after the Viking lander on Mars, and ten years after Lovelock's paper, with the first paper on the subject published in September 1977.

These were able to survive with almost no communication with the surface. The higher lifeforms there did use oxygen dissolved in the water. Some of the microbes however didn't rely on anything except chemical energy and did not need oxygen.

Hydrothermal vent

Tubeworms deep under the ocean living on the hydrothermal vent

Complex flow of chemicals for a hydrothermal vent. See Deep sea vents (Microbe wiki)

The interesting thing here is - that a planet could have communities like this at the depth of oceans - and they would have no effect at all on the atmosphere. Even the chemical byproducts just get dissolved in the ocean and do not return to the atmosphere.

Cold seeps

This is just the first of many such discoveries. Now we also have cold seeps, where microbes use methane and hydrogen sulfide as an energy source

See Cold Seep Communities

Life that can survive in tiny microhabitats

Then we have had discoveries of possible habitats for life on or near the surface in microhabitats. I've already mentioned Nilton Renno's discovery of a new way of droplets of water where salts and ice meet  in the section on UV light, cosmic radiation and perchlorates on Mars. There are several other promising ones such as the warm seasonal flows, more of this in a minute. So, what effect would something like this have on the Mars atmosphere? I'm not going to try any detailed analysis. But let's look at something quite crude, the amount of oxygen the lifeforms produce. I know that oxygen is not a sign of life by itself. But the amount of oxygen produced by photosynthetic lifeforms can give us a rough first idea of how much of a difference microbes can make to the atmosphere in these arid sparse microhabitats.

How much oxygen would surface life produce on Mars?

We can get some idea of this from measurements of microbial mats in ice covered lakes in the dry McMurdo valleys.
These measurements were taken in Lake Hoare

Lake Hoare (Antarctic explorers)

Benthic Microbial Mats Ice-Covered Antarctic Lakes

See Photosynthetic performance of benthic microbial mats in Lake Hoare, Antarctica

The researchers estimated oxygen production of the microbial mats at around 0.089 micrograms per square centimeter of surface  per hour. That's 0.0089 grams per square kilometer per hour, or about 89 grams per square kilometer per year of oxygen production.

For the next step we need to know the "residence time" - how long the oxygen remains in the atmosphere. I can't find figures for Mars - anyone know (for methane it is 430 years)? However, on Earth, atmospheric oxygen has a residence time of 4500 years. So, if oxygen lasts as long in the Mars atmosphere as it does on Earth, that takes the total amount of oxygen in the atmosphere up to 4500 * 0.089 or around 0.4 tons per square kilometer That is, of course, if the entire surface was as habitable as ice covered lakes in Antarctica.

How does that compare with the mass of the Mars atmosphere? Using the average atmospheric pressure of 7 millibars, gravity 38% of Earth's, so 10 tons per square meter requires 26 tons per square meter on Mars, so 7 millibars requires about 0.182 tons per square meter, or about 182,000 tons per square kilometer.

So our 0.4 tons per square kilometer would contribute about 0.0002% by weight of oxygen to the Mars atmosphere. As it turns out, Mars does have oxygen already, at far higher levels than that. Curiosity measured 0.145% of oxygen in the Mars atmosphere (mole fraction). We don't really need to convert that to percentage by weight - is clear that Curiosity could not spot oxygen at 0.0002%. And diurnal cycles are going to be less than 0.4 parts per billion, at the limits of its sensitivity and surely hidden in the noise.

That is for the entire surface of Mars as productive as the Antarctic ice covered lakes - and all that oxygen ends up in the atmosphere.

How much oxygen would life produce in rare habitats like the warm seasonal flows?

It's hardly likely that the surface has on average as much photosynthetic life as an ice covered lake in Antarctica. Suppose that it only occurs in the warm seasonal flows? These are exceedingly rare. Each asterisk on this map shows one of the sites

Sixteen sites in total marked, over entire surface of Mars. See Water seems to flow freely on Mars So there aren't many of them. And the habitats themselves consist of just thin streaks on the slopes like this

Suppose, for example, that we had a total of ten square kilometers surface area of these dark streaks on Mars - I mean just the streaks themselves. I expect that's an over estimate - but - this is just to give a very rough idea of what's involved here. Suppose that they really are liquid water, as they seem to be - and suppose they are all inhabited by biofilm of life to a depth of a few mms as in Antarctica. And suppose these are all photosynthetic lifeforms. Then that would be less than 1 kg of oxygen production over entire Mars surface, per year, and assuming a residence time of 4500 years, then a total of 4 tons of oxygen in the atmosphere.

At these levels, we have almost no chance of spotting Lovelock's traces of reactive gases.Many other habitats suggested on Mars are also rare, such as the Martian Geysers. And others may be common - but consist of tiny droplets, like the ones found by Niton Renno's team.

Searching for surface life from orbit with ESA's Trace Gas Orbiter

ESA's Trace Gas Orbiter is sensitive to up to 10 parts per trillion to many different chemicals in the Mars atmosphere. I.e. one part in 100,000,000,000. That would be enough to detect abundant subsurface life. But what about these surface habitats? With 182,000 tons per square kilometer of atmosphere as we saw, TGO could detect a signal of just 1.82 grams of organics released per square kilometer. So - if you had large very productive areas as productive as the biomats in Antarctica, covering an entire square kilometer somewhere on Mars, then TGO could detect the 89 grams per square kilometer it adds to the atmosphere. But to detect the faint signal from a sparse habitat such as the RSL's seems to be right at the edge of its capabilities and probably beyond it, if they are only surface habitats. This is just a crude approximation. I was surprised to work out that it can detect concentrations as low as a few grams per square kilometer. Especially if the processes are not continuous but produce larger quantities of organic gases in the atmosphere over a short period of time, perhaps built up over years then released all in one go,I wonder if it has a chance of detecting life there?

So these habitats may exist and if so, any life there would be hard to spot because it would have minimal effects on the atmosphere. We have high resolution images of Mars from orbit by HiRISE, with resolutions of tens of centimeters if needed. But even that would not be enough to spot microbes hidden inside rocks or in thin layers of brine just beneath the surface.

So - let's look a bit more closely at these possible surface habitats. How likely are they?

Habitats for life on the surface of Mars - warm seasonal flows

There are many other seasonal features on Mars but most are caused by dust, wind, or dry ice. The Warm Seasonal Flows or RSL's are the best known, of the ones that may provide habitats for life, indeed there is indirect detection of water flowing there through hydrated salts, those also seem a pretty sure bet for liquid brine but the question there is, is the brine warm enough, for life, and if it is warm enough is it too salty or is it fresh enough for life?

The better known warm seasonal flows. These form on equatorial facing slopes even close to the equator, but only on some of them, extending downwards from bedrock outcrops. The reason why they form on some of those slopes and not on others is not known at present. These examples are on the slopes in Newton crater. High resolution version and techy details here.

It's pretty much confirmed that they involve salty brines in some form flowing beneath the surface. The dark patches are not damp patches but rather some effect on the surface due to the brines flowing beneath. However it's not known yet whether the brines are habitable -they may be either too cold or too salty for life or both. These are very hard to study from orbit because the highest resolution photos we have of them can only be taken during the local afternoon, the worst time to detect the water. That's due to the orbit of the spacecraft taking the photos, which approaches Mars on the sunny side during the local afternoon. For details, see my Why Are Hydrated Salts A Slam Dunk Case For Flowing Water On Mars? And What Next?

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. That's because they only form on sun facing slopes, only form when the local temperatures get above 0°C, and they lengthen in the spring, and gradually vanish away in the autumn as winter progresses. 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 the wind, as there are many temporary wind formed changes on Mars. You can get dark streaks rather like this from avalanches of dust after a strong wind look. However, the RSL's are not associated with winds and the seasonal changes don't match what you'd expect from winds.

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. There are three main theories for how this could happen:

  • Ancient ice just been deposited there millions of years ago and not yet exhausted (hard to see how there could be enough of it)
  • It's replenished, indirectly, by deep down ice, possibly all the way from the hydrosphere
  • Deliquescing salts just below the surface feed water into the warm seasonal flows (again a bit hard to see how there could be enough water this way)

(for details, see Seasonal Flows on Warm Martian Slopes).

Southern hemisphere flow like features

In the case of the Richardson Crater flow like features - especially if they are indeed centimeters thick layers below clear ice - the water will definitely be both warm enough and fresh enough for life. The interfacial liquid layers also seem promising because of the way the models predict them to flow together into a liquid stream of water that then picks up salts on its way out. First this shows where it 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 - but little 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.

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. Those are widely known and many scientists would tell you how great it would be to look at them up close.

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. 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).

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

BTW it was 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.

I’ve done my best to register them with each other but I couldn’t figure out a way to do it automatically, indeed, they are taken at slightly different angles also so 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. The general idea is clear enough.

All the likely models for these features, to date, involve some form of water. Alternatives 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.

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).

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, then 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 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 equator facing slopes. 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 water) which forms as a thin layer over surfaces and can melt at well below the usual melting point of ice. In Mohlmann'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 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.

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.

Northern hemisphere flow like features

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.

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. 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.

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.

Other surface and near subsurface habitats for life on Mars

The RSL's and the Richardson flow like features are just two of many habitats suggested on the Mars surface. I like to draw attention to the flow like features particularly, because though the specialists have known about them for many years - his paper is from 2010, it is one of the least publicized, yet in some ways most interesting potential habitats because of the potential for fresh water at 0 °C. As far as I know it is the only surface habitat so far that has the potential to be so warm and also to have fresh water. For some of the others, see

Earth life that could contaminate Mars habitats

None of this would matter if Mars was so different from Earth that no Earth life could survive there. For Earth life to survive on Saturn's moon Titan would indeed be like sharks surviving in the Savannah. Temperatures there 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 would be no issues with contaminating Mars if conditions there were like Titan. But no, it's actually rather habitable for Earth life - for extremophiles that is. Though no animals or humans, birds, insects could survive on Mars, and most plants couldn't either there are some lichens and microbes from Earth that would fit in and be right at home there - in the right situation.

If these habitats do exist and are habitable, there are many Earth microbes which have been shown to be able to survive in Mars simulation conditions, and so could potentially survive there, contaminate them and make it difficult or impossible to study them to find out what was there originally.

Researchers at DLR (German equivalent of NASA) testing lichens in Mars simulation experiments. They showed that some Earth life (lichens and strains of chroococcidiopsis, a green algae) can survive Mars surface conditions and photosynthesize and metabolize, slowly, in absence of any water at all. They could make use of the humidity of the Mars atmosphere.

Though the absolute humidity is low, the relative humidity at night reaches 100% because of the large day / night swings in atmospheric pressure and temperature.

Here is a list of some of them, for the cites 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[200] - 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 elegansPleopsidium 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.

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, see my

(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)

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

Need for robotic exploration first

All these are places we can explore by telerobotics using increasingly capable robots, also explore using robots controlled from Earth. There is no need to send humans to these places as quickly as possible. It won't help to make us multiplanetary, but it may mean we miss out on discoveries about the origins of life, and other lifeforms. Imagine if you could learn about life on a planet or in the ocean of an icy moon around another star? Even if it was just 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. We don't know until we study them 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

It's the aspect of our exploration of the solar system that gets most interest of all from the general public I think. And if we did find an early form of life, or something significantly different, it would be the greatest discovery in biology since the discovery of evolution, or perhaps the discovery of the helical nature of DNA, of that order of importance. Who knows what implications it would have, if you think of how much of modern biology comes from those two discoveries.

If we introduce Earth microbes to them, accidentally or intentionally, this may well be irreversible. It's the irreversibility that's the issue here. If it is biologically reversible, not so much of a problem. But if irreversible, that means it would change those places for all future time, not just for us, but for our descendants and all future civilizations that arise in our solar system, they won't be able to make the discoveries they could make by studying these places as they are now, without Earth microbes introduced to them. They also won't be able to transform them in other ways if they decide they wish to introduce a different mix of microbes from the ones we brought there.

I think we just know far too little to make such a decision for all those future generations and civilizations and indeed for ourselves. At present anyway. Future discoveries of course can change this.

What we could learn - some examples

The exobiologists, who hope to fly in situ life detection instruments to Mars some day, design them to be as flexible as possible, to detect not just familiar forms of life. As an example, Chris McKay with his "lego principle" suggests a general way of looking for life not depending on any assumptions that it resembles Earth life. See his What Is Life—and How Do We Search for It in Other Worlds?

What we discover there could include any of:

  • Early life, e.g. tiny RNA world microbes without DNA or proteins. There are many ideas for early life that could perhaps still exist there, though extinct on Earth. These could fill in the huge gap between the organics and cell like structures resembling cells that turn up in laboratory experiments, and the immense complexity of modern life. One idea is an RNA world cell with no proteins, or ribosomes either, instead using RNA sliced into pieces and recombined to make a ribozyme, a tinier distant cousin of the ribosome. This is possible in theory, and some have suggested that present day Earth might have a "shadow biosphere" consisting of RNA world cells, but this has never been confirmed. Maybe we can find RNA world cells on Mars instead? 

    There are many other ideas for early life that could perhaps still exist there, though extinct on Earth, including the so called autopoetic cells that replicate just by producing daughter cells with a similar mix of chemicals when they get large, with no genetic code to regulate the process.
  • Unrelated life, perhaps based on some form of XNA (Xeno Nucleic Acid) instead of DNA. This would be the most amazing discovery of all. It would lift biology into a new dimension, show how life can exist based on completely different principles from DNA based life.

    There are many alternatives to DNA and RNA. RNA and DNA are both particularly fragile, DNA especially and hard to form naturally, need the environment of the cell or special conditions to keep them stable. RNA is more stable when it is very cold for instance, and ribose in its backbone is stabilized by the presence of borates, one of the points in favour of an origin on Mars. Some of the others are more robust and 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 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).

    The interior of a cell is so complex it's been compared to an entire ecosystem. So life based on different principles could be as revolutionary for biology as discovering a coral reef for your first time, when the only ecosystem you knew about before is the African Savannah. I make this analogy here: "Super Positive" Outcomes For Search For Life In Hidden Extra Terrestrial Oceans Of Europa And Enceladus
  • Life that is based on novel new principles that we haven't thought of yet. For instance, what if other life doesn't use a helix? Suppose for instance that the life used a sheet like two dimensional structure, planar rather than linear, and replication happened by a second layer forming on top of the original sheet?

    Or could it even be 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 - would you not find the idea of a helical structure that has to uncoil and unzip to replicate implausible and unlikely too?
  • Life that has evolved further than Earth life. Mars has had such harsh conditions in the early solar system, alternating ice 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, more robust cells, with more non redundant nucleotides, and more capabilities than Earth life, maybe even totally novel capabilities never explored here, even if it is just single cell life. 

    Present day Mars probably only has microbes, or perhaps lichens, if it is fair to make a comparison with similarly harsh environments on Earth. But the harsh environment may mean it evolved further on Mars than on Earth. Or 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 
  • Life with a capability Earth life doesn't have, 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.

    ET microbes might well use some fourth form of photosynthesis that has never been explored on Earth.
  • Life similar to Earth life in most respects, 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.
  • Uninhabited habitats - no life but with organics, and all the ingredients for life. This may seem boring, but 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.
  • Some major unexpected discovery that nobody currently is likely to predict.

All 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

Even 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. Perhaps that it has a new form of photosynthesis, a more efficient metabolism. We already have the C3 pathway used by maize which is more efficient than the photosynthesis used by wheat. But neither of those is anywhere near optimal. How efficient can photosynthesis be in a living organism? In a microbe? Or perhaps it has more diverse pathways so that it can cope with a wider range of condition. It's likely to be better adapted to ionizing radiation for instance, able to repair itself more rapidly, have more robust and longer lasting spores and resting states,, more resistant to desiccation, and 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 for a more advanced form of life on Mars than on Earth could be elusive. And of course it could be more advanced in some ways and less advanced in others. E.g. better at photosynthesis than Earth life, but less efficient metabolism, or vice versa.

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. Perhaps life never evolved, or it evolved but became extinct, or it just takes a long time for life to colonize a new habitat in the harsh conditions on the surface of Mars. Perhaps it takes hundreds of thousands of years or millions of years for life to colonize a newly formed habitat on Mars.

Uninhabitable liquid water on Mars

You also have the complication that water on Mars might not be habitable at all. Almost all places on Earth where you find water, or even water vapour from the atmosphere, you also find life, including salt lakes, concentrated sulfuric acid, permafrost, and places like the Atacama deserts and the McMurdo dry valleys. But you could get liquid water on Mars in conditions even more inhospitable for life than any of these.

A nice example of an uninhabitable water rich environment on Earth 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. Apart from that, about the only place where we have uninhabitable liquid water on Earth may be the extremely salty Don Juan pond in Antarctica - and even there, there is some doubt about whether it is completely uninhabited.

However it could be the norm on Mars to have liquid water which is not available for life to use. Reasons could include, too much by way of salts (including chlorates, and sulfates), too much acid, or lacking essential trace elements and nitrogen. Conditions were better in the past, even the recent times when the Mars atmosphere was a bit thicker on occasion. But It's a special challenge for present day life on Mars; because over much of the surface, ice sublimes directly to water vapour or is close to its boiling point right away. So 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 is possible for Mars to have habitable water. But we don't know if it does, yet, and 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 just 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.

But there are other ways that water there could be uninhabitable. It could be habitable to Earth life but uninhabitable to Mars life.

Early life not so versatile as present day 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 surely it must have had life 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.

Then, 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.

Pumice and ash floating on Lake Nahuel Huapi, Bariloche, Argentina

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?

Early pre-archaea type life on Mars could be extremely localised

As 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, it would be hard for such primitive life to transfer from one vent to another, to start with at least. So, even the hydrothermal vents of the Noachian period might not all have life in them. Or 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?

The first cells also might not have reproduced like modern cells with their complex transcription methods and error correction. 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. Perhaps remains of one attempt at life provided the raw materials for the next attempt until finally it succeeded long term. And protobionts might not have had any informational coding molecules at all.

The main problem is we have no timescale for this. Of course life must start somewhere, or several places at around the same time perhaps; 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. Nobody knows. We can't create these conditions in a laboratory and have no evidence at all from early Earth.

Life evolving from scratch many times - multiple genesis

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, Earth could 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. So, 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, then the search for past and present life there may be elusive.

The same could be true for present day life as well. How do we know that modern Mars life has evolved photosynthesis? If not, then where should we look to find it? We might find that it is only present in a few locations even if there are many places that modern Earth microbes could survive. If that was the case, and if it was an early form of life, RNA only, or even something not quite what we call life, not reproducing exactly, it could be so vulnerable to introduced Earth microbes, and also, hard to find.

Imagine if life on Mars is like that, not photosynthetic life, only occurs in particular habitats, perhaps only in particular RSL's, is early life less developed than DNA life. It could easily be extinct before we can find it. We might never know it was there.

So, at any stage in the history of Mars, it might be totally uninhabited. It might be inhabited but only in special locations, such as hydrothermal vents, or rock pools on ocean margins, or deep underground. Or it might be 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 in conditions suitable for preservation of the organics. It could be inhabited by a mix of forms of life with a different genesis. 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.

But will we get the opportunity? Perhaps it is already too late to find present day life on Mars? Maybe even the signal from past life is also already confused beyond recognition so that we will have endless discussion about whether Mars ever had life, and the question is never resolved?

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. 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.

Well, there is some truth in this, as 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 was a 1 in 10,000 chance of contaminating Mars per mission and a 1 in 1000 chance of contaminating it during the exploration period. It never was to be 100% sure we can't contaminate it. Of course ideally that is what we'd want it to be. But we can't do that at present. I think we should aim for 100% myself for Europa and Enceladus by sampling the plumes rather than landing. But for Mars the die is cast. However, the chance is probably something like 99.9% certain that it is not yet contaminated.

So even with Viking it was done on a probability level. The decision to stop sterilizing to Viking level was done on the basis that conditions on Mars are so harsh that they correspond to the heat sterilization stage of the Viking lander. 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.

What happened is that before Viking they didn't know quite how hostile conditions were there. After Viking, they came to the conclusion that such measures of sterilization were only needed if the spacecraft contacts regions in Mars that could be habitable for life - and Viking level sterilization is still the requirement for those "Special regions". For other spacecraft like Pathfinder, Opportunity, Spirit, Phoenix, and Curiosity, they sterilized them to the pre-heat treatment stage on Earth for Viking. Then they count on the harsh environment on Mars for the rest of it. They did give up on the use of probabilities pioneered by Carl Sagan et al, because of the impossibility of assigning a probability to life contaminating Mars, but the basic objective is the same to have a tiny chance of contamination, of the order of 1 in 1000 for contamination during the "exploration phase" of perhaps 57 ground missions an 30 orbiters (Carl Sagan's figures). Even though we've had crashes on Mars, they also were probably sterilized pretty much during the re-entry and crash itself.

So that question about what counts as too much contamination is something the exobiologists have already looked at and written many papers about.

The current guideline, for Curiosity and for all other missions to the surface (apart for those that search for present day life which have stricter requirements) is to reduce the bioburden to 300,000 bacterial spores on any surface from which the spores could get into the Martian environment. Any heat tolerant components are heat sterilized to 114 °C. Sensitive electronics such as the core box of the rover including the computer, are sealed and vented through high-efficiency filters to keep any microbes inside.

That is a level of protection we can do with rovers and landers. It is totally impossible to achieve that once you have humans on board. So, has it worked?

Could we have contaminated Mars already?

Mars has turned out to be a bit more hospitable than we thought. So that raises the prospect - what if it is already contaminated? I think the Phoenix lander is the most likely to have done so, or alternatively the Mars Polar Lander because it crashed in polar regions. After all Phoenix observed what seemed to be droplets of liquid salty water on its legs.

Possible droplets on the legs of the Phoenix lander

Also Phoenix got crushed by the advancing dry ice in winter, as was expected for its location

    Phoenix lander crushed by frost - layers of dry ice forming on the solar panel in winter snapped one of them of and it was not expected to last the winter - the right hand image shows it two years after the 2008 landing in 2010.

If any of our landers have contaminated Mars, I'd have thought Phoenix was a likely candidate. As usual it was sterilized to high standards, but before Phoenix nobody realized there was any possibility of liquid there, now we realize that liquid brines are a distinct possibility, also droplets of water on salt / ice interfaces. Most of those are probably either too salty or too cold for life but are there any that Earth life could survive in? We just don't know. Experiments show that it is possible to achieve habitability but it depends on the particular mix of salts.

    Jim Young (left) and Jack Farmerie (right) from Lockheed Martin, working on the Phoenix lander science deck under clean room conditions to protect Mars, following planetary protection guidelines. Credit: NASA /JPL/UA/Lockheed Martin.

    However nobody knew back then that liquid water could form on the surface in those regions. The entire polar regions of Mars are now declared a "Special Region" meaning that landers there need Viking level sterilization for anything that could potentially contact a habitat. Has Phoenix contaminated Mars? The consensus seems to be that probably it hasn't, but it's site would be an ideal one to visit to check how effective our measures to date have been.

I think myself that a priority mission for planetary protection is to send a lander to investigate one of these sites. If Phoenix, say has started to contaminate Mars we might find a small enclave of life around the lander. I think that it is high time that we actually had a mission to the surface to actually test to see how effective our planetary protection measures are. The mission could be dual purpose, first to search for life habitats, past and present life signs etc - so it would land some distance away from Phoenix - then it would travel up to the crashed lander, photograph it, and analyse the remains and test for liquid water droplets and for signs of life, and examine the spacecraft itself for viable life there.

What if we have contaminated Mars?

First, if there is Earth life there already, brought on our landers - the last thing we should do is to introduce new life. For instance if it has been contaminated by a photosynthesizing green algae, well perhaps that plays nicely with much of the Mars life. Even if what we have there is a vulnerable RNA world that has been made extinct on Earth, well whatever there is obviously well adjusted to oxygen, including the perchlorates and hydrogen peroxides. A little oxygen from green algae is not likely to bother it. The green algae as primary producers are not likely to harm it, may even be a source of food, creating new organics from just sunlight, CO2 and trace elements.

This doesn't mean that it is okay therefore to introduce all the microbes on a human occupied spaceship that would get there after a crash on Mars. That's like saying that if you introduce rabbits to an island, then that's the end of any attempt to protect it from invasive life, so you might as well introduce rats, cane toads, goats, cats etc. There may be many things that are vulnerable to rats, cats etc that are not harmed by rabbits.

Or it's like, if you are overrun by kudzu, the answer is to say okay, let's have Japanese knotweed, let's have Himalayan Balsam, let's have every single invasive species that ever causes problems as obviously it's all over now.

A gardener or farmer would not do that. Instead you'd minimize the effects of the kudzu as much as you can and do whatever you can to prevent the other species from invading.

In the same way if we find that Phoenix has introduced life to Mars, or any of our other landers or crash sites there - then the first priority would be to see if we can limit or reverse the damage. The life would be slow growing in such harsh conditions. Perhaps we could sterilize it with ionizing radiation or similar. We could take a high intensity gamma radiation emitter to Mars and use that to sterilize the immediate vicinity around the lander. Who knows, maybe it is not too late and we can sterilize and reverse the contamination completely. And if not, we manage it as much as we can, slow it down as much as we can, and make sure we don't introduce any other invasive microbes to Mars.

This is keeping our options open for the future. If we have introduced the equivalent of rabbits to Mars - well let's be careful not to introduce the rats and cane toads as well, until we find out what is going on there.

Idea of returning samples from Mars to Earth

When Curiosity's successor and the ExoMars rover land on Mars around 2021, we will see two different approaches to the search for life on the planet side by side. NASA's mission is the first stage of a sample return program. The ESA's ExoMars rover (in partnership with Russia) will explore Mars in situ for biosignatures as well as drill two meters below the surface. Which is the best approach? 

MSR ascent module

A sample return would be great for geology. But would it help with the search for life on Mars? Or is it more of a technology demo for this? NASA's decision was based on the last planetary science decadal survey in 2012, for the decade 2013 to 2022. In this survey, NASA asks for input from panels of space scientists. NASA do one high cost "flagship mission" in each decade.  The committees chose a sample return mission (over the Jupiter Europa Ocean mission), but with the funding available, they could only pay for the first half, sample caching on Mars. They left return of those samples to Earth as a decision for the next decade. So essentially, it's a double decade flagship mission. This makes it one of the most expensive decisions NASA has committed to in the field of planetary sciences in recent years. It would return less than a kilogram of material at a cost of millions of dollars per gram.

Surely someone needs to do a comparison study before such expensive decisions? It turns out that someone did, in a white paper submitted to the decadal survey itself. Surprisingly given the outcome of the survey, and the enthusiasm of many space scientists for the idea, this study comes out firmly in favour of in situ exploration and against a sample return, for astrobiology. A study like this would normally be followed up by more detailed studies, so it has to be treated as preliminary. But it's all we have at present. Why did these astrobiologists come out so strongly against the idea of a sample return? When they, of all scientists, are keenest to find out about Mars life, if it exists? And what are the implications for NASA's plans?

Well it's because they were early life enthusiasts rather than fossil optimist, I've covered the paper already above in Follow the nitrogen, dig deep and look for biosignatures. So they expect Mars past life to be hard to find, as hard as the search for past life in ALH84001. With that background and the complex geology of Mars, it's not at all clear that we can hope to find evidence of past life on Mars easily, or at all, by sample return missions unless we already know it is there, and where to look.

NASA are just first off the block. Other countries that may do a sample return in the near future include Russia, China, and indeed the ESA themselves who have explored the idea for many years. So the details in that white paper are relevant to them as well.

Impossibility of sampling everything

This region of the Mawrth Vallis area of Mars gives some idea of the complexity of the situation on Mars. Imagine trying to study this region by returning samples to Earth for analysis? And now, imagine that you also have to drill below the surface to find samples less affected by ionizing radiation?

Close up image of a region of stratified clays in the Mawrth Vallis region of Mars

With current ideas, the sample would only return small quantities, probably less than a kilogram in total. So there is no way we can do a complete survey of any moderately complex region of Mars and return samples from all the interesting points in the region.

An in situ search on the surface is not restricted in any way. We can continue studying new samples indefinitely. We can also home in on regions of interest.

If an in situ study finds that a particular band of rocks, or type of rock, for instance, is of especial interest - then the in situ rover can then focus the search on other rocks of that type. Perhaps it finds a chiral signature, or it finds amino acids or other biologically interesting molecules. Then it can focus the search on that layer or those rocks, and follow the signal.

Or it can drill into the layer to get deeper samples and so on. It can make decisions about where to go next based on the analyses already done. But if you have to return the rocks to Earth to search for biosignatures, this is impossible.

Tissint meteorite - a great example of what we might get in a sample return from Mars

Another example of a fascinating Mars meteorite is the Tissint meteorite, which was in the news recently. It's a witnessed fall, so one of the least contaminated of all the Mars meteorites, only been sitting around for a few days before it was collected. Again, for various reasons, some scientists see this as good evidence of early life on Mars.

This also is proving as controversial as ALH84001. See Meteorite May Contain Proof of Life on Mars, Researchers Say and Experts Cast Doubt on Meteorite Study's Claims of Martian Life

Here you can see a fragment of this meteorite from London's Natural History Museum, discussed by Caroline Smith, their meteorite expert.

It would be wonderful to have a few more samples like these two meteorites. And especially so, it would be great to have the context, the exact location they come from on Mars.

However, is it worth the price tag of millions of dollars per gram, to get more samples like this, even ones that are brought straight to Earth from Mars in a spacecraft? How much will this help with the study of exobiology and the possibility of life on Mars? You have to bear in mind the impact this has on other missions, which won't be flown because the funding is used instead for a sample return.

This was the question the exobiologists addressed in their study. And they came down strongly in favour of in situ exploration at this stage of our exploration of Mars.

"Two strategies have been suggested for seeking signs of life on Mars: The aggressive robotic pursuit of biosignatures with increasingly sophisticated instrumentation vs. the return of samples to Earth (MSR). While the former strategy, typified by the Mars Science Laboratory (MSL), has proven to be painfully expensive, the latter is likely to cripple all other activities within the Mars program, adversely impact the entire Planetary Science program, and discourage young researchers from entering the field."

"In this White Paper we argue that it is not yet time to start down the MSR path. We have by no means exhausted our quiver of tools, and we do not yet know enough to intelligently select samples for possible return. In the best possible scenario, advanced instrumentation would identify biomarkers and define for us the nature of potential sample to be returned. In the worst scenario, we would mortgage the exploration program to return an arbitrary sample that proves to be as ambiguous with respect to the search for life as ALH84001."

(white paper by Jeffrey L. Bada, Andrew D. Aubrey, Frank J. Grunthaner, Michael Hecht,Richard Quinn, Richard Mathies, Aaron Zent, andr John H. Chalmers)

Instead of a sample return at this stage, they recommend more thorough in situ searches, and increased mobility, to look at the many possible habitable environments on Mars. They also recommend drilling to depth, and searching for biosignatures.  The main difference in the perspective of the astrobiologists, and the geologists, is in the timing. They recommend a sample return at a later stage in exploration, once we have explored Mars more thoroughly and definitely identified biomarkers on Mars. Alternatively, if we never find biomarkers on Mars, we could return samples after we have exhausted all the in situ technologies available to explore for the biomarkers on Mars itself directly.

Paper: "New priorities in the Robotic Exploration of Mars: The Case for In Situ Search for Extant Life"

This is a paper from 2010 by four more astrobiologists, New Priorities in the Robotic Exploration of Mars: The Case for In Situ Search for Extant Life

They say

"Given the most updated knowledge we have about Mars’ environmental evolution, we call for a long-term architecture of the Mars Exploration Program that is organized around three main goals in the following order of priority: (1) the search for extant life; (2) the search for past life; and (3) sample return. We argue that this is the most efficient approach by which to address, with a high degree of certainty, the question as to whether life exists on Mars. "

(Alfonso F. Davila, Mark Skidmore, Alberto G. Fairén, Charles Cockell, and Dirk Schulze-Makuch)

In more detail they say:

"Priority 1: In situ search for extant life"

"The search for extant life should be conducted in environments with the highest potential to support active organisms or preserve dormant organisms. These environments should be selected based on studies conducted in the best terrestrial analogues on Earth, particularly the Atacama Desert, the Antarctic Dry Valleys, and basal ices of polar ice masses. We assume the martian biosphere is carbon based and follows nutrient requirements similar to those of terrestrial microorganisms."

  • "Rationale: Extant life on Mars, if present, is not currently detectable at a planetary scale but might be detectable at the local scale. This is observed in terrestrial analogues that are closest to martian conditions, where life is present and can be relatively abundant but is only detectable in specific niches with enhanced habitability potential (e.g., ice, interior of salts) and the use of microscopy or simple molecular techniques. "
  • "Approach: The in situ search for extant life should be dedicated, focused, and relatively inexpensive (Discovery-type missions), with the use of instruments that can provide indisputable evidence for the presence or absence of organisms, for example, via microscopy in addition to measurements based on activity or metabolic state.""

"Priority 2: In situ search for extinct life"

"The search for extinct life should be conducted in environments on Mars that are known to support active microbial communities in terrestrial analogues on Earth. "

  • "Rationale: Based on the above, niches with the highest potential to host extant life also have the highest potential to have fossil traces of a biosphere from the recent past. Two of the target sites proposed for in situ search for extant life, ice and salt, also have a high potential to preserve organic compounds for extended periods of time in the range of millions of years."
  • "Approach: The in situ search for extinct life could be carried out by independent missions or in tandem with missions that are searching for extant life. If neither extant life nor evidence for extinct life are found in these icy and salty targets, then it will be reasonable to refocus the search for traces of past life in the rock record."

"Priority 3: Sample return"

"After lander missions have satisfied Priorities 1 and 2, we will have an informed perspective of the potential for life on Mars. Sample return would be most efficient and logical once we have information from a variety of environments, particularly if evidence of extant or extinct life is found at any of these sites"

Decadal summing up doesn't discuss this issue

The decadal survey does list Bada et al's paper amongst the submitted white papers at the end of the report. But it is not cited in the body of this report or discussed. Nor do the speakers mention it in their final presentation (which is available as a video online).

This is what the decadal survey says:

The Mars community, in their inputs to the decadal survey, was emphatic in their view that a sample return mission is the logical next step in Mars exploration. Mars science has reached a level of sophistication that fundamental advances in addressing the important questions above will only come from analysis of returned samples.

The site will be selected on the basis of compelling evidence in the orbital data for aqueous processes and a geologic context for the environment (e.g., fluvial, lacustrine, or hydrothermal). The sample collection rover must have the necessary mobility and in situ capability to collect a diverse suite of samples based on stratigraphy, mineralogy, composition, and texture. Some biosignature detection, such as a first-order identification of carbon compounds, should be included, but it does not need to be highly sophisticated, because the samples will be studied in detail on Earth.

Vision and Voyages for Planetary Science in the Decade 2013-2022

If they included the astrobiologists in "the Mars Community", then it would be more accurate to say that they were emphatic in their view that a sample return is not the logical next step. Let's take some of the things they say in the summing up, and compare them with the statements in Bada et al's paper.

  • Summing up: "fundamental advances in addressing the important questions above will only come from analysis of returned samples"
  • Bada paper: "We have by no means exhausted our quiver of tools, and we do not yet know enough to intelligently select samples for return


  • Summing up: "Some biosignature detection, such as a first-order identification of carbon compounds, should be included, but it does not need to be highly sophisticated, because the samples will be studied in detail on Earth."
  • Bada paper: "We argue here that when in situ methods have definitively identified biomarkers, or when all reasonable in situ technologies have been exhausted, it will be time for MSR. We are not yet at that crossroad."

How did this happen? Given that the main objective of the sample return is to look for life, you'd expect the views of astrobiologists to have top priority, so why weren't they mentioned at all? After such a strong warning against a sample return, by the experts most knowledgeable in this topic area, why didn't it trigger an in depth study of some sort.

Will the sample return happen this time?

It's a two decade project, and given the expense and technical challenges, it may not happen. Many earlier plans for a sample return never came to anything.

1978 proposal for orbital Anteus receiving facilities for Mars Sample Return
Antaeus Orbiting Quarantine Facility (1978)

The idea of a Mars Sample Receiving laboratory was first studied in 1978. The idea then was for an orbiting quarantine facility called Anteus to receive the samples. 

Other proposals were explored in the 1980s, including direct entry of a sample container to the Earth's atmosphere, recovery by the space shuttle, recovery to the space station, recovery to a dedicated Antaeus space station, and several intermediate proposals. Mars Sample Recovery&Quarantine (1985)

Perhaps this time it will happen however? If so, what can we do to help make it a success, and a valuable part of our space program? Can we do anything to reduce the huge cost? And can it be done safely, given the issues for back contamination of the Earth? At reasonable cost?

Rhythms from Martian sands - what if Viking detected life?

There is one scenario that could mean that we return life from Mars right away, maybe even in the first samples from the planet. What if there is life there already in the sand dunes, and Viking detected it? Gilbert Levin has been saying this for decades, and recently some other scientists have found new evidence that may support him.

So, did our two Viking landers find life on Mars in 1976? Astonishingly, thirty seven years later, we still haven't sent anything to Mars able to answer this question for sure.  None of our spacecraft since Viking would be able to spot life which we now know exists in the driest deserts on Earth, and only one of the experiments on Viking had this capability. There are  hypotheses about what they found, but no definite proof.

A few years back, to everyone's complete surprise, Joseph Miller, specialist in rhythms of life, spotted smooth daily cycles in the data from 1976, strongly suggesting life processes. So did Viking spot life or are these smooth cycles signs of something else, perhaps some complex chemistry?

Nowadays most researchers think that there is some liquid water on present day Mars. Even so, our best analogs on Earth are the driest areas of the Atacama desert, or the McMurdo dry valleys in Antarctica. 

These are places with patches of hard to detect microbes. They are so hard to detect, that in some of the habitats, life was discovered for the first time in the last decade. Many of the microbes live beneath surfaces of rocks or below the surface soil, slowly growing; sometimes with individual microbe lifetimes of thousands of years.

If there is life on Mars it's likely to be like this. Hard to spot visually and difficult to detect with our instruments also. Curiosity, for instance, wouldn't have much chance of finding it.

This section comes from my Rhythms From Martian Sands - What Did Our Viking Landers Find in 1976? Astonishingly, We Don't Know, also available as a kindle ebook)

Carl Sagan standing next to a model of the Viking Lander - so far the two Vikings are the only spacecraft ever sent to another planet to search for life. 

Curiosity's main focus is on habitability of ancient Mars, and can search for organics, but it is not equipped to detect present day or past life itself, at least, not in the low concentrations you'd expect.

Two of the three Viking experiments drew a blank, but we now know that they were probably not sensitive enough to detect in these ultra low concentrations. (see also original paper- though, note that some scientists think that the other experiments were able to detect ultra low concentrations of life. - this whole field is controversial). Our later rovers such as Spirit, Opportunity and Curiosity are just not sensitive enough, and would have no chance at all of spotting the life that exists in the most inhospitable ares of the Atacama desert or McMurdo dry valleys. Then, another twist in the story, a recent re-analysis of the Viking data suggests that these two experiments might have detected organics after all. They detected chlorohydrocarbons which were dismissed as contaminants at the time, but might well be the result of reactions of perchlorates with the organics (Phoenix discovered perchlorates in the Martian soil in 2008).

The upshot is, we've only sent a single experiment to Mars with a decent chance of finding life there - and it came up with results that are strongly suggestive of life, though most would say, not conclusive. NASA, perhaps understandably, discounted its results at the time. But now that we know that the other instruments had almost no chance of spotting life there, their reasons don't seem as convincing as they did to many back in the 1970s.

Close up of the Labeled release experiment on its testing rig in the laboratory. Photograph from the original paper published before Viking landed on Mars.

According to modern understanding of conditions of Mars, this is arguably the only instrument humans have sent to Mars which had any chance of spotting present day life there in the sparse populations we'd expect in such dry desert like conditions.

I'd like to look a bit closer at the Viking results here, and also look at the possibilities of life able to survive in the equatorial regions of Mars. Then, let's look forward at experiments we could send to Mars to settle this question about the Viking results, and look at how the labeled release and other experiments could help with the search for life in other places on Mars if it turns out that Viking has another explanation, or to help find life in other habitats.

How the experiment works

It's a simple idea. You prepare food for micro-organisms with the ordinary carbon 12 replaced with radioactive carbon 14. They will normally give off some gases including carbon, such as carbon dioxide, and methane, when they digest the food (they may also produce some hydrogen and hydrogen sulfide). You then detect the radioactivity in the air.

Exquisite sensitivity to life - no need to reproduce - and not confused by non life organics

The great thing about this experiment is that it is very sensitive - and doesn't require the organisms to reproduce. Microbes from cold dry places such as Antarctic dry valleys will often have long lifetimes and reproduce only every few months, or may go for a thousand years in a slowly metabolizing almost dormant state before they reproduce. When you bring them into the laboratory then they are often reluctant to reproduce and keep this slow pace of life. You might need to wait several months, even years, to see a change. But if you test for evolved radioactive carbon, it doesn't need to reproduce, just metabolize. This makes it exquisitely sensitive, and the team were able to show that it could detect life even in places such as the dry Atacama deserts, and the McMurdo dry valleys. Yet at the same time it was not confused by the signals e.g. of organics in lunar soil. (Lunar soil does have some organics in it from the solar wind and from meteorites, in parts per billion).

If we find organics on Mars, that doesn't need to mean life. Even if we find chirality signatures, then again that's not a definitive proof of life as there is a chiral signature in meteorites, with some having a surprising excess, and both radiation and warmth can degrade the signal. In this 2006 analysis the EET92042 and GRA95229 meteorites 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 isovaline, while the EET92042 meteorite ranged from +31.8‰ for glycine to +49.9‰ for L-alanine. It's thought that these excesses are extraterrestrial and not due to contamination by Earth life.

You'd notice if it had 100% of one of the amino acids. But then amino acids can swap handedness in warm conditions (racemization) and any ancient organics may be thoroughly racemized if they have been warm for some period of time (you can work back if it's been kept at reasonably cool conditions by looking at the different rates different amino acids transform). The situation would get especially complex if you still had some of the original amino acids with, say, a meteorite chirality excess and some converted into life, or a mixture of ancient and present day organics. So the chirality signal may be clear, but it might be confused and need detective work to find out what is there.

If we find RNA or some complex molecule not found in meteorites, would be a reasonably good evidence of life. But still, strictly speaking, it's not proof, you'd still ask if there is any inorganic way of making it. One recent paper found that RNA strands of up to 400 bases can be made by freezing processes in sea ice - without any intervention of life. See Did Life Evolve in Ice? (Discovery Magazine) and for the original paper, Ice and the Origin of Life.

Hauka Trinka who studied sea ice in the Arctic regions, and came to the conclusion that RNA gets formed in the ice through inorganic processes, producing RNA chains of up to 400 bases. He thinks this may be a possible mechanism for origin of life.

Whether this is how life began or not, his experiment also shows that detection of RNA does not by itself prove that you have life. You might need other evidence even in such an apparently clear case as this, after all if you relied on RNA detection you'd conclude that his artificial sea ice experiment created life from amino acids, when all it created was RNA, a biosignature, but no life.

But if we find evidence of active metabolism, and transformation of organics into gases, that's a good sign of life, and if we can find evidence of circadian rhythms, and the rhythms go out of sync. with the Martian sol under constant conditions - that would be hard to explain as anything except life - or at least, some really really complex chemistry getting on for the complexity of life itself. If we could find all that - and at the same time, evidence of complex chemicals with chirality - then the two observations would complement each other.

Labeled release type experiments might help find forms of life that are novel, extremely different from Earth life (e.g. not based on DNA). It would also tell you something about its metabolism and capabilities, e.g., you might learn whether it can photosynthesize and what are its preferred temperatures, and in more refined versions of the experiment, you could learn what gases it produces during metabolism, what amino acids it uses, and so forth.

Joseph Miller's analysis of rhythms in the Viking data

Circadian rhythms are the rhythms of life. We get hungry in the daytime and sleep at night, and suffer from jet lag, all because of these rhythms. Micro-organisms have them too. They follow day and night cycles - but not exactly. Rather the rhythms run on their own but use heat, or sunlight or both as a reset, to keep them in sync with the days and nights. That's why you get jet lag as your body gets back in phase with the shifted rhythms. Microbes are the same; they also have a form of "jet lag", a delayed response to changing external cycles.

Joseph Miller is a specialist in circadian rhythms who spotted something unusual in the Viking data which nobody else had noticed, It seemed, to his expert eye, to strongly resemble terrestrial circadian rhythms, and with a period of a Martian sol. 

He was able to get hold of the original raw data. This was not easy because it had been stored in microfiche format but mixed up with data from the other experiments as well as engineering data - and some of the images not so easy to read, making it a long project to extract it. It was also stored on CD - but mixed up with huge amounts of engineering data - and the documentation of the CD didn't give enough information to restore it.

Luckily Gilbert Levin's co-researcher Pat Straat had kept extensive printouts of most of the LR data, and so finally the team were able to use these to get most of the data. They then used the microfiche data to fill in gaps.

Allison Lopez (left), Dr. David Williams (center), and Lois Hughes (right) who helped restore the missing LR data - using printouts from Pat Straat and the microfiche data

For the details of what he found, see his paper Periodic Analysis of the Viking Lander Labeled Release Experiment. Let's look at some of his main results. He then analysed the data using all the tools you use to detect Terrestrial circadian rhythms, and the results all came up positive. The experiment was isolated from the fluctuation of day night temperatures on the Mars surface, but imperfectly, so still had temperature fluctuations. His main alternative hypothesis was temperature dependent solubility of the CO2 in the soil. However this didn't seem to be able to explain all the effects seen.

First, it was not quite enough of an effect to explain the data. Then, the LR data follows these fluctuations, but doesn't follow every fluctuation exactly as you'd expect from a purely chemical reaction to the temperature change. You get changes in temperature with no changes in the CO2, and the curve is smoother than you'd expect from the noisy temperature data, which also has other regularities which the LR data doesn't respond to. The LR response is also delayed by 2 hours after the temperature fluctuations. From the design of the experiment he concluded that though a 20 minute delay was possible, with variation in CO2solubility, 2 hours seemed too much of a delay to explain by chemical reactions.

Then another thing which is suggestive, but unfortunately not conclusive, there seems to be a sign of a change of rhythm after the second nutrient injection. This is something that often happens with circadian rhythms.

Shows temperature fluctuations in red, and the radioactivity measurements in black. The black lines are almost synchronized with the temperature fluctuations, but delayed by two hours, and don't follow every variation exactly.

This is an actogram. Looks complicated perhaps but idea is simple. It's just a long timeline broken up into pieces stacked on top of each other. It's double plotted so the second half of each line overlaps the first half of the next line, and each line is two Martian sols long.

You read it from left to right and top to bottom. Each row starts one sol later, and is two sols long so the right half of each line is the same sol as the left half of the next line, and the black lines show times when the labeled release values are above the two sol average. 

The white circles show the moments of maximum LR for each sol. The white diamond at day 7 show the moment of the second nutrient injection.
If you draw a line between the white dots, it slants down towards the right to start with, delaying each day, with a period of 25.46 hours, but then at the second nutrient injection starts sloping to the left with a period a few minutes short of the Martian sol of 24.66 days. 

The new pattern is interpreted as due to the advancing sunset times on Mars, as he found that it follows them almost exactly. 

This change of slope is a common feature of circadian rhythms after nutrient injection - though with only six days, there is not enough data to be certain that the slope did change; it could just be a statistical anomaly. If it did, then apparently it's hard to think of a good explanation not involving biology.

They plan to do more analysis of the data.

To a trained eye, apparently this is all suggestive of a circadian rhythm, though not conclusive.

Another thing strongly suggests biology rather than chemistry as an explanation. The "active ingredient" whatever it is is deactivated, not only by high temperatures, but also by keeping the soil in darkness, at a temperature of 10°C for two months before it is used in the experiment. This was an accidental experiment, they used samples collected at the beginning of the experiment and stored, out of concerns about whether the scoops could stand up to repeated scooping. It turned out that after this period of two months, in cold and darkness, it lost whatever it was that gave LR activity. It's hard to think of some chemical explanation, why two months of storage would inactivate an active chemical ingredient of the soil - with the only difference from its original condition on the Mars surface, that the temperature was kept constant and that it's excluded from light.

Whatever is going on here, it's either something biological, or some complex chemistry. Either way, it's a highly unsatisfactory situation that nearly forty years on, we have no idea at all about what it is that Viking found.

Ice and water in equatorial regions on Mars

We know now (though it wasn't known at the time of Viking) that the surface equatorial regions are not completely devoid of water, so there must be something else going on. First, ground penetrating radar showed possibilities of ice below the surface, in the Medusae Fossae Formation - it's either miles deep layers of either equatorial dirty ice or very porous regolith all the way down.

Do you see how there is a faint double 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) - and powerpoint style slides.

Then more recently, we spotted what are believed to be actual damp streaks on the surface, or at least streaks in some way caused by water. Just a few places here and there, with other identical places that don't have them.

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

see section above: Habitats for life on the surface of Mars - warm seasonal flows .

From that section the three main theories are:

  • Ancient ice just been deposited there millions of years ago and not yet exhausted (hard to see how there could be enough of it)
  • It's replenished, indirectly, by deep down ice, possibly all the way from the hydrosphere
  • Deliquescing salts just below the surface feed water into the warm seasonal flows (again a bit hard to see how there could be enough water this way)

The first two seem unlikely explanations for water at the Viking site, not likely both sites were above a geothermal hotspot (must be very rare on Mars with no hot spots yet detected from orbit, and no active volcanoes though clear signs of geologically recent activity). It's also unlikely that both sites happened to be above deposits of ancient ice (as far as we know or expect, rare in equatorial regions). The last one, the deliquescing salts idea, might be quite promising as a possible way water could be present, for life to use, even at the Viking sites. Then there's another idea that could support life almost anywhere on Mars, without deliquescing salts, just using night time humidity of the air.

Equatorial frosts - source of water for the deliquescing salts

This is an observation that goes back to Viking. You get frosts even in equatorial latitudes. This shows that though there is hardly any water vapour in the Mars atmosphere (absolute humidity is low), yet at night it gets cold enough for 100% relative humidity.

Here relative humidity is what makes cumulus clouds form, for instance. As the warm air rises, it cools down. There is no more water vapour in the air than before, but the colder air can't carry so much water so it condenses out as the droplets of water of a cloud. It's also the reason you get morning dew, ground fog, and frosts. As the temperature falls, absolute humidity doesn't change, there is no more water vapour than before, but because of the reduced capacity to carry water vapour, the relative humidity rises until, when it reaches 100%, it has to come out of the air as a cloud, dew, fog, or frost.

On Mars then the water vapour forms frost, or ice crystal fogs or clouds (on Earth, cirrus clouds are similarly made of ice). Conditions are too cold and the air too thin for water vapour clouds (except just possibly at deep locations like Hellas basin).

Frosts on Mars - this photograph from Viking 2. Mildly enhanced to bring out the colour of the frost.

This shows that there is a hundred percent relative humidity of the air on the surface at night. In the day time, when it melts it probably evaporates instantly into the air. But deliquescing salts could trap this evaporating moisture, and retain it long enough to be useful.

When the frosts melt, then though they evaporate too quickly to be of use for life, this creates a temporary source of high humidity at soil level, which could be exploited by life.

Life in the Atacama desert without rain

First, you get communities of micro-organisms in coastal areas beneath quartz inclusions in the rocks. In coastal areas, 80% of the quartz inclusions can have life, which get their water from the fog, and all quartz inclusions over 20 grams were colonised.

This net is used to collect the fog in the Atacama desert for human use. The fog is called camanchaca by the locals. See trapping humidity out of the fog in Chile. Life in the Atacama desert can also use these fogs as a source of water.

Fogs on Mars (digitally enhanced) in the Valles Marineres - however these are fogs of ice rather than water. See Fog phenomena on Mars. (first page and five minute free preview available at DeepDyve).

This shows quartz inclusions in the coastal regions of the Atacama desert. The green in the photograph at bottom left shows algae underneath a rock, probably Chroococcidiopsis. Quartz inclusions on Mars could be a promising habitat for similar species. Chroococcidiopsis particularly is a prime candidate for a life form on Earth that could survive on Mars because of its multiple extremophile adaptations, and its ability to survive as primary producer in a single species ecosystem, and tolerance of ionizing radiation in quantities that would kill most microbes.

Though Mars has fogs of ice rather than water droplets, that's just because the air is so thin. It does have high humidity at night.

In the driest part of the desert, there's no fog and no rainfall for years on end. Yet you still get life. It can get this through salt deposits which take in water from the atmosphere, a process known as deliquescence. This makes the salt slightly damp, and microbes can make use of this.

Salt formations in the hyper-arid core of the Atacama desert, where microbes were found, seven to eight years ago 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.

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.

Hyper-arid core of the Atacama desert. Originally it was thought, conditions here are so inhospitable that it would be the "dry limit" of life. But in 2002 researchers found life here by DNA sequencing. Originally it was thought, you could only have dormant life that relied on the very rare rains here. But now it's known that life there can survive without any rain at all.

Life relying on deliquescing salt has also been found 2 or more meters below the surface of the Atacama desert. They use mixtures of halite and perchlorates for deliquescence, and they use sulfates, perchlorates (ClO4), and nitrates, together with organic acids such as acetate, or formate, as sources of energy. Quite Mars like conditions really. See A Microbial Oasis in the Hypersaline Atacama Subsurface Discovered by a Life Detector Chip: Implications for the Search for Life on Mars

Similar habitats such as colonies inside halite crystals, or beneath quartz inclusions, could occur in equatorial regions of Mars. Some of these micro-organisms can capture and retain water vapour at levels of relative humidity too low for salts to deliquesce. They manage this using capillary condensation in micro-pores less than 0.1 micrometers in diameter in the cell walls. This seems a likely adaptation for Martian life to develop. See Novel water source for endolithic life in the hyperarid core of the Atacama Desert

Trangressing sand dunes bioreactor

For the original paper here, see Habitability of transgressing Mars dunes. This is a paper written in 2013 just after the Curiosity landing, and looks at conditions in Gale Crater, and found that there's a possibility of present day life there.

This is an idea that we've had since Phoenix and its discovery of salts which we know can deliquesce, and discovery of the drops of what looks like water deliquescing on its legs. Unfortunately we didn't have the capability to analyse them, just visual evidence, but is hard to see what else they could be, dark drops that grew gradually, occasionally merged, and eventually drop off. When they dropped off they never formed again at the same spot. Most likely, this shows a process of deliquescence on salt thrown onto its legs during the landing.

Certainly it's a theoretical possibility. Also the Phoenix observation of isotope ratios in the atmosphere show that the CO2 in the atmosphere must have come from recent volcanic eruptions, and that the oxygen has subsequently changed atoms with some material on the surface, which almost certainly means, dissolved in water. That's good indirect evidence for reasonably abundant water, either thin layers over much of Mars or occasional meltings (e.g. meteorite impacts on the polar regions and high latitudes) or both. Of course both possibilities are of interest for present day life on Mars, but the idea of a permanent layer of liquid water is the one of most promise for the Viking results.

The paper suggests that the melting salts would create humidity which would be captured by salts in Gale Crater (similar processes elsewhere of course but the paper focuses on possibilities for Curiosity). They suggest magnesium perchlorate and calcium perchlorate as possibilities for capturing the water. They suggest ferrous and ferric iron, or ferrous iron and perchlorates as the energy source.

The moving sand dunes would churn up the soil and mix the reducing lower parts of the soil with the oxidizing surface layers and bring up nutrients from below the surface.

This striking image from Mars Reconnaissance Orbiter shows quite how much sand movement you can get on Mars. In places, sand dunes as high as 200 feet (61 meters) are moving over the surface of Mars - a surprising result with its thin atmosphere (but strong winds and months long dust storms sometimes).

Sand dune movement could churn up the soil and bring deliquescing salts to the surface, and also mix up reduced and oxidised chemicals, making conditions easier for life.

Desert varnish

Just mentioning this, as it could just possibly be evidence of life in equatorial regions of Mars - but won't spend much time on the topic here, as it is controversial, whether this is evidence of life on Mars or not. It's a shiny coating that forms on rocks in desert landscapes.

Petroglyphs carved in a desert rock, newspaper rock, Utah

Nobody is quite sure what causes desert varnish on the Earth. The curious thing is, it doesn't seem to be a weathering effect as the composition of the varnish is independent of the rock it forms on.

It might be life, but might also be non life processes. The problem is that it's easy to find life in the varnish on Earth - but it might just be coexisting with it. Since the varnish forms slowly over thousands of years, you can't run experiments for long enough to duplicate the exact conditions that lead to its formation.

Mars also has desert varnish.

An effect that looks like desert varnish on Mars, photo by Spirit rover

Curiosity made some accidental measurements which might possibly give information about its properties. But generally it's not well understood either on Mars or on Earth. Of course any dedicated life mission to Mars should examine the desert varnish and see if there are any organics there, or any signs of life.

Lichens and cyanobacteria able to take in water vapour without need for salts

This is research originally sponsored by the German aerospace company DLR. Lichens in tundra and cold high dry mountain conditions can often survive and grow without any source of water at all. Instead they take in water from the atmosphere, which has high relative humidity when its at its coldest. They are also protected from UV and drought, in conditions with higher than normal UV influx. 

Lichen P. chlorophanum on a Mars analog substrate for the DLR Mars simulation experiments.

Though not adapted to live on Mars particularly, of course, these adaptations turn out to make them astonishingly resistant to Mars conditions. In Mars simulation chambers, which reproduce the thin atmosphere, the UV radiation, and the day to night variations in humidity, they are able to grow apparently completely normally even metabolizing and photosynthesizing, for weeks on end. High CO2 concentrations at normal Earth atmospheric pressures reduce photosynthetic activity, but if you reduce the atmospheric pressure to Mars levels, then this has a compensatory effect, and the levels of photosynthesis return to the same levels they have at normal Earth atmospheric pressures and CO2 concentrations.

Five of the species of the lichen used in recent astrobiological studies - these are able to metabolize and photosynthesize in simulated Martian environments, and can withstand the UV light through protective chemicals such as parietin. They can take up the moisture directly from the atmosphere at times of high relative humidity, so have no need for deliquescing salts.

The lichens studied not only survive, metabolize and synthesize in Martian conditions - but in protected conditions (such as partially shaded or protected by a thin layer of dust) both algal and fungal components not only continued to metabolize, but quickly adapt to their new conditions. Within 34 days, the algal component of the lichen increased its photosynthetic activity to compensate for the harsher conditions. These lichens could survive almost anywhere on the surface of Mars provided you have sources of light, the CO2 from the atmosphere, trace elements from rocks, moisture from the air - and some source of nitrogen would also be needed, maybe from nitrates which we now know occur on Mars.

These extremophile lichens can also survive cosmic radiation, UV, and the vacuum of space. Perhaps some lichens might even be able to survive the journey from Earth to Mars or vice versa, if so might be a shared species between the two planets. Gilbert Levin has been saying for years that he thinks green patches on Mars might be lichens. Whether that's right or not, these results certainly vindicate the idea that lichens on Mars are a distinct possibility, and it does make sense to search to see if we find any.

There have been many researches of chroococcidiopsis which is a cyanobacteria with amazing resistance to space and martian conditions. These are ongoing experiments. A new one year mission called BIOMEX, part of EXPOSE-R2 is due to start on the ISS in April 2014 to explore their tolerance of Mars conditions further. This will fly a wide variety of organisms, including lichens, for 12 to 18 months. They will be exposed to a simulated Mars environment on the outside of the Russian module of the ISS.

Liquid brines beneath the surface of sand dunes at night - beneath the sand that Curiosity drives over

This was a surprise discovery announced in April 2015. Liquid brines that form through deliquescing salts (perchlorates) - the salts take in water from the atmosphere (same principle as the salts you use to keep equipment dry). They found it indirectly - when Curiosity drives over sand dunes, then the air above them is drier than it is normally - when it leaves the sandy areas the humidity increases. This shows that something in the sand dunes is taking up water vapour from the air, and rather a lot of it too. The perchlorates in the sand take up so much water at night that the liquid brines would be habitable, except that they are too cold. As the day progresses the brines warm up but they also dry out, and become too salty for life. The authors of the paper concluded that the conditions in the Curiosity region were probably beyond the habitability range for replication and metabolism of known terrestrial micro-organisms. . "Evidence of liquid water found on Mars"  "Transient liquid water and water activity at Gale crater on Mars"

However Nilton Renno, who is an expert on Mars surface conditions suggests that microbes may still be able to exploit this liquid brine layer through biofilms, see::

"Life as we know it needs liquid water to survive. While the new study interprets Curiosity's results to show that microorganisms from Earth would not be able to survive and replicate in the subsurface of Mars, Rennó sees the findings as inconclusive. He points to biofilms—colonies of tiny organisms that can make their own microenvironment."

Mars liquid water: Curiosity confirms favorable conditions.

Nilton Renno was one of the first to suggest that liquid brines could occur on the Mars surface after he noticed what look like droplets forming on the legs of Phoenix.

So we now have several ways that life could be possible in the equatorial regions

  • Somehow using the frosts that form at night for 100 days of the year as they evaporate in the morning
  • Using pores in salt pillars
  • The warm seasonal flows (or RSL's) since some of them occur in equatorial regions
  • Advancing sand dunes bioreactor
  • Micropores in salt pillars
  • The liquid brine layers found by Curiosity.
  • Directly using the night time humidity

The dust could also have spores in it from nearby habitats even if it is not actually inhabited.

How the future might unfold if Viking did find life in the 1970s

So, what if it turns out that Viking did find life. Many would say that it seems a remote possibility. The skeptics think that they have shown that what Viking found didn't have to be life, by finding alternative explanations. But even if that is right, which is disputed, they didn't prove that it wasn't life. What if Gilbert Levin, Joseph Miller and the others turn out to have the correct interpretation of the results after all?

If that is correct - then since Viking didn't land anywhere special on Mars, it probably means that life is present in low concentrations over much of the equatorial regions. It could be present as spores, or it could be that it somehow takes advantage of the evaporating dry ice / water ice frosts, or uses the 100% night time humidity, forms in salt pillars, or is sustained by the advancing sand dune bioreactors.

In this scenario ExoMars will probably detect biosignatures of life quickly, maybe right away. Its instruments are sensitive enough to detect life even in the Atacama desert (where levels of organics are too low for Curiosity and Viking to detect anything). In this scenario, ExoMars finds biosignatures in trace quantities almost everywhere it looks, in the Martian sand dunes. If not, then perhaps when the more sensitive SOLID3 or astrobionibbler gets sent to Mars, then it detects traces of present day life almost everywhere on the surface.

In this scenario, Curiosity 2020 would have life in the samples it collects too, and this perhaps then gets returned for analysis in the 2030. If so, then we may confirm that there is life on Mars rather soon.

But of course this doesn't go the other way. If the sample return in the 2030s doesn't contain life, it doesn't even conclusively prove that Joseph Miller and Gilbert Levin's interpretation of the Viking data is incorrect. Perhaps Curiosity's successor is unlucky or both Viking landers were very lucky. After all, the plan is to return just a few hundred grams of samples in total, with most of that rock rather than dust from the surface, so it would be easy to miss a very sparse signal in the dust. It certainly doesn't mean that there is no life on Mars, or even, no life in the equatorial regions! Unless you have a lot more context to interpret the result, all you can deduce from a sample return with no life in it is that there are some rocks and patches of dust on Mars which don't have life in them. That would hardly be a huge surprise.

Mars sample return as a geology mission and a technology demo

The decadal survey summing up motivates the Mars sample return using examples of previous returns of comet and interplanetary dust, and moon samples.

Tracks of particles from comets collected in the stardust aerogel, first sample return of a comet to Earth

These undoubtedly were hugely valuable in advancing our understanding. But those are all astrogeological missions, and their value was geological. Geological specimens don't deteriorate in the same way as organics. There is no problem of racemization, or of lifeforms eating them, and usually no problem of them being washed out by flooding. They are also easier to find. They are easier to preserve for transport back to Earth. They are also relatively easy to identify. The sample return from Mars may well be of great value for geology. Nobody controverts that.

Astrobiological motivation

The big difference is that a Mars sample return is motivated mainly by its value for exobiology. Geologists would love to get hold of a sample from Mars. However, it's hard to motivate a multi-billion dollar mission to return a kilogram or so of samples from Mars for the geology only. That's especially so since we are making great strides in understanding the geology of Mars robotically and we also have a wide variety of meteorites samples from Mars to study already. If the motivation is geological, you listen to the planetary geologists, and ask them how to design the mission and what to do. But if the motivation is astrobiological - surely you need to listen to the astrobiologists? Methods that work for astrogeology may not work so well for astrobiology.

Why did the decadal survey choose a sample return?

To return to our earlier question, why did the decadal survey choose a sample return mission over in situ exploration? Why didn't they listen to the astrobiologists? I don't know the answer to that, since they don't discuss the Bada paper at all as far as I can see (do correct me if you know of anything they say on this subject, anyone, or you have any more information on the background to this). But I can offer a few thoughts that might be relevant. NASA, of all the space agencies, is the one most focused on the aim of an eventual human landing on Mars. So could it be connected with that?

"Safe on Mars" - could a sample return tell us if mars is safe for astronauts?

This is just a guess, but I wondered if it is possible that they were motivated partly by the Safe on Mars report in 2002 (which is cited by the decadal survey). This recommended a sample return from Mars to check to see if there are any biological hazards for humans on the surface. This is a 2010 animation showing how a sample return from Mars might have been done with 2010 era technology (they show it returned to the space shuttle).

If so, it's interesting to note that Safe on Mars recommended a sample return only because at the time they wrote the report, there were no instruments sensitive enough to do a good search in situ, in their view. They say:

As stated above, there are currently no measurement techniques or capabilities available for such in situ testing. If such capabilities were to become available, one advantage is that the experiment would not be limited by the small amount of material that a Mars sample return mission would provide. What is more, with the use of rovers, an in situ experiment could be conducted over a wide range of locations.
(Page 41 of Safe on Mars)

So, actually, when you read it in detail, it's a similar recommendation to the one by the astrobiologists.

Well, now, these capabilities are available. Many instruments that were huge laboratory filling machines even as recently as this report in 2002, with no chance at all of sending them to Mars - they have now been miniaturized and a fair number also tested in space simulation conditions, and could easily be sent to Mars. These include DNA sequencers, electron microscopes, ultra sensitive biosignature detectors able to detect a single amino acid in a sample, and updated versions of the Viking Labeled release using chirality to eliminate false positives. Our instruments also include the exquisitely sensitive electrophoresis "lab on a chip" methods mentioned by Bada et al. Another new idea is the Solid3 approach of using polyclonal antibodies - which can detect, not just the organics you find in animal bodies, but a wide range of organics, again with exquisite sensitivity, and a "lab on a chip".

A sample return can only tells us that there are some rocks on Mars which are safe for humans

The geology of Mars is much more varied than realized in 2002 when that report was written, and conditions for habitability even more so. We have ideas now for potential habitats for life even in equatorial regions such as the advancing sand dunes bioreactor and the warm seasonal flows. These habitats could depend on things such as small local variations in the concentrations of various salts in the soil. Also there are ideas for ways that life could survive (perhaps just below the surface) using the night time humidity with no water at all.

It doesn't seem likely that a few samples returned from the surface even of a large plain of sand dunes, for instance, would be able to confirm or deny the advancing sand dunes bioreactor hypothesis. There might be only a few sand dunes with the right mixtures of salts to give conditions for life in the entire plain. Or life might have colonized some rocks and not others, just through chance (as happens in deserts on the Earth).

So, we can't hope to deduce that much from a small sample return about present day life on Mars. At least - not without a lot more context and understanding than we are likely to have by then.If it has no life in it, all you can say from a selection of samples like this is that there are some rocks and sand dunes on Mars that have organics, but don't have life in them. That's no great surprise. 

And if there is life in the sample - again - it doesn't tell us that much about the range of possible lifeforms on Mars. Mars may well have more than one species of life - so would they all be present in these first samples returned from Mars? If we find a cyanobacteria for instance - does that mean that the only life on Mars is cyanobacteria? Not likely we could conclude that from just a few samples returned from Mars.

In conclusion, given what we now know about the variety of conditions on Mars and the varied possibilities for habitats there - it doesn't seem likely that a sample return from Mars at this stage would settle anything about safety of surface conditions for astronauts.

Sample return needs to take account of the possibility of life in the sample even if it is a low probability, 1% chance or a tiny fraction of a percent

So, present day life on Mars has to be very abundant even in equatorial regions, for Curiosity 2020 to have a good chance of returning present day life. And as we've seen already, it's very unlikely to have evidence of past life unless the past life was detected already on Mars. But how likely is it to have present day life? If you ask scientists if Viking found life and if life is possible in the equatorial regions, I think you'd get a range of answers from impossible to close to certain for the few, like Levin, who think that Viking already found life. So from 0%, perhaps all the way to 100% probability. But if you do a sample return, you have to plan for the contingency where it does find life. After all the effects could impact on the whole Earth in the worst case.

Suppose you decide that the chance of life in the sample is 1% just to take an example for the sake of argument. Well that is a 1% chance of life that Mars has been continuously inhabited by life for billions of years since if there is life in the equatorial regions, the most difficult place for life to live on the planet, and in a particularly dry phase of its history - then it's probably been there continuously since whenever life first evolved on Mars.

So then - we have life from Mars that has evolved for as long as Earth life. But we don't know if it evolved at the same pace, faster, or slower. We have no basis really to decide the question so let's suppose that those three possibilities are equally likely. If it is less evolved than Earth life, especially if it is, say, RNA world life, then it would be very vulnerable to Earth life. If equally evolved or more evolved then Earth life could equally be vulnerable to it. It may be more capable, more versatile, have more resistant resting states, more efficient metabolism, more biochemical pathways, better at photosynthesis etc. After all there's no reason to suppose Earth life is the pinnacle of evolution - what will happen if our microbes continue to evolve for another several billion years? That could be what we find on Mars.

Or, perhaps the most likely possibility, it's advanced in some ways, less advanced in others. It doesn't need to be all round better. If it is better at photosynthesis for instance, that could be enough so that it can out compete the photobionts in our seas once it adapts to Earth conditions.

So, going with our 1% figure, now we have a one in three chance that it is evolved further than Earth life, perhaps. So then 1 in 300 chance that there is something returned to Earth that is more advanced than Earth life evolutionarily. This is of course just a very crude way of thinking about it. The idea is just to get you thinking about it. Can you come up with a better way of assessing whether or not it is going to be hazardous to the environment of Earth and to assign a probability to it?

Even if the chance of life is much less than that, and the chance that it is a danger to Earth is much less also, then if the potential effects could be devastating to the environment of Earth, then as Carl Sagan said, we can't take even a tiny risk with a billion lives.

At any rate the studies into risks of a sample return from Mars have all concluded, to date, that we do have to take care, for this reason. That the chance of harm to Earth is probably very low, but we have no way to assess the risk as a probability. Until we know what we are likely to return to Earth, we need to design the sample return to be sufficiently robust and safe to contain any possible form of exobiology that we might find on Mars. That is tricky to do, since we only have the example of Earth life. So how can we design to contain any possible form of extraterrestrial life when we have only the one example? It would be much easier if we knew what we were going to return first.

It is easy to protect from known hazards such as rabies, legionnaire's disease, Dutch elm disease, oriental fruit fly etc. But how do you contain a sample when you have to protect Earth from any possible form of extra terrestrial life and don't know anything about it, whether it is nanoscale, what range of temperatures, levels of ionizing radiation etc it can tolerate etc etc? That's the challenge that the experts face who were consulted about safe methods to return samples from Mars.

How can we protect Earth during a sample return mission?

So, a sample return is not only an expensive mission, it also raises unprecedented issues of planetary protection for the Earth, and especially so if the sample is returned at such an early stage, when the planners of the mission can have no idea what is in the sample of biological interest. If NASA does go ahead with the second half of their proposal, and they do a sample return, this is something that Carl Sagan and others have argued is something we should only do with great caution.

I have two new suggestions here that could, just possibly, help resolve this. First, though, for those of you who are new to this, let's just summarize the way that it would be done according current ideas.

Safe return of unsterilized sample

If the sample is unsterilized, we have to take precautions to protect the Earth. That is the conclusion of all the studies into the back contamination risks of a Mars sample return to date.

Easy part, sample return to surface in container

The easy part is to return a sample to the Earth surface. That's pretty much worked out. The idea is that you have to break the "chain of contact" with Mars. Make sure nothing that has contacted the Mars surface or contacted anything else that contacted the Mars surface is exposed to Earth environment. The easiest way to do that is to use nesting capsules. The Mars sample is placed inside a larger capsule in Mars orbit. On return to the Earth system, you could indeed put it inside an even larger capsule enclosing both. Make sure that there is no way those capsules can be broken even in event of a crash landing on Earth.

The main issues there are - that a micrometeorite could pierce the capsule - and human error, some mistake in design of the capsule that is never picked up all the way through the design process - and a bad re-entry that burns up the capsule so that the interior is exposed. But a carefully designed mission could deal with all those - those are addressable issues.

Hard part - what do you do next once you have a sample on the Earth?

The hard part is, what do you do when it returns to Earth? If you just wanted to keep the sample in its container for ever, simple, bury it deep below the ground. Maybe enclose it in synthetic rock. Or simpler still just sterilize it completely with ionizing radiation and all is safe and dandy.

But of course that's not what we want to do. We need to study it, in a laboratory, cut bits out of the sample, and move those fragments around and look at them in many different machines. Eventually to send those samples to other laboratories around the world. You’d think this was easy, but it turns out to be surprisingly complex and difficult. Back in the 1990s the general idea was that we can just return samples to a glove box facility in a biohazard 4 laboratory. Idea was - that since we know how to contain hazards such as the Ebola virus etc, surely there would be no problem containing a sample from Mars. Just use the same techniques we already use in biohazard laboratories.

After a series of studies, however, it was realized that it's not as simple as it seemed at first. The precautions needed got more and more complex. The most recent studies require a facility costing perhaps half a billion dollars or more, with capabilities never tested before. One problem is that it is easy to contain a known pathogen, say smallpox, or anthrax, or the Ebola virus etc. Because you know that it needs an animal host (maybe a human host), and know what kills it. But what can you do when you don’t know what is in the sample, what its capabilities are, what size it is, or even what biochemistry it has? And if it possibly doesn’t use DNA? And perhaps it is a spore in resting state, that is highly resistant to ionizing radiation, to oxidising agents like hydrogen peroxide, and other chemicals, able to survive vacuum conditions, etc etc - all of which are very likely to be the case for Mars life? And could be tiny far smaller than any Earth life?

The smallest size for early cells if they don't contain all the machinery of modern life, is generally estimated as about 40 nm. Successive studies by the National Research Council (NRC) in the USA (two studies) then the European Science Foundation (ESF) (one study) gradually lead to more and more stringent requirements. First came the reduction to 200 nm by the NRC after discovery of the ultramicrobacteria Then, that was reduced to 10 nm by the ESF. as a result of discovery of how readily archaea can share their DNA through the tiny Gene Transfer Agents (GTA's) Archaea can transfer genes between phyla that are as different from each other as fungi are different from aphids. It is an ancient mechanism and so may also be able to transfer genes from life that had last common ancestor with us in the early solar system. As we've seen already, In one experiment 47% of the microbes (in many phyla) in a sample of sea water left overnight with a GTA conferring antibiotic resistance had taken it up by the next day

So if the life is at all related to Earth life, you have the possibility of this exchange of DNA bringing new capabilities to Earth microbes from space. Even if the microbes themselves don’t survive.

Another thing that makes the design more complex is the need not just to contain the sample (which is usually done by a positive air pressure from outside) but also to protect it from outside organics (which needs a positive air pressure from inside). You end up with some kind of a double walled facility and they cite this as one of the main reasons why you have to have a new design of building, never tested before. This is one of the designs they came up with in 2008, with telerobotics.

The LAS sample receiving facility uses a fully robotic workforce, including robotic arms that manipulate samples within interconnected biosafety cabinets. Carrier robots would transport the samples around the facility. Credit: NASA/LAS

This is for just a less than 1 kg of samples returned most likely, yet you have to build something like this. And even then, it might not be sufficient. Every Mars sample return study to date says at the end that their conclusions have to be reviewed continually, based on new research. For the next study, whenever it is - well there is much active research at present into into a semi synthetic minimal living cell or an artificial minimal cell. Does the 40 nm size limit still apply based on the recent research? In the Programmable Artificial Cell Evolution project, the smallest artificial minimal cells were as small as 103 atoms, based on PNA instead of DNA, making it possible to simulate the whole cell as a quantum mechanical system in a computer. These “cells” were just a few nanometers across.

Also there's much more work been done on possible XNA based life, and I would expect that to feature more in a new study than in previous ones. And none of the studies to date address issues of human error, accidents, terrorism, a crash during transport of the sample to the facility, a plane crashing into the facility etc. The studies done to date mention these issues, but only to say that these issues were not part of their remit.

And then after all this work - we might find that the sample receiving facility wasn't even needed. The samples returned might be completely harmless. It seems a back to front way of proceeding to me. Wouldn't it be better to first characterize the sample before we return it? Then design the facility around the samples once we know what they are?

For more about this see my Need For Caution For An Early Mars Sample Return - Opinion Piece

Legal complexities

Margaret Race (of the SETI institute) covered these in an excellent paper. There’s far more to it than you’d think. Back in days of Apollo, the quarantine rules for the Apollo 11 return were only published on the day that they launched to the Moon, giving no opportunity at all for comment or peer review. That would simply not be permitted today. Also the Apollo regulations have lapsed.

Also, there are many domestic and international regulations to be negotiated and new laws to be passed. She considered the whole process likely to take ten years or more, and it can also potentially involve the domestic laws of nations that are not receiving the sample, because the potential effect of the worst case scenario could impact on all nations. It would be a process that would be carried out in an open fashion with public debate. See Planetary Protection, Legal Ambiguity, and the Decision Making Process for Mars Sample Return

Natural contamination standard - great for asteroids and comets but doesn't work for Mars

There is one argument, similar to Zubrin's meteorites exchange argument, which is used in planetary protection calculations. That's the natural contamination standard. If you can prove that what you are doing is equivalent to what happens naturally, then the mission is not a biological issue. That's used for sample returns from comets and asteroids to Earth. Since fragments of comets and asteroids hit Earth all the time, then it's not adding to the hazard, whatever there might be. If there is any life on comets or asteroids, which is not ruled out, then we've evolved to be able to cope with its influx into our atmosphere, so there's no problem returning those samples.

The problem with Mars is that sending humans to Mars or returning samples from Mars to Earth is not like the processes that happen naturally. The natural contamination standard would involve simulating somehow the equivalent of 100 years of interplanetary cosmic radiation, and vacuum and the cold of space. And even then, those are events that happen only every 100 million years or so. While in the reverse direction from Mars to Earth, it might not have happened at all for billions of years, depending on whether any meteorite that hit Mars not only hit a habitat for life there, but also sent life into orbit - given that the material that goes into orbit comes from a distance of a few meters below the surface, and the surface habitats that may have life consist of dust, clays, and ice which would most likely just be scattered back into the atmosphere.

We'll only be able to fill in the gaps in this picture once we have life detection from Mars, if there is life there. From the evidence so far, we might well find habitats on Mars without Earth life in it, habitats that Earth life could inhabit. There's another way also that Earth life might not be in those habitats. That is if they are very rare on Mars and form for a few centuries then go away. Maybe the Earth life just doesn't get there in time with a few spores spread in the dust storms before the habitat disappears again. That happens on Earth also in newly formed lava flows but it only takes a few weeks for Earth life to colonize them. On Mars maybe it takes centuries or millennia. It might be in some of the habitats and not in others. It might occupy them on Mars for some time, even perhaps occasionally for millions of years, then go extinct.

So anyway - Zubrin and a couple of exobiologists have put forward a thesis according to which they think that habitats on Mars will have exactly the same lifeforms that the sam habitats have on Earth. But they don't go into details about how it would happen. It's largely "hand waving" arguments, and it's by no means proven and is rather controversial. I think many exobiologists would be very surprised if that's what they found. And would be bound to be some differences which you'd want to explore and understand, if he was right, to learn which lifeforms got to Mars, how they got there, and how they survived when they got there, what was the first lifeform to get there, and how they evolved and changed after spending tens of millions of years, perhaps billions of years, in Mars conditions.

But could also be that it just never happened. Or that there's a mix of Earth and native Mars life that gets on fine on Mars but won't work any more after you introduce more Earth species. Or perhaps Zubrin is right and somehow all the Earth lifeforms that could survive in those habitats are already there. If so that would be an extraordinary event that you'd want to understand well before introducing more present day Earth life to Mars. Or there might be habitats but no life, as Charles Cockell talks about, and again you'd want to understand that well too. It could give us insights into exoplanets that don't have life, and into the role of life in geological processes on Earth as a "control" and tell us something about how far complex chemistry can get on its way to life on a planet without life.

One way or another, I think it is just far too soon to say that it is okay to introduce Earth life to Mars.

For more on all this see my:

Does Earth Share Microbes With Mars Via Meteorites - Or Are They Interestingly Different For Life?
Could Microbes Transferred On Spacecraft Harm Mars Or Earth - Zubrin's Argument Revisited 
No Simple Genetic Test To Separate Earth From Mars Life - Zubrin's Argument Examined

Why quarantine won't protect Earth or humans sent to Mars - if Mars life exists

Mars life could also be hazardous for Earth (this question about whether microbes from one planet can harm life on another goes both ways). If you haven't come across the scientific papers and workshops and studies on this issue before, chances are you're first thought will be of the "Andromeda strain" or some other science fiction scenario. In that case it's viruses from outer space. But viruses aren't a likely problem for humans going to Mars, because they have to be adapted to their host. Any life on Mars has never encountered humans before so can't be adapted to us.

There are many other ways though that Mars life could be hazardous to humans and also to the biosphere of the Earth.

  • Gene transfer agents. These are much smaller than viruses, and they can transfer small fragments of DNA from one species to another to give them new capabilities. It's an ancient mechanism, and works between distantly related species. GTA's can transfer capabilities between species as unrelated as fungi and aphids (example of a GTA that gave an aphid the capability to create carotene, from a fungus). Also it works very quickly between microbes in sea water, if the GTA's ever got into the sea. In one experiment a GTA conveyed antibiotic resistance on 47% of the microbes in sea water, all types, just the microbes you have in sea water naturally, after they left them exposed to the GTA's overnight. This is relevant if Mars life is distantly related to Earth life. Even if the life got transferred from Earth to Mars or Mars to Earth billions of years ago, it could still exchange capabilities with Earth microbes readily using GTA's 
  • Life not based on DNA which has chemical signatures that Earth based life is not designed to respond to. Our defenses would only respond to the trauma, not to the cause of it. This is a point made by Joshua Lederberg, Nobel prize winning microbial geneticist, in Parasites Face a Perpetual Dilemma and also in Exobiology: Approaches to
    Life beyond the Earth 
    and a nice quote here: 

    "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”?
     in his "Paradoxes of the Host-Parasite Relationship" (he also gives an interesting analogy there with symbiosis with mitochondria)

    In the worst case, of total naivete on the part of Earth microbes, lifeforms like this could live on our bodies, in our guts, and produce chemicals that are poisonous to us or take the place of microbes that we need to survive. Or just eat us. And our defenses might not respond. 
  • Life from Mars could harm us in many other ways, not just directly as diseases of humans. If the life from Mars has a major effect on microbes that we depend on (like the algae in the sea) or on the plants we depend on for food, wood and so on, or on the animals, it could be just as disastrous for the environments on the Earth. As an example, cyanobacteria produce toxins that kill cows. There's no evolutionary advantage in this as far as we know, the cyanobacteria can't eat the cows and it's unlikely to be a measure to deter predation by cows. It's just a case of toxins that are effective over a large evolutionary distance. See Alien Infection (Astrobiology magazine, 2008)

    For another example, cyanobacteria produce BMAA which is implicated in Alzheimer's. This is a chemical that resembles L-serine and can be misincorporated in its place and cause folding disorders in proteins, amongst other effects. Again there is no advantage to the cyanobacteria to cause Alzheimer's. in humans. And another nice example, cocoa plants produce theobromine which kills dogs if they eat too much chocolate. The cocoa plant doesn't need to defend itself against dogs. 

    In a similar way, microbes from Mars could easily produce toxins that have adverse effects on Earth life. 
  • Life that out competes Earth life. One example here, what if Mars has some fourth form of photosynthesis different from the three main types on Earth.

    Our three types are:
    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 CO2 into 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.

    What if Mars life uses a fourth form of photosynthesis is more efficient at making use of sunlight than the methods used by the green algae in our oceans? The space of possibilities is so vast, there is no way that DNA based life on Earth has explored even all the possibilities for DNA. For instance, over many millions of years, higher lifeforms in Australia never developed the placenta or anything resembling a modern mammal, and so it was vulnerable to introduction of rabbits, which were not at all adapted to Australian conditions, but still easily out competed the native Australian marsupials. Similar things could happen at the microbial level for transfer between planets instead of continents. Mars microbes could have capabilities never explored in the entire history of evolution on Earth. 

    For another example, if not based on DNA, it could be able to do more with less. The microbes might be smaller, the encoding more efficient, less need for error correction, enzymes much smaller to do the same thing, it could be all round more efficient, and so able to manage on less by way of resources, with a more efficient metabolism. It might out compete Earth life everywhere where it can survive on Earth, by making do with less. 
  • Life returned from Mars might have a different side to it, like harmless grasshoppers which some trigger can turn into locusts, some condition on Earth triggers a different behaviour or capability that never turned up when encountered on Mars or in transit. 
  • Life returned from Mars could be harmless at first, until it adapts to Earth conditions, but then evolve to be a major problem later. For instance it might need to adapt to warmth, or to water that is less salty than on Mars, or to lack of perchlorates (if it tends to depend on perchlorates for food) or the denser atmosphere.
  • It could be a slowly developing problem even without adaptation. E.g. back to that example of photosynthetic life that is just slightly better than Earth life, then it might take decades before sufficient numbers build up to replace the green algae and other photobionts in the oceans. Nevertheless, with exponential growth, there might be nothing we can do to stop its inexorable advance. 
  • Mars life could also be totally harmless, as Carl Sagan said in Cosmos"There may be no micromartians. If they exist, perhaps we can eat a kilogram of them with no ill effects. But we are not sure, and the stakes are high. If we wish to return unsterilized Martian samples to Earth, we must have a containment procedure that is stupefyingly reliable...here are nations that develop and stockpile bacteriological weapons. They seem to have an occasional accident, but they have not yet, so far as I know, produced global pandemics. Perhaps Martian samples can be safely returned to Earth. But I would want to be very sure before considering a returned-sample mission.”

With this background, then you can see that ideas for quarantine just wouldn't work to keep Earth safe. There's a great tendency to look back at Apollo and assume we'd handle it as they did, put the astronauts in quarantine for a few weeks on return to Earth. But those quarantine precautions never had any peer review. They were published on the day of launch. And they were not even applied properly at the time. Buzz Aldrin noticed ants found their way into the quarantine facilities while he was in quarantine.

“The unit was comfortable, but there was little to do and nowhere to go, so we got bored in a hurry.

"One day, I was sitting at the table staring at the floor, and I noticed a small crack in the middle of the floor, with tiny ants coming up through it! Hmm, I guess this thing isn’t really tightly sealed, I thought. Imagine, if we had brought some sort of alien substance back with us, those ants could have contracted it and taken it back out to the world!”

Earlier, the command module hatch was opened when they landed, and dust from the Moon surely went into the sea at that point, and there were other breaches of protocols as well. But even if it was done perfectly, it wouldn't have protected Earth from microbes from the Moon on the remote chance that it had any.

If we were to attempt to use quarantine today, for samples or astronauts returning from Mars, then problems with this approach include:

  • If any of the astronauts become seriously ill, they will be rushed to hospital and not permitted to die in the quarantine facilities. If you try quarantine in orbit, they will be returned to Earth as soon as they encounter any really serious health issue. This is clear from Apollo. The crane they had designed to pick the command module out of the sea had a problem. Rather than fix the problem and leave the astronauts bobbing in the ocean, getting seasick while the world watched, they sent a helicopter with divers, who took the astronauts out of the module, into an open boat, and put them into their suits. By then the dust from the module would be in the sea already and planetary protection was already compromised even assessed by the standards of their time. From this example to show how mission planners were ready to waive precautions just to prevent the astronauts from getting seasick for a few hours while they fixed the problem, it's clear that they would definitely not let their heroic astronauts die in the quarantine facilities if they became seriously ill. So the Apollo astronauts quarantine was a largely symbolic gesture which would have done nothing to protect Earth from any real hazard in the unlikely case that the Moon did have microbes hazardous to Earth. 

    It's not clear you can ethically keep them inside anyway if they get seriously ill in quarantine facilities designed to protect Earth from unknown dangers. At the very least it's a very tricky ethical and legal area. Even if they consent beforehand to be left to die there to protect Earth, it's not clear you can hold them to that in the event that it happens, especially if you have no idea whether there is some extra terrestrial cause - when it might well be some Earth based illness that needs to be diagnosed in the advanced facilities of a modern hospital to save their lives. 
  • It only protects from diseases that affect humans or other lifeforms in the facility. You can't take all the higher animals we depend on, the trees, grasses, sea water, etc. etc. into the facility for testing
  • You have to guess at the latency period. Some diseases of humans such as leprosy can remain latent for decades before anything happens. There was no scientific reason for choosing any particular quarantine period for the Apollo astronauts. It was just a guess and I haven't seen any reasoning to explain their choice. Quarantine does work fine when you know the maximum latency period, and if it is a reasonably short period. But it doesn't work if that period is very long or you don't know what it is. 
  • It gives no protection from problems that manifest later. E.g. quarantine won't help at all if the microbes that humans carry with them need some time to evolve to adapt - either to terrestrial conditions or indeed even, to adapt to humans.

For all these reasons I think there is almost no point in attempting quarantine to protect Earth or to protect humans on Mars. It's largely symbolic and would give a false sense of security. It doesn't matter where you do the quarantine either, on Mars, or the journey back, or in orbit around Earth, or on the Moon, or back on Earth, none of that helps.

Instead I think there is no substitute for knowing what is in the samples before they are returned to Earth. I also think that we shouldn't send humans to Mars until we understand Mars conditions very well indeed and have done a reasonably complete biological survey of the planet, or for some other reason have a high level of confidence that there is no life there, or that any lifeforms there are safe for humans and for Earth. That's for safety reasons alone, apart from any other considerations, to protect both the astronauts and Earth (after they return).

Returning samples from mars - unlikely to find life if not already discovered

NASA has made it a priority for the next twenty years to return a sample from Mars for analysis on Earth. ESA has also proposed it as a flagship mission.

Artist's impression of Mars sample return vehicle launching from Mars - credit ESA.

However, with this background that we don't know where to look yet, for both early and present day life, and since Mars is such a complex planet, with such varied terrain to such, there is quite a risk that such a sample might fail to return material of biological interest. Probably we can only be reasonably confident of success if we have detected clear biosignatures on Mars already. Even if it contains organics, the organics might not arise from life.

So, if we want to find traces of life on Mars, it seems pretty clear, that we need to find it in situ on Mars first, just for reasons of expense. The NASA plans would return a few hundred grams of crushed samples at a cost of billions of dollars, and this is too much to spend to return a sample that has only a small chance of containing any material of interest to exobiologists. It's true that once returned, we can analyse them over and again with more and more instruments - but that's only useful to exobiologists if we return the right samples in the first place. While we can send increasingly sophisticated instruments to Mars to study it in situ and explore a far wider range of possibilities there.

It's possible however, that ExoMars, or one of our other rovers, will find clear biosignatures of life on Mars at an early stage. If that happens then exobiologists will probably be keen to return these samples to Earth for analysis. Can this be done safely?

Suggestion for protection of earth during sample return - ionizing radiation

I won't go into this in detail, why protection of Earth is needed, as you'll find plenty about it in my other articles, see Need for Caution for a Mars Sample Return - and Could Microbes Transferred On Spacecraft Harm Mars Or Earth - Zubrin's Argument Revisited

But in brief, life from Mars could be benign, but it could also be capable of competing with Earth life. In the worst (but most interesting case) Martian life could be XNA, capable of setting up an independent self contained ecosystem on Earth. Martian cells could also be far smaller than Earth life, as the earliest cells before the archaea must have been at most a few tens of nanometers across, about a tenth of the size of the smallest known modern cells (the ultramicrobacteria). Or, if it has a common origin with Earth life, could have capability of transferring genetic material via Gene Transfer Agents, as archaea are able to swap material very readily in this way, and the GTA's are again only a few tens of nanometers in size.

In the XNA specifications section of this paper: Xenobiology: A new form of life as the ultimate biosafety tool The authors talk about biosafety requirements for this procedure

"The ultimate goal would be a safety device with a probability to fail below 10-40, which equals approximately the number of cells that ever lived on earth (and never produced a non-DNA non-RNA life form). Of course, 10-40 sounds utterly dystopic (and we could never test it in a life time), maybe 10-20 is more than enough. The probability also needs to reflect the potential impact, in our case the establishment of an XNA ecosystem in the environment, and how threatening we believe this is."

Since XNA from Mars could also potentially set up an XNA ecosystem in the environment on Earth, we need to be similarly careful when considering its impact.

This all makes it extremely hard to contain reliably, especially when you don't yet have a thorough understanding of what it is you need to contain. It is not too bad so long as you keep the specimen in the capsule, but as soon as you remove samples for analysis, it's hard to see how you can keep it completely enclosed to ten nanometers level - as the optical resolution for the best high powered microscopes is around 200 nm. There's also the risk of damage to the capsule, and loss, theft, natural events such as hurricanes, or airplane crashes, or human error leading to accidental release.

Current requirements for sample return and legal situation

The most recent ESF study on how to deal with samples returned to Earth from Mars recommends returning them to a new type of facility which has to contain them right down to the level of GTA's as well as the smallest size of microbe they think is possible using unknown extra terrestrial biology. Their recommendations are that it has to be capable of containing particles well below the optical resolution limit of 200 nanometers (ideally it shouldn't permit release of particles over 10 nanometers in diameter). In other words, the facility has to be able to contain particles only visible with electron microscopes or similar. This is well beyond the capabilities of a normal biohazard level 4 containment facility where the aim is to contain known hazards of known size and capabilities. It also has to protect the samples against contamination by Earth life, even by a few amino acids.

Also, there's all the extra legislation to pass. Margaret Race looked at it. You'd be astonished, there are many domestic and international laws, needing to be passed - which were not needed for Apollo because the world nowadays is legally far more complex. After reading her paper, I think it could easily take well over a decade just passing all the laws even if everyone agrees and there are no objections, and surely longer if there are objections.

The basic idea here is that we return unsterilized samples from Mars to Earth before we know what is in them. To do that safely we have to design the sample return in such a way that the facility is safe to handle any conceivable extra terrestrial biology, before we have discovered even one other example of life other than Earth life. I think that's the main thing that makes this so tough. If we knew what we were returning, it would be so much easier. If the life on Mars is early pre-DNA life and if we can show it was made extinct on Earth billions of years ago, for instance, we might not need to take any precautions at all. On the other hand if it is not based on DNA or RNA at all, or if we had evidence that the life is at a stage of evolution several billion years ahead of Earth, or has capabilities Earth life doesn't have (such as more efficient photosynthesis) then it might need extreme caution.

Surely there is no substitute to finding out what is there first. So how can we do that? Well one approach is to use in situ searches, which may be the best way to search for life anyway - and then perhaps once we find life on Mars, by sterilizing samples returned to Earth.

Using ionizing radiation to sterilize the sample returned to Earth

However one possibility is to use ionizing radiation. Ionizing radiation is not used for sterilizing spacecraft to Mars because gamma radiation destroys semiconductors. But with a Mars sample return, you don't need to sterilize an entire spacecraft. The only part that gets returned to Earth is the sample itself in its container. The rover remains on the Mars, just launches the container to orbit around Mars, and the container is picked up by a separate orbiting spacecraft.

So, my suggestion is, for the first sample returns from Mars, why not use ionizing radiation? Subject it to enough ionizing radiation to thoroughly sterilize even the most radioresistant microbes known such as radiodurans and chroococcidiopsis or halobacterium. Modern analysis techniques would still permit us to learn a lot from a sample sterilized in that way. Actually I'd subject it to more ionizing radiation than that. Radioresistant organisms on Earth don't seem to have adapted to high levels of radiation particularly, as they can never encounter those environments (except in very rare situations such as natural nuclear reactors in deposits of enriched uranium in the early Earth). Instead they probably developed radioresistance as a side effect of adaptations to extreme dry conditions and UV, which damage DNA in a similar way.

But on Mars any life would evolve radioresistance specifically in response to ionizing radiation. So, life on Mars may be even more resistant to radiation than the most radioresistant microbes on Earth. The most extreme example of radioresistance on Earth seems to be Thermococcus gammatolerans - an obligate anaerobe from hydrothermal vents which was able to continue to grow after irradiation by 30 kGy of gamma radiation (applied at a rate of 60 Gy per minute).

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 400,000 years worth of surface radiation on Mars at the radiation levels detected by Curiosity during the current solar maximum of 0.073 Grays a year - possibly it could survive surface radiation for longer than that when you include periods of solar minimum.

This micro-organism didn't evolve in an environment with high levels of radiation, but developed this resistance as a side effect of other effects that can damage DNA. Microbes on Mars would have evolved in an environment with high levels of radiation, and adapted specifically to that environment. They might be even more radioresistant than this.

So, it looks as if you'd need to use at least hundreds of kGy to be safe. There wouldn't be much left of complex molecules after that, sadly, such as the carotenoids, but the chiral signal of amino acids would be strong, even after 14 MGy. If these are samples of early life on Mars, then 14 MGy is approximately the dose the microbes received anyway from natural nucleotides in the rocks. So, for samples of ancient life, a radiation dose of a few MGy is maybe not much of an issue, except for the remote possibility of revivable ancient life, or life retrieved from pure ice, or from salt deposits with no radioactive isotopes in them.

So - that's my suggestion. For the very first sample returns from Mars, when we have very little idea of what is in them, the idea is to irradiate it with several MGy of radiation before you open it.

Safest of all would be to irradiate it in Mars orbit, before it returns to Earth, perhaps with gamma radiation, using Cobalt 60, as is standard in food and medical gamma ray sterilization. One idea, the Cobalt 60 could be included in a shielded container sent along with the spacecraft that picks up the returned sample in Mars orbit. That way, even before the sample leaves Mars orbit, it is put straight into the container along with the Cobalt 60 source, with the whole thing surrounded by thick layers of lead to protect the spacecraft itself from the radiation.

As is usual for Mars sample return proposals, you would do this in such a way as to break the chain of contact with the Mars surface. When the orbiter picks up the capsule, it carefully positions one capsule inside the other in the vacuum conditions of space without ever letting the exterior of the capsule touch any other part of the orbiter. So then only the interior of your return capsule, the part strongly irradiated with Cobalt 60 during the return journey, has any contact with material which has touched the Mars surface.

I don't know if that is practical; it is just a suggestion. The advantage is that it would be far less damaging to the sample than heat sterilization. Also sterilizing at Mars deals with the issue that the capsule could be damaged by a micro-meteorite during the return journey. It also replaces the immense complexity of the Mars handling facility on Earth with a relatively simple addition to the sample return mission.

You still want to take every possible care when handling it, and you might as well still return it to a biohazard handling facility (after all it might have harmful bioactive chemicals in it still). But the chance of release of extraterrestrial life with the ability to reproduce on Earth seem remote even at the 1 in 1020 level when you have a sample already thoroughly sterilized with gamma rays - and you no longer need to attempt the perhaps almost impossible task of containing it at the 10 nanometer level.

If you want to do DNA sequencing of present day life, or perfectly preserved early life - or even XNA sequencing, you can do that on Mars using the ideas for a miniaturized DNA sequencer to send to Mars (SETG, already built and pretty much ready to fly). If you want to revive revivable ancient life, again do that on Mars, and other experiments that need unirradiated specimens would be done on Mars to start with.

Subsequent sample returns

After the first samples are thoroughly studied, then the situation can be reviewed. But probably we should continue to apply extreme caution until we have a very thorough understanding of the situation on Mars, because there might be a variety of forms of life on Mars.

For instance if the first sample contains DNA based life, this doesn't rule out the possibility that Mars also has XNA based life. You can easily imagine a scenario where past XNA life co-exists on Mars with more recent DNA based life introduced on meteorites, for instance, either in different habitats or in the same microbial colonies - and the XNA life might be hazardous for Earth life, and never made the transition here via meteorite.

I'd suggest that we need to continue to take these precautions even if we think the chance of contamination of Earth is extremely low. After all even if there is as little as a one in a billion chance or less of returning XNA to Earth able to out compete Earth DNA and establish a separate ecosystem here or take over from some Earth life-forms - that would still be a completely unacceptable level of risk to take according to many ways of thinking.

This is just a suggestion which I present for discussion. Would this ionizing radiation, perhaps 2 or 3 MGy or so, be sufficiently sterilizing to make a sample return from Mars completely safe for Earth life even at, say, the 1 in 1020 level that seems necessary for novel existential risks? Would it also preserve the science value of the sample? What do you think?

Another possibility though might be to return unsterilized samples to cislunar space, but not to Earth itself.

Suggestion to return samples to above GEO

I think that given that we have no experience at all in handling extraterrestrial biology, that it's better not to return them to Earth at all, but what if we return it instead to a telerobotic facility above GEO - furthest in terms of delta v from Earth or the Moon of any point in cislunar space?

We could return some samples to Earth right away so long as we sterilize them first. So that should satisfy the geologists. I suggest using ionizing radiation to sterilize them, as that happens anyway on Mars, and would still preserve some evidence such as chirality and complex chemistry to show that there was life there before it was sterilized, if that was the case. And easy to take account of for the geologists, who already disentangle the ionizing radiation effects of the journey from Mars to Earth when studying Martian meteorites.

If they are shown to be harmless quite quickly, we just return them as is, much as we did with the Moon rocks. This saves years of legislation (probably a decade or more to pass all the laws), and hundreds of millions of dollars of expense for designing, building and operating a facility that is never needed.

Returning to above GEO simplifies all that as no new legislation is needed, can be done within all the existing laws. Also, you don't have any concern about the staff not using the right protocols because it is all operated from Earth and there is nothing the staff could do by mistake or laziness that would lead to life from the facility escaping into the environment of Earth.

Yes, a plan to return to a facility above GEO would add to the expense of the mission, but nothing like as much as to a surface facility. The orbital facility could just be a single spacecraft that receives the sample, and does preliminary studies. Since some of the plans involve sending a spacecraft up to collect the samples anyway, it might not cost that more at all.

Then it's an open ended future after that. So any stages after that, to study the sample once it is in orbit, can be treated as extended missions. So this also reduces the up front cost and makes it much more likely to be accepted for funding. So I think this idea that a mission to return it to a spacecraft in a safe orbit above GEO for preliminary study would be the simplest one and lowest cost and most likely to be approved.

While if we decide that the samples are potentially hazardous for the environment of Earth, then by the time we do this, in the 2030s at the earliest, then it should be easy to send hundreds of tons of equipment to above GEO to study the samples, and this could be the basis of an international operation to study them in orbit via telerobotics if they turn out to be potentially harmful to Earth.

In that case we would design the facility on Earth around understanding of what is in the sample, or maybe just continue to study the samples above GEO. Either way we save major expense on designing a facility to handle any possible form of exobiology, and instead design our facility, on Earth or above GEO based on whatever is needed to contain an already studied sample. And if we do decide that the material can be returned to Earth in viable form for study as living organisms on Earth, then this will be for a known biology, so the legislation needed could be passed more easily.

For instance if it is viable early life, based on RNA or even just primitive autopoetic cells, it might be easy to establish at an early stage that there is no possible hazard for Earth at all, in which case perhaps it doesn't need to be studied in a biohazard containment facility at all, but just protected to keep Earth life out of the sample.

About the only thing that could damage and release the sample above GEO is an impact but there wouldn't be any risk from spacecraft debris, as any debris in GEO or the graveyard orbit a few hundred kilometers above GEO wouldn't travel far - those spacecraft are pretty much stationary relative to each other.

You'd place it far enough above GEO can put it out of way of any debris from defunct GEO satellites. So the chance would be very low of an impact leading to release of the material from the sample, only from natural debris from asteroids and comets, and being a spacecraft it could also maneuver to avoid such hazards like the ISS.

For more details see my:

Will NASA's Sample Return Answer Mars Life Questions? Need For Comparison With In Situ Search
No Simple Genetic Test To Separate Earth From Mars Life - Zubrin's Argument Examined
How To Keep Earth Safe - Samples From Mars Sterilized Or Returned To Above Geostationary Orbit - Op Ed

Need For Caution For An Early Mars Sample Return - Opinion Piece
Concerns for an Early Mars Sample Return - background material
Mars Sample Receiving Facility and sample containment
Mars Sample Return - Legal Issues and Need for International Public Debate

If there is Life in Venus Cloud Tops - Do we Need to Protect Earth - or Venus - Could Returned XNA mean Goodbye DNA for Instance?

Or return samples to the Moon

Hazardous Biology Facility on the Moon, telerobotically attended, surrounded by vacuum - Artist's impression, illustration by Madhu Thangavelu and Paul DiMare © from The Moon: Resources, Future Development and Settlement

If you return samples to a human occupied base on the Moon, then it's got the same issues as returning them to a human occupied facility anywhere.

As with anywhere else, like the above GEO idea, quarantine simply can't work unless you know what is in it and what precautions are needed. Even if they agreed, it's not at all clear you can ethically or legally commit humans to stay there for the rest of their lives, should it turn out to be potentially hazardous for humans or any other creatures or the environment of Earth (e.g. carried to Earth on the skin or inside bodies of humans). What do you do if they become ill and Earth is the only place they can be treated effectively?

But if you return it to a robotic facility on the Moon - well now, it's far better isolated than anything we could achieve on Earth, yet perhaps easier to build and work with than a large facility in orbit, especially if we develop infrastructure on the Moon. As with the facility in orbit, then it's fine to build it first, and then to send instruments to it, so long as it only goes that way, and any materials are sterilized in the reverse direction.

It could be useful for any hazardous biology generally, like an extra biohazard level above biohazard 4. So for instance if we wanted to experiment with synthetic biology using XNA in place of DNA, then we could use a facility like this on the Moon, to minimize any risk of it affecting Earth.

Even if the life did escape from the facility, e.g. after a meteorite strike, where would it go? About the only way it could be transported is via the levitating lunar dust, but that would surely be thoroughly sterilized by UV radiation before long. You could also turn the region around the facility into glass and remove any dust that strays onto that glass regularly.

You would have to think about the effects of larger meteorite strikes. And it would need to be evaluated by exobiologists, but seems very promising to me for hazardous biology!

One other suggestion, what about putting the hazardous biology facility in a lunar cave? There are many cave entrances discovered on the Moon now, and some of them might be not needed for human habitats and just lead to a small cave the right size for the facility. It might have smooth walls like a lava tube. Ideal for the facility. Protected from impacts by all except the very largest of the near Earth asteroids. And you could use a liquid airlock for the entrance, to have air inside as would be needed perhaps for some of the machines, but no risk of dust / air getting out onto the surface.

Returning samples to the Moon is a lot safer than returning them to the Earth's surface. However, the COSPAR guidelines for category 5 (sample return) missions currently say that

"(The Moon must be protected from back contamination to retain freedom from planetary protection requirements on Earth-Moon travel)".

So before samples can be returned to the Moon, that would need to be discussed and the guidelines altered. One issue I can see that would need to be looked into in detail is - what if the sample return mission crashes on the Moon somewhere different from its intended landing site?

Search for early life on Ceres, our Moon, or the moons of Mars

Though Mars is the most obvious place to look for evidence of early life, there are other places we can look too. First there's a chance that Ceres was the origin of life for both Mars and Earth. It seems to have got off lightly in the bombardment by giant meteorites in the early solar system, and likely to have had hydrothermal vents, and large amounts of water. And Hubble has recently detected water escaping from Ceres, so it has liquid water as well, in the present day solar system.

With these discoveries, Ceres seems a prime target for the search for origins of life.

Earth, Moon and Ceres to scale, for comparison. One theory suggests that Ceres could be the origin of life for Earth and for Mars. The Moon could be interesting for the search for life also, as it would preserve meteorites from impacts on early Earth, also on Mars and probably Venus too from the earliest solar system.

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, in craters that have never seen sunlight for billions of years. They are colder even than Mars, so we could find organics there as well, preserved, hardly changed, since the early solar system. There may be organics back to soon after the formation of the Moon itself. Not just a few meteorites. There may be as much as 200 kilograms of material from early Earth per square kilometer of the lunar surface. We might find fossils also, fossil diatoms are still recognizable after a simulated impact on the Moon, indeed the smallest ones are intact, complete fossils. There must be a lot of material from the Chicxulub impact on the Moon - so perhaps there are fragments of ammonites and other sea creatures from the Cretaceous period, with organics still preserved, on the Moon. 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 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. See section 3.1.1 of Back to the Moon: The Scientific Rationale for Resuming Lunar Surface Exploration. Even today, Earth gets around 500 meteorites from Mars as large as half a kilogram every year. though most fall in uninhabited areas, or the sea. So the Moon also must have meteorites from Mars right through to the present. They may be easier to find there, without the erosion processes that quickly merge them into the landscape in most places on Earth. The Moon may perhaps have meteorites from early Venus too, from before its atmosphere became as thick as it is now. Early Venus might have had oceans and might have been as habitable as early Earth and Mars.

Then, during the Late Heavy Bombardment, large meteorites impacting on Mars, Earth, Venus, must have sent rocks throughout the solar system. After the Moon formed, it was a prime target for these rocks to land on. So we might well find meteorites from any of these places on the Moon. Perhaps a particularly good place to look might be the lunar poles, where the ice deposits would help to keep the meteorites from drying out - and search for meteorites deep below the surface, protected from cosmic radiation.

The same applies to Mars' moons. Phobos particularly might well have meteorite debris from early Mars which could possibly tell us things about the early Noachian period on the planet. See Why Phobos Might be the Best Place to go for a Sample Return from Mars Right Now

Myth of automatic terraforming

This is the idea that if you add microbes to a planet, no matter what they are, that it will automatically turn into a second Earth or the closest to Earth that's possible for the planet. I call that the "myth of automatic terraforming". To see why that is not automatic, think of a future Earth too hot for life, a billion years into the future. It would just have extremophiles.

Just possibly there might be some biological way to do something about this to cool down that future Earth using microbes - but why would just adding a lot of microbes from present day Earth cool it down automatically? If it could sort itself out, it would have done it already. Mars may well have life already, and if so, it has not terraformed it, and why then would life from Earth terraform it if its own native life has not?

Adding life to a planet could push it in many different ways and there is no way of knowing if it would make it better or worse. The one thing it definitely does do though is to close off future options. After you've done that, you can never roll back, if you later find that one of the lifeforms you introduced is a major problem on the planet. Not with microbes. It is hard enough to roll back higher lifeforms like rabbits, cane toads, rats, Kudzu or Japanese knotweed. Even camels are a problem in Australia since the continent is so huge. How could you roll back a problem microbe from a planet as large as the land area of Earth?

What will you do if you have introduced some problem microbe? Maybe you want to increase oxygen levels but you introduced aerobes that eat the oxygen? Maybe you want to increase methane levels but you accidentally introduced methanotrophs that eat it? Maybe you introduced secondary consumers that eat the algae that you want to use to introduce oxygen. Many things could go wrong as a result of microbes you introduced by mistake.

    Imagined colours of future Mars. This is just to suggest the idea that there could be many possible futures and accidental or intentional attempts to transform the planet could push it in many different ways, and we might not have much control on what happens after that especially if something takes it in an unexpected direction.

    The one in the middle is the aim of terraforming. But it could as easily be any of the others or something else altogether. And once we start to introduce life to Mars, there is no way to take any of it back again if it causes problems, or evolves rapidly into something problematical. See Imagined Colours Of Future Mars - What Happens If We Treat A Planet As A Giant Petri Dish?

As one simple example of how microbes introduced by mistake could mess things up quickly, some bacteria convert water to calcite, and if you introduce them by mistake, you might find that these microbes have converted all the underwater aquifers to cement. That's an example from Cassie Conley, current planetary protection officer for the USA - she is a microbiologist / astrobiologist.

Going to Mars Could Mess Up the Hunt for Alien Life

I think this is based originally on Lovelock’s Gaia hypothesis in its strong form, the idea that life makes planets more habitable for itself. The weak Gaia hypothesis that the Earth has many systems that work together to help keep it in a habitable state, mediated by life, is widely accepted. But the idea that such a system arises automatically on all terrestrial planets with life is not at all universally accepted. That’s the “strong Gaia hypothesis”. Some things about our own planet are puzzling, for instance, why did photosynthetic life evolve at just the right time to turn a CO2 into oxygen, to cool our planet to keep it habitable, instead of arising too soon, to make it too cold, or too late, leaving it too hot? Then in science fiction the strong Gaia hypothesis has been exaggerated to mythology, the idea that introducing life to a planet not only helps keep it habitable for that life, but that it also automatically makes it habitable for humans too. Why?

If life made Mars as habitable as it possibly could - the atmosphere would be methane, not oxygen

The way to make Mars the most habitable it could be for life would be for methanogens to evolve to convert all the atmosphere to methane, which is a strong greenhouse gas. That would make Mars nearly as warm as it could be, using natural methods, though if the strong Gaia hypothesis was true, then surely also the life would evolve to generate stronger and stronger greenhouse gases on Mars to keep it warm. That would make it more habitable, but not an environment humans could live in.

    We may have spotted methane on Mars. If so this figure from NASA / JPL shows possible sources. One possibility is methane clathrate storage. It's possible that early Mars had large amounts of methane in its atmosphere which helped keep it warm. The only natural way for a Martian version of Gaia to keep it warm today is through generating greenhouse gases. 

    If so, a methane atmosphere is one way it could do it, or some other stronger naturally produced greenhouse gas. The result would be habitable possibly for ancient Mars life, but not for humans. This could be a way to "Mars form" Mars to return it to conditions that it enjoyed in the early solar system. But if so, whatever lead to the methane disappearing would probably happen again. The idea that life on a cold planet like Mars would automatically produce methane to keep it warm would be a very strong version of the Gaia hypothesis.

That would be a very strong version of the Gaia hypothesis - the idea that life on planets like Earth evolve oxygen generating photosynthesis to make it colder as it gets too warm, and life on cold planets like Mars evolves methanogens to create greenhouse gases to warm it up. Mark Waltham has argued that it is probably much more a matter of luck, at least partly, on Earth that life converted carbon dioxide in the atmosphere in to oxygen at just the right time to cool it down.

If it was true, it would not be too promising for making Mars Earth-like as it would tend to converge back to a methane rich atmosphere.

With this background, then introducing Earth life to Mars would probably do nothing to make it more habitable, not without some long term plan, megaengineering, and careful selection of which lifeforms to introduce when. You can't just leave it "up to Gaia" to do it for you, as even on the strongest possible Gaia hypothesis, then it can't create an oxygen rich Mars because it would be too cold out there. It would probably need artificial greenhouse gases or large planet scale mirrors or both to remain warm enough long term. In a thousands of years project that then goes on and on, trillions of dollars a year keeping it habitable. And what do you do if it begins to go in some unexpected direction? It is a major issue on Earth just to keep the levels of carbon dioxide at the correct values from rising at levels of only 400 parts per million.

I think it is great to think about terraforming ideas, yes. It helps us learn a lot about our planet and exoplanets and Mars itself to do those thought experiments. But as for practical experiments, let’s start a lot smaller. We haven’t yet managed a closed system ecosystem the size of Biosphere II on Earth. Once we have very small closed system ecosystems on Earth, then we can try it in space also, for instance in the possibly vast lunar caves, as vast as an O'Neil cylinder.

    Artist's impression by Don Davis of the interior of an O'Neil style cylindrical space colony - from Space Colony art of the 1970s. The caves on the Moon may be as vast inside as this, in the low gravity, several kilometers in diameter. The Grail radar data suggests the possible presence of lunar lava tube caves over 100 kilometers long.

    So, lunar caves could potentially be as vast as an O'Neil cylinder . If so, maybe some day we could have colonies like this on the Moon, easier to construct than an O'Neil cylinder - though probably multiple tiered and of course nobody living upside down on the roof. The lighting for the caves could come from solar collectors on the surface channeled through optical fiber to the caves during the lunar day - and then from efficient LED lights at night powered either from stored fuel cells or power from strips or patches of solar panels that circle the Moon round to the day side - easy to make in the hard vacuum, solar panel paving rovers, see Solar cells from lunar materials - solar panel paving robot

    Though vast, such a project is nevertheless far far smaller than the planetary scale megaengineering needed for terraforming. It is also a project that could be completed in decades. A terraformed planet would take thousands, or hundreds of thousands of years to completion. On Earth the process took millions of years.

    If we can't make O'Neil cylinder type habitats or their analogues in lunar caves, we have probably got nowhere near the capability needed to terraform a planet.

Then we can work up to larger maybe city dome or Stanford Torus type ecosystems. Eventually we can try Terraforming and paraterraforming the Moon. Let’s leave off ideas to terraform planets until we know a bit more.

Pristine Mars

And - let’s keep Mars pristine for scientific study at least until we know what is there. Otherwise we may mess it up for future transformation, if we do try to change it, and we may also spoil the opportunity to make the next big discoveries in exobiology. It may be the equivalent of an exoplanet on our own doorstep in terms of the discoveries we could make there. So let’s keep it like that, not try to make it into a pale shadow of Earth before we know what’s there.

I fully understand how those who are keen on colonization of space want to land humans on Mars as soon as possible. They’ve been looking forward to this for decades some of them. They may be so keen on this that they think that it is far more important than any discovery in biology.

But we aren’t talking about preserving some obscure microbe only of interest to microbiologists. What we discover there could lead to the biggest discoveries in biology of this century. It could be as big a discovery as the discovery of evolution or the spiral structure of DNA.

It’s only because introducing life to Mars is irreversible that we are in this situation. Their keenness to colonize Mars doesn’t give Elon Musk or Robert Zubrin or anyone else the right to make an irreversible decision about Mars for the rest of humanity. We are in it together and we all have a right to a say in this decision. The situation is particularly acute because there is a significant risk of a crash of the first human missions to Mars if we do send humans to the surface. See Why Do Spacecraft Crash On Mars So Easily? A crash of a human occupied ship would be the end of planetary protection of Mars for science.

Searching for a non confrontational way ahead

At the moment, there's a tremendous impetus amongst Mars advocates to get to Mars as soon as possible. Elon Musk even hopes to send humans to the Mars surface as soon as the 2020s, recently suggesting a first human mission in 2024, with NASA talking about the 2030s. I think it would be wrong though to suggest that Elon Musk doesn't 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. But is that right?

I did a survey of the scientific literature, to see what there is by way of proposed habitats and to investigate the range of views on the topic:

Are There Habitats For Life On Mars? - Salty Seeps, Clear Ice Greenhouses, Ice Fumaroles, Dune Bioreactors,... (long detailed survey article with many cites)

It's also available as a kindle booklet, and also online here with table of contents

As you see, there's an almost bewildering variety of suggestions for habitats on Mars for life. The main ones are (these links take you to the online booklet)

There's a wide variety of views also on the topic of whether any of these are habitable, and whether they actually have life in them, from almost impossible to very likely, see Views on the possibility of present day life on or near the surface, and for the idea that they may be inhabitable but uninhabited, see Uninhabited habitats.

So when will we resolve this? Well not for some time. Most of these potential habitats would be hidden from view, a few millimeters or centimeters below the surface. Some of the habitats might be quite productive, for instance methanogens in warm humid locations deep below the surface heated by geothermal processes. There might be enough life there to cause obvious effects on the atmosphere, such as the methane plumes. But as Mars changed from a warmish wet planet to a cold dry planet, any surface life would probably become more and more sparse, and have less and less effect on the atmosphere.

As Mars slowly changed from the warmish humid planet on the left to the dry cold planet on the right, then any surface life may have become more and more sparse, and had less and less effect. Image from NASA (Goddard space center).

So, if the life from early Mars still lingers but is sparse, it might easily have almost no effect on the atmosphere by now. The most habitable areas of Mars such as the warm seasonal flows, if we are lucky, might be about as habitable as the Antarctic dry valleys or the high Atacama dry desert. If that's the way of it, life in those few square kilometers of the Martian surface would have almost no effect on the atmosphere. Mars already has small amounts of oxygen (0.145% as measured by Curiosity). The signal of oxygen from photosynthetic life on the surface, at such low levels, would just be hidden in the noise.

Indeed, even if the entire surface of Mars is as productive of oxygen as Antarctic ice covered lakes - and even if all that oxygen ends up in the atmosphere, the signal from all of that photosynthetic life would still be lost in the noise and not noticeable in the atmosphere (I made it about 0.0002%, in a very rough calculation, by just assuming a residence time of oxygen in the Mars atmosphere of 4500 years, the same as for Earth - at any rate it would be a tiny, surely undetectable, signal).

Why Mars Surface Life May Leave No Traces In Its Atmosphere: Our Rovers May Need To Go Up Close To See It

also my Our Spacecraft Could Look Straight At an Extraterrestrial Microbe - And Not See a Thing!

Value of a non confrontational approach

You might think, why not go out and out, confront the issue head on, and whoever wins the confrontation gets the prize? Well, yes, sometimes confrontation can be good. Sometimes you have to do it. Or sometimes it's good to tackle an issue head on and you get more clarity from clearly exposing your differences from another person. If you are lucky, you may find that something new comes up that transcends either of your views or the things you knew, which you could only get to by clearly exposing the differences. Or you might be able to go different ways after the confrontation, with a bit more clarity. But sometimes you are "in it" together and can't just continue your separate ways, and sometimes after battling away at a confrontation, you find it is going nowhere.

At other times, you can compromise, find an approach that lets you accommodate both views at once to some extent. But sometimes a compromise satisfies nobody. It would satisfy nobody to pass a compromise law to let fruit importers import Hawaiian fruit into California on the first day of each month. The fruit importers would be severely restricted in what they can do, and every month there would be new opportunities for crop infestations by the oriental fruit fly, so the law wouldn't be much help to the fruit growers.

Sometimes then, confrontation isn't wise, as it only entrenches views, and it makes it harder to look at the good points of what the other person is saying. And sometimes compromise is impossible because it is a situation that just doesn't have a natural compromise that would satisfy both. When that happens, it's time to look for a non confrontational approach, a way ahead that while accepting the differences of views, leads maybe in an unexpected direction or in some way just takes a detour around the confrontation that was looming up. That can then be satisfactory to both.

That is what I'm attempting here. I think it's a situation where direct confrontation will only polarize positions and entrench ideas, I don't myself think that the compromise approach of sending humans to Mars with some extra precautions is adequate (highlighted by the problem of a human crash on Mars which would effectively end all possibility of planetary protection). But I do think it's a situation where a non confrontational approach is possible, satisfactory for both. That can give us some breathing space, which can lead to new ideas, discoveries and solutions for the way forward in the future, whatever it is.

Moving your house to avoid a pond for great crested newts

So, to try to see this in perspective, first lets try to look at something much smaller. Suppose you want to build a house and need to fill in a pond. You get an assessment done, and you are told that this pond is the breeding ground of a rare form of amphibian. In the UK it could be the great crested newt.

You might not give it a second glance, but this is a European protected species. Your would not be permitted to just build on its pond, but would have to preserve the pond and build somewhere else.

Of course some people couldn't care less about whether it goes extinct. But others do, and it's accepted, that we have to have laws like this to protect endangered species. It's not a big deal if you don't care for great crested newts. You accept that others do, so you just build somewhere else. I gave the example also of the oriental fruit fly which makes fruit unfit to eat and so you can't import some fruit and flowers into California from Hawaii. It's an annoyance I'm sure for fruit importers, but it is something they understand the need for, and so most will just keep to the regulations.

For another example, the Kakapo, a flightless parrot, is very trusting and vulnerable to cats, dogs, etc.

I think most people would understand and accept that you can't have cats and dogs on islands inhabited by the Kakapo. And similarly it's easy to understand why you wouldn't be able to get permission to melt through the ice above the Vostok lake in Antarctica to put a human occupied submersible into it and cruise around. That's perhaps the closest to Mars planetary protection, because there, the aim is to keep microbes from the surface out of the lake.

What if there seems to be no alternative? - we must have Mars!

But where it gets much harder to cope with is if there seems to be no alternative. The Mars colonization enthusiasts want to colonize Mars. If the planetary protection rules were enforced as strictly for humans, as they are for robots, it would certainly keep humans away from Mars altogether. I think everyone would agree with that much. There is just no way you can sterilize a human occupied lander to robotic standards, because of the trillions of microbes that live in and on the human body, also in our food, and in the air.

Also, if you assessed human landings on Mars in the same way you do for a robotic mission, you'd have to do planetary protection assessments of the effects of a "hard landing", i.e. a crash on Mars, as I looked at in this booklet:

Can We Risk Microbes From Human Crashes - On Mars? If Not, What Happens To Dreams To Colonize The Planet?

The only way humans could be permitted to go to Mars surface under COSPAR recommendations in the near future would be if they had a consultation that reduced the planetary protection requirements to much less than that needed for robotic spacecraft. Also they would have to just choose not to investigate the effects of a crash of a human spacecraft on Mars (because that would count as an immediate fail of planetary protection). And if a human occupied spacecraft did crash on Mars, I think that would pretty much be the end of planetary protection for Mars.

The result seems inescapable to me -, if humans go to the Mars surface, we would probably have to relax the requirement of biological reversibility. Even if the microbes did not encounter any habitats on Mars, the spores would be spread over the surface in the dust, and it would probably be impossible to "put the genie back in the bottle".

Spores last for a long time, especially if they can get into a shadow, protected from UV light, and even more so if they get into a cave. They can sometimes last for millions of years on Earth. Eventually, in the global dust storms, some of those spores would encounter habitats, if there are any at all on Mars. They'd still be there thousands of years in the future also, to potentially cause problems with plans to transform Mars. For instance, if we try to roll back to early Mars, or to do step by step terraforming, or other transformations based on introducing some species before others (ecopoesis), these pesky spores could scupper all our plans.

It would still not be a confrontation if you could land humans somewhere on Mars isolated from everywhere else. But the Martian dust storms turn the whole planet into one connected system, apart from a few places perhaps, like the crater at the summit of Olympus Mons (on short timescales of thousands of years anyway, so long as it doesn't erupt). And even if you aim for the crater at the top of Olympus Mons, there's a possibility that the spacecraft crashes somewhere else on Mars during the landing attempt. And it might not be totally isolated even at that height surrounded by the rim of the crater.

Ponds and flightless parrots again - back to the Moon

So - it's like the example of the island of the flightless parrots, the Kakapos, or the pond for the great crested newts, except that it is now a planet sized "pond", and a planet that some humans want to attempt to colonize. And there seems to be nowhere on Mars that would be truly isolated.

Anyway - this becomes a confrontation when you think there is no way ahead. If you can just move your house to avoid the pond with the great crested newts, no problem. So that's when I realized, that what is needed is an alternative vision, somewhere else in the solar system that is as good as Mars. You can use the asteroids, and Phobos and Deimos for materials to build habitats, and some space advocates are very enthusiastic about such ideas. But for others similarly minded to Elon Musk, the asteroids don't quite cut it. Mars may seem a lot easier in some ways than building habitats from asteroids. At any rate, it may seem a rather different direction of space settlement, a different kind of vision.

But what about the Moon? Could that help defuse the situation? I was already a "Moon firster" and was aware of some of the material on the subject. But until I wrote this book, as I've said, I had no idea quite how many points there were in favour of the Moon as a place for human habitats and ISRU. Depending on what we find when we explore, study and prospect further, the Moon might actually be better than Mars for this. So - like the house builder moving the position of their house to deal with the issue of the great crested newt pond - is it possible that the Mars enthusiasts can move their base to the Moon, and use much of the same ideas for ISRU there, instead? For the first few missions at least?

Meanwhile also of course, we would explore Mars, and eventually send humans there, to explore it from orbit. We can also use many of the ideas for Mars Direct, and other Mars architectures to send robots to the surface - highly capable fast moving rovers fueled by the methane fuel that are used for humans in those designs. Similarly, use the Mars stationary satellites over the base to relay signals back to Earth via broadband.

In this way we can get some breathing space, of a few decades, hopefully, to find out about Mars on a scientific level. To find out if there are habitats there for Earth life and search for exobiology. Meanwhile, you are also building up an infrastructure on Mars and in Mars orbit that would be useful if we did ever decide to send humans to Mars. Or indeed, it could be useful for other things too, anything we might do on Mars. Perhaps you decide to try ecopoesis (duplicate the biological transformations of early Earth on much faster timescales), or turn the clock back to early Mars, or transform it in some other way, or even grow plants there (plants could be grown on Mars using sterile hydroponics without impacting on any native Mars life, since seeds can be sterilized). There would be many possible futures still open to you at that point.

Also meanwhile we can work on space habitats, closed systems, eventually build city domes on the Moon and large closed systems in the lunar caves, continue to explore ideas for creating larger and larger self sustaining habitats. Whether we eventually get to the point of terraforming entire planets, I think can be left to later, until we have much more understanding than we have now, with these early experiments. So, then it becomes an open path, where instead of closing off futures, we open out to more and more possible futures, and wider vistas at every step. These vistas don't just include Mars either but many destinations for humans in the solar system.

If this approach is valid, I'm sure it will still be a slow process. People don't change their ideas overnight, especially if they have been working for decades to try to get humans to Mars. And this is just one vision, which has to be part of a debate, to explore possibilities. But I hope perhaps that with this book I'm helping provide a greater diversity of visions for the future. :).

How many years are needed to do a biological survey of Mars?

This is a bit like asking how long a bit of string is. The surface of Mars is similar in area to the land area of Earth. If you had a couple of missions each to each of the Earth's main continents, how much would you learn about the makeup of Earth at ground level?

However, to make a start on it, Carl Sagan had to come up with a number of biological exploration missions, for one of his calculations. Writing in the mid 1960s, he assumed that about 60 missions to the Mars surface would occur before a human landing, giving enough time to get a first idea of the exobiology of Mars. He assumed 54 of those successful, and 30 flybys or orbiters, in a twenty year exploration phase before a human landing. So that's averaging nine missions for every opportunity to send spacecraft to Mars (See his "Decontamination Standards for Martian exploration programs").

So far we've had seven successful landers in the six decades since he wrote that: the two Viking spacecraft, the Mars Pathfinder lander (with its tiny Sojourner rover), Spirit, Opportunity, Phoenix and Curiosity. Of those seven, we have three landers able to travel kilometers, the Spirit, Opportunity and Curiosity rovers, and one tiny rover able to travel of the order of meters, the Sojourner rover. In addition, we've had 13 successful orbiters if I counted it right. And we have one extra lander and one orbiter on the way there right now (the ExoMars Trace Gas Orbiter and the Schiaperelli lander which is mainly just a technology demo and test for ExoMars, only able to survive a few days on the surface).

However, only two of those missions could really count as biological exploration - the two Viking missions. I think you could call the Trace Gas Orbiter, and Curiosity early stage pre-biological exploration missions. They could in principle discover life, but both would need a strong signal to have a chance to distinguish it from non life easily.

So that's only 4 missions so far with a strong biological focus: two stationary landers, one rover, and one orbiter. And only the Viking landers are really strongly focused on in situ life detection. You could say that Carl Sagan's planned "biological exploration phase " really started and then stopped immediately with the two Viking spacecraft. We have done no direct life detection tests on Mars since then, so I'm not sure if you can really include any of the missions since then as part of his biological exploration phase.

ExoMars in 2020 will probably be the first mission to be able to search for present day life in situ on Mars to some extent, and still is not as sensitive for this task as Viking was designed to be (with controversy of course about whether it discovered anything because of the unusual Mars chemistry). And even with ExoMars, its main focus is past life, with its search for present day life as a bonus. The area it's going to is not high on the list of candidates for present day life.

That's mainly because for about thirty years, most scientists thought that present day surface life on Mars was impossible. This is now changing, and on the plus side, our tools for investigating it have moved forwards by leaps and bounds since Carl Sagan's time, but the habitats on Mars have also turned out to be much more elusive than Carl Sagan could have imagined.

So I think we have to say that the biological search on Mars is just restarting, after a long pause after Viking in the 1970s. So, to venture a very rough guess, just based on Carl Sagan's numbers, it's at least a few more decades required. If we send, say, a couple of new landers for each launch opportunity, it would be 54 years to complete his preliminary survey, unless Mars exploration is stepped up hugely. If we can get it up to the levels envisioned by Carl Sagan or further, perhaps ten or more missions every two years, maybe we can do it in twenty years or less. Though, of course, in the other direction, if there is present day life on Mars, we might find it with the first mission to go to a promising habitat there. You never know.

Speeding the search up with miniature robots - what could we do if we had funding for a preliminary exobiological survey of the whole of Mars from earth?

Miniaturization may speed it up also, if we can scatter lots of smaller robots over the surface of Mars. See my Soaring, Buzzing, Floating, Hopping, Crawling And Inflatable Mars Rovers - Suggestions For UAE Mars Lander.

However, a swarm of 54 identical probes all sent to explore a single cave or a single region on Mars, and 30 identical orbiters similarly wouldn't hack it. Also, Sagan was thinking surely of missions with capability similar to Viking, so several instruments and very sensitive. We can certainly do with a lot less mass than Viking, today, and maybe we could get dozens of landers into a single launch of something like the Falcon Heavy, but they need to be carefully planned missions.

Imagine trying to get a clear view of the biological diversity of Earth with 54 landers, so about eight per continent, or in terms of countries, one lander for every three countries. When you think about it that way, it's not a lot to try to find out about a planet with a total land area similar to Earth, and with a complex and diverse geology.

There are many places we need to explore and dedicated missions for each. But if small, many of them could go on the same launch perhaps, then sent to many different locations on Mars from a mother spacecraft in orbit around the planet. Perhaps it could even send later missions in response to results of earlier ones.

This is a survey I did of some of the proposed surface habitats on Mars as well as some of the near surface ones. Are There Habitats For Life On Mars? - Salty Seeps, Clear Ice Greenhouses, Ice Fumaroles, Dune Bioreactors,... which may give an idea of the variety of possible surface habitats that have been suggested, most of them new suggestions in the last decade or so, and of course, the RSL's now confirmed to have liquid water (almost certainly) though not yet settled whether they are habitable or not.

So, what could we do if the funding was available to do an exobiological survey of Mars from Earth? I'd think you need a few missions to each of these targets myself:

  • The Recursive Slope Lineae (RSL's). You would need to send missions to more than one of those as they are especially interesting, geographically isolated and it's possible some have life and some don't.
  • The flow like features in Richardson's crater near the south pole. This may consist of fresh water trapped under ice, so are especially interesting for viability
  • Equatorial sand dunes. Levin thinks Viking discovered life already, and recently with discovery of circadian rhythms in the re analysed experimental data, others think there is a possibility of that also. Then, whether Viking found life or not, there are ways they could be habitable. So we need to check up on that to be sure. Also Curiosity found a liquid water layer a few cm below the surface of the sand dunes indirectly. Nilton Renno has suggested that microbes could find a way to create a niche in it, by transforming the environment as it can do on Earth even though the data suggests it is always either too salty or too cold for life. [ExoMars may give the first ideas about this - though it's not quite as sensitive as Viking's labeled release, it could find life in the Atacama desert core which Curiosity couldn't]
  • Salt / ice interfaces. Nilton Renno's "swimming pools for microbes" in droplets of water that form where salt touches ice.
  • Salt pillars and salt deposits, for deliquescing salts, and for water that can form in fine pores in salt pillars.
  • Both of those could be combined with a visit back to Phoenix's landing site - a study from ground level there able to detect life could also detect whether any of the Earth microbes from Phoenix have been able to replicate as Phoenix was crushed. Hopefully not, but if they have best to know at an early stage. And gives us some ground truth for robotic exploration sterilization to show our measures are adequate.
  • The Hellas basin because of the icy mists that form there and because it is the densest atmosphere on Mars which could make a difference to habitability.
  • Caves need a visit. Not so much lava tube caves as other types of caves, for instance ones formed by slippage, or ones that formed through erosion by water or dry ice. The difficulty is, they are hard to spot form orbit. We do need survey not just orbital images, which are also limited to particular times of day and such like. E.g. miniature planes to fly along the Valles Marineres to photograph it up close.
  • We should explore the surface itself for life, for lichens and cyanobacteria that might be able to grow in partial shade, using just the night time humidity, according to the DLR experiments.
  • Study the Martian dust as it might have spores in it from anywhere on Mars.
  • Check the polar ice caps for deep subsurface water, using radar, and if any is found, to drill down to search for life (if liquid water forms at depths of over 900 meters after a meteorite impact or through geothermal heating, it will remain liquid indefinitely through the flow of heat from Mars itself insulated by the ice above).

There may be other places to target, but those are the main ones I can think of right away.

The orbiters would be like the Trace Gas Orbiter, searching for traces of gases produced by life on Mars as well as photographing the surface. For instance our photographs of the RSL's from orbit are all taken in early afternoon, the very worst time to spot effects of liquid water on Mars. That's because the spacecraft that takes those photographs is in an orbit that takes it closest to Mars when locally it's early afternoon there. We need orbiters to photograph Mars close up at other times of day such as early morning. We also need orbiters dedicated to broadband communications with Earth (probably doing their own observations of Mars as well). These would certainly be in place for human missions for Mars, best done right away at the biological survey stage.

With broadband communications, then instead of communicating with Mars once a day as is the current situation you have delays of between 8 minutes and 48 minutes there and back. So, between 15 and 90 times a day, or if you have powerful lights and are fully powered at night, between 30 and 180 times a day. When Mars is closest to Earth then with broadband you could do as much communication and control in one day as we currently do in a year. Or even more if you use artificial real time.

There would be some building on previous expeditions so I think you definitely can't do it all in one mission. You'd have survey missions and preliminary missions first, but if we had the funding, say a dozen missions every two years for twelve years :). Each wave of missions building on the previous ones, refining the search. Or some other combination including maybe building up to more and more missions as we get an idea of which places to target.

You'd also be looking for habitats with no life. If there are surface habitats, whether there is life in them or not, those are vulnerable to Earth microbes meaning that sending microbes there is irreversible so you want to know that too even if they are uninhabited. They could be of great interest for exobiology indeed, especially if they have complex organics, but no life, or "almost life". So it is specifically a search for present day surface habitats and life - or ones that are reasonably easily accessible from the surface.

If it's possible to get humans to Mars orbit, then they could oversee all these rovers on the surface, rather similarly to the game of Civilization. Many of those 54 landers would be doing routine tasks at any time, things they can do autonomously. They could be controlled remotely from Earth using broadband. But from time to time they'd be doing something more challenging and interesting and that's where astronauts in orbit would step in. So in that way, a half dozen astronauts in orbit could work with all of those 54 landers and more using telepresence.

For more about the potential habitats, see my Are There Habitats For Life On Mars? - Salty Seeps, Clear Ice Greenhouses, Ice Fumaroles, Dune Bioreactors,...

A rapid survey of Mars does seem possible - however none yet planned to guide our decisions by the 2020s

So, it's possible we could survey Mars more rapidly, but there aren't any planned missions yet to suggest we are going to do this in the very near future, as in, the next decade (say).

In the circumstances, if we send humans to Mars in the 2020s or in the 2030s, unless there is some big change, and someone does a large number of robotic missions first, we can't have anything like enough information by then to know what effect their microbes would have on Mars.

If you share Elon Musk's certainty that there is no life on the surface - which to be fair was the scientific consensus right up to 2008, then you may agree with his conclusions there. But ideas about Mars are changing fast, and we can't be so sure now about the apparent certainties of the early 2000s.

Clash and confrontation?

For this reason, I foresee a possibility of some kind of a confrontation, where experts who meet to make planetary protection guidelines for COSPAR just don't have enough information to say for sure if there is present day life or habitats for Earth life on Mars or not. So then it would come down to personal judgement. Experts who are skeptical about life on Mars might say to the Mars colonist advocates like Elon Musk, "Sure, go ahead, you probably won't do any harm". While those who are optimistic about the proposed habitats are quite likely to say "Slow down, we need more data". I wouldn't be surprised if the workshop was inconclusive with some saying it is okay and others saying it is not.

This potential confrontation was highlighted recently in my guest appearance on David Livingston's "The Spaceshow" on 3rd May 2016. I said, during the show, that it's possible that we might not have enough information for COSPAR to approve humans to Mars soon enough for Musk's plans, and also said that it is still a possibility that we could find out that there is vulnerable life on Mars. I was saying much the same things I've been saying in this book.

Anyway I expected this to be a controversial thing to say, knowing that many keen Mars advocates would be listening to the program - but I was surprised at quite how controversial it seemed. From the questions we got by email, it seems that many people would be very upset if their plans to attempt to colonize Mars were even delayed a few years because of issues such as this. David said that the possibility that planetary protection issues could delay their plans is never raised in the Mars colonization conferences at all, which are held every year in the States. So, if I'm right about this projection, it would be a great surprise and shock for them. What can we do to help defuse and resolve this possible future confrontation? My Case for Moon First book actually came out of my deliberations after thinking over that show.

This approach doesn't mean that humans can never land on Mars ever

The idea isn't at all to prohibit humans on Mars. The humans are not the problem; it's only their microbes that are. And the idea is to do it step by step and to make sure we understand Mars and understand the implications of our actions before making a decision about whether it is okay to have human boots on Mars.

I'm a spaceflight and science fiction enthusiast myself and I'd love to be able to cheer on humans on an expedition to Mars. Just for the childlike wonder of seeing humans doing things like that in space. So it would be fun to see humans go to Mars. And at least we can send them to Mars orbit whatever we might discover about the surface - so long as it is done with care to make sure that they can't crash on Mars.

What about humans on Mars later on?

We could decide what to do later on, based on what we find out. If we find that there is some vulnerable early RNA based life on Mars for instance, I think that public opinion might well swing in the direction of saying we need to go slow here, and study it first before doing anything that could make it extinct on Mars. The scientists would be on the TV talking about how exciting it is, and I think nearly everyone would soon understand the importance of what we had found.

In the other direction, there might be other findings that show that microbes would have minimal impact on Mars. For one example, suppose that none of the proposed habitats turn out to be habitable for Earth life? I think that's an unlikely scenario myself, and it would be a disappointment for exobiologists, but it's a possible future as of writing this.

Or maybe new technology gives us the capability to send humans to Mars in a biologically reversible way. Again, it's hard to see that with present day technology, at least not for an interesting mission for the humans involved. But the human in a metal sphere idea shows that it is at least possible in a minimal rather uninteresting way.

Could there be some other more flexible and more interesting ways to achieve the same thing? I can't imagine how that would happen but there are many technologies today that I couldn't have imagined in the 1960s when I watched the Apollo landing on the moon on TV as a child of 14. Indeed right up to not long before the landing, the science fiction writers never imagined that it would be watched on global TV as it happened. So sometimes your ideas about the future can be upturned like that, suddenly, in just a couple of years.

If you have any other ideas for biologically reversible human exploration of Mars, do share in the comments!

When will we know enough about Mars?

I don't think we can answer this at this stage in any definitive way. It's asking us to predict future science. You can't know what direction it will go and what we will learn about Mars. So it can't be timetabled. Carl Sagan's 60 rover missions and 30 orbiters searching for life, mentioned above, was just for the sake of having a number to work with for his probability estimates.

Let's take it step by step, and send humans to the Moon and asteroids and Mars orbit first, and make the next decision based on what we find out from that first phase. The main thing right now should be not to close off future possibilities. If we make that decision in the future, it will be an informed decision.

We wouldn't know Mars completely, as that would take for ever. We don't understand Earth completely yet. But we'd know a lot more about Mars than we do today. We need to leave it to our future selves or descendants to evaluate what they know then, and to decide whether they know enough yet to make this decision. I think that with the rapid pace of science, you can only foresee the future perhaps ten or twenty years ahead in detail, and even on that timescale, surprises are likely.

Two futures - humans landing on Mars or a habitable Mars surface with humans in orbit only

I see two possible futures here. One, where we find that the Mars surface is uninhabitable and humans land there just as they did on the Moon. That would be fun and exciting just as it was for the Moon. The young space geek, science fiction loving kid in me would just love that!

In this future, humans could perhaps land early on - though there are still issues to think over about introducing earth microbial spores that could interfere with future plans to transform Mars. It is after all, an entire planet, connected through the dust storms, and the dust able to protect spores from UV light. Also, after the initial excitement, it might well turn into a place like any other with humans cramped in habitats, complaining about food and the difficulties of being cut off from Earth, sending video clips talking about how they never get to see a blue sky or sunlight or trees or grass any more, saying how homesick they are, and how they can't get out of their habitat - I'm sure there'd be some grumbling from people who didn't realize quite what they are getting into. It might not be as glamorous as it seems in advance.

The other future is one where the Mars surface turns out to be habitable. In the most exciting future here, perhaps nearly all those suggested habitats turn out to be habitable. Nilton Renno's salt / ice interfaces, the seeps of briny water, the pure water at 0 C in Richardson crater due to solid state greenhouse effect through pure ice, and on the surface using the 100% night time humidity, all of those inhabitable.

Almost anything that could happen in this future is exciting. Even Zubrin's picture of a Mars with the same lifeforms as Earth would be exciting, how could that possibly happen? How did they all get there, and when, and why didn't they evolve in some different direction from their cousins on Earth? The future of uninhabited habitats would be interesting too. We'd learn a lot from both these futures.

The most thrilling future of all here, though, would be the discovery of indigenous life or early life precursors. Like the possible life forms in ALH84001, with the debate going both ways about whether it is life or not. Mars has such a different past from Earth, at its most habitable for just a few hundred million years, with oceans, early life could have evolved there and still be there. Those early life forms might never have gone extinct on Mars. That would be the most exciting of all, life that was made extinct on Earth, and they could be extremely vulnerable to Earth life. That would fill in a huge gap in our understanding of life.

Either an early form of life on Mars, or some form of life that's followed a totally different direction of evolution from Earth life. Even perhaps more than one form of biology, different directions explored and perhaps none of them have yet taken over as the only form of biology on Mars.

That would be like discovering an exoplanet, complete with its own extraterrestrial life, in our own solar system. Ok, humans can't land on the planet, not early on anyway. But to compensate, they can explore it by telepresence, and we can all participate, looking at the streaming feeds from Mars and walking via virtual reality through the landscapes they uncover in their explorations from orbit. It would be exciting to follow the expeditions of the telenauts exploring Mars from orbit. And there's something also fascinating about a place you can't go to in person, for whatever reason. It would add to the interest and mystique of Mars.

For me, that's by far the most exciting future here. So I'm rooting for that future where the habitats turn out to be inhabited - and most exciting of all, a future with some form of indigenous life, early or life precursors or alternative biology. This is a scientific possibility at present, something that could turn out to be true. I hope this is what we'll discover on Mars in our near future.

There might be other possibilities that we can't see yet. I hope we don't end up in a future where we accidentally introduce Earth life to Mars however. That would be so sad, to do that by mistake, if that's not what we want to do.

Only one Mars - no warp drive yet

These planetary protection issues arise so acutely for us because there is only one Mars within reach.

  • If Mars has early pre-DNA life on it, then for all we know, it may be the only place in our entire solar system with early life on it.
  • Or if for instance Europa or Enceladus also have early pre-DNA life, Mars could be the only terrestrial planet with this form of life in our solar system.
  • If there are habitats there, but no life, again it is the only terrestrial planet in our solar system which is habitable at all on its surface and has no life on it, a situation that could teach us a lot about the role of life in planetary processes as well as help us distinguish exoplanets with and without life.
  • If it has complex life precursors but no life, again it is the only terrestrial planet in our solar system like that.

And so on for all the other various possibilities for what we may find there. Mars is our only opportunity to study a terrestrial planet of that type. And because we can't travel outside of our solar system easily then this means it is the only terrestrial planet of its type in the habitable zone of its star in the entire universe that is accessible to us to study close up.

If we could travel at warp speed to distant star systems within days or week, maybe we might know of thousands of Mars like planets with pre-DNA life, or whatever it is that makes Mars unique in our solar system. Then, it might perhaps be a matter of less consequence what we do to Mars. We could try experiments then with some of those many planets.

Even then I think we have a fair bit of responsibility for those planets. Even though we would have many planets at our disposal, it would be an irreversible change, still for any of them. We'd have responsibility for the future beings that might evolve on those planets which we transform, maybe millions of years into the future. We'd have to think through whether they would be able to keep their planet habitable in the long term future, especially future intelligent lifeforms that may arise there which we may not even be able to predict at present. For instance, to start a new habitable planet that would unterraform a few thousand or a few million years into the future might well be seen as irresponsible even if we have thousands of planets at our disposal to experiment with. We might still have some kind of a "prime directive" that applies even to planets with only primitive life.

But - it would be a different situation. We would still have ethical dilemmas and responsibilities, but we wouldn't have so much to lose by transforming the planet. As it is though, we have only the one Mars in our solar system, just as we have only the one Earth.

There is no immediate prospect of developing a realistic warp drive as far as we know, ideas yes but nothing concrete. There are issues with warp drives also as if it is possible, it may permit travel backwards in time which is an idea that challenges causality. Faster than light travel is common in science fiction, but though we do have theoretical possibilities for it in our own universe already, such as the Alcubierre drive, this requires exotic matter with negative energy which we don't know how to make. It's not yet at all clear that we will ever be able to do it in practice. Certainly we have not got any spacecraft able to do this yet, and I don't think many would say we should plan on the assumption that we will be able to do this in the near future.

So we need to proceed carefully with actions that may change Mars irreversibly. We may never be able to access another planet like this, within a travel distance less than decades even traveling at a tenth of the speed of light.

Even in future if we ever do develop a warp drive, it may still be unique. It might be that only Mars has early life that's related in some way to Earth life, to give some example (the hypothesis that Earth life originated on Mars first). There may be many other such connections, including just that it is a planet of exactly the same age (to within a few million years) around the same sun and with a shared history.

Other planets may have other forms of early life in other solar systems but that close connection to Earth could make Mars of especial interest to us. That's just one example. It's not impossible that Mars, and Earth are unique in various ways in the entire galaxy. Also, on the knowledge we have so far, it's not impossible that our solar system is the only solar system that has life in it in the entire galaxy.

So one way or another, Mars could be unique and very precious to us, as the only planet of its type accessible to us, right now, and quite possibly also for many or all future civilizations that arise on Earth in the future.

Precautionary principle and super positive outcomes

The main principle here is that we should advance by increases of knowledge until we have know enough to make informed decisions. And for as long as we have fundamental gaps in our understanding,with significant unanswered questions about whether this course of action could cause problems for us and our children and future civilizations on Earth, we just shouldn't introduce Earth life to Mars irreversibly, or indeed, to anywhere else in our solar system.

Colonization advocates will often argue that since we don't know that we will cause problems on Mars, we might as well just go ahead and see what happens, and "learn on the job" what impact we will have on Mars. But for me that's not nearly good enough reasoning to back up an action that risks potentially irreversibly introducing Earth life to Mars. This section comes out of attempts to make this clearer, and to explain why I think it is so important that we don't proceed here out of ignorance, but make sure we have a reasonably clear understanding of our situation first.

It's similar to the Precautionary principle guideline in International Law

"When an activity raises threats of harm to human health or the environment, precautionary measures should be taken even if some cause and effect relationships are not fully established scientifically.

"In this context the proponent of an activity, rather than the public, should bear the burden of proof.

"The process of applying the Precautionary Principle must be open, informed and democratic and must include potentially affected parties. It must also involve an examination of the full range of alternatives, including no action."

My suggestion for a new principle here is based on the idea of a "super positive outcome" - a potential but not certain outcome which could have transformative effects on us, our children and all future generations and civilizations. In this case discovering some alternative form of life or early life on Mars could revolutionize biology, could potentially benefit medicine, agriculture, and indeed anything that we do that uses products of life, also nanotechnology. It could potentially, in the best case scenario be a hugely positive transformative discovery.

The precautionary principle can't be applied here, if we start an irreversible process that makes some unique or early form of life extinct on Mars .There is no risk of harm to human health and the environment, at least not on Earth. But there's a risk of destroying a potential future benefit of immense value. The consequences would be so positive if it exists, and it would be so tragic if we found that there was something unique like that on Mars right up to the first human landing or crash on Mars, and that we made it extinct. And as with the precautionary principle, there may be no way we can establish the cause and effect relationships thoroughly before it happens. We don't even know what the possible effects are until we find out whether Mars has habitats and whether those habitats contain life.

I suggest we should have similar guidelines for super positive outcomes like this - things of potentially overwhelming positive value that we could discover:

"When an activity impacts on a potential super positive outcome, precautionary measures should be taken even if some cause and effect relationships are not fully established scientifically.

"In this context the proponent of an activity, rather than the public, should bear the burden of proof.

"The process of applying the Precautionary Principle must be open, informed and democratic and must include potentially affected parties. It must also involve an examination of the full range of alternatives, including no action."

So I'm not saying we should never land there. But that if we do, then as with the precautionary principle, it has to be on the basis of knowing clearly the consequences of our actions, with public debate and open and informed democratic discussion of whether we should do it.

But to have informed public discussion, first we have to know what is there. For that we need to explore and discover first, before making any irreversible decisions such as whether to introduce Earth life to a planet.

For more about this see my "Super Positive" Outcomes For Search For Life In Hidden Extra Terrestrial Oceans Of Europa And Enceladus

For discussion of how we could explore Mars first, in detail, see my section How many years are needed to do a biological survey of Mars? (above)

Searching for life on Europa and Enceladus

The main focus of this book is on the search for life on Mars. However, many of the same ideas apply to searches for life on Europa and Enceladus, and in the other direction too, with Europa and Enceladus, we have a chance to start from scratch and design new ways to search for life, learning from our experiences on Mars. Perhaps we can find a way to do it which doesn't need us to do these probability calculations such as Sagan did for Mars. Can it be possible to search for life in a 100% safe way with no risk of contaminating the places we study at all? And if so, can this perhaps be applied to Mars too, for future searches of the present day habitats on Mars?

The life searches to Europa and Enceladus will be especially challenging, because the target is ice. When ice is melted, life can survive there readily. But some of the targets on Mars, such as the flow like features in Richardson crater, may be similar. They are like tiny cm thick subsurface oceans. Maybe we need to use similar care in our search for life there as we need for Europa and Enceladus.

Hubble's confirmation of probable geyser activity on Europa

Hubble has found new evidence of possible plume activity on Europa. In a series of ten observations, they saw them on three occasions. Here are the images they created.

The possible plumes are in the seven o'clock position, not far from the South pole - though the central image here has another possible plume that's close to the equator.

Interestingly, they spotted them in the same position as the previous plume detection in 2012:

Hubble Sees Evidence of Water Vapor at Jupiter Moon

Note that in all these images, the photograph of Europa was not taken by Hubble. It just made the observation of the water plume - those large blue pixels in this last image. 

Europa is tidally locked to Jupiter, so every time Europa does a transit of Jupiter, we see the same face of it in the same orientation. This shows how it works:

So this could well be a recurring plume in the same place as for the 2012 observations.

This is potentially exciting news because the last time it spotted a plume of water, it seemed to be a once off observation and didn’t get repeated. After the initial excitement many astronomers concluded it was probably just a rare asteroid impact on Europa sending water up into space. Europa's Elusive Water Plume Paints Grim Picture For Life - Astrobiology Magazine

Now that Hubble has spotted it again, this makes asteroid impact a very unlikely explanation, especially as it's been observed in the same position approximately, multiple times. This suggests that it is probably a huge geyser. This is  very exciting news for astrobiologists, because such a huge geyser would suggest the water is coming from deep below the surface, maybe even indirectly or directly from the subsurface ocean 100 km below the surface. This image shows some of the possibilities.

In the case of Enceladus we have evidence that suggests that the material that Cassini observed was perhaps even in contact with hot rocks only a few months before the observations. However the ice cover for Europa is 100 kilometers which makes it rather difficult for tidal effects to keep a channel open all the way down to the subsurface ocean against the immense pressures of the ice closing the channel.

This seems a more likely scenario

There a hot plume of water from the ocean rises very slowly through the ice. We have evidence on the surface of chaotic terrain, which may be the result of one of these plumes reaching the surface. The water, denser than ice, would cause the surface of Europa to dip as it approached it, and then once it reached the surface, the water would freeze and break up into giant icebergs that would turn over and form the chaotic terrain. At least that's one explanation of how it might work. If so, these geysers could come from one of these giant rising hot plumes of liquid water.

This in turn is very interesting because this could make it one of our best chances in the solar system for finding extraterrestrial life and even possibly, complex multicellular life such as we have on Earth. There is almost no chance of life getting transferred from Earth to Europa or vice versa over the entire history of the solar system as models suggest that only a few meteorites have ever made that transition in the entire history of the solar system. And if the ocean material is indeed sent into space in a geyser, one of our spacecraft could sample it just by flying through it with no need for a lander.

How can we find out more?

NASA has plans for a Europa multiple flyby mission to be launched in 2022 on the SLS to get there in just 3 years. If you do a type II Hohmann transfer, spanning less than 180 degrees around the sun, then you can get from Earth to Jupiter in under two years as Voyager 2 did, taking one and a half years to reach Jupiter from Earth. So it is not a particularly long journey time from Earth.

Because of the strong radiation around Jupiter close to Europa, it actually makes most sense to do flybys of Europa rather than to orbit it. After each flyby, they will have plenty of time to send back the vast amounts of data they can collect with each close approach. The end result is much more data sent back and a mission that can last for years instead of just a month or two.

Keeping safe - sample geysers

I think the best solution here is to focus on sampling any geysers as our main priority. We can definitely do that with a mission to Enceladus, and now it seems we may be able to do it for Europa as well. Enceladus is less known amongst the general public, but it also may have life.

Geysers on Enceladus (moon of Saturn). A spacecraft could fly through these geysers (Cassini has done so many times now). It could do a detailed analysis and even a life search as according to some theories, the water in these geysers was in Enceladus’ ocean as recently as a few months before they are ejected into space. Europa may have geysers also but with its larger gravity they may not go so high into space, so may be harder to spot.

With these new observations this now becomes a top priority. As with Cassini for Enceladus, a Europa flyby mission should be able to do multiple flybys of Europa. Cassini found out a lot about Enceladus's subsurface ocean from analysing the plumes, and that is with scientific instruments built 20 years ago (it was launched in 1997) and a mission that was planned at a time that we didn't know that geysers were even a possibility. 

A flyby mission can go through the plumes at different heights, and at different times in the orbit and gradually build up a picture of what is in the material. It can actually sample the material directly and analyse it on board the spacecraft. The Europa flyby will do around 45 flybys of Europa so will have plenty of opportunities to fly through the plumes.

So then what about landing on Europa?

It’s actually quite a challenge to land on Europa. I’m not at all sure we should be doing it now even. Rather controversially, NASA have a mandate to include a lander on this mission. This is a possible version of it, see A Lander for NASA’s Europa Mission

It's controversial because this is an idea put forward by Congress. This is not usually how you plan science missions, that a politician tells you that you have to do it in a particular way. That's more like the way that human missions are done, where the objectives are often to a large part political. Normally it's a case of asking the scientific community for suggestions, then detailed proposals, fully worked out with costings, and then they are compared with each other based on their scientific merits rather than their political merits. It seems odd to do it the other way around, for a pure science mission. 

The original plan was to have both lander and orbiter on the same mission. Now Congress have mandated NASA to do them as separate missions, and have also said that both missions have to use the Space Launch System (SLS) - a heavy booster being developed in the States, which itself is rather controversial, since it is going to be high cost, it's going to fly infrequently, and each launch is going to be extremely expensive. It will be a remarkable vehicle able to send large masses into space and send human crew on deep space missions. However some think it will be overtaken by the private sector who have their own independent ideas for ways to achieve heavy lift such as the Falcon Heavy. The Europa mission could cost upwards of 2-3 billion dollars not including the launch, possibly as much as 3-4 billion dollars. The SLS launches themselves will add $500 million to $1 billion apiece just for the launch. The whole thing is quite controversial. See Two SLS to Jupiter in the Space Review.

Anyway this is now a very unique mission, one of the few to have a launch vehicle selected by congressional mandate. All this does have its advantages though for a Europa mission.

First, the spacecraft can be far more massive. It can have more instruments, a much shorter transit time to Jupiter of only three years, and it can have more radiation shielding to protect it from Jupiter's ionizing radiation. It can get there so quickly because with such a capable launcher, there is no need for a gravity assist.

For the orbiter, the main controversy would be about the cost. The mission itself will be much more capable than it could be without the SLS. 

However for the lander, then in the case of Europa, there are additional reasons why a lander makes it more tricky. The first issue is that the surface of Europa has not yet been imaged in the detail needed to choose a landing site, and is thought to be very rugged in detail. Landing on Europa might be as risky and fraught with unknown quantities as landing Philae on comet 67p, though for different reasons. There is no risk of it bouncing off the surface, but it could crash on rugged terrain.

Another issue is that you have to sterilize the lander sufficiently for planetary protection, because the very last thing we want for Europa is to go there just to discover life that we brought ourselves. Is that something we can actually do at this stage? It's a huge challenge when the target is icy. A hard landing (crash) on the Europan surface could heat the ice to melting point, or even potentially contact liquid water beneath thin ice and the ice could then shield microbes from the ionizing radiation of Jupiter.

The strange thing here is that congress actually have mandated a launch date for the lander too. NASA has to launch it in 2024. That means they have to send it to Europa a year before the orbiter gets there, and so before they have any new data on the Europan surface conditions.

So let's look in detail at some of the issues with sending a lander to Europa based only on the knowledge we have about it so far. Is it something that we can actually do, realistically, in this time frame - with a launch in 2024, and do we know enough to design it before detailed observations of Europa from the orbiter?

First, the surface is unknown at the scale of meters, most of it. As an example of how little we know, one theory that has not yet been disproved is that parts of the surface might be covered in closely spaced vertical “ice blades” or “ice knives” which would make a landing there hard to achieve. On Earth these blades form quickly, in special conditions On Europa they would take millions of years to form, but it’s the same basic process. As Daniel Hobley said: "Light coming in at a high angle will illuminate the sides of the blades, causing them to retreat away,"

These are called Penitentes. See Penitentes: Peculiar Spikey Snow Formation in the Andes

This video shows how they form on Earth and decline, time lapse:

Here is a photo from the European Southern Observatory site high in the Atacama desert:

Planetary Analogue, see also their Icy Penitents by Moonlight on Chajnantor, and Iconic, Conical Licancabur Watches Over Chajnantor

Ice knives on Europa

On Europa, if they exist, these structures can potentially be meter scale or higher. With no atmosphere, the conditions on Europa might well be ideal for their formation. Our missions to Europa so far haven’t taken high enough resolution photos to see them. Ice blades threaten Europa landing - BBC News

They wouldn’t be the result of ice or snow subliming into an atmosphere, obviously. It’s a slightly different process. Instead they’d be the result of the sunlight causing the ice to sublime to water vapour in a vacuum at very low temperatures well below 0 °C. Also they would form slowly over much longer timescales, of millions of years.

The surface of Europa is about 50 million years old, so when we ask if penitentes can form on Europa, one of the main questions is, how much can the ice there erode under the influence of sunlight in 50 million years? The answer to this question is extremely sensitive to the peak temperatures on Europa, to the extent that twenty degrees can make a difference between formations that are meter scale and ones that are on the scale of millimeters.

In the paper: HOW ROUGH IS THE SURFACE OF EUROPA AT LANDER SCALE? Hobley et al produce this table

So, for a surface temperature of 132 °K (about -150 °C) it loses about 5.66 meters over the average age of the surface of 50 million years. For a temperature of 128 °K (-154 °C) it loses 1.28 meters in 50 million years, tailing off to 1 cm at 116 °K (-166 °C), and only millimeters at 114 °K

So this is very sensitive to the peak surface temperatures of Europa. Also, the surface is eroded by sputtering from the Jupiter radiation and from bolide (meteorite) impacts. That would counteract the effects of the ice blade formation at temperatures of 126 °C downwards. They conclude in the paper that the knives could be from one meter to 10 centimeters in height, probably restricted to within 15 or 20 degrees of the equator.

However Europa also has “true polar wander” by which the entire crust moves over the subsurface ocean. This could reduce the size of the blades but also move the ice blades away from the equatorial regions.

Upturned icebergs - for regions like Thera Macula - amongst the most interesting regions on Europa

Other issues could include a frozen landscape consisting mainly of upturned icebergs. According to some ideas, then hot plumes of melted water rise from the deep subsurface sea and eventually reach the surface and produce these irregular landscapes, as icebergs form on the freezing surface, and then turn over.

One of the most interesting regions, thought to be most likely to have thin ice over liquid water by the “thin icers” is the Thera Macula

This might be a region of overturned icebergs with, perhaps, liquid water still present only a short distance below the surface. Most of these chaos regions are raised, which suggests the ice below them that lead to their formation has frozen. But Thera Macula is actually a dip in the surface of Europa which may be a sign that it has the denser melted water still beneath it. See Is Europa's ice thin or thick? At chaos terrain, it's both!

Possibility of liquid water close to the surface or breaking through

So there could also be liquid water close to the surface. Geysers are another possibility. So again there may be a small chance of our lander crashing through thin ice or a soft surface, especially if we land it on the most interesting regions such as Thera Macula. Or it could fall into a crevasse and be unable to communicate.

I know the plan is to orbit Europa for a while before the lander gets there, but what if the orbiter doesn’t find any suitable spot for the design of lander, and decides a different design of lander is needed, or no lander at all? Maybe the lander has to land somewhere uninteresting, or they have to hold back from landing at all for planetary protection reasons?

Can we sterilize a spacecraft 100%

Then the other problem is that we don’t know how to sterilize a spacecraft 100%. Or more accurately, we can sterilize a spacecraft completely, but the methods that do this, such as prolonged heat, or ionizing radiation, also destroy the electronics so it won’t work any more. That includes of course the ionizing effect of Jupiter’s radiation - although the surface of Europa is riddled with ionizing radiation that would quickly kill any human, any spacecraft there has to survive this, at least up to the landing, which would mean that it is protected sufficiently that microbes could survive also.

If there are some microbes on the lander, and they survive to the landing, then it might impact into liquid, or create a liquid area due to a crash on Europa which might be deep enough to shield microbes so they can reproduce there. Or microbial spores brought to Europa with the lander could eventually in the future over thousands or years find their way into the ocean.

Fast follow up landers

They plan to send the mission to Europa possibly as soon as 2022 to get there by 2025, using the SLS, which lets gets there, then our technology may be so advanced we can send a follow up orbiter or lander within months or a year or two. In any case I think we simply should not risk a lander at this stage due to planetary protection issues unless we can sterilize it 100%, or somehow can prove that there is no significant possibility of it irreversibly introducing Earth microbes to Europa. Even a 1 in 10,000 chance of contaminating Europa with Earth life, I think would be too high, given what we may be risking there, some unique discoveries that we could never do anywhere else. E.g. it could be some early form of life, not as far evolved as DNA or evolved in a different direction, which might be very vulnerable to DNA based life. And it’s probably impossible to do an accurate assessment of how likely it is that we could irreversibly introduce Earth life to Europa by mistake, we just don’t know enough yet about Europa or about exobiology with no examples yet of any known exobiology to base our decisions on.

Again by the 2030s we may have the technology to sterilize a spacecraft 100% without destroying the electronics. I hope so!

Mission to Enceladus geysers - and perhaps identical mission to Europa

Meanwhile one thing we can do right away is to send a mission to Enceladus to analyse its geysers close up, and it would be reasonable I think to send life detection instruments on that mission too. Instruments that would help with analysing whatever is in the particles, able to detect complex organics, and also able to find indications of life too if present.

If funding permitted, perhaps we could also send an identical orbiter geyser fly through mission to Europa “on spec” just in case we find geysers there, to save time. I think that would be less risky than a lander, no danger of crashing, and likely to add to our understanding of Europa even if it has no geysers, by examining the region around Europa just as Cassini did for Rhea etc.

There’s some evidence already of possible water plumes from Europa - though it’s a one off observation by Hubble which hasn’t been repeated. It might have just been a meteorite impact. If it is evidence of geysers, that could be very exciting for search for life on Europa. Water Plumes on Europa: What Lies Beneath?

In any case as I said, I think we should equip any Europa orbiter with similar instruments to Cassini which would help with analysing any dust or ice particles or gas around Europa with the capability of detecting complex organics, which may be in them whether or not Europa has life, and I think we should add chirality detection at a minimum. There’d surely be some dust or gas to analyse even if there are no plumes.

Safe and easy landings for Europa - “ice breaking” instead of “aerobraking”

Huygens was an easy experiment yes, for Titan. We can’t do aerobraking on Europa.

However you could do equally easy experiments for Europa - one idea is a penetrator, using what we could call "ice breaking" to slow it down. I'm not a fan of that myself for planetary protection reasons unless the penetrator can be sterilized 100%.

Planetary protection friendly version - artificial geyser - Bernd Dachwald’s idea

However there’s a planetary protection friendly version of it. You could use two spacecraft - a dumb penetrator consisting of just a metal slug, easily sterilized. This sends a plume of ice into space. You could use two “dumb penetrators” with the second one closely following the first for more effect.

In effect, you are creating an artificial geyser here. This would be followed by a low flying orbiter to capture the sample for analysis.

That would have minimal planetary protection issues if the dumb penetrators can be 100% sterile - e.g. just lumps of metal heated beforehand to temperatures where no Earth microbes could survive or otherwise 100% sterilized before impact. This is an idea Bernd Dachwald (head of the German IceMole project) once suggested to me in conversation, which I think is an interesting one.

However if Europa is indeed producing geysers naturally, we don't need to do this, we can just observe the plumes "as is".

Chipsats for Europa - could they be 100% sterilized?

Another interesting idea, here is an old mission idea to send “chipsats” to Europa’s surface, each one rather “dumb” but lots of them, each one consists of just a few sensors on a flat chip. Some would fail but enough would get through, and they would be able to survive impacts that a larger more complex lander couldn’t.

That sounds like a kind of a lander that is so minimal, perhaps it could be 100% sterilized by supercritical CO2 snow or something similar? That’s a technique that can remove all the organics from the surface of an electronics chip without damaging the chip. It’s been shown to work with USB drives. So though it might be tricky to scale up to a complete spacecraft, I wonder if it is good enough to 100% sterilize chipsats? It would have to be 100% reliable.

"ATTEMPT NO LANDING THERE" --New NASA Mission to Europa will Ignore Arthur C Clarke's Warning (2014 Most Popular)

Can we achieve 100% sterile electronics for a Europa or Mars lander?

There’s no in principle reason to prevent 100% sterile electronics. You just have to find some process that electronics can withstand and life can’t. If you heat metal to hundreds of degrees C for instance, no life will survive and the result will be 100% sterile. The problem is that this will destroy the spacecraft electronics too. So can we find a way to sterilize it of Earth microbes without destroying the delicate equipment? That’s the big question here.

Also all this might be far easier to do with a chipsat than with a large conventional spacecraft.

First one method being explored by the European Space Agency is Deep cleaning with carbon dioxide. and Science Daily article about it.

  • CO2 a liquid at 100 atmospheres and 50 C.
  • And then on release of pressure turns to snow and takes the dirt, organics, everything away leaving the surface dry.
  • Mixed with Hydrogen peroxide and other chemical to increase effectiveness.
  • Can be used even with sensitive electronics. Was used to clean USB drives in testing and they functioned afterwards.
  • Surface is left with no trace of organics, not just with dead micro-organisms. Major plus!

Could you remove all traces of organics from the exterior in this way? And - can you also keep exterior and interior separate so there is no chance of leaking contamination from inside the mole?

High temperature sterilization

Then also, if you can make the whole thing able to withstand high temperatures, you can just heat it up to a high enough temperature to sterilize all life.

The main issue with sterilizing modern spacecraft is that many instruments are quite delicate, also they can go out of alignment,so even the sterilization temperatures used for Viking of 111 °C for 40 hours is too much for them.

But there are electronic circuits now designed to operate at up to 200°C . High-Temperature Electronics

And there are other developments that should permit temperatures of 200°C upwards :).High-Temperature Electronics Operate at 300 degrees C | EE Times and Designing for extreme temperatures

There’s an economic incentive for developing these electronics, as they are useful in oil wells and motor cars.

I’ve never seen this suggested for a way to keep Europa landers sterile, but it sounds as if it should work!

Back to the drawing board probably for a lot of the designs to make the whole thing uses chips and solders etc that work up to high enough temperatures for 100% sterilization. But it seems like it may be possible! Thanks to Adeel Khan for the suggestion

Is this right? Is it possible to achieve 100% sterilization by heating electronics that’s capable of resisting temperatures of up to 300 C. I wonder if anyone working in the field of spacecraft sterilization has investigated this, either experimentally or in theory. Or is there some other way to achieve 100% sterile electronics such as the CO2 snow approach?

I think we need to look into that myself before we consider sending any probes to habitats that may include liquid water habitable to Earth life. Except of course for the plume flybys. They are safe so long as the ice particles they collect can’t dislodge microbe spores and return them to the liquid water in the subsurface oceans. That sounds likely to be for all practical purposes, zero risk though you’d need to examine it carefully of course.

Multiple methods at once

Perhaps for the best results both can be used one after the other. High temperature to make sure there is nothing viable. Then CO2 snow to remove the organics as far as possible. Heat it up again before it is released from the orbiter for a final precaution to make sure.

Especially for electronics in an impactor / penetrator as that would have to withstand high g force and perhaps high temperatures too, so it would need to use specially hardened electronics. And it needs to be hardened for the ionizing radiation for Europa as well so you are hardly talking about “off the shelf” electronics here.

A rather more far out idea - 3d printer on Europa plus raw materials for some of the components

Another idea, just for fun for now - but: land a sterile 3D printer + some raw material feedstock for it, also sterile. The surface would be high vacuum, ideal for electronics. First thing it does is to 3D print a shelter for itself or dig below the surface for protection from the cosmic radiation. Then it sets about printing out whatever you need, including a Europa submarine from the sterile components you supplied it with. If it is a nanoscale printer it can do circuit boards as well. So all you need to do is to send it some sterile chips to attach to those circuit boards, and other hard to print out components pre-sterilized. Most of the rest it does itself.

This is a bit far future perhaps.But perhaps some element of 3D printing could help for an idea of partial in situ construction of devices for helping to study Europa in a sterile way? Especially small chipsat type devices. Sterile electronics plus 3D printing of some extra components to help with mobility or sampling or some such.

If we can’t achieve 100% sterile landers for Europa

If we can’t do it, I think we simply should not send a lander or submarine to Europa until we can, and should not risk introducing Earth microbes to a habitable environment on Europa.

It is just risking too much to do that. Not just for us, not just for the mission that goes to Europa right now, but for our descendants and indeed all future civilizations on Earth also. It would be just tragic to find some interesting form of exobiology on Europa only to know that we have seeded Europa with microbes that will eventually make it extinct.

It could be very vulnerable to Earth life. The example I like best there is the idea of some primitive early life, for instance RNA based, or even an RNA ocean or autopoetic cells. If Europa was like that, then introduced Earth microbes in a globally connected ocean through exponential growth would surely do short work of converting it all to DNA based life.

Why not just send Earth life there?

Some enthusiasts suggest we just send life to Europa to seed it with Earth life. The problem with this idea is that then we won't be able to find out about the life that is already there, if there is any - or pre-biotic or non biotic chemistry - or whatever there is there right now. Especially since our life could make it extinct. About half of Earth's biological history in terms of gene complexity is unknown to us. We just have no idea how the early organic chemicals developed into lifeforms as complex as the simplest microbes. Lot's of sketched out suggestions but no answers and it is way beyond any attempt to simulate in a laboratory.

Well one likely thing to find in the Europa ocean, if life is common, is some early form of life. Maybe RNA based life. Maybe just an RNA ocean. Or maybe autopoetic cells. Or some primitive lifeform that reproduces, sort of, but not nearly as accurately as DNA life does. Or perhaps it's RNA based using ribozymes in the place of ribosomes, everything done in RNA. And that's just a few examples based on what might have happened in our own planet's past. Europa life may well not be related to Earth life at all. In the entire history of the solar system, at most a handful of rocks may have made it from Earth to Europa. So it could be something else as well.

As those examples show, it could be very vulnerable. An RNA ocean say, or RNA only lifeform could perhaps become extinct after just a few years of exponential growth after the first contamination by Earth life throughout the entire ocean, especially if it is all connected and its ocean has food sources for the life to use. And however quickly or slowly it happens, there is no way we could reverse something like that once it got started. It would be the worst possible anticlimax to all our searches for life in our solar system, to know that Europa was such a biologically fascinating place, until the first probes from Earth landed there, and is no longer like that.

Until we know what's there, I think we have to treat every potentially habitable planet or moon or other habitat in our solar system as if it was the only one of its type in the solar system. Because a lifeform that evolves in Europa's ocean may well not evolve in Enceladus, or Ceres or on Mars or whatever place you study next. It could be our only opportunity for light years in every direction, to study such a lifeform.

  • Perhaps they all have different unique lifeforms or types of pre-biotic life.
  • Perhaps they all have almost identical independently evolved life (very surprising I think).
  • Perhaps life from a previous star seeded them all or most of them.
  • Perhaps only one of them has life, or none of them do.
  • They are sure to have complex chemistry and we can learn from that also, maybe learn that life evolves only with great difficulty and find out what happens when it doesn't evolve too. We won't know until we find out.

As for experiments in Earth based life in space - we can do closed system habitats to try that out anywhere. For instance the Moon may have vast caves kilometers in diameter, so maybe we do it there. Or in free flying space habitats. There's enough material in the asteroid belt alone to create habitats with a total land area a thousand times that of Earth. There may be many opportunities to do that. We don't need to have as our first priority to turn everything into the closest possible approximation to Earth we can imagine, especially a very poor imitation of it, an ocean covered in kilometers of ice with the harsh environment of Jupiter's radiation on the surface, and too far from the sun for most photosynthetic life to be practical and not at all in its oceans (except for life that uses the heat radiation from hydrothermal vents for photosynthesis).

And meanwhile constructed habitats from asteroid materials can be designed with whatever environment you like, tropical gardens if you like, depending how much sunlight you reflect into it using space mirrors or solar collectors, or simulate conditions on Europa or Mars or other places in our solar system if that's your aim. Or you could simulate some the conditions on an interesting exoplanet. You can use spinning habitats with artificial gravity for whatever level of gravity you want, too.

That's looking forward a bit there - but only decades, centuries at most. You could build a Stanford Torus habitat within a decade or two with the funding and political will to do so even with present day technology. If we want to explore setting up habitats with Earth life in it outside of Earth, I think things like that would be the way to go - starting on a much smaller scale first probably. You could start with small exovivaria in LEO or on the Moon, and experiments with closed system recycling.

While there’s no way we can duplicate the billions of years of Europa’s history and the vast oceans larger than Earth’s oceans. If we mess it up, then the nearest “Europa” analogue may be light years away. And even then, chances are that if Europa and some Europa analogue both have life, even then most likely it has its own unique lifeforms, probably not even the same informational polymer in the place of whatever Europa has - not at all likely that it has the same lifeforms or proto life that evolved on Europa.

So why did they send a lander to Titan instead of Europa?

I think it might be partly that they were sending a spacecraft to the Saturn system anyway. In the case of the Jupiter system, then it’s much harder to visit Europa for more than a short time because of the ionizing radiation. Still you could do a penetrator with a fast flyby and that would work much like Huygens. It could communicate back to Earth during the flight to Europa and if it survived the landing, do some experiments and report back during its design life whatever it is.

But it would have many more planetary protection issues to work through than a Titan mission. I think myself it is best to wait for the orbiter mission first before we decide what to do next. We might well confirm the plumes on Europa and that would make it really easy to sample it’s ocean with a low flyby or orbiter and then we might not need a lander at all for the first missions there.

In situ instrument capabilities

During the Q / A, the team mention the idea of sending in situ life detection instruments to Europa in the future. We could use these on a flyby or a lander. So what instruments could we send? Actually there are many such already developed. Some of them have exquisite sensitivity, and could find life based on the minutest of traces, even able to detect a single molecule in the sample of biochemical interest.

Most of these instruments were developed for Mars. Whether they can be used as is for Europa, or need more modification, this shows the range of instruments we can send. I've no idea about the engineering challenge, to examine materials captured in an aerogel "in situ". One issue is to ensure that the readings are not confused by the material that makes up the aerogel so the composition of the aerogel is important. It also helps if you can do a slow flyby, so that there is less damage during impact into the aerogel. Anyway here are some of the instruments we can send, and some are exquisitely sensitive and would surely detect life if it is there.

I think if astrobiologists were asked, maybe in a competition to devise astrobiological instruments to send to Europa, they would rise to the challenge to devise instruments for a Europa flyby and you might get some surprises, neat ideas that you didn't expect. With the huge mass of the Europa missions on SLS, you could fly many of them - a lot of them are "labs on a chip" that weigh hardly anything.

Here are some ones that are already at quite a developed state, with an eye to eventually fly to Mars:

Rapid non destructive preliminary sampling

  • Raman spectrometry - analyses scattered light emitted by a laser on the sample. Non destructive sampling able to identify organics and signatures for life. It's sensitive, can measure the distribution of the organics and other compounds by pointing the laser at different points on the surface - and is non destructive so it can be applied first before any of the other tests.

Detection of trace levels of organics and of chirality

Dallas Ellman fine tunes a component of the astrobionibbler. It uses ideas from the larger UREY instrument, using high temperature high pressure subcrtiical water as a solvent for non destructive extraction of organics. Now miniaturized to a "lab on a chip", low power requirments, low mass. 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, so sensitive that it could detect a single amino acid in the sample. The target goal is a mass of 2.5 kilograms, a quarter of the mass of UREY. Astrobiologists have designed many exquisitely sensitive instruments that they want to send to Mars to do in situ searches for biosignatures. So far none have yet flown. UREY was approved for ExoMars but was descoped when NASA pulled out of the partnership.

Solid 3 - detects biosignatures using polyclonal antibodies. This version of it consists of a sample preparation unit weighing 5.5 Kg and a sample analysis unit weighing less than 2 kg. In tests, it's predecessor detected a previously undiscovered habitat two meters below the surface of the Atacama desert. Astrobiologist want to send this and other very sensitive instruments to Mars to search for minute traces of degraded ancient organics from life and for present day life which is also likely to be hard to detect in such hostile conditions. Some are tiny labs on a chip - instruments that a few years ago filled an entire lab and now can be miniaturized to a single chip. These in situ searches could be confused by the minutest traces of organics from Earth brought by astronauts.

An earlier version of the same technology, the Life Marker Chip, mass of 4.7 Kg, was approved for ExoMars but later descoped.

Direct search for DNA

These can detect life on Mars if it is DNA based so related to Earth life. As DNA sequencers, they can sequence the entire genome of any lifeform found.

  • Miniaturized DNA sequencer could work if we had a common ancestor right back to the very early solar system whenever DNA first evolved. This is in a reasonably advanced state. They say it could be ready to fly by 2018.

Electron microscope

Search for life directly by checking for metabolic reactions

These can detect life even if it doesn't use any recognized form of conventional life chemistry. But requires the life to be "cultivable" in vitro when it meets appropriate conditions for growth.

  • Microbial fuel cells, where you check for redox reactions directly by measuring the electrons and protons they liberate. This is sensitive to small numbers of microbes and has the advantage it could detect life even if not based on carbon or any form of conventional chemistry we know of.
  • Levin’s idea of chiral labeled release, where he has refined it so you feed the medium with a chiral solution with only one isomer of each amino acid. If the CO2 is given off when you feed it one isomer and not with the other, that would be a reasonably strong indication of life.This has the advantage that the life just needs to metabolize amino acids, and to produce a waste gas that contains carbon (such as methane).

There are many instruments like this we could send, and several of them are already space qualified but never flown.

Optical microscopy

I also wonder about an optical microscope. Why not send, not just a "geologist's hand lens" but a diffraction limited optical microscope? With resolution of 200 nm. It could tell us things about the behaviour and structure of micro-organisms or protocells we might not be able to find out by other methods. 

Ideally you want to see the structure of protocells if they exist, and other sub-optical limit structures, so I do wonder also about the microscopes that go beyond the diffraction limit, but I'd have thought they are probably too complex to send into space? Probably won't verify life or protolife unless it is actually still viable and active. But could give interesting data in combination with the other instruments.

What if we get ambiguous results from Europa just as we did for Viking?

Yes we might well get get ambiguous results. That's how science works. But if we are so scared of ambiguous results that we never fly anything unless we are sure it will give a clear cut result - surely that's going to slow down the pace of discovery? I think that the natural response to the ambiguous Viking results was to do a follow up experiment such as the labeled release, to find out what happened. If we'd sent such an experiment, say, on Sojourner, it would have weighed very little and we'd now know for sure if Viking found life or not, and there'd be no more discussion about it, either way.

The safest approach is to search for things you know you can discover, with proven instruments you have already sent into space. And it is natural for mission planners to want to do that as it is the least risky thing to do. But you may be missing out on new discoveries that perhaps could be made easily with different instruments.

Null and ambiguous results are important. If you send a DNA sequencer to test for DNA - well it is one hypothesis that Europan life could be related to Earth life through cells from before the origins of our solar system. If you find other indications that may indicate life, or strongly indicate it, but no DNA that's a significant null result. Based on that one can decide what to fly in the next mission for a follow up. If you get ambiguous results from an experiment that is expected to decide the question - that shows there is something interested and unexpected going on. It might even be more interesting than the thing your experiment was designed to test for.

Europa is only two years travel time from Earth via fast Hohmann transfer, and by the 2020s we may have more heavy lift capabilities that will make it easy to send follow up missions to resolve the questions that early missions raise.

As well as all that - just sending the instruments at all gives exobiologist experience in sending their instruments in space, which then become space rated for future missions, so adding to exobiology experience. If the DNA sequencer gives a null result for Europa, still, it's now space rated, and you can fly it to Ceres, say, if we find habitats on Ceres, or to Enceladus, and indeed to Mars.

Also the process helps inspire a whole generation of astrobiologists, to see their instruments fly in space. At the moment only geological instruments have flown, apart from the very early Viking instrument. You have to start somewhere with the astrobiological in situ searches.

See also

Part of this article originated as my answer to If there is a possibility of life on Europa, then why did NASA land a craft on Titan and not Europa? on Quora

Other places in the solar system - Titan, Triton, Io, etc

So, now let's look further afield at some of the other places in the solar system that life could be found, or prebiotic chemistry. Most of these are places where the conditions are so different from Earth life that there may be no issue at all even sending human beings to do in situ searches there, once we have that capability. An international committee of scientists, COSPAR, meets every two years to discuss issues to do with planetary protection amongst other things. They publish the results using a classification scheme, described in detail in the wikipedia article on Planetary Protection:. So let's just quote from that article (Actually I'm the main author of this section of it).

Planetary Protection classifications

Category I

“not of direct interest for understanding the process of chemical evolution or the origin of life.” 

  • Io, Sun, Mercury, undifferentiated metamorphosed asteroids

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.”

  • Callisto, comets, asteroids of category P, D, and C, Venus, Kuiper belt objects (KBO) < 1/2 size of Pluto.

Provisional Category II

  • Ganymede, Titan, Triton, the Pluto-Charon system, and other large KBO's (> 1/2 size of Pluto) Ceres,

Category III / IV

“…where there is a significant chance that contamination carried by a spacecraft could jeopardize future exploration.” We define “significant chance” as “the presence of niches (places where terrestrial microorganisms could proliferate) and the likelihood of transfer to those places.”

  • Mars because of possible surface habitats.
  • Europa because of its subsurface ocean.
  • Enceladus because of evidence of water plumes.

Category V

Unrestricted Category V: “Earth-return missions from bodies deemed by scientific opinion to have no indigenous life forms.”

Restricted Category V: "Earth-return missions from bodies deemed by scientific opinion to be of significant interest to the process of chemical evolution or the origin of life."

In the category V for sample return the conclusions so far are

  • Restricted Category V: Mars, Europa, Enceladus.
  • Unrestricted Category V: Venus, the Moon.

with others to be decided.

There may be issues though with the Venus clouds. As you see from that list, they were classified by COSPAR in 2006 in the same way as the Moon, so that you only need to document what you did when exploring them, and sample return was also classified as for the Moon, so no precautions are needed. However some astrobiologists challenge this classification and think it needs to be revised.

Callisto and other places with subsurface oceans not connected to the surface.

Callisto is thought to have a subsurface ocean now, like many of the ice moons. But it's classified as category II, "ere there is only a remote chance that contamination carried by a spacecraft could jeopardize future exploration”

That's because they think there is no connection between its subsurface ocean and the surface. It's got a cratered surface with no sign of any modification. Callisto could be a valuable stopping point for humans in the outer solar system. It's just outside the hazardous radiation belts of Jupiter, so an ideal place to stay while studying the Galilean moons and has plentiful ice.

So, there is no problem at all sending humans to Callisto. However if they want to study its subsurface ocean they would have to drill deep. The ocean, if it exists, is at least a few kilometers thick and is less than 300 kilometers below the surface. Also not too likely to be habitable.

It's ocean might have life. It's probably salty. But it's only heated by radioactive materials in Callisto's core so wouldn't have the energy flux of the Europan ocean which is heated by tidal heating as well. This possibility was first announced in Nature in october 1998

Life in liquid ("supercritical") CO2

This first habitat is unusual because it is an "extra terrestrial" type habitat that's actually accessible here on Earth, yet we know little about it to date. It's also probably present in other places in our solar system, particularly on Mars, and (not so easily habitable), on Venus too.

We are used to CO2 as "dry ice" which turns instantly into a gas when it is heated. But, did you know that CO2 is liquid under high pressures and at normal temperatures? You can actually find liquid CO2 in its natural state on Earth, at the high pressures of the ocean depths. Indeed if you explore the oceans, then anywhere below around 0.8 kilometers below the ocean surface,, CO2 is a liquid. This video is taken at a depth of 1.6 kilometers at a white smoker vent system on one side of the small undersea volcano Eifuku off Japan

The "bubbles" in this video are in fact bubbles of liquid CO2. And there's life there, not actually living in the bubbles but apparently not bothered by it at all. See "Life in liquid carbon dioxide". The first discovery of natural liquid CO2 in the oceans goes back to 1990. Interestingly, as you raise its temperature, liquid CO2 at around 31.1°C, and 73.8 atmospheres in pressure becomes a supercritical fluid. That's a phase where the distinction between a gas and a liquid disappears. Often the properties change also.

Table from the paper Supercritical Carbon Dioxide and Its Potential as a Life-Sustaining Solvent in a Planetary Environment by Ned Budisa and Dirk Sculze Makuch

This is important for habitability. CO2 at these temperatures (above 31.1°C) and pressures (above 73.8 atmospheres), is actually a solvent like water. Indeed, the researchers Ned Budisa and Dirk Sculze Makuch found that it has advantages over water for life processes. Enzymes are more stable in liquid CO2 than they are in water, and it makes enzymes more specific about the molecules they bind to. This supercritical liquid CO2 is often used for sterilizing, because of its affinity for organics. However, some microbes and their enzymes can tolerate living in it. This was a very surprising recent discovery reported in February of this year. The researchers found six strains of microbes, isolated from three sites targeted for geological carbon dioxide sequestration - that have the astonishing ability to grow on the interface between water and supercritical CO2. See Microbial growth under supercritical CO2.

There are fairly large reservoirs of liquid CO2 beneath the Earth's oceans. For instance a CO2 lake found off the coast of Taiwan at a depth of 1.4 kilometers.

At that depth, liquid CO2 is lighter than water. That is how they found it through the rising bubbles of liquid CO2  rising through the sea water. For this to work, most of it must be kept in place somehow or the whole reservoir would empty. In this case they think it is probably trapped by an overlying layer of clathrates, because they observed clathrates. They found life there, even in low numbers within the liquid CO2 layer itself. In this case, the liquid is below the temperature at which CO2 is supercritical.

Below a depth of around 3 kilometers, CO2 becomes heavier than water. There probably are large reservoirs of liquid CO2  below that level in the floor of our oceans, but they would be harder to detect, as no bubbles would escape from them. So - do these reservoirs exist? Are any of them warm enough for supercritical CO2? And if so, are they perhaps inhabited by novel forms of life? That's a lot of "if"s but it's fun to think that we could possibly have life as novel as that even on our own Earth, and if so, we wouldn't know yet.

You get supercritical CO2 on other planets too. Indeed the whole of the Venus atmosphere is supercritical at ground level and for a short way above. However - it's not a likely place for life. It's too hot for organics to be stable. As well as that it's more of a gas than a liquid at those temperatures. That's because it is below the "Frenkel line" - the line above which a supercritical fluid has more liquid like than gas like properties.

Diagram from Structural Evolution of Supercritical CO2 across the Frenkel Line

However it may have a layer of supercritical liquid CO2 a short way below its surface, and in the past, when Venus's atmospheric pressure was probably several times higher (getting into the hundreds of bars), and the temperatures cooler, it might have had supercritical liquid CO2oceans, which might have flowed like a fluid and carved out some of the features found on the surface.

You don't need a thick atmosphere or oceans for pressure though. Surface ice and rock layers would also lead to layers of liquid CO2, and this could happen on Mars. Liquid CO2 would be stable on Mars beneath about 100 meters of rock in the cold conditions there. At one point this was the favoured explanation of the Mars gullies as many of them start at a depth of about 100 meters below the top of the cliff. Combine those conditions with high enough temperatures, say from hydrothermal heating, and again you would have supercritical CO2 on Mars. Which might be a habitat for an exotic form of life.

Life in atmospheres of gas giants

This is an idea suggested by Carl Sagan in his Cosmos series.

For Carl Sagan's scientific paper on this, see Particles, Environments and Possible Ecologies in the Jovian Atmosphere.

And more recently by Stephen Hawking in his "Into the Universe"

In Cosmic Biology: How Life Could Evolve on Other Worlds, Louis Irwin and Dirk Schulze-Makuch look at this idea in some detail. First they note that the upper layers with the visible clouds on Jupiter seem to be too cold for liquid water. Lower down, where the temperatures are warm enough, then there is little water. See page 166. They thought it unlikely that Earth life could survive there, not being pre-adapted to the conditions. That's why, Galileo at the end of its mission was sent to crash into Jupiter, for planetary protection reasons, and it's also why Cassini will be sent to crash into Saturn. The idea is that Earth life can't survive in either of the gas giant atmospheres, and by doing this we avoid the possibility of contaminating the interesting moons of these giants with Earth life.

However, what about life based on different principles from Earth life? The main problem in that case, for Carl Sagan's gas creatures, is how the life could evolve there in the first place. There are trace amounts of hydrocarbons, nitrogen compounds, and sulfur complexes. But there's a lack of oxygen. Also, there's no mechanism for concentrating the organics into one place. Any region where life could form is liable to be torn apart by the strong winds, and turbulence, and affected by strong radiation. They thought that the conditions there could lead to complex organics forming structures made up of large scale chemical networks of life precursors. But without a hard and fast physical boundary or exact replication, then it couldn't develop to the next step and would not fit the criteria to count as life.

They did find a way that the gas giants could have life though, and that is if the gas giant atmospheres could be seeded by life from their own planetary moons, such as Io for Jupiter, and Titan for Saturn. Unlike Earth life, this life may be similar enough in chemistry to the planets for their life to survive there. In case of Io, which could seed Jupiter, it would be pre-adapted to radiation and very low temperatures and to a sulfur based chemistry that could be found in the Jupiter atmosphere. And in the case of Titan, which could seed Saturn, it would be adapted to a dense, cold, organic atmosphere, again conditions that could be found in pockets in the Saturn atmosphere.

For details see page 168 of their book, most of which is available online.

Supercritical liquid hydrogen layers in gas giant atmospheres

Another possibility is life at a much lower level in the gas giant atmosphere. At high enough pressures, hydrogen becomes supercritical, and, like liquid CO2, it becomes a solvent for organics. On Jupiter this zone is very narrow but on Saturn then it is quite wide. This is suggested as a possibility in The Limits of Organic Life in Planetary Systems which was produced by the Space Studies Board.

Life in liquid nitrogen - Pluto or Triton

This photograph of Neptune's largest moon Triton shows dark streaks thought to be nitrogen geysers.

Triton's south polar terrain photographed by the Voyager 2 spacecraft. The dark lines are the trails of plumes from volcanoes, thought to be caused by eruptions of liquid nitrogen from below the surface. Some of the plumes were actually observed erupting during the flyby- by anaglyph projection, which made them easy to pick out as they were closer to the spacecraft than the other features.

Triton - and perhaps Pluto also - could have thin layers of liquid nitrogen, between a surface of solid nitrogen ice and a subsurface of water ice. One researcher Jeff Kargel suggested that Pluto could have rivers of liquid nitrogen or neon. If not on the surface, it could flow below the surface since solid nitrogen is "a fantastic insulator". So could there be life in these layers of liquid nitrogen? Liquid nitrogen would be a non polar solvent (there is no separation of charge in its molecules), which means it can't dissolve organics (which are polar). But it might be just the thing for polysilanes, a complex molecule that uses silicon in the place of carbon. Maybe Triton and Pluto could have silicon based lifeforms?

It turns out that silanols - a kind of silicon version of alcohol - can dissolve in liquid nitrogen. And - in these conditions, silicon has as diverse a chemistry as carbon, as William Bains has argued in his hypothesis paper Many Chemistries Could Be Used To Build Living Systems. The silanols have weaker bonds than carbon based organics - but this is just the thing you need in very cold conditions.It's "silicon based life" but not in the sense that rocks are made of silicon - any more than humans are diamond or graphite or charcoal based life. The silicon of course is combined in long chains with many other atoms. See also Peter Ward's chapter on this idea.

Life in the oceans of Titan

Saturn's moon Titan is, so far, the only known place in the solar system with liquid water on its surface apart from Earth. Most of the lakes are around the north pole, with one lake, lake Ontario, at the south pole.
Glint of sunlight on the lake region around the northern pole of Titan.

Here is Chris McKay talking about prospects of life in the Titan lakes

He mentions there the provisional observations of hydrogen and acetylene and ethane depletion near the surface of Titan, which they'd predicted as a possible sign of life on Titan.  Also he mentions that oxygen would be in short supply, and need to be extracted from water ice "rock" by microbes. This is related, a possible way that life on Titan could make cell membranes without use of oxygen.

Possible oxygen free cell structure made of organic nitrogen compounds that could function at the low temperatures of Titan's ocean.

Titan is the only place in the outer solar system which we have sent a lander to, the Huygens probe. Sometime maybe we will send some more probes there to explore it further. This was a recent idea for a submarine to explore Titan:

See also Life on Titan (wikipedia).

This is such an extreme habitat, that it seems first of all, impossible that Earth life could survive there. You could land humans on the surface of Titan - and in some ways it might be one of the easiest places for us to live, as suggested by Charles Wohlforth and Amanda Hendrix, authors of Beyond Earth: Our Path to a New Home in the Planets. Their idea is described in brief in Let's Colonize Titan in the Scientific American. Some of the advantages are:

  • Atmosphere at the same pressure as our Earth's atmosphere. Your spacesuits wouldn't need to be pressurized. They would of course have to be insulated from the cold, but they would be much more flexible, without the enormous outwards pressure of tons per square meter needed in vacuum conditions. You could build conventional houses too, equal pressure inside and out. They would only need to be air tight.
  • Protection from cosmic radiation by the atmosphere
  • Plenty of ice for water.
  • Earth life would probably have no impact on any Titan life
  • Abundant resources for making plastic which could be used to make housing. So easy to construct, with just a thin covering to keep out it's atmosphere, equal pressure inside and out - so like the Venus cloud colonies, you could have huge internal spaces filled with oxygen and nitrogen for very little by way of mass
  • Terminal velocity a tenth that on Earth so if you fall out of a plane, no problem.
  • Human powered flight - could fly in the light gravity

You still need to think about whether molecules from Earth life such as RNA and DNA could be incorporated by Titan life. Also, whether organics from a human occupied habitat would confuse the search for life there, if the life is elusive.Also, though it's easy to see that Earth life couldn't survive on Titan except in cryovolcanoes - can we be sure the other way around, that Titan life can't survive on Earth or in human occupied habitats? It seems unlikely that life adjusted to those habitats could survive here. But what if the Titan life evolved from previous life that first originated in its deep subsurface oceans? Is there any chance it could retain capabilities that would let it survive as spores, and then maybe reproduce in oil deposits or in some other way survive in the much hotter conditions of Earth? It seems unlikely, but can we rule it out before we know what is there?

Also, Titan probably has a subsurface ocean. If so, if there is communication between the subsurface and the surface, e.g. cryovolcanoes with liquid water in place of lava, then that's an obvious contamination issue for the subsurface oceans. Titan is currently categorized as "Provisional category II" with "only a remote chance that contamination from Earth could jeopardize future exploration" - the same as our Moon. But it is provisional because they say that more research is needed. Other places categorized as provisional category II are Ganymede, Triton, the Pluto Charon system, Ceres and the larger kepler belt objects (up to half the size of Pluto).

It would surely be studied robotically first so you'd know all that before you land humans there..

Sulfur based life on Io

There might be life on Io, sulfur based, in underground pools of liquid SO2, with the life chemistry probably also using H2S, because, though less abundant, it forms hydrogen bonds easily, and is more suitable for a solvent within the cells. So life might seek out the liquid H2S and use this as their intercellular solvent, while living in pools of the more abundant liquid SO2.

Volcanoes erupting on Jupiter's inner most moon Io, photographed by the Galileo spacecraft. The surface is rich in sulfur, and one suggestion is that Io could be host to an exotic sulfur based biochemistry.

Louis Irwin and Dirk Schulze-Makuch explore this in some detail in Cosmic Biology: How Life Could Evolve on Other Worlds, The life molecules would have backbones of sulfur, nitrogen, phosphorus and other elements and would take advantage of the complex chemistry of sulfur compounds. Sulfur has a wide variety of oxidation states, even fractional oxidation states such as -0.4 or -2/3. It also forms a variety of ring compounds, and polymers.The temperature range over which H2S and SO2 are both liquid is quite narrow, from -75°C to -60°C. And because these temperatures are quite low, the reaction rate is likely to be slower.

A plume ejected from same general region as one of the regions imaged by Voyager, called Masubi, it erupts 100 km into space from Io. The eruption comes from different places in this region and leaves plumes of SO2 on the surface.

Louis Irwin and Dirk Schulze-Makuch say in their concluding section of this chapter:

"It would be all underground and able to thrive only when temperatures reached an appropriate narrow range. But that would happen, for at least a brief period, in local pools or over short stretches of ground, every time a hot spot erupted or a sheet of lava advanced."
in Cosmic Biology: How Life Could Evolve on Other Worlds - Chapter 9, Fire and Ice (not available online).

Io is classified as category I, no problems of planetary protection at all. It would be a hazardous place to land humans because of the high levels of ionizing radiation from Jupiter. But it seems unlikely that Earth microbes could be harmful to any Io life, if present, and it's not likely in the other direction either, that any putative Io life could harm Earth.

What about life in lava flows on Venus?

One of the most challenging forms of life would be silicon based life living in liquid lava. If this was possible, we might find it on Venus - which has entire river channels that travel for huge distances across its surface.

The arrows show the two ends of this section of Baltis Vallis on Venus - the longest known channel of any kind in the solar system, total length of 7,000 kms. Though there is no liquid in it now, it may have been carved out by rivers of liquid lava, which, if not too hot, might have been a suitable place for life that relies on silicones.

These could use Silicones - organosilicon polymers with a silicon-oxygen backbone. These are stable at temperatures so high they would destroy any organics. But - would they remain stable at the temperatures of even cooling magma pools on Venus? That's the big question. Unlike water chemistry which is easy for us to explore in our laboratories - it's not so easy to explore the chemistry of silicones in magma pools.

Venus has no continental drift so doesn't have volcanoes all the time as Earth does - instead its entire surface may "turn over" every few hundred million years. But it shows signs of "young" features and may still be geologically active.

Recent observations show that some parts of the Venus surface may still be active today, with hot spots that appear and disappear, perhaps pools of lava, superheated rocks, or plumes of hot gas.If life is possible in magma pools - then why hasn't it evolved on the Earth also? Why have we never found silicon based life fossils in lava flows on the Earth? Silicon life seems unlikely to us. But is it just that it is a low probability life form? After all - for all we know organics based life, evolving in oceans, might also be low probability.

Maybe, just as we have no silicon based life in our lava, as far as we know - maybe, on other planets, there's no water based life in their water. Maybe on other planets there are silicon based life forms reasoning in the same way that we do, that carbon based life is impossible. Perhaps those silicon based life forms, if they exist, living in molten magma have lifeless seas of water, just as we have lifeless flows of lava (as far as we know anyway). To them, water might well seem too cold to sustain life which they think can only occur at the temperatures of molten lava :).

Life in clouds of Venus

The idea here is that Venus started off Earth like in the early solar system. But at some point it dried up, lost its ocean due to a runaway greenhouse effect, which didn't affect Earth in the same way because continental drift on Earth continually buries and circulates the carbonates. Though the surface of Venus is amazingly inhospitable, the layer at the top of its clouds is in many cases the most habitable location in our solar system after Earth - almost Earth like in temperature, pressure, and atmospheric composition (without the oxygen of course). It has one major drawback, droplets of concentrated sulfuric acid.

However we do have acidophiles on Earth that survive in conditions not far off the acidity of Venus clouds - in sulfuric acid outflows from mines on Earth. The UV light at the cloud tops could be hazardous also, but the life in the Venus clouds could be protected by pigments or by an external layer of solid sulfur (the allotrope S8). So - it's possible that there is sulfuric acid tolerant life in the Venus clouds. The other main problem with the high Venus clouds is that there are no solid surfaces of course. But it could have evolved in the early solar system and then migrated to the clouds as the surface got drier and hotter.

The main question is, could the life find some way to stay aloft? The residence time of particles is months rather than days - so - that makes it easier, and turbulence could return some of the life to the tops of the habitable layer after it reproduces - but it's still quite a challenge.The surface of Venus is totally hostile to Earth life, a dim, hot furnace, with temperatures well over 400°C. But conditions are different at the Venus cloud tops. Temperatures are ideal, with plenty of light. There are several factors that suggest life may be possible there:

  • The atmosphere is not in chemical equilibrium, with H2S and SO2 present together, also oxygen and hydrogen, which life could use as a source of energy.
  • Detection of carbonyl sulphide (OCS) - a clear sign of life here on Earth (though it could be created inorganically on Venus, by volcanoes), and the levels of H2S are also higher in the upper atmosphere, a suprrising result which might mean it is produced by some process in the upper atmosphere rathre than from volcanoes, possibly by life.
  • Doesn't have the expected concentrations of carbon monoxide. That could be a sign that life is using the CO for its metabolism. On Earth, 450 strains of photosynthetic bacteria rely on carbon monoxide for their carbon. Schulze-Makuch suggests that they could be combining carbon monoxide with sulfur dioxide and possibly hydrogen. The anoxic acidic envieronments in the Venus clouds are similar to those in the hydrothermal vents in some ways. See http://www.astrobio.net/news-exclusive/venusian-cloud-colonies/
  • Atmosphere super rotates giving a day night cycle of a total of 4 days making photosynthetic life easier because the night is shorter than the 117 day cycle on the planetar surface
  • Continuous presence of thick clouds more stable than those of Earth, with water present in the form of sulfuric acid in the clouds - this is the region of the Venus atmosphere with most water present.
  • It takes more than 200 days for a 1.1 micron particle to fall out of the clouds , which is more than enough time for several generations even of rather slow growing microbes, to reproduce and form spores ( page 10 of Charles Cockell's Life on Venus. ) ,
  • Most of the ingredients of life are easily accessible in the atmosphere. Five of the six main elements life needs, hydrogen, oxygen, carbon, nitrogen and sulfur are present in large quantities . The main limitation on life similar to Earth life may be the requirement for phosphorus which is present in trace quantities. Chlorine is present in low qunatities. Calcium, potassium, sodium, mangesium and iron are not yet detected though they are likely be present in the form of dust since surface rocks include MnO, MgO, CaO, Na2O,K2O,FeO, TiO2 and SiO2 - See Geoffrey Landis's Astrobiology: The Case for Venus and page 10 of Charles Cockell's Life on Venus.
  • Detection of particles which are non spherical like microbes and the right size for them. This was a rather surprising discovery, as there is no obvious vapour source that would condense as particles of this size and shape. Some scientists have suggested that these large particles may be a result of miscalibration of the equipment, with the conclusion: , "This allows a simple understanding of the source of all the cloud particles, but at the cost of disbelieving some of the measurements.". .However others say that there is good evidence that the equipment was operating under nearly optimal health conditions. This is the "Mode 3 particle controversy", for details of the controversy see section 4.2 of this paper.
  • Detection of dark streaks in the atmosphere of unknown composition which absorb UV. The absorption spectrum is broad without distinct peaks which suggests it is due to particles or droplets rather than gas, Could this be due to absorption of the UV by life either to protect itself or for photosynthesis or both?

Of those, the three main lines of evidence which just possibly could indicate the presence of life are the UV absorption (possible photosynthesis or UV protection), the chemical disequilibrium, especially the presence of OCS, and the possible presence of large non spherical particles. All of these could also be due to non life processes but are not easy to explain in that way.

For more on this:

If we do find life there, it probably didn't originate in the cloud tops. Instead, it's probably a relict of surface life in the early solar system, which migrated to its upper atmosphere as the conditions became harsher. (See also section on Venusian clouds in "Cosmic Biology - How Life could Evolve on Other Worlds").

Artist impressions of Venusian clouds, credit ESA
Artist impressions of Venusian clouds, credit ESA. The surface of Venus is utterly hostile to Earth like life, at temperatures of well over  400°C is. It is also dim, not much light filters through the clouds. But high in the atmosphere above the cloud tops, then conditions are far more conducive to life, at temperatures around  0°C. The cloud droplets themselves are the main challenge, concentrated sulfuric acid, with acidity similar to battery acid.  There are intriguing signs that just might indicate life, in the upper atmosphere though they can also have other interpretations.

Venus probably started off similar to Earth. It's surface actually gets less light than the Earth, because though closer to the sun, it has highly reflective clouds. It is so hot, not so much because it is closer to the sun, but because of a runaway greenhouse effect. Earth has similar amounts of carbon dioxide locked up in limestone, and could look the same in the future as the sun heats up further. Most scientists think that Venus was a near twin of Earth in the early solar system, with oceans like Earth. We don't have quite the same confidence about this that we have for Mars because its entire surface was resurfaced a few hundred million years ago which would erases any clear signs of the ancient oceans such as the deltas and shore lines of Mars. But there are still hints that suggest it did have them.

Evidence for early oceans on Venus is indirect.

This is a temperature map of Venus. Observations from orbit are consistent with the idea that Venus had earlier oceans, with suggestions that it might have granite land masses. If so these may be the remains of ancient continents

"The eight Russian landers of the 1970s and 1980s touched down away from the highlands and found only basalt-like rock beneath their landing pads. The new map shows that the rocks on the Phoebe and Alpha Regio plateaus are lighter in color and look old compared to the majority of the planet. On Earth, such light-colored rocks are usually granite and form continents.

"Granite is formed when ancient rocks, made of basalt, are driven down into the planet by shifting continents, a process known as plate tectonics. The water combines with the basalt to form granite and the mixture is reborn through volcanic eruptions. If there is granite on Venus, there must have been an ocean and plate tectonics in the past," Nils Muller said.
See Oceans on Ancient Venus - Study suggests (space.com)

This life could also survive in high pressure subsurface habitats with supercritical liquid water.

If Venus did have oceans in the early solar system, life could have evolved independently from Earth. Or, it's possible that Venus was seeded by life from Mars or Earth, billions of years ago. Or the other way around, it could have seeded Earth or Mars, or both. If so then this is a really exciting possibility for biology. We may make amazing discoveries from studying life that's been isolated from Earth for billions of years, or possibly evolved independently.

Venus and Earth (ESA)
Venus (left) may have had oceans like Earth (right) in the early solar system, and life could have evolved there, or been seeded by Mars or Earth. If so it might still exist in the clouds.

However this may also have planetary protection implications. Some time back, in 2006, an international team of scientists for COSPAR (Committee on Space Research) examined the situation for Venus, in "Assessment of Planetary Protection for Venus Missions" (you might find that the easiest way to read this report online is to get free membership of NAP and then use the download button and read it as a pdf). They came to the conclusion that even in the Venusian cloud tops, conditions are so different from Earth conditions that there is no need for planetary protection. As a result of this report, Venus is currently classified as Category II, and sample return as unrestricted Category V. This means that you simply need to document whatever it is you do.  (For the current planetary protection categories, and policies, see Planetary Protection (Wikipedia) )

This also means that you can return a sample of the Venus atmosphere to Earth for study, with no need to contain it or act in any way to protect the Earth environment. The only requirement is that you have to keep detailed documentation of whatever you do. However, there was a dissenting voice at the time, by Dirk Schulze-Makuch who was not part of the team. See Planetary Protection Study Group Mulls Life On Venus

Everyone seems agreed that there are no planetary protection issues for the Venus surface, with temperatures well over 400C. But should the Venus atmosphere perhaps be re-categorized as category III, meaning that you have to sterilize spacecrafts that visit it? Should sample return from Venus be re-categorized as restricted Category V, meaning that you have to take precautions to protect Earth in event of a sample return?.

Some of the material here comes from my article If there is Life in Venus Cloud Tops - Do we Need to Protect Earth - or Venus - Could Returned XNA mean Goodbye DNA for Instance?

Possibility of earth life able to survive in the Venusian clouds

At first sight it certainly seems unlikely that Earth life could survive in the concentrated sulfuric acid droplets in the clouds. These droplets have pH less than 0, similar to battery acid. This is the main reason the COSPAR team gave for their conclusion that no Earth life could survive in them. However, in 1991 researchers found some Earth microbes able to survive sulfuric acid with pH 0 or lower, close to the Venus cloud top conditions. These researchers also wrote that it is  possible that we might find organisms able to tolerate even lower pH levels. Their most acidophilic (acid loving) microbe was  Picrophyilus, which grows optimally in sulfuric acid at pH 0.7 and is capable of growth (not just survival but growth) down to pH -0.06 (1.2 M sulfuric acid). This is a microbe which you can find living naturally in highly concentrated sulfuric acid in the wild, in acid mine drainage and in solfataras (sulfur emitting fumaroles).

So perhaps some Earth micro-organisms could live there after all. Only 1% of the bacteria on Earth can be readily cultivated in culture media. There are various reasons why this might be the case. See Strategies for culture of ‘unculturable’ bacteria for an overview.

Great plate count anomaly
The "Great Plate Count Anomaly" - if you cultivate cells in a medium and count the number of Colony Forming Units (CFU's), and if you then take the same sample and count individual cells in a  high powered optical microscope, typically you find that there are about 100 times as many cells as you detected with the CFU method. This is because biologists currently can only cultivate 1% of living cells, typically.

Also, only a tiny percentage of all species have been studied in any detail. So it is hard to say for sure what the capabilities are of the micro-organisms we haven't yet studied, such as the majority of the archaea. This is the issue of "Microbial dark matter". For instance a recent study found that - "Of the 100 major branches, or phyla, of microbes, less than one-third have any described species", see How Many Microbes Are Hiding Among Us?

The microbes carried by humans can have hidden extremophile capabilities - because microbes do not lose their capabilities, usually, when they move to a different environment. Some are polyextremophiles able to live in a variety of extreme environments as well as in much more ordinary ones (for humans). A typical human has 100 trillion microbes in 10,000 species - and the species mix varies from one person to another. Many of these will be unknown to science, and some may well have extremophile capabilities. For example a recent study of microbial populations of human belly buttons found a couple of species able to thrive in extreme cold and extreme heat. Another example is the discovery of a microbe on a human tongue able to thrive in conditions of very low pressure.

Possibilities of indigenous life in the Venusian clouds

The other way then there have been suggestions of possibilities for life in the Venus clouds, indigenous life. There are one or two interesting hints, observations that could be interpreted as evidence of indigenous life. The most intriguing of these are, the presence of OCS which on Earth would be strong evidence for life. On Venus however it is just suggestive, not conclusive. There are processes that could create the observed levels of OCS without life, and detailed models of these processes are compatible with the observed levels. The atmosphere is also not in equilibrium, as it has both H2S and SO2. This disequilibrium is something that life could exploit. The upper atmosphere of Venus also has been shown to contain particles that are microbe sized, and non spherical, which might be an indication of life in the clouds. Particles in the Venus atmosphere stay suspended for months, rather than the days for Earth. Still, they will eventually fall to the lower layers; so that makes it an issue, how do the microbes stay aloft? 

I haven't yet seen a worked out answer to this, so here are a couple of suggestions to explore. First, perhaps microbes in one droplet, descending, could send out spores (explosively perhaps) that land in other droplets that ascend, and so continue the reproduction?

Another idea is based on the observation that some microbes form gas vacuoles on Earth more or less permanentThey are used by cyanobacteria to regulate buoyancy in water, not that far off the idea of using hydrogen vacuoles to regulate buoyancy in CO2, evolved over billions of years. Is it possible I wonder? The main difference is, that the gas vacuoles in cyanobacteria take up only a small part of their bodies (and are made up of smaller, rigid, gas vesicles). Apparently Anabaema has gas spaces occupying up to 9.8% of their volume (see page 124 of the paper "Gas vesicles"). But this is far below the levels needed for the Venus atmosphere.

Gas vesicles
Gas vesicles. These are filled with ordinary air, and are used by cyanobacteria to regulate buoyancy in water, several of these cluster together to make a gas vacuole. The gas can occupy to 9.8% of the volume of the microbe.

The vesicles would need to be filled with hydrogen instead of air. Then with the density of CO2 of 0.001977 (and hydrogen, 0.000089) compared with water, at 0°C, they would still need to have so much hydrogen in the vesicles that they occupy approximately 98% of the volume of the microbe. I'm not sure if this is possible. However, life solutions are often surprising.

If we do find life in the Venus upper atmosphere, then we would need to find out next, how it managed to stay there and reproduce. For instance, could the cells provide hydrogen filled bubbles, or external vesicles filled with gas, which remain attached to their bodies, somewhat like the bubble nests created by some insects, and use those to float in the Venus atmosphere? Or, very speculatively, indeed might there even be higher plants or animals that do this in the Venus atmosphere?

Froth of Spittle Bug, or Frog Hopper - Larval form
Froth of Spittle Bug, or Frog Hopper - Larval form - could a similar technique be used in the Venus cloud tops, using bubbles filled with hydrogen, attached to the microbe or higher life form as a type of froth or foam, for buoyancy?  It would need to have less than 98% of the volume for the bubbles and the body of the creature, with the rest all hydrogen, to float at the 1 atm level on Venus. 

This idea that Venus cloud life could use hydrogen for buoyancy is my own suggestion. I've not seen it published anywhere. Do say if you know of anyone who has published a paper exploring it, or any research into it.

The main questions still unanswered for planetary protection for the Venus clouds

  • Could indigenous life colonize earth after a Venus sample return - the study came to the conclusion that due to the high acidity then these life forms if they exist are unlikely to be able to colonize Earth. But  Dirk Schulze-Makuch  was not convinced by this conclusion - so that suggests there is room for discussion here. I am not either.
  • Acid environments on Earth - could the lifeforms, if acidophiles, colonize some of the most acidic environments on Earth?
  • Retained capabilities - also - microbes often retain capabilities that they no longer need. Even though it would be billions of years since these microbes lived in Earth like environments,could some might retain capabilities from those times to live in less acidic environments than for the Venus clouds.
  • Could there be micro habitats in the Venus clouds created by life? We haven't studied the clouds in detail close up, only from orbit, or with instruments with limited capabilities. Especially if there is life there, then we know that life can transform habitats and form micro-habitats. If that's so, there could be micro-habitats in the clouds caused by life processes that are not so acidic, inhabited by symbionts which would perhaps need the capability to live in non acidic environments. If so, then these habitats could be colonized by Earth life, and in the reverse direction, the life in those habitats could be pre-adapted to less acidic environments so more likely to be able to survive on Earth.
  • Could gene transfer agents transfer capabilities? If Venus and Earth shared life late enough in the solar system, after the evolution of modern DNA life, you have the possibility of Gene Transfer Agents, those tiny virus like particles, well below optical resolution, able to transfer DNA from one micro-organism to another, very readily, as we've seen already, in that striking experiment where 47% of the culturable microbes in sea water had taken up antibiotic resistance from GTA's. after they were left overnight in bags filled with sea water and the GTA's.

    In this way, Venusian cells even unable to survive on Earth; even if they are dead, might be able to transfer some of their genetic material to Earth archaea - so transforming them and adding new capabilities. This is also a potential issue for forward contamination of Venus clouds as well. Earth archaea could transfer genetic material to microbes in the clouds in the same way even if they can't survive there themselves.

Does it matter if life from the Venus clouds gets established on Earth

You might wonder, okay we are required by the Outer Space Treaty to protect Earth from harmful contamination from Venus. But does it really matter if life from Venus gets established on Earth, or genetic material gets transferred to Earth archaea via GTA's.? Would it indeed be harmful if this happens? If we can show that it is not harmful, there is no cause for concern, and also, we don't need to worry about the OST either (as the clause refers to "harmful contamination").

So, just to go over it quickly, some of the things that could happen are similar to those for Mars.

  • Pathogen of humans or our crops or animals, sea creatures, plants, trees. Doesn't need to be adapted to us. A disease of microbes for instance can directly infect humans without any adaptations, as happens with Legionnaire's disease, a disease of amoeba that happens to be able to reproduce also in human lungs. Indeed pathogens most usually adapt to keep their host alive for longer, not to kill their host.
  • Is able to live on our skin (e.g. fungal infection), in our lungs, in our sinuses, or perhaps in our stomachs. That last possibility is an obvious microhabitat for an acidophile from Venus - though it would be used to sulfuric acid rather than hydrochloric acid of gastric acid (the Venus atmosphere does have HCl as well however).

      Perhaps just possibly, it would be able to reproduce in the stomachs of animals, and ourselves, and interfere with digestion, or create byproducts poisonous to us or the animal?
  • Takes the place of some other organism in an ecosystem but behaves differently so disrupting natural cycles
  • Is an allergen or creates allergens as a byproduct
  • Creates a poisonous byproduct that interferes with human or other animal biological processes. For a simple example, green algae produce a chemical that it's thought, may cause Alzheimer's disease in humans. The way it works is interesting also, it creates a non protein amino acid, β-N-methylamino-l-alanine (BMAA), which substitutes for the protein amino acid, L-serine, and this leads to cells to cluster together and die. A microbe from another planet might well create chemicals that resemble ones in our body but are not identical and get substituted for them, and so disrupt the way our cells work or the way cells of other organisms work. 

    It is of no benefit at all to the green algae to cause Alzheimer's and is not part of its natural cycle. It doesn't even colonize humans. Just creates a chemical which gets concentrated in shellfish and the like, and may possibly (not totally confirmed yet), when eaten by humans, cause Alzheimer's

Life returned to Earth from another planet may well be harmless, but there are many ways that it could cause harm, also. We can't know with reasonable certainty until we know something about the form of life and how it works.

XNA based life in the Venus clouds

Finally, there is the possibility that Venusian life is not based on DNA but some other basis such as XNA (change of backbone) or something more radical than that. If so then we can't really generalize from DNA to capabilities of XNA.

Rotating DNA animation. Could life on Venus have a different backbone from DNA , using PNA, HNA, TNA, GNA or other XNA?

Here XNA is a general term for nucleic acid analogues - with the same bases as DNA but a different "backbone", in place of the Deoxyribose of DNA. These include HNA, PNA, TNA or GNA (Hextose, Peptide, Therose or Glycol NA). The PNA world hypothesis for instance suggests that life on Earth went through an earlier stage where it used PNA (peptide nucleic "acid") before it started to use RNA or DNA. That's because DNA and RNA are so complex it is a little hard to see how they arose from non living chemicals alone.

Life on Venus could have done the same, but maybe didn't end up as DNA, may still uses PNA or evolved to some different form of XNA.

That raises the possibility that XNA based life could be better at coping with Earth conditions than DNA itself. This could be possible, if it is really a completely different form of life with different metabolism, cell machinery, etc. and has never had any previous contact with the Earth environment. If there does turn out to be life in the Venus clouds, then, the situation is not that different from the situation for Mars.

Venus cloud life might give the best chance for XNA in our solar system

The Venusian clouds indeed might give us one of our best chances of finding XNA in our solar system - in the remote case where there is life there. That's because for billions of years it has been almost impossible for Earth life to be transferred to Venus. The surface of Venus is so hot that Earth life would be destroyed soon after it got there, if it made it all the way to the surface of Venus.

The other way around also, then it is almost impossible for the cloud top life of Venus, if it exists, to be ejected through the thick atmosphere as the result of meteorite impacts on the surface of Venus, with enough velocity to leave the strong Venus gravity and get transferred to Earth or Mars. A huge asteroid impact on Venus would disturb the cloud deck for sure, but could even a giant impact send significant amounts of the high Venusian atmosphere into space? 

Chandra has put forward a controversial theory that the solar wind could transfer microbes from the upper Venus atmosphere (high above the cloud decks) to Earth at times when the planets are aligned. See Microbes Could Travel from Venus to Earth However other scientists find his research unconvincing, so far, with many details to be filled in. For instance, it doesn't seem that the solar wind would have enough energy to remove a microbe from the Venus gravity well, since it is far heavier than the ions it can transport. Also, any dormant microbes that did get ejected from Venus would also be vulnerable to cosmic radiation and high levels of UV, which they might not be adapted to.

So, it seems at least possible that life could have evolved independently on Venus, and has been there ever since. If so, it would probably be a form of XNA. In that case all bets are off as far as planetary protection of the Earth. We can't say much by analogy with DNA life even about its size, or its properties or its adaptability to different environments/.There are other places that could have XNA, including Mars, or comets. But Venus has been more isolated from Earth than any of those. Even the Europan oceans could potentially share DNA with Earth through impacts on Earth sending debris all the way to Europa. This probably was only be possible for Venus in the very early solar system. The Venusian surface might also have been too hostile for Earth life already by the time Earth was habitable.

Hazards of XNA from Venus

If it turns out that Venusian life is based on XNA, this does not make it safe for Earth life. Yes, as some say, the XNA would not be adapted to Earth life. But the other way around, Earth life might not recognize XNA as potentially harmful (as in the Lederberg quote above). And adaptations of microbes are usually in the direction of keeping the host alive for longer; it is of no benefit to a microbe that infects a human for the human to die. It might also be able to out compete Earth microbes in their own habitats, and yet behave differently from them, transforming ecosystems. It could damage crops or animals, or change the balance in the seas. In the worst case, XNA based life might prove to be better than DNA based life all round. For instance it might be more efficient at metabolizing and reproducing. The very worst case is goodbye DNA. For these and other reasons, then researchers in the field of synthetic biology, who are actually contemplating the possibility of creating new life based on XNA instead of DNA (by substituting XNA for DNA in a cell, complex process but most of it is now worked out) - they are exceedingly cautious about the research.

In the XNA specifications section of this paper: Xenobiology: A new form of life as the ultimate biosafety tool The authors talk about biosafety requirements for this procedure

"The ultimate goal would be a safety device with a probability to fail below 10-40, which equals approximately the number of cells that ever lived on earth (and never produced a non-DNA non-RNA life forms). Of course, 10-40 sounds utterly dystopic (and we could never test it in a life time), maybe 10-20 is more than enough. The probability also needs to reflect the potential impact, in our case the establishment of an XNA ecosystem in the environment, and how threatening we believe this is."

So, the idea is that the experiments need to be designed so that there is less than a 1 in 1020 chance of the XNA reproducing in the wild outside the laboratory (most likely by making it dependent on some substance not available "in the wild" outside of the laboratory).

Impossibility of containing XNA at sufficient probability levels

XNA returned from Venus could not be contained at those sort of probability levels. It would more likely be a one in a million type containment such as is suggested for the Mars sample return proposals. One in a million containment is already potentially a major engineering challenge if the particles to be contained are small, such as 0.01 microns across in the case of the Mars sample receiving laboratory. (Incidentally, my own view, I also think the plans for one in a million level containment of a Mars sample are totally inadequate levels of probability for Mars, if the sample happens to contain XNA or substantially different life forms). For the issues for a Mars sample return see Could Microbes Transferred On Spacecraft Harm Mars Or Earth - Zubrin's Argument Revisited and  Need For Caution For An Early Mars Sample Return - Opinion Piece

COSPAR study of the Venus atmosphere didn't consider XNA or gene transfer agents

Here the situation is similar to the studies of risk for Mars sample return. Often new planetary protection studies bring up the possibility of new risks not considered in previous studies. The 2009 Mars sample return study by the US National Research Council brought up the new possibility that Mars life forms might be smaller than previously thought and added a new recommendation to contain ultramicrobacteria at 0.2 microns across. The 2012 Mars sample return study by the European Space Foundation added another new recommendation, this time to contain Gene Transfer Agents only 0.01 microns across if possible - it was published just after the discovery of easy transmission of GTA's. between unrelated species of microbes in sea water.

Both studies of Mars sample return mention XNA but they do not go into it in any depth, particularly, they don't mention the researches into safety considerations for XNA in Earth laboratories. Also neither study considered the possibility that the life forms to be contained are smaller than the smallest known Earth microbes. This seems at least possible since, though 0.2 microns seems to be the smallest organism that could contain all the cell machinery of modern life, early cells on Earth must have been smaller than the ultramicrobacteria of the order of tens of nanometers across. Also, we have no way to be sure of the size of XNA lifeforms.

The Venus planetary protection study didn't consider GTA's. or XNA, and doubts were raised about their conclusions about the possibility of Earth originated acidophiles to survive in the Venus atmosphere. Also the study was not based on experimentation and we have limited knowledge of the Venus upper atmosphere. We don't know enough yet to make an accurate simulation of it in a laboratory on Earth for testing. The clouds may well turn out to be so utterly hostile to Earth life that there is no chance it could survive there. It may well have no Venusian life in it either. But I'm not sure we can conclude this for certain yet. I think it is possible that a new study, taking account of these ideas, would change the provisional classification of the Venus atmosphere for both forward and backward contamination.

Need to study in situ first

For all these reasons, putting my own personal view here for discussion, I think the wisest approach in the case of the Venusian clouds is to study them "in situ" to start with. Perhaps we shouldn't bring unsterilized samples back to Earth quite yet.

We may get this in situ search soon. Some scientists working on designs for the next Russian mission to Venus, Venera D which hopefully will launch some time in the 2020s. Provisionally 2026. The original plan was for a balloon (as well as a lander and orbiter). They want to include ideas from the Northrup group VAMP project for an unmanned aerial vehicle for Venus. This would actually deploy outside of the Venus atmosphere and do a hypersonic entry. Because it is so large and light, it decelerates very high in the Venus atmosphere, and so does not need an aeroshell as it decelerates more slowly and the skin is not raised to a high temperature


It inflates before it enters the atmosphere (see Patent). Because it is so low in density (low ballistic coefficient), it decelerats slowly in the very thin upper atmosphere, so generating much less heat. So it doesn't need an aeroshell, though, its outer envelope is reinforced to withstand up to 1200 C along leading edges

They hope it can be used for Venus, and also Titan, possibly Mars.

Eventually we can send humans to the clouds. The HAVOC idea is to do this. Their airship expands after it enters the Venus atmosphere, but the rest of the design is very similar. This is a video showing how it would work.

Need to treat the Venus atmosphere as a category III destination for COSPAR

Apart from classification issues, I feel personally that we should sterilize spacecraft and instruments designed to study the cloud tops of Venus, until we know a bit more about it, as the classification is not certain enough that it might not change in light of future discoveries.

Okay this may add 10% to the cost of the mission (sterilizing Viking added an estimated 10% to the mission cost). That is a big increase when margins are tight, I understand. But that is well worth it to be totally sure that e.g. if you do detect apparent signs of life in the clouds, such as DNA or amino acids, that it comes from Venus and not your spaceship. Also to make sure you do not contaminate Venus samples or the clouds themselves with reproducing life, including the probably remote chances of some archaea with pH 0 acidophile capabilities getting transferred to Venus on our spacecraft, or some of the archaea able to share their DNA with Venusian organisms via GTA's., or Venusian XNA able to out-compete DNA. If we go into this with our eyes open, and debate all the possibilities, however extreme, we will be able to explore the solar system safely.

I think also that we shouldn't think about sending humans there until we've studied it a bit in situ to see if there is life there, remote though the possibility probably is. This should be regarded as an exciting possibility. In my own view again, then if there is life in the Venus clouds, especially interestingly different, or XNA based life, this is such a wonderful and interesting result for biology and science and evolution - and in the long run for humanity generally - that it far outweighs the disappointment that we need to postpone colonization of the cloud colonies for a later date

We should celebrate the discovery of other forms of life anywhere in the solar system. Also, if discovered, proper study of exoplanet life should take priority over colonization, in my view, if there are any conflicts of interest. What do you think?

Venus may eventually be one of the best places to send humans to in the solar system, at least in terms of habitability. You can argue a surprisingly good case for it. See also: Will we Build Colonies that Float Over Venus like Buckminster Fuller's "Cloud Nine"? But I'd say, let's explore it with sterile robots first, if there is any chance of finding life in the atmosphere, even if we think the chance seems rather remote.

Objective for humans to Mars

If we decide we shouldn't send humans to the Mars surface quite yet, there are many exciting missions we can do to the Mars system. You can explore Mars by telepresence from orbit, or from its moons Phobos and Deimos.

Telerobotics as a fast way for humans to explore Mars from orbit

Telerobotics lets us explore Mars much more quickly with humans in the loop. And you'd use an exciting and spectacular orbit for early stages of telerobotic exploration of Mars, following the HERRO plans. It comes in close to the poles of Mars, swings around over the sunny side in the equatorial regions and then out again close to the other pole, until Mars dwindles again into a small distant planet - and does this twice every day.

Imagine the view! From space Mars looks quite home-like, and the telerobotics will let you experience the Martian surface more directly than you could with spacecraft, actually touch and see things on the surface without the spacesuit in your way and with enhanced vision, blue sky also if you like. It's like being in the ISS, but orbiting another planet.

12th April 2011: International Space Station astronaut Cady Coleman takes pictures of the Earth from inside the cupola viewing window.- 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.

This is a video I did which simulates the orbit they would use - in orbiter. I use a futuristic spacecraft as that was the easiest way to do it. Apart from that, it is the same as the orbit suggested for HERRO.

It would be a spectacular orbit and a tremendously humanly interesting and exciting mission to explore Mars this way. The study for HERRO found that a single mission to explore Mars by telepresence from orbit would achieve more science return than three missions by the same number of crew to the surface - which of course would cost vastly more. Here is a powerpoint presentation from the HERRO team, with details of the comparison.

Then, you'd also have broadband streaming from Mars. As well as being very safe, also comfortable for the crew, you'd also have wide-field 3D binocular vision. It's amazing what a difference this makes, I recently tried out the HT Vive 3D recreation of Apollo 11. We'd have similar 3D virtual reality experience of the Mars surface. Also, it would actually be a much clearer vision than you'd have from the surface in spacesuits, digitally enhanced to make it easier to distinguish colours (without white balancing the Mars surface is an almost uniform reddish grayish brown to human eyes)|.

Here is this hololens vision again, which though it's not telepresence, I think gives a good idea of what it might be like for those operating rovers on Mars in real time from orbit, some time in the future with this vision.

It's safer too. No need to suit up. No risk from solar storms - at worst you have to go to a storm shelter in your spaceship, not rush back to your habitat as fast as you can to get out of the storm in time. No risk of falling over and damaging your spacesuit. And when you need to take a break, have your lunch, or whatever, you can just take it up again where you left off, indeed leave the robot doing some task while you have your lunch or sleep.

This is an artist's impression from ESA showing a telerobotically operated rover on Mars

The ESA are currently testing rovers controlled from orbit - which will eventually use binocular vision and haptic feedback. in a program called IMPACT. This is their artist's rendering of such a rover on Mars.Tim Peake's recent test of controlling another rover on Earth from the ISS was part of this program. Previously, Andreas Morgeson drove the rover shown in this rendering, on Earth from the ISS, using binocular vision and haptic feedback to remove and plug a pin into a small hole with a tolerance of 0.15 mm, less than a sixth of a millimeter. He did this successfully, video of the actual event here.

Telerobotic drilling versus human drilling

Drilling is especially important for the search for past life, as well as any present day life that may live deep underground in geological hot spots, or kilometers below the surface, or beneath ice sheets. However, in Mars conditions, robots can drill as easily as humans in spacesuits, and probably more so. The Apollo astronauts had a lot of trouble drilling by hand by just two meters, sometimes falling over. So humans have problems too.

If you want a deeper hole, you can't use water as a lubricant on Mars, so you can't use a conventional drilling rig. The best technology is likely to be the robotic self hammering mole, has potential to drill much deeper than either of those, and certainly to the ten meters depth needed to find organics not degraded by cosmic radiation and solar storms. These moles may eventually drill for tens and hundreds of meters, even for kilometers in Mars conditions at ten to twenty meters a day.

These moles don't need an astronaut to operate them. ExoMars will be able to drill down two meters, using a different approach. The Insight lander was going to drill three to five meters using a robotic mole. The regolith thickness varies, here is an estimate for the site of the Insight lander - they estimate that ~90% of the Insight landing region is covered by a regolith that is at least ~2 to 3 m thick. ... and that the regolith is 5 to 6 m thick over ~50% of the region. Honeybee robotic are working on a drill that will be able to drill through gypsum and hard ice to a depth of hundreds of meters. They say that their inchworm mole will be capable of drilling up to tens of kilometers through soil, ice and rock without need for a tether and then return to the surface.

Drilling is very slow at present - it takes a week on Curiosity. However that is with a huge amount of latency. The team send commands to Curiosity typically the next day after they get the images back. Also it's not really fair to compare that one week with the time it takes to actually drill out a sample on Earth because it also includes the time needed to take it back to the laboratory and analyse it, which is done in situ on Mars. So it's not as bad as it might seem, but it could be done a lot faster via telerobotics. For more on this see this presentation by Marc Boucher (slides and audio).

Mobility for humans and robots

Our rovers travel very slowly on Mars but that's mainly because of the turn around time of 24 hours between getting the images and sending the next day's instructions, and limited autonomy. The Apollo lunar roving vehicle had a rated top speed of 8 km / hour (though it could go faster), weighed 210 kg, for the entire vehicle, and had a range of 92 km, nearly twice the total distance Opportunity covered in ten years. So it's not lack of power that limits our rovers - they could go much faster, and further, if there was the need to design them to do so.

We can use the same methods to power robots on the surface as you can use for human driven rovers. So, for instance, we can use Zubrin's ideas to generate fuel in situ using hydrogen feedstock from Earth. Or we can use solar power and batteries, adapting the Mars One idea to spread a large area of thin film solar photovoltaics over the Mars surface for power. Perhaps we could use batteries with enough charge to last for a few hours at 8 km / hour, as for the lunar rover, and then when the charge runs out, then it leaves the battery to charge up, and uses power direct from its own solar panels until it is ready to do another tens of kilometers long journey again.

In this way, the methods designed for human missions to the surface can be used for our robots also, so that they can travel faster, and explore more in each day. The robots don't have to go back to a base, are not limited by the amount of oxygen and food they can take with them, can travel right up to habitable regions if sufficiently sterilized. Humans can start telerobotic control within seconds, in a shirt sleeve environment, "teleport" from one rover to another from orbit. And of course when not controlled by humans in orbit, they can be controlled by teams of scientists on Earth as before. So it's not at all clear even that the robots are less mobile than humans. Once the technology is well developed they might even be more mobile.

Imagining a telepresence mission in the HERRO molniya type orbit

Imagine yourself in orbit around Mars - in a Molniya orbit - comes round to the sunny side of Mars twice a Martian day - you go really close to the surface - and spend some time there controlling rovers on the surface - driving them around - with reality headsets like the Occulus Rift -

- the Mars astronauts in orbit could explore the surface with headsets like this - and haptic feedback gloves so you can feel what you are doing, and omni directional treadmills like the virtuix omni

and automatically enhanced vision with everything you see on Mars streamed back to Earth so everyone back here can join in and see what you see exactly as you see it whenever you explore the surface of Mars.
Then after a few hours of that you see that Mars is now getting further away, becomes smaller, and then 12 hours later you come in again for another close approach and real time exploring - you can continue to explore all the time - but when you are really close you can control things on the surface in real time as if you were there.

Small planes and entomopters etc

You could fly planes around on Mars, small planes, or entomopters - same design as a bumble bee. Lightweight, you could carry many of these along with the humans in a human mission to Mars orbit or the Mars moons to send on to the Mars surface.

Many other ideas like that - surely much more fun, to operate those from orbit around Mars, in a shirt sleeves environment than living a troglodyte existence on the surface under meters thick layers of soil, going out only rarely to keep down your lifetime radiation dosage - and knowing all the time that just by being there you have contaminated Mars and made it far harder for scientists to find out interesting things about biology and alternative forms of biology and the early history of evolution.

Also all this would be great for collaboration - probably need a big international expedition to send the humans out to orbit around Mars. But as well as that, anyone who can send a spacecraft to Mars (probably many countries by then) can send landers, for them to operate telerobotically. The more the better really. So it is something that all countries with interest in space could work on together, each contributing different things depending on their expertise.

Safe ways to get humans to Mars orbit or its moons to avoid any risk of crashes on the surface

You couldn't do aerocapture in the Mars atmosphere as a way to get into orbit. It would be far too risky. Also Hohmann transfer with insertion burns are too risky also, as the insertion burn is done as close to Mars as possible to reduce the amount of fuel needed due to the Oberth effect. So you would need to be very sure that the insertion burn can't go on too long and end up on an impact trajectory with Mars.

I suggest ballistic capture is a far better method for human missions to Mars. The idea is that you launch the spacecraft to arrive ahead of Mars at just the right point for it to capture you as a temporary satellite. Once you leave Earth, you are already on a trajectory that ends up with your spaceship getting captured temporarily in a distant Mars orbit when it gets there, with no need for an insertion burn. Then once you are in that orbit, you use ion thrusters to spiral down to lower permanent orbits around Mars.

This is surely the safest of all the ways proposed to get into a Mars orbit, and the best way to prevent a crash of a human occupied spaceship on Mars.

Then you also have the flybys. Flybys are safe because although they involve precision targeting, you have months to set the target up. Also, the ones that are of most interest for Mars are free return, so even if your rocket fails, you are still on an orbit that will take you back to Earth again. You would use trajectory biasing of course, so that as you leave Earth you are biased away from Mars rather than towards it and use fine adjustment then to target the flyby orbit.

We have done many flybys, delicate ones, repeatedly for Saturn's moons with Cassini, and get them right every time, so it is obviously one thing we know how to do reliably. This has no time critical insertion burns. Just gentle thrusts nudging until you are in the right trajectory, which you set up long in advance of the actual flyby.

Interesting flyby and orbital missions for Mars

Especially Robert Zubrin's double Athena flyby - a very interesting mission - is safe for humans to Mars. This has two flybys of Mars. The first diverts you into an orbit that closely parallels Mars for half of its year, so a full Earth year. The second flyby takes you back to Earth 700 days after the launch. It's free return - once you leave Earth you are already on a trajectory that will take you back to Earth 700 days later even if your rocket motors fail completely.

It's a great orbit for telerobotics as you spend several hours close enough to Mars for direct telepresence with each flyby, and days close enough for significant advantages relative to Earth, and over the entire one year period when you are almost paralleling Mars in its orbit, your crew are much closer to it for controlling robots on the surface than anyone on Earth. Advantages of this are

  • Least delta v of all the proposals, so also the cheapest
  • Astronauts are close enough to Mars for direct telepresence control for several hours for each fly by, still remain close for days after the first flyby and before the second fly by, and close enough to be a major asset over Earth based control for the entire year.
  • It's a "free return" mission - you don't need to do any extra delta v to get back to Earth. The second flyby, an Earth year later, takes you back to Earth even if you don't do anything.

Robert Zubrin's plan has several advantages over Denis Tito's Inspiration Mars, which is the other free return mission proposal.

  • You can do it more often, roughly every two years,
  • It doesn't have the high speed return of Inspiration Mars so less issues with aerobraking into the Earth atmosphere on return
  • You have far more time for near to Mars telerobotics - with Denis Tito's mission you just have the one, very fast flyby of Mars and then it is over.
  • But is a longer mission by 200 days.

I'm not sure that the Inspiration Mars 500 days is such an advantage over 700 days. I think if we can manage 500 days safely (surely not for a while yet) - then we can probably do 700 days also. (it is an extremely poor safety margin for Inspiration Mars if most astronauts are close to death at the end of 500 days). Also the double Athena can be done every two years, doesn't need any special alignment with Venus, and it has a much slower return trajectory relative to Earth - can just do normal aerobraking much like the return from the Moon, which Inspiration Mars can't do because it has a far faster return velocity delta v relative to Earth. Anyway its great advantage is that you are close enough to Mars for hours during each fly by - and in between you spend days really close, close enough to do things like drive rovers in real time and supervise experiments.

The HERRO mission suggested a near sun synchronous Molniya orbit, which is elongated, similar to a Mars capture orbit. This is the easiest orbit to get to in terms of delta v, needing a similar amount of rocket fuel to a mission to the Moon for the same payload. This has the advantages that

  • Less fuel to get there
  • Approaches the sunny side of Mars twice a day, each time visiting the opposite hemisphere,

    So the entire surface of Mars is available for close up direct telepresence control of the landers, in full sunlight, every single day, by just the one crew.
    See:  HERRO mission to mars using telerobotic surface exploration from orbit

Turns out, if you choose just the right orbit, it's ideal for studying Mars by telepresence. You use a slowly precessing sun synchronous Molniya type half sol orbit. This is a precessing orbit that automatically keeps your spacecraft approaching Mars on the sunny side twice a day, all through the Martian year.

This shows how you get into this orbit - just directly from the Earth-Mars transfer orbit.

With this orbit, you have several hours of close up telepresence every 12 hours over opposite sides of Mars each time, also always on the sunny side of Mars. The delta v is the same, to all intents and purposes as a Mars surface using aerobraking. But without the dangerous descent to Mars and without the expense of developing human rated landing equipment -I think pretty clear it would cost less than a surface mission.

All of these missions have the disadvantage that you have to take all your shielding for the entire mission with you. You'd need "storm shelters" for solar storms also. This would be worked out with the lunar precursor missions to L2 etc - I don't think we should do a mission like this without experience of similar missions close to Earth. And any Mars mission needs to spend around a year combined (there and back) fully exposed to solar storms and the cosmic radiation anyway.

Its moons are actually, rather interesting places for a human to visit. In some ways they are more exciting than the surface, as we'll see, especially when combined with telerobotic exploration of Mars. Also, they are far easier places to land on safely and return. And there is almost no risk of introducing Earth life to Mars accidentally, so long as you get there without use of aerobraking. I think it may be some time myself before we can send humans even this far safely. There are many more challenges than there are for a lunar landing. The task of physically getting to Mars orbit is perhaps the least of many issues. But when we do get there, then these could be really exciting places to visit,

So which of the two moons should we visit? The most detailed proposal is by Lockheed Martin in their "Red Rocks project"  as part of their "Stepping Stones to Mars" program. And in their comparison study, Deimos beats Phobos on almost every score - though Phobos does have a couple of major advantages. 

Advantages of Deimos

So - the advantages of Deimos first

  • It has a South pole almost permanently shadowed - amongst the coldest places in the inner solar system. This also is a place where you are completely sheltered from the dangerous radiation in solar flares (which emanate from the direction of the sun). And it has natural cooling for any rocket fuel that needs to be kept at cryogenic temperatures (it is also ideal for infra red observation).
  • The permanently shadowed craters are probably a good place to study (just as for the Moon's craters of eternal night), with an ice record of the early solar system and materials from early Mars. It may also be a source of ice for colonists.
  • Close to these places of permanent shadow, you have a site of continuous summer sunlight to place your solar panels.
  • You also have permanent line of sight communication with Earth 24/7 for nearly all the year except when Mars is behind the sun.
  • Deimos is almost synchronous with the surface of Mars - and tidally locked - so you can have continuous line of sight access of Mars for telerobotic communication.
  • Any site on the surface of Mars is accessible for 60 hours at a time from Deimos. By comparison, from Phobos, any site is only accessible for four hours at a time.
  • A typical site on Mars is visible from Deimos for 45% of the time, and 97.5% of Mars is directly accessible via telepresence at some point or another.
  • The required delta v to get there is somewhat less than for Phobos.
  • Surface materials can be used for resource utilization and to cover the habitat to protect from cosmic radiation.
  • It's an interesting place to study in its own right.

Advantages of Phobos

There are two major advantages of Phobos,

  • because it is closer to Mars, it has a two way time delay for telerobotic exploration of only 40 ms, while Deimos has a two way time delay of 134 ms.
  • It has a crater, Stickney crater, on the Mars facing side. A party in Stickney crater would be protected from about 90% of cosmic and solar radiation, possibly more, at least according to this article (if they got it right). That's because it is protected by the crater walls, Phobos, and Mars itself which is overhead, and for solar storms it would be out of the direct line of the storms for most of the time. It is one of the places in the inner solar system most protected from cosmic radiation, even more so than the poles of the Moon. Only the lunar caves and Martian caves are more protected from cosmic radiation - or the high Venus cloud decks (and of course, Earth itself).

Stickney crater on Phobos. This large crater faces towards Mars. A base sited here would be protected from solar storms, and also from cosmic radiation. It's blocked by Mars overhead, Phobos below and the crater rim to all sides, and so gets only 10% of the cosmic radiation of an unprotected base. 

Phobos also probably has a high percentage of material from Mars in its regolith, and larger meteorites surely as well. So you can study the geology of Mars by looking at fragments of rocks on the surface of Phobos. Possibly the biology of Mars also.

Phobos was the target for the Planetary Society workshop in 2015. They base it around the SLS (Space Launch System). We send cargo to the Mars system in advance before any humans get there:

  • Two spacecraft are waiting in Mars, with chemical propulsion. One to deliver the crew to Phobos and back, and the other, to return the crew to Earth.
  • A habitat waiting for them on Phobos.

These are put in place using two SLS launches and then solar electric propulsion once in the Mars system. Solar electric propulsion is slow but it means they can deliver the cargo there outside of the normal launch windows to Mars (using ballistic transfer ideas).

Then two more SLS launches put all the equipment the human crew need for the journey to Mars into Earth orbit. Then crew then launch from Earth in a two and a half year mission which includes a 300 day stay on Phobos. When they get to Mars orbit they dock with the Phobos transfer stage which is already there and leave their deep space habitation module and dock with the Phobos habitat for their mission. They use the same transfer stage to get back again. Details on page 19 following of their report

Mining Phobos and Deimos

Deimos is also an asset for mining - it's not known for sure yet, but is similar in composition to meteorites that can have a large ice content. So it may have large deposits of water ice. If so that's of great value for fuel - and the delta v budget is such that it's actually one of the easier places to mine ice to return to LEO. As well as use in Mars orbit.

David Kuck in 1997 suggested starting up a Deimos Water Company to supply Earth orbit with water from Deimos. The Kuck mosquitoes are small unmanned craft that drill into Deimos and extract water from below the surface, use part of it as fuel to transport the rest back to Earth.

See also Mining Phobos and Deimos

This makes it a project that could be commercially viable, unlike Mars surface, through sale of ice to LEO.

Lack of gravity on the Martian moons

There is one major issue of course - the gravity, almost none. Can humans stay healthy with such low levels of gravity? Or - can we create artificial gravity on the surface (spinning habs)?

But perhaps this can be done with either a carousel type approach or with centrifugal sleeping quarters for the crew. I think this all depends on what turns out to be the requirement for artificial gravity to keep a human body healthy, and on what our tolerances are for spinning motions in AG environments - which we don't know anything about as yet. See also my sections in Case for Moon First:

Free flying habitats

Artificial gravity is probably an easy matter to arrange in a free flying settlement in orbit. You can use a tether system to generate gravity for the earlier settlements. You can use materials from the Martian moons for shielding. Long term, you could use materials from the Martian moons eventually to build Stanford Torus type settlements in Mars orbit. Deimos has a mass of 1.48 * 10^12 metric tons, which at 15 metric tons per square meter is enough to make Stanford Tori with about 100,000 square kilometers of living area.

So, if you were to mine the whole of Deimos - not suggesting we do - but if we do - that's cosmic radiation shielding for roughly the size of Iceland, larger than Scotland, or Norway, more than twice the size of Switzerland, which could be useful for Mars orbital colonies. In terms of US states, that's about the size of Oregon or Colorado That's just the outermost hull which you can build on top of - and larger habitats might well have multiple "shells" within it - so that's a lower bound on the total land are you could create from Deimos using it for cosmic radiation shielding.

Anyway that's obviously for some way down the road, if we ever do that.

Planetary protection issues for Phobos and Deimos

When Russia proposed a sample return from Phobos, then it was classified as an"unrestricted category V" mission, meaning that no special precautions are needed. Despite that, Russia did plan to take some precautions on return of the sample, though not required for planetary protection according to the classification their mission received.

The Russian Fobos-Grunt spacecraft, which was designed to return a sample from Phobos to Earth. The spacecraft never got there, it failed to separate and fell back to Earth and was burnt up. This sample return mission got an "unrestricted category V" classification for planetary protection on the basis that Phobos couldn't be habitable for Earth life - but the Russians did plan to take some precautions even so.

There is no chance of present day life surviving on Phobos or Deimos to the best of our knowledge, no chance of habitability on the surface or below the surface. These moons are just too small to have any chance of liquid water even below the surface, so it's thought. But - there could be a very minute chance of dormant life, because Phobos receives material sent into orbit from Mars after meteorite impacts. Any life at least on the surface of Phobos would be sterilized over long time periods such as millions of years. But the most radioresistant life on Earth can resist the equivalent of four hundred thousand years of Mars surface cosmic radiation, although not evolved in the presence of ionizing radiation (as a byproduct of heat and desiccation resistance probably). For more on this: UV&Cosmic Radiation On Mars - Why They Aren't Lethal For The "Swimming Pools For Bacteria" Life on Mars, evolved in presence of cosmic radiation could conceivably resist that much and possibly more. And Stickney crater gets just a tenth of those levels of radiation, so dormant spores there there could survive ten times longer, and more so if somewhat buried.

So given that material is sent into orbit from Mars every one or two million years, with the last known impact large enough to send material as far as Earth a little over 700,000 years ago, there would seem to be a small chance of dormant life on Phobos - that is if life from Mars can be transferred on meteorites at all, which we don't know yet. But theoretically it would be possible. I think myself that this suggests it would be worth while doing robotic in situ exploration of Phobos and Deimos first before we send humans there. And if signs of life are found there, dormant life, to re-evaluate the situation and take appropriate precautions depending on what is found.

In the forward direction also - then though to best of our knowledge, there is no chance of life from Earth surviving on Phobos or Deimos, there could be regions there of especial interest for the study of organics in the early solar system and dormant life. For instance what if there are ice deposits at the South pole of Deimos - including samples of organics from early Mars itself? Then we would need to keep humans well away from these until we have a chance to study them in situ and work out what effect a human base would have on them.

Could a human occupied base there be kept so clean that there is minimal impact on the study of ancient organics in the ice surrounding the base? If not, what can we do? Deimos is rather tiny, only 12.4 km in diameter. After a year or two, then the entire region around the base would probably be covered in footprints unless you take special precautions to minimize the impact of humans on the Moon. And what about refuse piles, human wastes etc? If Deimos is interesting in its pristine state - well that's something we need to think through before we send humans there. Anyway, by then we probably have experience already of bases on the Moon so may be able to use that to assess what impact a human base has on the area immediately surrounding a human habitat.

More ideas for these early orbital or flyby missions

With of HERRO, and indeed for the other missions also, you could send supplies to Mars in advance in separate duplicate spaceships before the human mission gets there. Most of the cost of an innovative mission is in the design, so it often adds little to the costs, percentage wise, to make several duplicates of the spaceship.

So, you have a habitat there already, in orbit around Mars,and with all the systems functioning including life support. Preferably, have two such ships filled with extra supplies, before you send the first humans there.

They would be fully fueled lifeboat ships able to get the crew back to Earth, or for them to survive in if systems in the main ship fail. You can also use them as extra living space at Mars during the mission, and as long term assets in Mars orbit.

Since these lifeboat ships don't need crew or provisions for the journey out - they could be filled with extra supplies, fuel and spare parts instead. These supplies could then be transferred to the main ship and used as extra shielding for the stay at Mars. In the worst case you can cannibalize the other ships themselves, for repairs, or if the main ship fails, transfer the mission to another ship.

And - if we were looking forwards towards such an expedition - all rovers to the surface of Mars could be fitted with binocular vision and hands with haptic feedback by default. Anyone who sent a spacecraft to Mars would be sure to set it up so that it can be controlled easily by telepresence whenever there are astronauts in close orbit around Mars.

Suppose we had a lead time, say of a decade in the run up to the first human missions to Mars orbit (during which we have human missions to L2 etc). Then by the time humans get there, we'd have a decade worth of Mars rovers and landers, all equipped to be controlled via telepresence, ready for use when the first human missions get to Mars orbit.

Later orbital missions could mine Deimos for materials, using the likes of the Kuck mosquitoes - dedicated small spacecraft to shuttle materials back and forth from the moons to the settlements. If there is ice in Deimos; you could use this as rocket fuel to export this extra shielding to the habitat,

Artificial real time

But there is another thing we can do - and that's to do autonomous exploration from Earth, using "artificial real time" which lets you drive a rover around on Mars even with a huge time delay of minutes. At the moment the way we control our rovers on Mars is hugely time inefficient. We could as easily control rovers on Pluto, because they download the data for one day, and use that to direct the rover's operations for the next day.

There's no point in trying to speed that up though, because it is hard to get a data link from Mars to Earth. Once a day is about all we can manage easily, because our orbiters have their own work to do.

If we have a dedicated link though between Mars and Earth, satellites in orbit around Mars just to relay signals to Earth - our rovers could be hugely speeded up.

And - in a situation like that, we could also speed them up so much that using this idea of "artificial real time" from computer games, we could control them almost as easily as a rover on the Moon (say).

Telerobotics with humans in orbit compared to robots controlled from earth

That's not to say that humans to orbit controlling robots on the surface would be better than robots controlled from Earth, bearing in mind the costs of the two types of mission. I don't know if anyone has done a comparison study there.

You might be able to compensate for the advantage of humans in orbit by having many more robots on the surface for the same cost, especially if broadband communication is possible, better robotic autonomy, and techniques from gaming such as artificial real time (building up a copy of the Mars surface explored by your robot in your computer on Earth and navigating that to help speed up movement from a to b on Mars).

But a human expedition might well capture the public imagination and so permit a much faster exploration of Mars from orbit. And would be an exciting and fun expedition to follow, and interesting for the crew too.

As a later mission you could then go on to explore Phobos and Deimos. They have many advantages for exploration. For instance Phobos has meteorites and micrometeorites throughout its surface layer of regolith, from the entire history of Mars, back to when Phobos first formed or was captured. This probably includes meteorites from the time when Mars had global oceans and then later on, lakes. Our Mars meteorites on Earth all left Mars no more than twenty million years ago (because the terrestrial planets clear their orbits so NEOs have to be replenished over a twenty million year time period).

Deimos also has a Mars facing crater which helps protect it from cosmic radiation, and solar storms - Mars obscures it from the sun in its local daytime, except for a few hours a day. Deimos may well have ice too, as it is related to a type of asteroid that often does have ice in its constitution.

There are many other advantages and points of interest of Mars' two moons.

For more on this, see my: 

Exploring Mars By Telepresence From Orbit Or Phobos And Deimos

So, how soon can we do such a mission? I suggested that while we explore the Moon robotically, we work on closed systems research, and also artificial gravity in LEO. That makes sense for a Moon base which you plan to keep occupied for years on end. But what about a first flyby of Mars? When could we try that?

Need for new comparison studies of the various ways of exploring Mars

The HERRO comparison was just a small scale study, done several years ago. But I don't know of any other. It's surely high time that we had a much more thorough and detailed comparison study of the various possible ways of exploring Mars. We may get practical experience of telerobotics in space with lunar missions in the near future. When that happens I think we'll find that machines are far more capable than they were in the days of lunakhod, operated from Earth most of the time, semi-autonomous, route finding on their own, able to do many things just by themselves with occasional help from Earth.

In a situation like that - operated remotely from Earth, or semi-autonomous, doing a lot of their own driving from place to place and then the crew in orbit around Mars step in to control robots that need particular help. I think that it would be much more than a 3 to 1 ratio compared with them working directly on the surface in spacesuits. Also, everything they saw would be streamed back to Earth in HD meaning that after an astronaut has just walked past a place and maybe glanced at a rock via telerobotics, amateurs and experts back on Earth can explore that footage with the same direct telepresence, binocular vision etc. experience, and maybe alert them to something they missed.

Also, yes humans are great at "on the spot" decision making. But they are also able to make very rapid bad decisions, or uninformed decisions.

If the mission is a scientific one, then you need on the spot scientific expertise. To really take advantage of the low latency on Mars, you need astrobiologists in orbit or on the surface, and astrogeologists. They are the ones with the expertise that will let them make fast on the spot decisions. An astronaut without that expertise is still dependent, much like a rover, on instructions from their team of experts back on Earth. That's the same for both surface and telepresence missions from orbit. But the orbital missions do have the advantage that just through their nature they require everything the astronaut sees to be streamed back to Earth. So if the "on the spot" mission specialists miss something, it's not a big deal because some expert back on Earth will spot it instead. The orbital missions also are safer missions to send the astrobiologists and astrogeologists to, once we get to the point where we can send missions safely as far as Mars. You don't have to ask an expert in astrobiology to take the huge risks of a landing on Mars and exploring the surface with spacesuits. If the mission is extremely risky as a surface mission is likely to be, that then restricts it to those who are willing to take huge risks with their lives, such as test pilots and those who like taking part in risky activities. But if it is reasonably safe then you can send scientist who wish to spend a couple of years of their life on the mission.

I think a proper comparison study has to take all of this into account. Enthusiasts for one or the other approach are bound to be biased somewhat to what they consider to be the best method to explore Mars. So,. I think a proper comparison study is probably best done by neutral parties or best perhaps, a workshop / panel that includes proponents of both sides in the debate as well as neutral parties. The cost of such a panel or workshop would be peanuts compared to the costs of the missions that we might commit to in the future for the exploration of Mars.

Compared with Mars surface missions

First of all, whatever the cost, I don't think that COSPAR should pass a humans to the Mars surface mission for planetary protection reasons.

Artist's impression of a human astronaut on the Mars surface holding Oskar Pernefeldt's proposed International Flag of the Earth - the linked rings symbolize how the different parts of Earth are linked together. (This is the latest of several proposed "Flags of the Earth"). 

Before a mission like that could be approved, a COSPAR workshop would need to show that it is consistent with planetary protection requirements, and would not risk introducing Earth life to Mars surface habitats.

Either that or there would need to be international agreement that Mars no longer needs to be protected from Earth microbes. To my mind, seems unlikely that either could happen before the 2020s or 2030s. As for the idea of a compromise based on humans contaminating only part of Mars, I find it hard to see how that could be approved by COSPAR either. How could the experts in the COSPAR panels sign their name to a statement that they know could lead to Earth life being irreversibly introduced to Mars? I don't really get it, how that could happen.

Meanwhile we could use telerobots to plant flags on Mars if that is the main aim of the mission or to touch Mars. Or if humans touching somewhere else other than Earth and the Moon are considered vital to this mission, we can plant flags on Phobos or Deimos and touch those moons instead.

In more detail there - the Outer Space Treaty is the only treaty we have to prevent siting weapons of mass destruction in orbit, or nations laying military claim to the Moon, etc - it's the main reason that we are able to do peaceful co-operative exploration of space. As well as the outcry from space scientists, the international upheavals resulting from something like this would be enormous. There is no way that the US or NASA could do this.

So, it's the same for planetary protection provisions based on the Outer Space Treaty. They are like quarantine laws; it doesn't matter how you get into space, you are still bound by them as a citizen of your country, which in turn is a signatory of the OST. The US has agreed to make sure that any US citizen or anyone using US hardware will keep to the provisions of the Outer Space Treaty. and the same applies to any other signatory of the OST which includes just about all nations either space faring or with space faring ambitions. The United Arabic Emirates hasn't yet ratified the OST but they will still keep to the provisions.

Cost savings compared with surface missions

It's interesting to notice that these orbital missions would cost less than a surface mission. Especially HERRO and the double Athena which Robert Zubrin proposed as a lower cost precursor mission. In a comparison made for HERRO then the orbital mission does three times as much science for less cost. Here is a powerpoint presentation from the HERRO team, with details of the comparison. That's just one study, but it surely needs to be followed up with more detailed studies to check it. Also with the stimulus from 3D virtual reality computer games, the technology for telepresence has moved on hugely since then, so a new study would probably find it is even more of an advantage.

The reason the orbital missions can do so much more in the same time period compared with a surface mission is that

  • You can control rovers anywhere on the surface of Mars, so can explore multiple sites at once.
  • There is no need to suit up and travel to the area of interest, you can do it all with shirt sleeves environment within the spacecraft.
  • When you take account of the reduced mobility of a human in a spacesuit, with clumsy pressurized gloves, then there's no great advantage of humans over telerobotically controlled rovers on the surface as far as mobility is concerned. Spacesuit technology of course will advance, but so also does telerobotic technology.

Then as well as that, there is no need at all to develop technology to land a human mission on the surface of Mars. That's not just a matter of delta v. You can land on Phobos or Deimos with a gentle use of delta v over a long period of time, and right up to the last minute, as for the Moon, if anything is wrong with your trajectory, you just abort and move away from the moon a bit, figure out what went wrong and try again, with hardly any waste of delta v due to the low gravity of these moons.

With a landing on Mars surface, everything has to go exactly right during the "eight minutes of terror" of the Curiosity landing. There's also almost no chance of humans intervening to save the mission if something goes wrong, as everything happens so quickly.

So - that's a whole new technology needed for a Mars surface landing that isn't needed at all for a Mars moon landing. And major human safety issues with a Mars surface landing that again are not issues at all for a Mars moon landing. You can study the Mars surface in a Molniya style Mars capture orbit, which requires less delta v than a surface mission. Even if it weren't for the planetary protection issues then telerobotic missions would seem to be the way to go for more science return and indeed a more immersive way to explore Mars than a surface mission.

Telepresence would be immersive, not using primitive mouse and keyboard control, and need to be compared to humans in a pressurized spacesuit

First the robots controlled by humans would have some autonomy of course our rovers already have. For instance they may be able to drive autonomously or avoid collisions. On the spot decisions would be done by the humans. I don't think it is at all established that humans would have a significant advantage in a spacesuit over telepresence. It depends how you do the telepresence. Also, it's hard to simulate spacesuits on Earth - they need to be pressurized to the extent that the gloves are stiff and it requires significant effort to close your hands or move your fingers, for instance. Described as like wearing a garden hose over your fingers.

Also, if you navigate by using mouse clicks and keyboard presses and a computer screen it is very different from doing it with binocular vision, an omnidirectional treadmill, and haptic feedback. There are many more programmers working on computer games than on space systems and I think a way forward here will probably involve a fair bit of use of software developed for computer games. We've had several experiments in telepresence from orbit operating robots on the ground and each one is more advanced than the previous one - most recent one was a rover controlled from orbit by Tim Peake. But if we had all out effort to get this working then we could do much better.

The habitats themselves are almost the same in space or on the surface. The Mars atmosphere is not enough to make a difference. We have no idea what the effect is of Mars gravity - you can't just draw a line between zero g and full g. It could even be worse than zero g or better than full g and you can't join them with a straight line based on two data points. But even if Mars gravity is better for human health than full g, not impossible, we could generate it in orbit using artificial gravity. Yes you could have mobile habitats on the surface in a pressurized rover, but that is just adding to the complexity and remember those habitats still can't go anywhere near spots on Mars that are potentially habitable, again the question is, why all that extra difficulty, danger and expense to get those habitats to the surface when they could be in orbit, if the actual search for life has to be done remotely to avoid contaminating the habitats they search for and study?.

Summary of advantages of telerobotic exploration

Of course this needs a proper comparison study but there are many advantages to consider

  • Best solution for planetary protection. It is hard to see how you could send humans to the surface of Mars without a risk of a hard landing which would contaminate a random area of Mars with all the hundreds of trillions of microbes in tens of thousands of species that accompany humans. If you introduce Earth life to Mars there is a major risk that you will detect life on Mars only to find that you brought it there yourself.
  • Costs far less for more science return - one mission does as much as three to the surface, no need to develop the technology for humans to get to surface, or return from surface to orbit, no need to design for the Mars dust, and to keep the perchlorates out of the habitat.
  • Safer . The landing on Mars is the most risky landing almost anywhere in the inner solar system.
  • Crew don't endanger their lives in spacesuit EVA's. Crew at all times remain in shirt sleeves environment in the orbiting spacecraft. All they can endanger is the "avatar" rover they control on the surface. And that, if damaged, can be repaired potentially. While if e.g. you damage your air supply to a spacesuit you die.
  • A rover can spend days, weeks, even months just at one spot on Mars using only electricity from sunlight while a human explorer has to return to base for provisions, oxygen etc frequently. For safety reasons a human astronaut would need to be close enough to a habitat or pressurized rover at any time to get back there on foot before their oxygen runs out. Even then they risk dying as a result of a sprained ankle or similar impairing their mobility.
  • A rover doesn't have to put on a spacesuit every day, which takes up an hour or two of every day.
  • Easily given capabilities beyond any human - easily given enhanced vision, can be made any desired size, can be given super-human powers such as great strength or dexterity or the ability to fly in the thin Mars atmosphere, because they can be made far lighter than humans.
  • On the surface crew are exposed to the Mars dust, and perchlorates - they can use the suitport to keep them out of the habitat but this is another thing you have to design for which you don't need to think about in space
  • Surface habitat and equipment has to be designed to cope with huge fluctuations of temperature. The Mars surface at night also often gets so cold that the carbon dioxide freezes out at night as frosts even for 100 days of the year in equatorial regions and the habitat has to cope with huge fluctuations of external temperature between day and night.
  • Solar power is reliable in space - on the surface then every two years there's a possibility of Mars dust storms blocking out the sun, turning night to day
  • Crew can explore several parts of Mars simultaneously, and "teleport" instantly from one experiment to another - leave one rover doing routine analysis while they drive another, or direct sampling for another - so the crew do all the interesting stuff and the rovers do all the dull stuff by themselves.
  • Mars from orbit looks quite Earth like, an interesting planet and the elongated HERRO Molniya orbit is especially stunning with close flybys of the spectacular landscape and the polar caps every twelve hours, with the landscape skimming past below your spaceship followed by a long fly out so far that Mars becomes quite small. Every day you have that experience, twice, and each time coming in over a slightly different part of Mars. On the surface you'd be stuck in a single spot from then on and probably not see that much, and in the dust storms, nothing at all.
  • When you drive the rovers on the surface with telepresence and haptic feedback, and virtual reality goggles to see the Mars landscape in 3D - you'd experience the surface vividly, far more so than if you were really there. Our eyes are not adapted to the Mars light and everything would seem dim and reddish brown, with colours hard to discern and a dull butterscotch sky. Exploring via avatars we can colour adjust automatically to resemble Earth lighting conditions, indeed with a blue sky if you like.
  • Whatever you see is digital streamed (haptic feedback also), can be recorded and streamed back to Earth. Anyone on Earth can experience it as you did, and examine the images to see if we spot anything you missed.
  • If anything goes wrong on the surface, you have everything recorded and streamed, so we can figure out what happened. There is no possibility of an accident where someone falls and dies and nobody is sure why it happened.

The main thing they lack is dexterity but this is rapidly improving with advancing technology. They are used for telerobotic surgery and deep drilling and many applications. You couldn't do surgery while wearing a spacesuit which I think helps highlight how telepresence could give you more precision of control and accuracy than hands on study of Mars in a spacesuit.

NASA organized a recent telerobotics symposium in 2012 and the conclusion of the conference was that telerobotics has great potential value for Mars exploration. The conference recommended their use during early orbital missions to Mars by humans, saying that a great opportunity would be missed if telerobotics was not used. But - why not just use telerobots from now on until we are sure of what we want to do on Mars?

Artist's impression of telerobotic exploration of Mars for the 2012 Exploration Telerobotics Symposium

Humans on the surface could start a new geological era on Mars - if they introduce Earth life irreversibly.

The eras so far, as we saw, are the earliest Noachian period of high meteorite bombardment and seas, the Hesperian period of volcanic activity and huge floods, the Amazonian period, as it is now, dry with some liquid water, a bit volcanic activity and occasional floods. On Earth geological epochs are named according to the prominent biology on the planet. So why not on Mars?

So, if we introduce Earth life irreversibly to Mars, we'd need a new name, maybe call it the Anthropocene again, as on Earth - of a Mars with Earth life on it, introduced by humans.

From then on for all future time, our civilization and all future civilizations on Earth would never have the opportunity to study the Amazonian period, with whatever unique lifeforms it might have. As we've seen in this book, it is well possible that Earth life could make Amazonian period Mars life extinct. After all, according to most theories of the origins of life, DNA life made its precursors on Earth, whatever they were, extinct. And later forms of DNA life made many earlier forms extinct. So it's certainly possible for one form of life to make another extinct over an entire planet.

Idea that we should exploit the small precious opportunity before humans land on Mars but shouldn't delay human landings

NASA's planetary protection office say that their job is to work out if the planet can be protected in the case of a successful human landing. So, they don't consider crashes of human occupied ships in their assessments. That is for NASA to look at, at a later stage. In their list of knowledge gaps for human extraterrestrial missions, they cover such things as leaks of microbes from spacesuits in EVA, and transport of microbes in the dust storms. But there is no mention at all of the effects of a crash of a human occupied spaceship anywhere in the list. They have to assume a 100% success rate for humans landing on Mars as without that assumption they would not be able to recommend any measures that could protect Mars from Earth life, even temporarily. Also, they no longer aim for biologically reversible exploration of Mars in the case of a human landing.

Their approach is that we will have a small precious window to find out as much as we can about Mars before humans introduce Earth life there by landing on the surface. Emily Lakdawalla, planetary geologist who often reports for the Planetary Society, expresses a similar sentiment in this article

"NASA recognizes that the potential for contamination is a problem, so there is a Planetary Protection Office that is specifically charged with overseeing how missions avoid contaminating Mars with Earth biota. There are two main approaches. One approach is to sterilize the heck out of anything that will actually be touching Mars. That's why Curiosity's wheels were specially wrapped throughout its final assembly, and why it was such a scandal that the drill bits were handled after sterilization. The other approach is to avoid landing in any location where you might encounter -- or accidentally create, should you crash -- a present-day habitable environment where Earth microbes could thrive. For instance, current rules prohibit NASA from targeting a mission containing a hot radioisotope thermoelectric generator (such as Mars 2020) anywhere near a place where a failed landing might place that generator close enough to subsurface ice that the heat of the decaying plutonium could melt it.

"But all bets are off once you send humans to Mars. There is absolutely no way to make a human clean of microbes. We are filthy with microbes, thousands and thousands of different species. We continuously shed them through every pore, every orifice, with every exhalation, and from every surface. True, almost all of our microscopic friends would fail to thrive in the radiation-baked, intensely cold and arid Martian environment. But life is incredibly tenacious. Sooner or later, humans will get to Mars; even if they die in the attempt, some of their microbial passengers will survive even the worst crash. Once we've put humans on the surface, alive or dead, it becomes much, much harder to identify native Martian life.

"This is one of many reasons I'm glad that The Planetary Society is advocating an orbit-first approach to human exploration. If we keep our filthy meatbag bodies in space and tele-operate sterile robots on the surface, we'll avoid irreversible contamination of Mars -- and obfuscation of the answer to the question of whether we're alone in the solar system -- for a little while longer. Maybe just long enough for robots to taste Martian water or discover Martian life."

The Planetary Society organized a workshop Humans Orbiting Mars in 2015 to explore the idea of exploring Mars from orbit first. Their report proposes a stepping stones approach to Mars, with missions to the Moon and asteroids, followed by missions to the moons of Mars and then boots on Mars towards the end of the 2030s.

Stepping stones approach proposed by the Planetary Society. See page 18 of their summary. Lockheed Martin proposed a similar approach previously.

Cassie Conley, NASA's planetary protection officer was David Livingston's guest on the SpaceShow in March 2016, and gave a very interesting talk about planetary protection. I posted some questions to her via email, so you can listen to her answers to them and get NASA's perspective on these issues in more detail. You can listen to her talk here.

Cassie Conley has also said she thinks Elon Musks' ideas have planetary protection issues, in an interview just before his big announcement here: Cassie Conley. Going to Mars Could Mess Up the Hunt for Alien Life There are no detailed guidelines yet for humans to Mars. These would be made by the international COSPAR committee which meets every two years, and all of their discussions to date have ended without any firm recommendations, saying that more information is needed.

Prestige or dishonour, first footsteps on Mars

Yes it would be an exciting day when the first human steps on the surface of Mars, and a matter of prestige for those concerned. This is something I have read about since a child, and would be as excited as anyone. But it might quickly change to dishonour if it is found that they introduced reproducing Earth micro-organisms to Mars. They might enter the history books as the people who contaminated Mars irreversibly. Why not let those first steps be taken by a telerobot instead, operated by a human in orbit around Mars?

Inspiration value of telerobots on Mars

Advocates for Mars surface colonization often talk about the inspiration of a human surface mission. But we have seen from the rover missions what fondness the general public have in their hearts for our robotic emissaries on Mars such as Pathfinder, Spirit, Opportunity and now Curiosity. Also look at how much interest and excitement there was for the Dawn mission to Ceres and the New Horizons mission to Pluto.

I am sure this would apply even more so to telerobots on the Mars surface operated by humans in orbit around Mars. For many I think that telerobots on Mars would be more exciting and interesting than a surface mission, especially if the reason for it is also well understood.

Biologically reversible exploration of Mars

The astrogeophysicist Christopher McKay has talked about the need for all exploration of Mars to be biologically reversible in the sense that, if necessary, we can remove all the microbes we brought to Mars, at least until we have a better understanding of Mars. He has also suggested that if we find a second genesis of life on Mars, biologically unique and different from Earth life, we might want to adapt Mars, to make it more habitable for native Martian life even if the result does not make it into a planet suitable for Earth life.

The idea is that if we find interesting life on Mars, we can remove all our contamination from the planet and leave it for the Martians instead - so that we can study the biology there, like having an exoplanet on our own doorstep. Maybe even get to restore the early Mars climate. Here is the same idea in his paper from 2007.

"Perhaps the most interesting and challenging case is that in which Mars has, or had, life and this life represents a distinct and second genesis. The discovery of a second genesis of life has profound scientific, as well as philosophical and ethical importance. Philosophically, the discovery would directly address the question of life in the universe, and would strongly support the idea that life is a naturally emergent phenomenon and is widespread and diverse in the universe. Scientifically, having another 11 example of life expands the scope of biology from one to two. There may well be significant advances in medicine, agriculture, pest control, and many other fields of biological inquiry, from having a second type of life to study. I would argue that if there is a second genesis of life on Mars, its enormous potential for practical benefit to humans in terms of knowledge should motivate us to preserve it and to enhance conditions for its growth. Observations of Mars show that currently there is no global biosphere on that planet and if life is present it is in isolated refugia or dormant. It is possible that life present on Mars today is at risk of extinction if we do not alter the Martian environment so as to enhance its global habitability.

"The utilitarian arguments presented above indicate that we should alter Mars to allow any indigenous life to expand and form a global biosphere even if the resulting biosphere is never a natural home for life from Earth or humans. If there is no indigenous life, these utilitarian arguments indicate that we should alter Mars to support life from Earth even if this never results in a biosphere that can be a natural home for humans.

"...This discussion has implications for near-term exploration of Mars by robots and humans. Until we know the nature life on Mars and its relationship – if any – to life on Earth, we must explore Mars in a way that keeps our options open with respect to future life. I have argued elsewhere that this means that we must explore Mars in a way that is biologically reversible. Exploration is biologically reversible if it is possible and practical to remove all life forms carried to Mars by that exploration. Because of the high UV and oxidizing conditions on Mars, biological reversibility is achievable.

"... Previous missions to Mars, such as the Pathfinder mission and the two MER rovers, have carried microorganisms to the martian surface where they remain dormant as long as shielded from ultraviolet radiation. To reverse this contamination already present on Mars, it would be necessary to collect all metal objects within which microbes could remain viable. Furthermore, the soil at crash sites and in the vicinity of landers that had come into contact with the spacecraft would have to be thrown up into the atmosphere where it would be exposed to sterilizing ultraviolet radiation. A similar approach can be used to reverse the contamination from human bases."

Quote from his Planetary Ecosynthesis on Mars: Restoration Ecology and Environmental Ethics

This was written some time back, in 2007, before the discovery of the many potential surface habitats we now know about, and before discovery of microbes capable of withstanding hours of the UV radiation on Mars. As you see he thought that it would be possible to reverse the effects of biological contamination after a human landing on Mars. He no longer thinks this is possible. It may still be possible for the robotic missions however.

Could we can send humans to the Mars surface in a biologically reversible way?

The only way I can see to do this in the near future, that could be given some reasonable guarantee of planetary protection, would be to use a metal sphere - that enters the Mars atmosphere at a shallow angle, so slows down to terminal velocity before hitting, with a human being inside. Then even if all systems fail, it would hit the surface at at most a few hundred miles per hour - in that case the human would not survive, but perhaps a sphere could be guaranteed to remain intact, or at least, not breached, after a crash on the surface?

Hollow spheres (rocket parts) that re-enter the much thicker Earth atmosphere survive intact to the surface. This hydrazine propellant tank from a rocket:

survives to the surface looking like this

(Space ball found in Namibia)

If all goes well, the human would land on Mars and then could be lifted off again - but of course can never leave the sphere or even look outside directly (as it is opaque), so I'm not sure if this would be thought worth doing. Another suggestion is to send humans to Olympus Mons.

Olympus Mons Caldera Region This might be the area on Mars most biologically isolated from the rest of Mars of anywhere, due to the thin air, high altitude, and the caldera walls. But it is a difficult place to land technically, and also - would even this be a biologically reversible place for a human base on Mars?

The idea is that it is so high above the surface that the air is very thin and there is almost no dust. This was suggested as a planetary protection measure in an article in The Space Review. It's a major challenge to do this with present day technology though, see Rob Manning's talk on the Space Show. In the space settlement article, they are suggesting a future with new technology with human bases already on Deimos. Some of the technologies Rob Manning mentions could be relevant such as deployable extending heat shields, or using larger parachutes than any of the ones tested supersonically to date. In any case, there's the same problem as with other human landings - we are unlikely to have 100% reliable landing systems. Even with your target the caldera at the summit of Olympus Mons, a failure during approach to Mars, entry, descent or landing could easily land you somewhere else. And would a human party - say inside the Olympus Mons caldera - really be biologically insulated from the rest of Mars? Also after a crash there? And would such a landing be biologically reversible in the future, if we need to remove the spores from Mars?

Yes, if there is anywhere on Mars, where humans can land in a biologically reversible way - or at least in a way that keeps the landing site separate from the rest of Mars, with only one area is contaminated - then Olympus Mons might be it. I think though, it would take a lot of research to be sure of this. If not biologically reversible, you have the possibility of an "oops" moment where you realize you have introduced Earth life to Mars, can't remove it, and have found a lifeform there you want to preserve or a biology such as ancient RNA based life, and can't do anything to prevent its eventual extinction.

Plants on Pristine Mars

You can put plant seeds on the surface, as these can be sterilized. You can grow plants with hydroponics and aeroponics. The difference here is that with hydroponics the plants grow directly in water, and in Aeroponics you use a mist, so it needs less water (which may be useful on Mars). There are different versions of these technologies. Some depend on micro-organisms but some do not. Instead of having micro-organisms you supply whatever the plant needs in chemical form in either water, or if it is aeroponics, in the mist.

The only life you have is the plant seed. You may use the Mars soil, or just use mist, depending on what you want to do. This microbe free version of hydroponics / aeroponics  introduces no risk of contaminating Mars so long as you do it with great care. The only thing  that will grow on Mars as a result of this experiment are these plants that you introduced to Mars. Here is a rather charming designer's concept for plants on Mars, called the "Little Prince Rover".

"Little Prince" rover designed to support a single plant on Mars. Book cover of "The little Prince" by Antoine de Saint-ExupérySince seeds can be sterilized (unlike humans or animals), these could be grown without any risk of contaminating Mars with Earth micro-organisms.

Named after the "Little Prince" who looked after a single rose on his asteroid in the fictional book by Antoine de Saint-Exupéry

It's possible that plants may be the first living Earth colonists of another planet.

So that introduces the possibility that you could have greenhouses on the surface of Mars and these could grow food for the colonists in orbit. They may have plenty to eat in their habitats anyway by then, but you could grow food on Mars too, maybe delicacies or things that grow particularly well on Mars or medicinal plants or whatever. Also you could have large plants, maybe trees (perhaps growing far larger in the Mars light gravity) or whatever else grows best on the surface of Mars.

Sadly, we can't apply these same principles to humans. If only we were like plants and could be grown from sterilized seeds. Well in principle, in the future, it could be possible by adapting the far future idea of "Embryo space colonization". If you could grow a human from an embryo in a sterile environment, and somehow supply them with all the nutrients they need without the symbiotic microbes we all have. Or else, engineer the microbes to make sure they can't survive on Mars. That is a far future science fiction idea however at present. And supposing it were ever possible, the Martian colonists would be vulnerable to Earth microbes if they ever left the planet, not being adapted to them, and anyone who left would not be able to return.

Perhaps there may be other future ways to do it. E.g. an impervious spacesuit or rover that can't be damaged even in a crash on Mars, that is also a biobarrier so that no microbes can get in or out and with the outside 100% sterile. This is way beyond anything we have at present however.

Biologically reversible exploration gives us a breathing space and leaves our future options open

The idea of biologically reversible exploration is to give us some breathing space, of a few decades, hopefully, to find out about Mars on a scientific level. To find out if there are habitats there for Earth life and search for exobiology. Meanwhile, you are also building up an infrastructure on Mars and in Mars orbit that would be useful if we did ever decide to send humans to Mars. Or indeed, it could be useful for other things too, anything we might do on Mars.

Perhaps you decide to try ecopoesis (duplicate the biological transformations of early Earth on much faster timescales), or you follow Chris McKay's proposal to turn the clock back to early Mars, or transform it in some other way, or even grow plants there (plants could be grown on Mars using sterile hydroponics without impacting on any native Mars life, since seeds can be sterilized). There would be many possible futures still open to you at that point.

Also meanwhile we can work on space habitats, closed systems, eventually build city domes on the Moon and large closed systems in the lunar caves, continue to explore ideas for creating larger and larger self sustaining habitats. Whether we eventually get to the point of terraforming entire planets, I think can be left to later, until we have much more understanding than we have now, with these early experiments. So, then it becomes an open path, where instead of closing off futures, we open out to more and more possible futures, and wider vistas at every step. These vistas don't just include Mars either but many destinations for humans in the solar system.

What if the decision is to keep Mars biologically pristine for ever?

What if we find independently originated life on Mars, or amazingly interesting evidence of early stages that almost reached life but not quite? Should we leave the planet pristine to avoid contaminating it? That would be an exciting prospect seems to me. It's like having our own exoplanet, with its distinct biology, in our own solar system. The nearest terrestrial planet like that, other than Mars, may be light years away. Depending on future technological progress, it might be centuries before we have a similar opportunity - or if life is rare in our galaxy, maybe even millions of years.

I would say why not? Let's go all the way to Mars, and set up colonies in orbit around the planet, but never set foot on it at all, to avoid contaminating it. It is a bit like mountains that are left unclimbed out of respect for the mountain or local beliefs. Not too many of those but the mountains in Bhutan over 6000m are unclimbed.

 shea-tang-la), Bhutan
This is possibly the highest unclimbed mountain in the world Gangkhar Puensum with an elevation of 7,570 m. All mountaineering is prohibited in Bhutan since 2004 out of respect for local religious beliefs.

This would be a future where you have agile rovers on the surface. and increasingly sophisticated humanoid avatars on the surface as well, directed and teleoperated by colonists in orbital colonies. It is a future where the Martian past and present turn out to be amazingly interesting, so much so, that humans never land on the surface in person, in their physical bodies, to preserve a biologically pristine Mars. I, for one, would find that an inspiring future to live in.

Collective sense organs for humankind on Mars

The idea that humans on the surface of Mars would contaminate it with Earth life is not much mentioned in the news. Out of dozens of news stories about ideas for human missions to Mars, perhaps only one or two will ever even mention it even as a topic for discussion. But it's frequently mentioned in the academic literature on spaceflight, with many publications debating the issue, and several planetary protection workshops on human missions to Mars. It's just that their deliberations rarely get into the news.

Generally those discussions are focused on the idea that we need to minimize the impact of humans on Mars. It is much rarer to suggest that it is a serious and significant issue which could be a reason to delay sending humans to the surface of Mars, or as a reason to go somewhere else first such as the Moon or the moons of Mars first. I think myself that it is just not good enough to do our best to exploit the brief window of opportunity of a few years before the first human landings or colonization attempts. We simply shouldn't risk destroying such a precious opportunity to make scientific discoveries, on the basis of ignorance, and there is no way we can do it on a basis of knowledge if the humans land on Mars in the 2020s or the 2030s. It is rare to suggest that, depending on what we find, and our decisions, we have a possible future where humans never land on Mars at all but explore it via telepresence instead.

In 1964, George Simpson wrote:

"There is even increasing recognition of a new science of extraterrestrial life, sometimes called exobiology-a curious development in view of the fact that this "science" has yet to demonstrate that its subject matter exists!"

That was before the Apollo landings. But though we have learnt so much by way of astrogeology, and have made great strides in understanding many things in the field of astrobiology, we are yet to shake off that major central issue. Andrea Rinalid in 2007 writes about astrobiology:

"Despite growing attention, the field is still haunted by the curse that evolutionary biologist George Gaylord Simpson voiced more than 40 years ago: “[T]his ‘science' has yet to demonstrate that its subject matter exists!” 

Now, 52 years after George Simpson wrote his paper, this could be our first opportunity to show that the subject matter of astrobiology exists. It could also possibly be the most major discovery in biology of the 21st century. How can we pass up on such an amazing opportunity and potential "super positive outcome" and ignore the potential issues of Earth microbes introduced to Mars?

Perhaps what we need is a change in the public's perception of how we explore space? Here is a quote from "When Biospheres Collide":

"One of the most reliable ways to reduce the risk of forward contamination during visits to extraterrestrial bodies is to make those visits only with robotic spacecraft. Sending a person to Mars would be, for some observers, more exciting. But in the view of much of the space science community, robotic missions are the way to accomplish the maximum amount of scientific inquiry since valuable fuel and shipboard power do not have to be expended in transporting and operating the equipment to keep a human crew alive and healthy. And very important to planetary protection goals, robotic craft can be thoroughly sterilized, while humans cannot. Such a difference can be critical in protecting sensitive targets, such as the special regions of Mars, from forward contamination.

Perhaps a change in the public's perspective as to just what today's robotic missions really are would be helpful in deciding what types of missions are important to implement. In the opinion of Terence Johnson, who has played a major role in many of NASA's robotic missions, including serving as the project scientist for the Galileo mission and the planned Europa Orbiter mission, the term "robotic exploration" misses the point. NASA is actually conducting human exploration on these projects.  The mission crews that sit in the control panel at JPL, "as well as everyone else who can log on to the Internet" can observe in near real-time what is going on. The spacecraft instruments, in other words, are becoming more like collective sense organs for humankind. Thus, according to Johnson, when NASA conducts it's so-called robotic missions, people all around the world are really "all standing on the bridge of Starship Enterprise". The question must thus be asked, when, if ever, is it necessary for the good of humankind to send people rather than increasingly sophisticated robots to explore other worlds"

See When Biospheres Collide

What do you think?

My Moon First books

This book has focused on the question of what happens if humans touch Mars and I have only briefly touched on what humans do if they don't land on Mars. But I go into this in a great deal of detail in my other books. I cover things such as the astronaut gardener on the Moon, the possibly vast kilometers wide lunar caves, ice at the poles with sunlight 24/7 nearly year round, and the value of the Moon for Earth amongst many other things. I see the Moon as a gateway and natural starting point for exploration of the solar system. That exploration can be open ended and lead eventually to human outposts throughout the solar system, in a collaboration where robots go to places where robots do things better, and humans play their part in ways where humans do it better, and the robots, as in "Where Biospheres Collide" are the mobile collective sense organs of humankind.

I've also been guest on David Livingston's The Space Show several times to talk about these ideas as well as the planetary protection issues for humans exploring Mars. See the list of my guest appearances on the show here.

If you are interested to learn more about this, please see my books:

"MOON FIRST Why Humans on Mars Right Now Are Bad for Science", available on kindle, and also to read for free online.

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

Facebook group

I've made a new facebook group which you can join to discuss this and other visions for human exploration with planetary protection and biological reversibility as core principles. Case for Moon for Humans - Open Ended with Planetary Protection at its Core

See also

Robert Walker's posts - on Quora

And on Science20

Robert Walker's posts on Science20

Kindle bookshelf - for my author's page

And I have many other booklets on my kindle bookshelf

My kindle books author's page on amazon

Change log

6th January