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.
The original painters touched the caves. Many of us would love touch them also, feel the texture of the rock that they were painted over. But not only is nobody permitted to touch them - we have to take care even about going into the caves at all. The warmth, humidity and carbon dioxide from the breath of visitors have all taken their toll. Fungi and black mold are attacking the paintings.
The purple markings in this photograph show damage to the paintings resulting from human presence
The Lascaux cave was first found in the 1940s by four children with their dog, and opened to the public immediately after WWII by the owners who enlarged the entrance, added steps and replaced the cave floor sediment with concrete. The humidity, carbon dioxide and warmth of all the visitors took their toll leading to microbes, fungus and black mold growing. Even though the cave has been closed to all except occasional specialists, it is too late now to restore it completely to its original condition.
Attempts to fix the many issues lead to one more misstep after another. For instance, after a white fungus spread over the floor and up the walls, the 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 for the photographs were themselves damaging the cave, encouraging the growth of black mold, which is now a major issue there with black spots spreading over the cave. 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 environomental 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 has to be to find an equilibrium which incorporates the new species of microbes introduced to the cave by human visitors.
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, always push beyond frontiers, whatever they are".
You ask 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 can't be stopped. 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 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 to the Lascaux cave 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. However much you might want to visit the lake Vostok in Antarctica, kilometers below the surface of the ice, you can't 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.
So, could we harm Mars as much as we did with 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 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" in the Ethics of Space Exploration.
Whatever ones views on that, our present reason for protecting planets from Earth life is a much more practical one. We do it to protect the science value of other planets. 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 later, but first, let's look at how microbes from Earth could confuse the search for past life on Mars.
In 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.
Image by Pat Rawlings, courtesy of NASA "20/20 Vision," illustrates search for life on Mars
After all that is how fossils of earlier lifeforms were first found on Earth. Here is a drawing of Mary Anning - the Victorian fossil hunter who is described in the popuolar tongue twister
"She sells sea shells on the sea shore"
She used to dig up fossils of ammonites and belemnites and sell them in her fossil shop at Lyme Regis.
And indeed, if we found something like this, the search would probably be over :):
Sketch of Mary Anning by De la Beche, gathering fossils. Her hammer is made of wood clad in iron. It's displayed in Lyme Regisís Philpot Museum. Details from page 78 of this World Heritage assessment of the Dorset and East Devon fossil beds.
Cast of Plesiosaurus fossil collected by Mary Anning, photo by FunkMunk
However the fossils which are large enough for us to see on Earth mostly date back to the last 500 million years, out of over four billion years of evolution. Mars 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. There are three main periods of Mars geology:
Did life ever evolve on Mars? We don't know. If it did, was it ever abundant? It's quite possible that it could evolve yet never be abundant, for instance if it only evolved near hydrothermal vents and never developed photosynthesis or any other way to spread any further. Did it ever develop to macroscopic life?
What if Mars life was abundant enough to develop thick deposits of oil rich shales? Or the equivalent of chalk which is made up entirely of shells? Could it have deposits consisting of meters thick remnants of ancient life in some form or another?
Mars may have been habitable in the early solar system for hundreds of millions of years in relatively stable conditions. So, if evolution got off to a rapid start, and evolved very rapidly, 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. There are plenty of craters but that would only unearth them if the deposits are very abundant.
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 if Mars had birds, and fish, 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.
The other problem is that we don't know what to look for on Mars. If we found a fossil archaeopterix 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. So, what if we find these?
These are now known to be early stromatolites. 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.
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.
Baryte Rose from Cleveland County, Oklahoma, photograph by Rob Lavinsky
"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.Ē
Oliver Morton, author of Mapping Mars: Science, Imagination, and the Birth of a World
This criterion is named after Andrew Knoll, author of ďLife 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 hase 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.
So, if there is life on Mars, how will we find it and recognize it? If we can't expect to identify it conclusively by recognizing fossils, 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.
So, past life on Mars is likely to be identified through organic biosignatures initially (the same is also true for present day life as we'll see). Once recognized that way then we may be able to identify them as fossils too, but it's unlikely that we recognize them first through their macrostructures. The enthusiasts who want to send humans to Mars tend to brush this off, the question of how we identify past or present organics from life on Mars, 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."
"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."
Zubrin 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 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, those are just three of numerous possibilities and indeed perhaps rather unlikely ones at that. So remarkable that you'd want to keep Earth life away from Mars while you study the remarkable phenomenon.
I go into this in a lot of detail in my Moon First books. But let's look at them briefly here.
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.
If you ask the astrobiologists, many of them will tell you that 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, and it may not be possible to cultivate it. Past life may have been destroyed long ago except in a few favoured patches which may have only a few trace amounts of organics.
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.
Many astrobiologist think that ALH84001 is a much more likely model for what we may find in the search for life on Mars than clear fossils, or shale oil deposits.
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).
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 chlorophyl 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 chlorophyl 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
All present day life on Earth uses RNA and DNA and it all uses the same complex translation method to convert DNA to RNA and then the RNA to proteins and many other biochemical pathways are identical. Modern life all depends on ribosomes made up of a mixture of RNA and protein, as catalysts. The main reason modern cells can't be any smaller than around the optical resolution limit of 200 nm is because the ribosomes are so huge and because they have to be able to translate DNA to RNA constantly and RNA to proteins, which all adds to the complexity of the cell and so to the minimum amount you have to have in a cell to make it function..
Early life just couldn't have started like this as the whole thing is far too complex to form spontaneously. It probably didn't have DNA. It may have had only RNA (or some other biopolymer). It may have used the far smaller ribozymes (which are made up of fragments of RNA) for the catalysis. Based on those ideas they suggest that early life could have had cells as small as 50 nm across. It may not have needed proteins at all. It may have consisted largely of RNA in different forms - the so called "RNA world hypothesis".
This leads to the idea of a shadow biosphere on Earth. This idea was quite popular a while back. It got tied with nanobes, structures that visually resemble 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 our tests for life 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. 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 accomodate ribosomes (Benner 1999). The shapes in meteorite ALH84001 just might be fossil organisms from a Martian "RNA world".
If we find early life, precursors to Earth life, then it can't possibly work in the same way. Transfer the genes for carotene 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?
We can't make a living RNA world cell. There is no way we could make modern DNA based life either, if we didn't have it already. We can tinker with it, even add an extra base pair, but the simplest living cell is way way beyond anything we could make from scratch from inorganic chemistry, if we didn't have it already. 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. There is another way also to see that we must be missing a huge amount of knowledge about early life.
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.
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.
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, like the African savannah with its ants, grasses, mice, trees, elephants, gazelles, lions and so on.
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.
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 Austrailian 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. 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.
WRITTEN UP TO HERE - REST IS FRAGMENTS TO JOIN TOGETHER
But if that is something waiting for us to discover there, most astrobiologicsts 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
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.
If our samples from Mars have any terrestrial contamination it may make it equally hard to tell if they originally contained life or not. But they are also likely to be contaminated by non life Martian organics as well. This hugely complicates the situation..
Some of the organics made through natural processes on Mras 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.
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. In that context - and supposing you aren't bothered about what happens to present day life on Mars in the long term so long as you are able to discover it before it goes extinct - 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.
Tthat basically is the plan for Mars exploration that NASA is following. They have given up on the idea that humans could explore Mars in a biologically reversible way. Instead, the idea is to contain the contamination as much as possible so that the humans are able to study Mars before it is irreversibly contaminated by the life they bring there inevitably.But the work of astrobiologists suggests it is likely to be far more difficult to find past life than this suggests. 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 they select to return to Earth are almost certainly going to come from those as well. Because 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 removewhatever organics are there already.
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.
GRA95229 - another chrondite, collected in Antarctica, had chiral excesses of +31.6Č for a-AIB to +50.5Č for isovaline, while the EET92042 meteorite ranged from +31.8Č for glycine to +49.9Č 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.
FOR A CLEAR SIGNAL OF PAST LIFE
So, for a clear signal, for past life, we have to look for life in the right place (e.g. hydrothermal vents, or salt lake deposits or the warm seasonal flows or whatever turns out to be best). And then your sample needs to be:
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 opportunities to look for this life on Mars. Surely somewhere on the surface of Mars we will find the ideal conditions leading to preservation of past life, and optimal conditions for present day life.
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.
The earliest lifeforms, if we can find them are also likely to be smaller than modern cells, of the orders of tens of nanometers rather than the hundreds of nanometers of modern cells. It's impossible that the modern cell in all its complexity arose in one go, That would make it an order of magnitude smaller than the smallest known cells on Earth and well beyond optical resolution.
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.
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:
The 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.
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. 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 thatís 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 it during the landing, except possibly in the last few minutes, it is just totally dependent on whatever the atmospheric conditions are as you land. The Mars atmosphere is very thin, 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.
Thereís also the uncertainty of the speed and position of the spacecraft as it enters the atmosphere, thereís always going to be some error there too.
So unlike a lunar landing where 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. Or you take a risk that if you hit a boulder, thatís 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. Thatís why Curiosity had to drive for so long before it got to Mount Sharp. It wasnít 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. Thatís what Elon Musk plans to do for SpaceX.
Conceptually it is about as simple as you can get. The rocket doesnít have an aeroshell or parachute or anything. It just decelerates.
Early artistís 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. And then 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 whatís 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 Earthís 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 - 14:00This is a summary of my Why Spacecraft Crash on Mars.
Every two years Mars has its dust storm seasons
OTHER THINGS TO COVER: