What sort of things might we hope to discover on Mars? This is another "synthesis" section. What might we find on Mars? Surprisingly, I can find hardly anything by way of detailed speculation about this - as usual do say if you know of a good paper to cite here. So I've based this section on various ideas for early life and alternative forms of biochemistry, and especially on any ideas that seem particularly relevant to Mars. I've also added a couple of speculative ideas of my own, clearly labeled (the idea of life based on replicating sheets instead of replicating strands is one of them).
Here are a few ideas.
Early life or protolife, to fill in the huge gap between the organics and cell-like structures that turn up in laboratory experiments, and the immense complexity of modern life. We might find the much simpler RNA world cell with no proteins, or ribosomes, or DNA. Instead of the huge ribosome it could use RNA sliced into pieces and recombined to make a ribozyme, that tinier distant cousin of the ribosome. For more on this see RNA world and the shadow biosphere (above). Or, we might find the so called autopoetic cells that replicate just by producing daughter cells with a similar mix of chemicals when they get too large. They might have no genetic code yet to regulate the process, or they might be able to regulate it, but only achieve very approximate, imperfect reproduction, leading to questions about whether they count as life or not. For many other ideas of what we could find, see What else could have come before modern life? Alphabet soup of "XNA", Ostwald ripening organic crystals, "naked genes", or almost alive "autopoetic" cells (above)
Evidence that life originated on Mars. DNA and RNA are both particularly fragile, and DNA especially is rather hard to form naturally. It needs the environment of the cell or special conditions to keep it stable. RNA is not quite so fragile and is more stable when it is very cold. Also, the ribose in its backbone is stabilized by the presence of special chemicals, borates. As it happens, these are found on Mars in useful quantities.
Steve Benner presented an interesting theory in 2013. He thinks that the borates prevented early organics from turning into tars, instead forming carbohydrates. He thinks that molybdenum in the form of molybdate played a role too, as it helps catalyse the formation of ribose, the "R" in "RNA". Both of these are common on Earth now, but being soluble in water, would have dissolved in our early oceans. Early Earth may have been an almost entirely ocean world, with far less dry land than today. Meanwhile Mars being drier could have created better conditions for this process and so might be favoured for the origins of life.
"Mars is the most Earth-like of our
neighbouring planets and enjoyed a
number of advantages during the early
history of the solar system. Though a
freeze-dried desert today, Mars was warm
and wet before about 3.6 billion years
ago
. Being a smaller planet, it cooled
more quickly, making it suitable for life
sooner than Earth. Gene sequencing
indicates that the oldest and deepest
branches of the tree of life are occupied
by hyperthermophilic archaea and
bacteria , hinting that the earliest life
forms dwelt deep beneath the oceans
near volcanic vents, or even kilometres
underground in the crust itself. "
"The deep subsurface zone remains
populated on Earth today, and probably
offers the most promising location on
Mars for finding any extant life. It would
have become cool enough for
hyperthermophilic microbial life on Mars
perhaps as long ago as 4.5 billion years,
when the Earth’s crust was still sizzling. Ensconced in this Hadean niche, shielded by a kilometre of two or rock, Martian life could have withstood the ferocious early bombardment that afflicted Mars just as it did Earth."
He has an interesting calculation based on this. He assumes that Mars was favourable for the emergence of life before Earth and had a longer window of opportunity - as he suggests there. He also assumes that life was an unlikely event (if life starts very easily, then it would develop quickly on both Earth and Mars, so is likely to have made a fresh second genesis on Earth). Finally he asssumes that the length of time needed for life to transfer from Mars to Earth in the early solar system via meteorite transfer after a large impact is a million years, and the window of opportunity during which both Mars and Earth had conditions ideal for life, so that transfer was easy, was a hundred million years.
Based on that, he finds that Mars is hugely favoured over Earth as an origin for life. This is especially so if the origin of life required a succession of several different improbable steps to happen. It is also especially likely to originate on Mars if early life is fragile and goes extinct easily. The calculation is in Does Life’s Rapid Appearance Imply a Martian Origin?
Techy note:
He works out that pM/pE = 100.8nm(τE/τM)mn
pM is the robability of life originating on Mars, pM/
pE is the probability of it originating on Earth,
n
is the number of successive improbable steps needed for life to evolve,
m is the number of genesis attempts before a robust form of life is evolved
τM is the average length of time you'd expect it to take for each of those improbable steps of evolution necessary for life in early Mars surface conditions.
τE is the average length of time you'd expect it to take for each of those improbable steps of evolution necessary for life in early Earth surface conditions.
100.8 = 6.3 is his estimate for how much longer life had available to evolve on Mars than on Earth. For this, he assumes that life was present on Earth 100 million years after it became habitable, and that, by then, Mars had been habitable for around 600 million years.
For those who want to read his calculation in detail, note, that for the deduction from equation (3) to (4) he is using a mathematical trick that some readers may not be familiar with. By the McLauren series expansion of the exponential function - you can express ex = 1 + x + x/2! + ... If x is very small (positive or negative, then this simplifies to 1 + x.
He calculates that the chance of an interstellar origin is very small. However that part of his calculation doesn't take into account of the possibility of life around another star seeding another star in our sun's stellar birth cloud, see Distant cousins with last common ancestor from a planet around another star (above)
Unrelated or very distantly related life, perhaps based on some form of XNA (Xeno Nucleic Acid) instead of DNA. This would be one of the most amazing discoveries we could make. It would lift biology into a new dimension, by showing that life can exist based on completely different biochemical principles from DNA based life.
It's a reasonable hypothesis, because DNA is very fragile indeed, and RNA is fragile also. Perhaps what that's telling us is that life started as one of the many alternatives to DNA? Some think we may have started with a PNA world for instance, as it is far more robust than RNA and forms more easily.
Perhaps we find one of these forms of life on Mars and assume that it is unrelated life at first - but later we find it is an ancient predecessor to Earth life that still remains on Mars barely changed since those early times. Or perhaps it has no connection to Earth at all and we discover that life originated separately on Mars. Or perhaps Earth life originated as an early form of life on Mars, yes, but it hasn't just stayed "as is" but has evolved and elaborated hugely in some different direction on Mars, into a different modern form of life, just as complex as our DNA and mRNA based life, but organized on different principles altogether. Perhaps life on Earth and Mars both began as PNA, for instance, with a common ancestor but on Mars it evolved in a different direction, for instance a more elaborate form of PNA based life, or to something else. Maybe we are such distant cousins that the Mars life is not DNA or RNA based at all.
Life with mirror helixes, triple helixes quadruple helixes, or side by side molecules that don't wind round each other. The first idea here is life based on an exact mirror of DNA. To get that to work, all the molecules in the cells would be reflected as if the whole thing was just reflected in a mirror. As far as we know aa mirror image cell like that would function perfectly well in its mirror image world. That's mirror image life, a trope in countless science fiction stories - but it is one of the more sensible ones, as the idea is perfectly feasible for extra terrestrial life, as far as we know.
The idea that we could flip someone or even a single cell into their mirror image is pure science fiction at present, and way beyond anything we know about. But idea of life that evolves independently using mirror image chemistry seems to be eminently sensible and perfectly possible as far as we know.
However there is also a form of ordinary DNA here on Earth that spirals in the opposite direction in a rather zig-zaggy way. It's called "Z DNA. It is the right most of these three models.
A, B and Z DNA - image by Richard Wheeler . The one in the middle, B type is "normal DNA". The A type, on the left, is wider, with a shallower pitch on the outside, and occurs in dehydrated samples but may also occur in cells in pairings of RNA with DNA. The Z DNA, on the right, spirals in the opposite direction from ordinary DNA in a rather zig-zaggy way - it does this when it is methylated (has the methyl group CH3 added) and can be stabilized in this form by using special Z-DNA-binding proteins and may play a role in transcription.
Could life be based on Z DNA?
Then what about a helical structure with three strands instead of two? Well, Earth life does that as well in some situations
Before the double helix structure of DNA was discovered, triple DNA was one of the hypotheses they were looking for. They found it was difficult to get it to fit the theory and observations. But now we do have an actual form of triple DNA that we know is stable. Could ET life use a triple helix for some reason?
Or could ET life even use a quadruple helix? Earth life uses those sometimes, it sometimes, forms four strand helixes, a recent discovery!
If life on Earth can use DNA in those different forms in special situations - is it possible that extra terrestrial life could use something like one of these as its "normal" form of DNA?
Or what about Rodley's "Side by side DNA"? This is just a hypothesis. There is no evidence at all that Earth life ever uses it, but if it doesn't exist here, perhaps ET life could use it? It spirals one way for a while, then changes direction and spirals the other way then back again in such a way that the two strands aren't actually twisted around each other. This could help them to separate more easily, which may perhaps be an advantage for replication.
Life that is based on completely novel principles, for instance replicating sheets instead of replicating strands. Are we being blinkered in some way because all we know of is life with double helix DNA and single helix RNA (along with a few variants like the triple helix DNA mentioned above)?
Suppose it is much more radically different than that? To help stimulate a few ideas here, suppose for instance that ET life uses a sheet-like two dimensional structure, planar rather than linear, and replication happens by a second layer forming on top of the original sheet? This is one of my own "fun speculations" to stimulate the imagination and get you thinking a bit "out of the box" perhaps.
It's formula is (Na, Ca)0.33 (Al, Mg)2 (Si4O10) (OH)2·nH2O. There the comma in (Na,Ca) means sodium or calcium can substitute for each other - they are not shown in this diagram but are attached to the outside layers of oxygen - the layers have a net negative charge which is balanced by the positively charged sodium or calcium ions attached to them. Similarly the (Al, Mg) shows that either Aluminium, or Magnesium can substitute for each other in the Alumina layer. Other atoms not shown in this formula can also substitute in the Alumina layer, such as iron and phosphorus.
The layers are separated by water molecules and when you add a lot of water they can slip relative to each other which is what makes clay slippy.
Anyway the idea is that defects, variations in the choices of ions, and irregularities in one layer somehow propagate through from the layers below to the weakly bonded layers on top of them. Some might be better at replicating than others, breed more true by this stacking method - and the ions in the layers could speed up catalysis of various reactions including polymerization of RNA.
The main problem with this model is that nobody has ever got anything like this to work in the laboratory - breeding true from one layer to another in a consistent way and doing something useful as a result. So, you just have to wave your hands and say "perhaps after millions of years of evolution, Nature will find a way". So though it does seem very promising as an idea, with a lot about it that is in common with the way that DNA works, it is hard to test it as we have nothing that works like this at present. Sadly, we can't leave oceans to evolve for millions of years to see what happens.
Graham Cairns-Smith's idea is that it would be a very early stage of life, with no cell walls, just these replicating sheets of clay, which would transition to RNA and DNA chemistry and eventually lead to us. As just said, we don't have good evidence for it at all.
But as a purely speculative idea, for fun, just to stimulate ideas, what if we run with it? What if ET life similarly started off as essentially two dimensional sheets of clay just a few atoms thick, separated by water only loosely bound together and they replicated in such a way that later forming layers duplicated the structure of the layer below? And then suppose they went a step further than Graham Cairns-Smith suggested, and incorporated those sheets inside their cells and used them to replicate? Eventually, could they perhaps build up a whole complex biochemistry, as complex as our helix based life, but based on using 2D sheets for replication instead of linear or helical strands? As with the "side by side DNA" it would have the advantage that the 2D biopolymer doesn't have to uncoil to replicate - and what's more, it could hold a high density of information in a small piece of material.
Techy aside for sci. fi. geeks, this is not quite the same as Greg Egan's fun but very speculative science fiction story "Wang's Carpets" based on the mathematical idea of "wang tiles" - in his short story the lifeforms themselves are entirely two dimensional, evolving patterns of tiles within larger two dimensional sheets, exploiting the complexity of the patterns you can make with a few simple shapes. These can form in a "Wang carpets", with patterns so complex to define that questions about them can be mathematically undecidable. Here though, it's just a 2D sheet of "instructions" standing in for the one dimensional sheet of instructions we know as a strand of DNA, within a larger 3D microbe otherwise similar to Earth life.
Or could the genetic instructions even be preserved somehow in a 3D informational polymer? Is there any approach that avoids the need to uncoil to read it? We can do this mechanically through laser scanning, in prototypes for future memory devices, so the idea is not so far fetched as to be totally impossible.
This is just fun speculation at present. But suppose that you are an ET biologist and your life uses 2D sheets to replicate or even, 3D informational polymers scanned in some way without altering its structure. Would you not find the idea of a helical structure that has to uncoil and unzip to replicate implausible and unlikely too?
Could anything like that be possible? What do you think?
Life that has evolved further than Earth life. Mars has had such very harsh conditions in the early solar system, alternating between a frozen "snowball Earth" type ice phase and more habitable phases. It's also been subject to strong ionizing radiation, extremes of cold, and near vacuum atmosphere. Some think that we have multicellular life on Earth as a result of a snowball Earth phase. If that's true, you could make a case for Mars life to be more highly evolved than Earth life - more complex, with more robust cells, extra non redundant nucleotides, additional amino acids, anything that it can exploit to add to the complexity and make it more capable than Earth life, maybe even with totally novel capabilities never explored here.
Present day Mars probably only has microbes, or perhaps lichens, at least if it is fair to make a comparison with similarly harsh environments on Earth. Microbes and lichens seem to be better adapted to such conditions than complex multicellular life. But the harsh environment may mean it evolved further on Mars than on Earth. Microbes yes, but microbes with the equivalent of several billion years of evolution advantage over us. Could it be vastly more sophisticated than Earth life, even if it is just a single cell lifeform?
The harsh conditions on early Mars could mean it didn't get as far and is an early form of life. It's hard to say in advance which way this would go
Life with a capability Earth life doesn't have at all, e.g. a new form of photosynthesis
Normal photosynthesis which splits water to make oxygen, also taking up carbon dioxide in the process. (basic equation 6CO2 + 12 H2O → C6H12O6 + 6O2 + 6 H2O where the oxygen atoms in bold are the same ones on both sides of the equation - see Plants don't convert CO2into O2, and Notes on lamission.edu)
The photosynthesis of the haloarchaea which works similarly to the receptors at the back of our eyes, based on a "proton pump" which moves hydrogen ions across a membrane out of the cell using bacteriorhodopsin similar to the rhodopsin in our eyes, with no byproducts such as sulfur or oxygen, just creates energy directly from the proton gradient. For more on this see Surprising distant cousins.
ET microbes might well use some fourth form of photosynthesis that has never been explored on Earth.
Life that is better adapted to some conditions than Earth life e.g. better able to resist ionizing radiation, or perchlorates, able to use hydrogen peroxide, better more efficient metabolism, more efficient photosynthesis.
As an example, C4 photosynthesis as used by maize is more efficient than C3 in conditions of drought, high temperatures and limitations of nitrogen, and is a later development over C3 photosynthesis, though it requires more energy so not so good in cold conditions. But it's not the most efficient possible. It still relies on the enzyme RuBisCO which helps catalyse conversion of CO2 into glucose which is not the fastest of enzymes.
Scientists have found a way to increase the efficiency of this carbon dioxide fixation step 20 fold, - though only in a test-tube, not in an actual microbe. They combined enzymes from the human body and gut bacteria, plants, microbes that live in the sea and microbes from the surface of plants, in total they used 17 different enzymes from 9 different organisms, which they combined into a new 11 step cycle. They worked out and synthesized the gene sequence to reconstitute this pathway in a test-tube. More details here.
Nature never discovered this cycle on Earth. What about on Mars? Might Mars life have found this pathway, or if not this one, some other novel CO2 fixation cycle? The scientists didn't invent it from scratch, but rather obtained it by serendipity, combining enzymes that already exist in nature. What else might nature be able to do, given hundreds of millions of years of evolution in a planet spanning sea punctuated by numerous impacts and changes of climate?
Life similar to Earth life in most respects - this is Zubrin's idea - and it would raise many questions. How has it evolved in such a different environment, since last transfer from Earth, surely at least tens of millions of years ago. How did it get there? We can test the theory of panspermia, find out in practice how easy it is for life to be transferred to another planet. Or test ideas of convergent evolution on microbes that have evolved on different planets.
Some think that all life has to be essentially similar to DNA in almost all respects, that DNA life is optimal. If so, we might find something that closely resembles Earth life, but independently evolved. Based on DNA and RNA. With a similar number of amino acids. With many of the amino acids the same as with Earth life. It's hard to imagine it could be identical in all respects, but it might be so similar that the differences require specialist studies to discern them. There's a short survey with cites to various authors presenting these views in section 4 of this paper. This incidentally gives another way that life from Mars could look so similar to Earth life that it's not easy to tell it apart. It might even use DNA, mRNA, and similar or even the same nucleotides, and proteins, again similar or even most of the same amino acids, if the ones used by Earth life are optimal in some sense. Depending how optimal Earth life is, we might discover the result of a convergent evolution of many characteristics of Earth life, yet be independently evolved.
Uninhabited habitats - no life but with organics, and all the ingredients for life. This may seem boring at first sight. But it could also be wide reaching in the consequences. It would tell us a lot about how hard it is for it to evolve on a planet, and about the paths it follows on the way to life. If not life itself, there has to be some complex organic chemistry going on, and cell like structures surely form, as that happens even in short term laboratory experiments. So how far did it get and what exactly happens on a world similar to Earth in many ways (especially in the early solar system), but without life?
Also, on Earth it's impossible to study uninhabited habitats, except for a very short time after a volcanic eruption. Life appears rapidly on any uninhabited habitat here. On Mars, we might have the opportunity to study uninhabited habitats on a planet that hasn't been inhabited for billions of years. This could help us to understand exoplanets and the origin of life and maybe find out that life is harder to evolve than we thought. It can also help to disentangle effects of life and non life processes on Earth.
Indeed, if no life ever evolved on Mars, that would suggest that evolution of life is hard and often doesn't happen. If so, Mars could be an example of one of the most common types of planet in our galaxy, and the only one of its type close at hand for us to study.
Some major unexpected discovery that nobody currently is likely to predict. We need a catch all wild card here as there's no hope of being exhaustive surely :). Not when all our experience is based on trying to generalize from a single example of biochemistry based on a single origin for life.
If you think of anything I've left out here, do say!
All the possibilities here are of exceptional interest for biology. If there are habitats for life at all on Mars, whether inhabited or uninhabited, then biologists world wide will want to study them as they are now, and the results in the best case could be revolutionary for biology. Let's look a bit closer at some of these ideas.