In my article Our Spacecraft Could Look Straight At an Extraterrestrial Microbe - And Not See a Thing! we found out that present day extraterrestrial microbes in our solar system would be sparse and hidden from the harsh surface conditions, and ancient life would deteriorate over millions of years in the surface conditions. Our search is likely to be long and to involve digging and drilling in many different places in our solar system. But where should we look for these traces of life?
Let's start with the most likely and then move through to the less likely places. Our search will take us to Mars of course, but also the Moon for traces of ancient life from Earth, Phobos, Europa, Encladus, clouds of Venus, poles of Mercury, comets, large asteroids like Ceres, and then the possibility of exotic life on Io, Titan, and Triton (possibly even Pluto) using liquid sulfur dioxide, or hydrogen suflide, liquid ethane and methane, and liquid nitrogen, and finally, Carl Sagan's idea of life in the clouds of the gas giants.
First of all, a certainty, we already have evidence from our meteorites of amino acids and other organics. So it's a certainty that we'll continue to find out more about this, from comets and meteorites, about the chemistry of the solar system before life began. Also we'll find out more about the organics that got delivered to early Earth when we get a chance to drill down to the ancient ocean deposits on Mars. Mars unlike Earth has never had continental drift and salt deposits from the early oceans must have gone into a deep freeze and been kept unchanged for billions of years.
Then, again a certainty, there was life on Earth in the early solar system. Is there a chance that we can find traces of this life as a result of debris from meteorite impacts on Earth, in other parts of the solar system, especially on our Moon?
Then next, a near certainty. When we are able to explore Mars in detail, we will surely find, either traces of ancient life from billions of years ago, or almost alive protobionts, or find out what else can happen to an ocean rich in organics left for hundreds of millions of years. On Mars, without continental drift, and in frozen dry conditions, some of those ancient deposits must have gone into a deep freeze with exquisite preservation. But they are likely to be as hard to find as well preserved fossils on Earth, so where should we look?
Then, let's look briefly at revivable ancient life from billions of years ago. Microbes have been revived after 750 million years on Earth, so in the colder more stable conditions of Mars this seems at least a possiblity.
Then let's look at present day life based on DNA, or XNA or other Earth like organic chemistry. We could find these on Mars, Europa, Encladus and other subsurface oceans, the clouds of Venus, and then, less likely, the poles of Mercury. I'll look closely at Mars as it is perhaps our best chance of finding present day life in the near future.
Then, in the search for present day life, what about comets, and the larger asteroids such as Ceres? All of these have been suggested as possible places to look for traces of life.
Finally let's look at more exotic life in seas or lakes of various liquids such as liquid ethane/ methane on Titan, liquid sulfur dioxide on Io, and liquid nitrogen on Triton and just possibly Pluto - all of these have been proposed as possibilities for life by scientists. I'll also find out what the latest scientific opinion is on Carl Sagan's ideas for life in the atmospheres of gas giants. These seemed a likely habitat in the 1980s, but now seem perhaps the least probable of all of them.
Then I'll look at ways that we could search for this life with our spacecraft.
We might find life in all these places, or none of them. It won't be easy for us to see, and we may need to look in many different places to find it.
In some ways the obvious place to start would be Mars. But let's start with Earth life first. There we have total certainty that there was life on Earth at least in the Archaean period and most likely in the Hadean period as well. So, might there be traces of this life elsewhere in the solar system?
TRACES ANCIENT LIFE FROM BILLIONS OF YEARS AGO - FIRST, FROM EARTH, TO THE MOON
We know that Earth had life in the early solar system, so that is a certainty. So let's start there. We have almost no evidence of the early Hadean phase, just a few zircons found in Australia, which have also trapped small quantities of the ancient Earth atmosphere. But surely there must have been some form of life before the complex modern cells. Even the archaea, the most ancient forms of life we know of, are 200 nm across or larger. It's totally impossible that one of those could just arise by chance from a mixture of amino acids and other chemicals. So there must have been earlier living or almost alive cells before them. The cells themselves are so complex, that surely there must have been something like the modern cell but smaller, perhaps a few tens of nanometers across. Perhaps these reproduced exactly, but at an earlier stage before the modern cell machinery got developed, there must be cells that didn't reproduce exactly.
Here is a rather fun experiment you can do to create something that resembles an early cell, and its metabolism, but with just a few chemicals, far simpler than anything we would call life. It can't reproduce exactly like life, but it can split into almost copies of itself.
Perhaps early life was like this. Or it could have been purely chemical, a soup of RNA or more likely some precursor of RNA such as PNA. RNA is such a complex chemical that many think that replication must have started with some simpler chemical.
There are many ideas about where life might have started on Earth, and about what might have come first. But with only these few zircons by way of evidence, there is absolutely no way to decide between them. We have no evidence at all of this "almost life".
That is - unless these nanobes are still surviving relics of those first steps towards life?
They are hard to explain by non living processes and they do resemble life visually. The problem is, they are so small that we can't study them easily. We can't watch them split and reproduce, if they do that. All we can do is to get snapshots of moments of their lives, in electron microscopes - that is if they are alive. They might just be complex static structures that form in some way that we haven't been able to model yet, and that don't reproduce or metabolize or do anything resembling life processes.
Perhaps we can find out on other planets and places in the solar system. Perhaps some places we search may have no modern DNA based life, but only the more ancient forms, so they might well have these nanobes. Or if the Earth nanobes are non living - still, other places in the solar system may have traces of the precursor forms of life tens of nanometers across that are extinct here on Earth.
We can send electron microscopes to find out, and if there is no large scale 200 nm or larger life found, we can analyse for large molecule organics and DNA or XNA without the complications of modern large scale life, and test to see if these structures replicate in a culture on Mars - start with a culture with no nanobes in it, then expose it to the Martian nanobes, over a period of months or however long we expect it to take for them to reproduce - and then analyse it to see if it now contains many new nanobes.
But even more useful, we might be able to find traces of life on Earth itself, on the Moon. In the "Late heavy bombardment" many kilotons of material would get ejected from the Earth and much of that ends up on the Moon. Some of these would be large chunks that get buried deep below the surface, and others would be smaller pieces that might just be resting on the surface, like the meteorites we find on Earth. The Moon has no atmosphere to slow down the impact, but on the other hand it has low gravity, and it has a soft regolith to cushion the fall. At the poles it has deposits of ice from comets in the "craters of eternal night". Any meteorites that landed there would be kept in a deep freeze even on the surface, though they would be damaged by cosmic radiation. If we can dig a few meters down though, we might find even undamaged pristine organics from Earth in its Hadean period on the Moon.
We could find traces of this life in other places as well, as a small percentage of the rocks sent into space after an impact on Earth get sent throughout the solar system. Rocks from Earth could even, in principle, have seeded life on Europa according to a recent study. So that means, any of the Jupiter moons could in principle have meteorites from ancient Earth. So of course, Mars could, also the larger asteroids such as Ceres, and so on. Any of these places could hide ancient rocks with traces of life from Earth.
TRACES OF LIFE FROM ANCIENT MARS AND VENUS
Any of these places could have rocks from Venus or Mars. Venus is especially interesting here, as itis might be our only chance to find out about whether life existed on Venus in the early solar system, when it might well have had seas like the Earth - that is unless the life also survives to this day in the Venus cloud tops, which we'll come to later.
TRACES OF ANCIENT LIFE ON MARS' MOONS
We can do similar searches on Mars' moons Phobos and Deimos. Phobos as the moon closest to Mars is the moon most likely to have significant quantities of ancient Mars material on its surface. In some ways it might be a better place to look than the surface of Mars itself, since there are no dust storms and no surface chemistry to weather away the traces of life. Many rocks must have hit Phobos from meteorite impacts on the ancient Noachian seas on Mars. If these contained life, then the traces may still be there to find especially if buried deep below the surface.
Deimos is interesting for this also, as it has craters at its poles which, like the Moon's poles, are continually shadowed and so would be extremely cold. This could make these good places to look for ancient well preserved life from Mars.
TRACES OF LIFE ON MARS
Mars must have had, at the very least, protobionts, various precursors of life, like the ones we get in laboratory experiments, but surely more complex, further developed towards life, maybe almost reproducing. So I would count this also as a near certainty that we find interesting things eventually, if we look long enough.
Okay -the standard answer is first Mars - could be surface - either everywhere at extremely low concentrations - or perhaps more likely, in a few "special regions", such as salt deposits and the warm seasonal flows.
It could also be a few meters below the surface in caves or geologically heated "hot spots" of trapped water - or deep underground in its "hydrosphere" a perhaps 100 meter or more layer of water trapped kms below the surface - or all three.
Then, Europa (subsurface ocean with water vapour plumes) and Encladus (ditto - with plumes sent into space).
Also the subsurface oceans of various icy moons. So there may also be life deep underground on Ganymede, even Callisto. And Titan also may have a sub surface ocean as well as its methane / ethane surface oceans.
Europa most interesting of all because it's ocean is thought to be oxygen rich through dissociation of the ice by the intense radiation from Jupiter.
The larger asteroids like Ceres also may have life though this is less likely - but it is likely enough so that Ceres is a borderline category in the Planetary protection workshops - marked with a note that when we get to study it close up - then the situation needs to be re-evaluated.
And - the larger Kepler objects and Pluto could potentially have life underground if they are warm enough beneath the surface.
Another possible location for life is in the upper Venus atmosphere. This is a major challenge - in many cases the most habitable location in our solar system - almost Earth like in temperature, pressure, and atmospheric composition (without the oxygen of course) - but with 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.
So - it's possible that there is sulfuric acid tolerant life in the Venus clouds. The planetary protection workshop on Venus came to the conclusion that this was unlikely - but their conclusions were questioned later by some exobiologists. So it's possible a new workshop held today might come to different conclusions.
The other main problem with the high Venus clouds is that there are no solid surfaces of course. So could the life find some way to stay aloft? The residence time of particles is months rather than days - so - that makes it easier - but it's still quite a challenge.
Very remote possibility - but might be life in the Mercury ice caps - perhaps beneath a protective layer of organics that would hold in the water and prevent it from evaporating.
For larger comets - this is like the earlier suggestion of life in interiors of the larger KBOs. Possible. And some smaller comets maybe traces as result of fragmentation of larger bodies?
Then - moving to more exotic forms of life not exactly like DNA. There might be life on Io, sulfur based, in underground pools of liquid SO2, with the life chemistry probably also using H2S.
There might be life on triton using polysylanes instead of polycarbonates.
Life on Titan is a challenge for us to model because of the cold, and the novel chemistry - no oxygen to speak of, no water, no CO2. But - nitrogen might take the place of oxygen - or it might use reactions involving the energy in double and triple bonded carbon.
With all these exotic chemistries - the thing is - that nearly all our experiments on Earth are conducted in a very limited temperature and pressure range. So we know a huge amount about organic chemistry at around room temperature - and then extrapolating to do selected experiments at higher and lower temperatures. But - we don't know much at all about what happens at ultra low temperatures and using long chain chemicals that are rare on Earth.
After all many of organics we study were originally formed only through life processes, though of course many are synthesized nowadays. Would we even know about many of the more complex organic molecules if we were not surrounded by plants and animals and microbes that use them?
It's hard to figure out how much weight to put on this observation. Does it mean we know almost nothing about complex chemistry outside of the range of temperatures humans are comfortable in, and outside of the range of chemical processes carried out by life itself? Or do we know a lot, through the extrapolations we can do from the experiments that are easy for us, and occasional experiments in more extreme conditions?
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 rivers of liquid lava flowing in lava tubes - for huge distances across its surface.
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 it remain stable at the temperatures of even cooling magma pools on Venus?
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?
So - that last one seems unlikely - but still worth mentioning.
For more about the more exotic ideas here, see Cosmic Biology
Some extracts available online here:
