There is no evidence yet of life on Mars - but we haven't really looked either. None of our rovers since Viking in the 1970s, including Curiosity, have the capability to detect life signatures on Mars unless really obvious. ExoMars in 2018 will be the first modern rover with that capability.
For past life - the complication is - that over billions of years, organics buried deep below the surface deteriorate through natural radiation of the rocks. And over far shorter time periods, anything in the top meter or so deteriorates from cosmic radiation - so much - that there probably won't even be any amino acids left - the primitive building blocks of life - unless it was exposed recently.
There could be present day life on Mars in low concentrations - almost everywhere is one possiblity - though much more likely to be located in special favourable spots only.
Could be past life also - but buried deep below the surface.
Fossils also even on the Earth are hard to find and need special circumstances to preserve them. Is a possibility that Mars did develop multi-cellular life - but doesn't mean that everywhere you go on Mars you will find fossils of it -any more than you expect to find ancient fossils in the Sahara desert immediately if you just land a rover somewhere there.
So - not impossible that there were fish there, and just not been discovered yet. After all, Curiosity just drilled into one rock amongst many, didn't split any rocks open, or dig below the surface or do any of the things you do to search for fossils.
And - if they are more like jellyfish - or the strange creatures of the Burgess shales - they may have no hard parts at all - and may need exquisite preservation to spot them at all. As it is - the surface of Mars is covered in dust - and then fine layers of transformed material that cover the rocks. It may not be easy to spot fossils if they need such exquisite preservation to see them.
Except that is for the problem of oxygen - more of that in a bit.
The other thing is, we don't know what fossils would look like on Mars. So for instance, are ancient stromatolites that - for a long time - nobody knew if they were living things or just unusual rock formations - until - not so long ago, they were finally proved fairly conclusively to be ancient life, amongst the most ancient life forms found on the Earth so far.
Searching for ancient life on Mars may be just as hard. We do have the opportunity to find organics more easily - without the obscuring effect of all the present day life on Earth - but on the other hand - the organics if near the surface have been hugely deteriorated by cosmic radiation and will be far harder to recognize than similarly ancient organics on Earth. May be literally nothing left of them, all turned into CO2 and water and gone into the atmosphere.
So - we are searching for - not just organics - but organics that got buried quickly, stayed deep underground at least 10 meters below the surface for billions of years - and then were unearthed in the very recent past. Curiosity is currently heading towards rock formations that just possibly might fit that description. But there again they might not. And in any case its sensors are not sensitive enough to spot life in them unless it is very obvious.
So - unless it spots a macro fossil, or some deep layer of organics (say like an organic shale deposit), Curiosity is not likely to spot life.
But ExoMars might do. Because ExoMars has very sensitive biosignature detection capabilities - so would spot signs of life even in badly deteriorated samples - and it can also drill 2 meters below the surface -so able to dig down to a layer where the samples would be somewhat less deteriorated from cosmic radiation effects - though it can't drill the 10 meters needed to get below all effects of cosmic radiation.
ExoMars is just the first of what will probably be many future rovers searching for past and present day life on Mars..
And - Mars wasn't as habitable as Earth except in the very early solar system. And back when it was very habitable, when it had oceans - then evolution had probably just started - so any life there might still be very localized - say to hydrothermal vents - or wherever it is that the life started (which nobody knows). So we might need to look very carefully at many different past habitats on Mars before we find life.
But - since we now know that ancient Mars did have an ocean, pretty much confirmed, and in early solar system was like early Earth - with hot oceans, dense CO2 atmosphere, nitrates, seems ideal for life to evolve - and then gradually cooled down but still liquid water for some hundreds of millions of years - seems that it very probably did have - either life - or at least, must have had "protobionts" - as you get those even in laboratory experiments, Things that resemble cells for instance, but not quite living yet. So - Mars must have had those at least, if it didn't have life as such.
So - sure to be exciting discoveries ahead whenever we do start to explore Mars as exobiologists rather than geologists.
And there is an excellent chance of present day life on Mars also nowadays- been re-evaluated since the 2008 Phoenix discoveries and various other new results - and especially the warm seasonal flows - our best evidence to date that - in a few very rare special spots on Mars - that water does still flow - just as a seep of salty brine probably - certainly no streams or lakes - but - seems quite possible that there may be present day life there.
Most likely just single cell life - evne on Earth then our most similar habitats are mostly colonized by single cell life. But some form of multi-cellular life, especially microbial films - maybe building up over time to make visible macro-structures - a bit like stromatoliates.
Also, maybe multi-cellular plants and lichen type things also could survive on present day Mars. That wouldn't be too surprising especially after the DLR experiments showing that some lichens from arctic and high apline regions can survive in a simulated Mars atmosphere - with simulated Mars UV radiation - at least for a month or so, and not just survive, but also metabolize and photosynthesize using just the humidity in the night time air on Mars.
Animals - a bit harder - the problem with those - is that as far as we know, Mars has never had any oxygen in its atmosphere or oceans at anything like the levels that fish and animals need.
Still there are some tiny multi-cellular "animals" that manage their entire life cycle without oxygen. Recent discovery that. So it's not impossible that Mars did have some kind of animal life as well.
And there is always also the possibility of finding something on Mars that doesn't use the same biochemistry as Earth does - and maybe found other ways to survive and live.
For past life - the complication is - that over billions of years, organics buried deep below the surface deteriorate through natural radiation of the rocks. And over far shorter time periods, anything in the top meter or so deteriorates from cosmic radiation - so much - that there probably won't even be any amino acids left - the primitive building blocks of life - unless it was exposed recently.
There could be present day life on Mars in low concentrations - almost everywhere is one possiblity - though much more likely to be located in special favourable spots only.
Could be past life also - but buried deep below the surface.
Fossils also even on the Earth are hard to find and need special circumstances to preserve them. Is a possibility that Mars did develop multi-cellular life - but doesn't mean that everywhere you go on Mars you will find fossils of it -any more than you expect to find ancient fossils in the Sahara desert immediately if you just land a rover somewhere there.
So - not impossible that there were fish there, and just not been discovered yet. After all, Curiosity just drilled into one rock amongst many, didn't split any rocks open, or dig below the surface or do any of the things you do to search for fossils.
And - if they are more like jellyfish - or the strange creatures of the Burgess shales - they may have no hard parts at all - and may need exquisite preservation to spot them at all. As it is - the surface of Mars is covered in dust - and then fine layers of transformed material that cover the rocks. It may not be easy to spot fossils if they need such exquisite preservation to see them.
Except that is for the problem of oxygen - more of that in a bit.
The other thing is, we don't know what fossils would look like on Mars. So for instance, are ancient stromatolites that - for a long time - nobody knew if they were living things or just unusual rock formations - until - not so long ago, they were finally proved fairly conclusively to be ancient life, amongst the most ancient life forms found on the Earth so far.
Searching for ancient life on Mars may be just as hard. We do have the opportunity to find organics more easily - without the obscuring effect of all the present day life on Earth - but on the other hand - the organics if near the surface have been hugely deteriorated by cosmic radiation and will be far harder to recognize than similarly ancient organics on Earth. May be literally nothing left of them, all turned into CO2 and water and gone into the atmosphere.
So - we are searching for - not just organics - but organics that got buried quickly, stayed deep underground at least 10 meters below the surface for billions of years - and then were unearthed in the very recent past. Curiosity is currently heading towards rock formations that just possibly might fit that description. But there again they might not. And in any case its sensors are not sensitive enough to spot life in them unless it is very obvious.
So - unless it spots a macro fossil, or some deep layer of organics (say like an organic shale deposit), Curiosity is not likely to spot life.
But ExoMars might do. Because ExoMars has very sensitive biosignature detection capabilities - so would spot signs of life even in badly deteriorated samples - and it can also drill 2 meters below the surface -so able to dig down to a layer where the samples would be somewhat less deteriorated from cosmic radiation effects - though it can't drill the 10 meters needed to get below all effects of cosmic radiation.
ExoMars is just the first of what will probably be many future rovers searching for past and present day life on Mars..
And - Mars wasn't as habitable as Earth except in the very early solar system. And back when it was very habitable, when it had oceans - then evolution had probably just started - so any life there might still be very localized - say to hydrothermal vents - or wherever it is that the life started (which nobody knows). So we might need to look very carefully at many different past habitats on Mars before we find life.
But - since we now know that ancient Mars did have an ocean, pretty much confirmed, and in early solar system was like early Earth - with hot oceans, dense CO2 atmosphere, nitrates, seems ideal for life to evolve - and then gradually cooled down but still liquid water for some hundreds of millions of years - seems that it very probably did have - either life - or at least, must have had "protobionts" - as you get those even in laboratory experiments, Things that resemble cells for instance, but not quite living yet. So - Mars must have had those at least, if it didn't have life as such.
So - sure to be exciting discoveries ahead whenever we do start to explore Mars as exobiologists rather than geologists.
And there is an excellent chance of present day life on Mars also nowadays- been re-evaluated since the 2008 Phoenix discoveries and various other new results - and especially the warm seasonal flows - our best evidence to date that - in a few very rare special spots on Mars - that water does still flow - just as a seep of salty brine probably - certainly no streams or lakes - but - seems quite possible that there may be present day life there.
Most likely just single cell life - evne on Earth then our most similar habitats are mostly colonized by single cell life. But some form of multi-cellular life, especially microbial films - maybe building up over time to make visible macro-structures - a bit like stromatoliates.
Also, maybe multi-cellular plants and lichen type things also could survive on present day Mars. That wouldn't be too surprising especially after the DLR experiments showing that some lichens from arctic and high apline regions can survive in a simulated Mars atmosphere - with simulated Mars UV radiation - at least for a month or so, and not just survive, but also metabolize and photosynthesize using just the humidity in the night time air on Mars.
Animals - a bit harder - the problem with those - is that as far as we know, Mars has never had any oxygen in its atmosphere or oceans at anything like the levels that fish and animals need.
Still there are some tiny multi-cellular "animals" that manage their entire life cycle without oxygen. Recent discovery that. So it's not impossible that Mars did have some kind of animal life as well.
And there is always also the possibility of finding something on Mars that doesn't use the same biochemistry as Earth does - and maybe found other ways to survive and live.
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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