Nobody can really say for sure what is likely to be on Mars - if we had just one other form of life with a totally independent origin from Earth...
So anyway the best we can do is look into life from Earth that could perhaps survive on Mars. There are several species of microbial Earth life that can probably survive on Mars just as they are, also some lichens - but not much more.
So, it seems most likely to have microbes, and possibly plant life. With hardly any oxygen in the atmosphere, animal life like ourselves is impossible.
It isn't likely to have many large animals because we can see its entire surface from space - with meter levels of resolution for any points we choose to focus the cameras on - so if there were many herds of large animals meter sized or larger, moving around on the surface we'd have seen them by now, so - no matter how bizarre the physiology - that doesn't seem too likely.
Stationary plants are another matter, or very slow moving animals - or if the animals are very rare - can't rule that out just from the visual images.
However, it seems that only extremely small trace amounts of liquid water can survive on the Mars surface, because it boils almost instantly in the thin atmosphere,
There is indirect evidence, especially the "Warm seasonal flows" of some water - perhaps a fraction of a cm or so thickness - thin films most likely. There could be more water trapped underground, but - if so, would evaporate quickly unless it is shielded from the surface in some way, e.g. like an oil well trapped below an overlying layer.
So - if there is Earth like life there - then on the surface at least, it's severely limited by the lack of water.
The nearest places we have on Earth as dry as that are the Atacama desert and the McMurdo dry valleys in Antarctica.
Where they are most like Mars, those areas of Earth tend to have microbes only, often just a single species of microbe. So - since even on Earth with all its species we get only microbes there - by analogy - but of course nothing proved - then the chances seem to be that we'd only get microbial life on Mars - maybe microbial mats in places where there is a lot of life.
The DLR experiments in Germany showed lichens growing in Mars like conditions using the night time water vapour - on going experiments but promising so far. If those are a good guide, it's possible there might be lichens on Mars. So that opens the door for somewhat more complex life forms.
But - that's all analogy from Earth - and Mars is very different from Earth - with the radiation, thin atmosphere, CO2 with no oxygen, very little by way of nitrogen, and so on, even the partial gravity might make a difference as microbes are influenced by gravity (surprisingly).
As for life deep belowa the surface of Mars, the most likely is really deep down, kilometers deep, below the cryosphere, heated by geothermal heating from the centre of Mars - might well be liquid water and a rich layer of life there - but again - if judging by Earth life deep down - expect it to be microbes, or possibly tiny worm like creatures, which we get here on Earth deep down.
These creatures on Earth do depend on oxygen, but only a thousandth of the levels you get in our oceans, The species is called Halicephalobus mephisto
Deepest-living land animal found
Lords of the Underworld: Radiation Eating 'Worms from Hell' Discovered a Mile Below Earth's Surface -"A Preview of Life of Mars"
Could they exist on Mars? That's the sort of question biologists tend to ask - not so much about ETs or large animals, but more, could there be little mm scale worms living deep below the surface of Mars. Or could there be lichens there?
The one place in our solar system that might have complex life is Europa, it's covered with ice but below that they think there's a global ocean, and with oxygen rich water - for a surprising reason. The intense radiation of the Jupiter ionosphere breaks the ice into hydrogen and oxygen, and over millions of years, the oxygen finds its way into the Europan sub surface oceans, so they think.
If so, again by analogy with Earth life, it might be a similar environment to our oceans, which here at least supports complex life like fish, sharks, octopuses etc. There could even be intelligent life beneath the surface of this moon of Jupiter - and we wouldn't know about them, and they wouldn't know about us - so long as they don't have the technology to cut through the ice or transmit radio waves. Fish, however intelligent, evolving in an ocean with no access to the surface probably, no lightning, no natural fires, except for lava flows perhaps - would probably find it hard to build radio telescopes!
It's hard to say how likely that is, perhaps low probability - but not impossible, that we might have ETs within our solar system and not know about each other.
That's on current scientific understanding, which might be overturned with new discoveries, but on our present knowledge,
Mars is probably of most interest for microbial life - and maybe lichens and primitve plants - and evolution of life from the early days. While Europa just possibly could have far more complex life. But that's hardly more than a somewhat informed guess, we won't know for sure until we can study these places properly - first attempt to study Mars biologically since the 1970s is ExoMars in 2018.
Curiosity doesn't have the instruments or capabilities to detect life in the low concentrations you'd get, e.g. in the Atacama desert, as well as exploring the area of Mars perhaps almost the least likely to have present day life (intentionally because it's not sterilized enough to visit any place likely to have present day life, and it's main mission is to find out about geological conditions and organics on Mars in the early solar system).
Mars could have microbes that are different in origin from Earth life. If so - in worst case - it could set up a different self contained eco-system if returned to Earth - could take over part of our ecosystem, or some niche, or even take over the whole thing. And Earth life might not recognize it as hazardous until it's too late. So worst case is "bye bye DNA".
Most would say that's very unlikely - but how can we say it's impossible? It's the most interesting case, if Mars has XNA instead of DNA, that's what we'd all like to find.
But - on Earth it's recognized that if we make XNA based life able to reproduce, in labs, then we have to be very careful to make sure that it can't reproduce in the wild in case it does form something like that. So same should apply to Mars XNA, even more so as it's not just modified DNA but could be anything, we have no idea what it would be like, maybe not resembling DNA but some other type of molecule for the genes for instance.
The other way around, if DNA proves better than XNA all round, it could be goodbye Mars XNA before it can be studied.
So we have to be very careful both ways. Other things that could happen as well, but that's the most dramatic if perhaps least likely.
Earth has plenty of anaerobes - unable to live in oxygen rich atmospheres - and Mars has strong radiation and also hydrogen peroxides, it's life forms may well not be too much affected by the oxygen in our atmosphere. So I think not right to say it won't be able to survive here. And - just because it does well in Mars cold conditions - yet - it gets above 0C often in equatorial regions, and who is to say it can't do even better in warmer wetter conditions.
Same the other way - we have several microbes on Earth that do well in Mars analogue environments, and lichens also in the DLR experiments.
See my science20 articles for more including Need For Caution For An Early Mars Sample Return - Opinion Piece
So anyway the best we can do is look into life from Earth that could perhaps survive on Mars. There are several species of microbial Earth life that can probably survive on Mars just as they are, also some lichens - but not much more.
So, it seems most likely to have microbes, and possibly plant life. With hardly any oxygen in the atmosphere, animal life like ourselves is impossible.
It isn't likely to have many large animals because we can see its entire surface from space - with meter levels of resolution for any points we choose to focus the cameras on - so if there were many herds of large animals meter sized or larger, moving around on the surface we'd have seen them by now, so - no matter how bizarre the physiology - that doesn't seem too likely.
Stationary plants are another matter, or very slow moving animals - or if the animals are very rare - can't rule that out just from the visual images.
However, it seems that only extremely small trace amounts of liquid water can survive on the Mars surface, because it boils almost instantly in the thin atmosphere,
There is indirect evidence, especially the "Warm seasonal flows" of some water - perhaps a fraction of a cm or so thickness - thin films most likely. There could be more water trapped underground, but - if so, would evaporate quickly unless it is shielded from the surface in some way, e.g. like an oil well trapped below an overlying layer.
So - if there is Earth like life there - then on the surface at least, it's severely limited by the lack of water.
The nearest places we have on Earth as dry as that are the Atacama desert and the McMurdo dry valleys in Antarctica.
Where they are most like Mars, those areas of Earth tend to have microbes only, often just a single species of microbe. So - since even on Earth with all its species we get only microbes there - by analogy - but of course nothing proved - then the chances seem to be that we'd only get microbial life on Mars - maybe microbial mats in places where there is a lot of life.
COULD THERE BE MULTICELLULAR LIFE
The DLR experiments in Germany showed lichens growing in Mars like conditions using the night time water vapour - on going experiments but promising so far. If those are a good guide, it's possible there might be lichens on Mars. So that opens the door for somewhat more complex life forms.
But - that's all analogy from Earth - and Mars is very different from Earth - with the radiation, thin atmosphere, CO2 with no oxygen, very little by way of nitrogen, and so on, even the partial gravity might make a difference as microbes are influenced by gravity (surprisingly).
As for life deep belowa the surface of Mars, the most likely is really deep down, kilometers deep, below the cryosphere, heated by geothermal heating from the centre of Mars - might well be liquid water and a rich layer of life there - but again - if judging by Earth life deep down - expect it to be microbes, or possibly tiny worm like creatures, which we get here on Earth deep down.
These creatures on Earth do depend on oxygen, but only a thousandth of the levels you get in our oceans, The species is called Halicephalobus mephisto
Deepest-living land animal found
Lords of the Underworld: Radiation Eating 'Worms from Hell' Discovered a Mile Below Earth's Surface -"A Preview of Life of Mars"
Could they exist on Mars? That's the sort of question biologists tend to ask - not so much about ETs or large animals, but more, could there be little mm scale worms living deep below the surface of Mars. Or could there be lichens there?
ONE PLACE IN OUR SOLAR SYSTEM WHERE COMPLEX LARGE LIFEFORMS COULD HIDE
The one place in our solar system that might have complex life is Europa, it's covered with ice but below that they think there's a global ocean, and with oxygen rich water - for a surprising reason. The intense radiation of the Jupiter ionosphere breaks the ice into hydrogen and oxygen, and over millions of years, the oxygen finds its way into the Europan sub surface oceans, so they think.
If so, again by analogy with Earth life, it might be a similar environment to our oceans, which here at least supports complex life like fish, sharks, octopuses etc. There could even be intelligent life beneath the surface of this moon of Jupiter - and we wouldn't know about them, and they wouldn't know about us - so long as they don't have the technology to cut through the ice or transmit radio waves. Fish, however intelligent, evolving in an ocean with no access to the surface probably, no lightning, no natural fires, except for lava flows perhaps - would probably find it hard to build radio telescopes!
It's hard to say how likely that is, perhaps low probability - but not impossible, that we might have ETs within our solar system and not know about each other.
That's on current scientific understanding, which might be overturned with new discoveries, but on our present knowledge,
WON'T KNOW ABOUT MARS FOR A WHILE YET
Mars is probably of most interest for microbial life - and maybe lichens and primitve plants - and evolution of life from the early days. While Europa just possibly could have far more complex life. But that's hardly more than a somewhat informed guess, we won't know for sure until we can study these places properly - first attempt to study Mars biologically since the 1970s is ExoMars in 2018.
Curiosity doesn't have the instruments or capabilities to detect life in the low concentrations you'd get, e.g. in the Atacama desert, as well as exploring the area of Mars perhaps almost the least likely to have present day life (intentionally because it's not sterilized enough to visit any place likely to have present day life, and it's main mission is to find out about geological conditions and organics on Mars in the early solar system).
MICROBES COULD BE HAZARDOUS EITHER WAY
Mars could have microbes that are different in origin from Earth life. If so - in worst case - it could set up a different self contained eco-system if returned to Earth - could take over part of our ecosystem, or some niche, or even take over the whole thing. And Earth life might not recognize it as hazardous until it's too late. So worst case is "bye bye DNA".
Most would say that's very unlikely - but how can we say it's impossible? It's the most interesting case, if Mars has XNA instead of DNA, that's what we'd all like to find.
But - on Earth it's recognized that if we make XNA based life able to reproduce, in labs, then we have to be very careful to make sure that it can't reproduce in the wild in case it does form something like that. So same should apply to Mars XNA, even more so as it's not just modified DNA but could be anything, we have no idea what it would be like, maybe not resembling DNA but some other type of molecule for the genes for instance.
The other way around, if DNA proves better than XNA all round, it could be goodbye Mars XNA before it can be studied.
So we have to be very careful both ways. Other things that could happen as well, but that's the most dramatic if perhaps least likely.
Earth has plenty of anaerobes - unable to live in oxygen rich atmospheres - and Mars has strong radiation and also hydrogen peroxides, it's life forms may well not be too much affected by the oxygen in our atmosphere. So I think not right to say it won't be able to survive here. And - just because it does well in Mars cold conditions - yet - it gets above 0C often in equatorial regions, and who is to say it can't do even better in warmer wetter conditions.
Same the other way - we have several microbes on Earth that do well in Mars analogue environments, and lichens also in the DLR experiments.
See my science20 articles for more including Need For Caution For An Early Mars Sample Return - Opinion Piece
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
4.8m answer views110.4k this month
Top Writer2017, 2016, and 2015
Published WriterHuffPost, Slate, and 4 moreHuffPost, Slate, Forbes, Newsweek, Business Insider, and Science 2.0