It's a bit hard to say for sure as we don't know what habitats exist on Mars. Our rovers and landers -the successful ones - have only been to the most inhospitable areas of Mars for present day life - the dry equatorial regions - and not the deepest areas of Mars either where the atmosphere could be a bit thicker. That's all except for Phoenix, which landed in an icy area - actually crushed by the advancing dry ice sheet as winter approached - but its capabilities were rather limited - and it couldn't move around to explore.
That's for technical reasons. Partly that it is far easier to land in the equatorial regions, as it happens - for reasons of orbital dynamics and amounts of fuel. And partly because it is going to be a major challenge to sterilize a lander or rover sufficiently to explore the more habitable regions of Mars. And partly because some of the best possible habitats are on steep somewhat inaccessible slopes (though others are probably more accessible).
So this answer has to be theoretical - we don't have much evidence from the ground on Mars yet about conditions there in the most habitable regions.
Some lichens though have survived in Mars simulation conditions on the Earth for several weeks, apparently growing normally, though they are slow growing. Both photosynthesizing and metabolizing. Without any water, using the night time humidity.
This is one of them - a lichen that's also able to live in warm climates, where it spreads over rocks, but in Antactica, at high altitudes, then it kind of scrunches up into cracks, a habit which would help it on Mars. It also has inbuilt protection from UV light. This lichen did very well in the Mars simulation experiments
What is rather remarkable is that a lichen is a symbiosis of a fungus and an algae - and the fungus is an aerobe which needs oxygen. So how does it survive in a Mars simulation atmosphere with almost no oxygen? It must be getting its oxygen from the algae component of the lichen.
This was in an environment without ice. The photosynthetic activity coincided with the beginning and end of the simulated Martian day - when atmospheric water vapour could condense on the soil and perhaps create cold brines.
Black fungi and black yeast are also able to do this same trick. Quite surprising, until recently nobody thought this was possible. The big question is whether they could do this in the real Mars environment - how good is the simulation - and how long can they survive, can they survive and grow endlessly on Mars. It's at least possible that they can.
There are also various potential habitats with liquid water. Most well known perhaps the Recursive Slope Lineae - recent confirmation of hydrated salts associated with them when they are at their broadest, suggests quite strongly that there is water involved somehow. Also Nilton Renno's "Swimming pools for bacteria" - they found that liquid water forms very rapidly on interfaces between salt and ice in Mars simulation conditions. Both salt and ice are abundant on Mars.
Some lifeforms are "prime producers" which means that unlike us, they don't depend on any other form of life to survive. For instance some cyanobacteria such as chrooccocidiopsis
The big question for Mars though, is whether any of these liquid water habitats are in the right salinity and temperature ranges for life. If they are, then Mars may be very habitable for microbes. Where very habitable here means - as habitable as, perhaps, the Atacama desert core, or the McMurdo dry valleys on Earth. I.e. as habitable as the harshest deserts on Earth, but that's very habitable for Mars.
There are other ideas as well. For some of them, see
It originates as an article I wrote for Wikipedia but the editors there are rather conservative on this topic, stuck in the past, a little under a decade out of date. They are totally convinced that Mars is not habitable for present day life, despite all these experiments I just mentioned. I don't know how anyone can look at the DLR lichen experiments, for instance, and conclude that the researchers think that Mars is not habitable for present day life - but that's what they say and are not convinced by any of the quotes I give them from the articles where they authors themselves talk about the implications for the possibility of Earth life living on Mars and planetary protection issues arising from it.
So the Life on Mars article on wikipedia just says - at least last time I checked - that life on present day Mars is not possible - they say that it's because of cosmic radiation.
But the radiation levels on Mars are similar to the interior of the ISS (except during solar storms - the ISS is protected by Earth's magnetic field while the Mars surface is not). Of course almost any form of life can tolerate that so long as it can metabolize. Even humans can. Humans need protection from solar storms but many microbes are far more hardy and can tolerate them just fine.
So, though written in encyclopedic style, I have made it available as an online book instead. I'm interested if anyone has any thoughts of good places to publish it - I think it's the only reasonably comprehensive online survey of all the potential habitats for life on Mars.
It's easy to find lots of statements by present day astrobiologists and the like saying they think there is at least a possibility that present day Mars is habitable, and a decent chance that it is actually inhabited already by Martian life. Slow growing surely, except in very unusual conditions, probably small populations - but still - it could be there. Some are very optimistic about it. Others are skeptical - say that these lab experiments are all very well but we don't yet know what conditions are like on Mars itself, for instance are the liquid water habitats too salty or too cold? And with the humidity experiments, well you can be skeptical about those easily enough as they are in the early stages yet.
About ten years ago now, the consensus was, amongst all except a few scientists, that life is impossible on present day Mars on the surface. But that was before all these ideas for habitats there were found. Back then the idea was that Mars is habitable occasionally when its atmosphere is thicker- but that any life there was around then would need to survive for several million years without any water at all. Over those long periods cosmic radiation is indeed deadly for any dormant microbes on the surface. But the effects of cosmic radiation act as an exponential process. Suppose, by way of example, say, that 10% of some organism can survive 1000 years, that means that only 1% survive 2000 years (another 1000 years so 10% of 10%), only 0.1% survive 3000 years, and so on. So it's easy to have microbes that can survive even a few thousand years of dormancy with no problem at all, but that can't survive a million years of dormancy.
The most hardy microbes could survive a few hundred thousand years on Mars. But doesn't seem anything could survive millions of years there, of Earth life - but that's only if it is dormant all that time.
Some microbes can repair their DNA with just a few hours of activity, no need to reproduce, remarkable. These would only need to wake up a few hours every few millennia to do just fine on Mars.
The recent discoveries suggest that it's at least possible that there are a few habitats on Mars where microbes, and even perhaps lichens, could survive, reproduce, metabolize, for probably many hours of every year. They wouldn't be bothered by ionizing radiation. As for perchlorates, also cited often as a problem - for some forms of microbes, perchlorates are a food. They are very harmful to humans but less so to microbes. That's partly because they are less active at lower temperatures - the perchlorates - and microbes can survive and grow at low temperatures unlike humans.
If it is DNA based that would mean it almost certainly has a shared ancestry with Earth life and then the big question would be whether it originated on Mars or Earth and of course biologists would be very interested in how it differs from Earth life. And - it could for instance have developed photosynthesis independently - did it hit on the same solutions for anything it evolved independently from Earth life - or did it find a different solution?.
If it isn't DNA based, that suggests independent origin and could be very exciting, like a Crick Watson, Franklin and Willikins moment all over again trying to figure out how it works. Sexism in science: did Watson and Crick really steal Rosalind Franklin’s data?
Also why not both? I think discovery of life that is related ot Earth life doesn't rule out also finding life that is unrelated as well. That's a question that has been raised for Earth life - could there be a "shadow biosphere" of unrelated life on Earth - not been proved that there is - but - why not therefore on Mars?
Then - as we explore Enceladus and Europa - I think those are the ones least likely to be related to Earth life. If there is life there - then either it is independently evolved - or it must have been shared in the very early solar system but even that is low probability, studies show (because there have been almost no meteorites shared in either direction because Europa is so deep in Jupiter's gravitational well, and Enceladus is beyond Jupiter).
If even Europa or Enceladus life had a shared origin with Earth life, it would suggest life starts very very early on in a new solar system and evolves extremely rapidly - to be present already so far back and evolved to the point where it is hardy enough to be shared between planets and also abundant - or else is seeded from another star. That is, if Enceladus or Europa also have DNA based life with the same bases and everything. Almost anything we find about Enceladus or Europa is likely to be extraordinary. Even no life, if we find organics there but no life - what do we find instead? And why no life?
there are many possibilities. First DNA can spiral clockwise or anticlockwise. It can have many different bases. Even the existing bases can be interpreted in many different ways - the language by which a string of three bases is turned into an amino acid fragment of a protein chain - that's pretty much arbitrary - Earth life has redundant coding. A computer search turned up 4,000 biologically reasonable amino acids. Earth life uses only 20 of those (maybe up to 23 depending how you count them).
As well as that, life can have different backbones. For instance, Earth life uses ribozomes to translate RNA into proteins, large complex structures incorporating RNA fragments with amino acid chains. But early life could be purely RNA based without any DNA and use ribozymes which just uses RNA fragments without any amino acids - much smaller permitting much smaller cells. And may have not needed to translate DNA into RNA.
So that takes you back to the idea of a precursor RNA world - perhaps less robust than DNA life, but still, it's life. Incidentally theoretically you can have a triple helix as well.
Then - many think that RNA didn't come first, but some precursor. Such as TNA, or PNA often suggested, because they are more robust than RNA and yet also easy to form from basic building blocks. Could also be a hodgepodge of several different backbones in the same molecule -the "hodge podge world" hypothesis.
So - perhaps Mars has less evolution than Earth, if so it might still have PNA or TNA or RNA world. Or hodgepodge world. Or it might have followed another direction. We are nowhere near simulating the origins of life in a flask, probably never will. And not likely we would have invented DNA based life if we didn't have it already as an example. So all our ideas are genearally by taking DNA life and tweaking the ideas.
But what if Mars life is in some way totally different? Then about the only thing it might have in common is that it uses infoRMational polymeres - some form of molecule that can encode information by repeating patterns of smaller units from a fixed tiny set of smaller units. Or is even that much essential? Is there some other way that life can encode patterns and reproduce
That's for technical reasons. Partly that it is far easier to land in the equatorial regions, as it happens - for reasons of orbital dynamics and amounts of fuel. And partly because it is going to be a major challenge to sterilize a lander or rover sufficiently to explore the more habitable regions of Mars. And partly because some of the best possible habitats are on steep somewhat inaccessible slopes (though others are probably more accessible).
So this answer has to be theoretical - we don't have much evidence from the ground on Mars yet about conditions there in the most habitable regions.
Some lichens though have survived in Mars simulation conditions on the Earth for several weeks, apparently growing normally, though they are slow growing. Both photosynthesizing and metabolizing. Without any water, using the night time humidity.
This is one of them - a lichen that's also able to live in warm climates, where it spreads over rocks, but in Antactica, at high altitudes, then it kind of scrunches up into cracks, a habit which would help it on Mars. It also has inbuilt protection from UV light. This lichen did very well in the Mars simulation experiments
This was in an environment without ice. The photosynthetic activity coincided with the beginning and end of the simulated Martian day - when atmospheric water vapour could condense on the soil and perhaps create cold brines.
Black fungi and black yeast are also able to do this same trick. Quite surprising, until recently nobody thought this was possible. The big question is whether they could do this in the real Mars environment - how good is the simulation - and how long can they survive, can they survive and grow endlessly on Mars. It's at least possible that they can.
There are also various potential habitats with liquid water. Most well known perhaps the Recursive Slope Lineae - recent confirmation of hydrated salts associated with them when they are at their broadest, suggests quite strongly that there is water involved somehow. Also Nilton Renno's "Swimming pools for bacteria" - they found that liquid water forms very rapidly on interfaces between salt and ice in Mars simulation conditions. Both salt and ice are abundant on Mars.
Some lifeforms are "prime producers" which means that unlike us, they don't depend on any other form of life to survive. For instance some cyanobacteria such as chrooccocidiopsis
Another likely candidate for Mars, haloarchaea - salt loving bacteria, they turn the Red sea Red - and they use photosythesis for energy, and are tolerant of high levels of salt.This shows quartz inclusions in the coastal regions of the Atacama desert. You can see green algae beneath rock surfaces in the coastal regions of the Atacama desert here. Quartz inclusions on Mars could be an especially good place for them to survive. They need little more than sunlight, CO2, some source of water, and a few trace elements to survive, and as prime producers, don't rely on any other lifeforms.
The big question for Mars though, is whether any of these liquid water habitats are in the right salinity and temperature ranges for life. If they are, then Mars may be very habitable for microbes. Where very habitable here means - as habitable as, perhaps, the Atacama desert core, or the McMurdo dry valleys on Earth. I.e. as habitable as the harshest deserts on Earth, but that's very habitable for Mars.
There are other ideas as well. For some of them, see
- Where To Search On Mars For Droplets, & Shallow Flows Of Liquid Water - Where Microbial Life May Flourish
- Or more techy: Are There Habitats For Life On Mars? - Salty Seeps, Clear Ice Greenhouses, Ice Fumaroles, Dune Bioreactors,...
which is also available as an Amazon booklet (for kindle, you can also read it online in your browser or download a kindle app)
You can get it here:Places On Mars to look for Life (Amazon, for kindle)
It originates as an article I wrote for Wikipedia but the editors there are rather conservative on this topic, stuck in the past, a little under a decade out of date. They are totally convinced that Mars is not habitable for present day life, despite all these experiments I just mentioned. I don't know how anyone can look at the DLR lichen experiments, for instance, and conclude that the researchers think that Mars is not habitable for present day life - but that's what they say and are not convinced by any of the quotes I give them from the articles where they authors themselves talk about the implications for the possibility of Earth life living on Mars and planetary protection issues arising from it.
So the Life on Mars article on wikipedia just says - at least last time I checked - that life on present day Mars is not possible - they say that it's because of cosmic radiation.
But the radiation levels on Mars are similar to the interior of the ISS (except during solar storms - the ISS is protected by Earth's magnetic field while the Mars surface is not). Of course almost any form of life can tolerate that so long as it can metabolize. Even humans can. Humans need protection from solar storms but many microbes are far more hardy and can tolerate them just fine.
So, though written in encyclopedic style, I have made it available as an online book instead. I'm interested if anyone has any thoughts of good places to publish it - I think it's the only reasonably comprehensive online survey of all the potential habitats for life on Mars.
It's easy to find lots of statements by present day astrobiologists and the like saying they think there is at least a possibility that present day Mars is habitable, and a decent chance that it is actually inhabited already by Martian life. Slow growing surely, except in very unusual conditions, probably small populations - but still - it could be there. Some are very optimistic about it. Others are skeptical - say that these lab experiments are all very well but we don't yet know what conditions are like on Mars itself, for instance are the liquid water habitats too salty or too cold? And with the humidity experiments, well you can be skeptical about those easily enough as they are in the early stages yet.
About ten years ago now, the consensus was, amongst all except a few scientists, that life is impossible on present day Mars on the surface. But that was before all these ideas for habitats there were found. Back then the idea was that Mars is habitable occasionally when its atmosphere is thicker- but that any life there was around then would need to survive for several million years without any water at all. Over those long periods cosmic radiation is indeed deadly for any dormant microbes on the surface. But the effects of cosmic radiation act as an exponential process. Suppose, by way of example, say, that 10% of some organism can survive 1000 years, that means that only 1% survive 2000 years (another 1000 years so 10% of 10%), only 0.1% survive 3000 years, and so on. So it's easy to have microbes that can survive even a few thousand years of dormancy with no problem at all, but that can't survive a million years of dormancy.
The most hardy microbes could survive a few hundred thousand years on Mars. But doesn't seem anything could survive millions of years there, of Earth life - but that's only if it is dormant all that time.
Some microbes can repair their DNA with just a few hours of activity, no need to reproduce, remarkable. These would only need to wake up a few hours every few millennia to do just fine on Mars.
The recent discoveries suggest that it's at least possible that there are a few habitats on Mars where microbes, and even perhaps lichens, could survive, reproduce, metabolize, for probably many hours of every year. They wouldn't be bothered by ionizing radiation. As for perchlorates, also cited often as a problem - for some forms of microbes, perchlorates are a food. They are very harmful to humans but less so to microbes. That's partly because they are less active at lower temperatures - the perchlorates - and microbes can survive and grow at low temperatures unlike humans.
WHAT IF THERE IS LIFE ON MARS ALREADY?
If it is DNA based that would mean it almost certainly has a shared ancestry with Earth life and then the big question would be whether it originated on Mars or Earth and of course biologists would be very interested in how it differs from Earth life. And - it could for instance have developed photosynthesis independently - did it hit on the same solutions for anything it evolved independently from Earth life - or did it find a different solution?.
If it isn't DNA based, that suggests independent origin and could be very exciting, like a Crick Watson, Franklin and Willikins moment all over again trying to figure out how it works. Sexism in science: did Watson and Crick really steal Rosalind Franklin’s data?
Also why not both? I think discovery of life that is related ot Earth life doesn't rule out also finding life that is unrelated as well. That's a question that has been raised for Earth life - could there be a "shadow biosphere" of unrelated life on Earth - not been proved that there is - but - why not therefore on Mars?
Then - as we explore Enceladus and Europa - I think those are the ones least likely to be related to Earth life. If there is life there - then either it is independently evolved - or it must have been shared in the very early solar system but even that is low probability, studies show (because there have been almost no meteorites shared in either direction because Europa is so deep in Jupiter's gravitational well, and Enceladus is beyond Jupiter).
If even Europa or Enceladus life had a shared origin with Earth life, it would suggest life starts very very early on in a new solar system and evolves extremely rapidly - to be present already so far back and evolved to the point where it is hardy enough to be shared between planets and also abundant - or else is seeded from another star. That is, if Enceladus or Europa also have DNA based life with the same bases and everything. Almost anything we find about Enceladus or Europa is likely to be extraordinary. Even no life, if we find organics there but no life - what do we find instead? And why no life?
HOW COULD LIFE DIFFER FROM EARTH LIFE
there are many possibilities. First DNA can spiral clockwise or anticlockwise. It can have many different bases. Even the existing bases can be interpreted in many different ways - the language by which a string of three bases is turned into an amino acid fragment of a protein chain - that's pretty much arbitrary - Earth life has redundant coding. A computer search turned up 4,000 biologically reasonable amino acids. Earth life uses only 20 of those (maybe up to 23 depending how you count them).
As well as that, life can have different backbones. For instance, Earth life uses ribozomes to translate RNA into proteins, large complex structures incorporating RNA fragments with amino acid chains. But early life could be purely RNA based without any DNA and use ribozymes which just uses RNA fragments without any amino acids - much smaller permitting much smaller cells. And may have not needed to translate DNA into RNA.
So that takes you back to the idea of a precursor RNA world - perhaps less robust than DNA life, but still, it's life. Incidentally theoretically you can have a triple helix as well.
Then - many think that RNA didn't come first, but some precursor. Such as TNA, or PNA often suggested, because they are more robust than RNA and yet also easy to form from basic building blocks. Could also be a hodgepodge of several different backbones in the same molecule -the "hodge podge world" hypothesis.
So - perhaps Mars has less evolution than Earth, if so it might still have PNA or TNA or RNA world. Or hodgepodge world. Or it might have followed another direction. We are nowhere near simulating the origins of life in a flask, probably never will. And not likely we would have invented DNA based life if we didn't have it already as an example. So all our ideas are genearally by taking DNA life and tweaking the ideas.
But what if Mars life is in some way totally different? Then about the only thing it might have in common is that it uses infoRMational polymeres - some form of molecule that can encode information by repeating patterns of smaller units from a fixed tiny set of smaller units. Or is even that much essential? Is there some other way that life can encode patterns and reproduce
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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