Okay, the answer here is a Yes and a No. First, liquid water can't form in the open on Mars in ordinary form such as, say, a puddle or a stream, anywhere except in the deepest parts of the Hellas basin and the Valles Marineres. And even there, the water would be close to boiling point as soon as it melts and would quickly evaporate.
That's because the air is so thin. You may know that mountain climbers when they climb Mount Everest can't make a really hot cup of tea because the water boils at a lower temperature at altitude. So it's the same on Mars - but more so. With the atmosphere a fiftieth of the pressure you get at the summit of Everest, then the water over much of the surface boils as soon as it melts.
Indeed leave a chunk of ice anywhere in the equatorial region of Mars, even sheltered from the sun and well below zero and eventually it will just evaporate away like dry ice, because the air is so thin. It does it more slowly than dry ice but does eventually, over a period of a few days
Here are a couple of photos taken by the Phoenix lander which shows this phenomenon on Mars.
As you see, there is rather less of that white stuff in the right hand image than the left hand one - in a scrape it dug. Though they couldn't analyse it, this was regarded as proof that the white stuff was water, and not dry ice (dry ice would have evaporated much more quickly) or, say, salt (which wouldn't evaporate). See Proof! Water Ice Found on Mars
So - rather than go liquid, ice on the edge of the polar cap would just sublimate like that into the atmosphere in summer.
So, that's the "No".
But now for the "Yes".
This may be the "smoking gun" of liquid water in the polar regions. The only explanation for them so far involve water in some form or another. Interestingly also, some of the ideas involve pure liquid water, with not much salt in it, at a little above 0C - it might be the warmest liquid water close to the Mars surface, surprising, with it being in the polar regions.
(most of the liquid water suggested for Mars habitats is salty so a little colder than this).
They form in the debris of the Martian Geysers which erupt earlier in the spring.
Not observed directly, just indirectly by the dark marks they form on the surface but the evidence is reasonably conclusive that these geysers do exist on Mars, but they are dry ice geysers, not liquid water ones.
But after they erupt, and spread a lot of debris over the surface in dark patches - those patches then gradually extend with these flow like features.
This is very hard to understand as anything else except liquid water in some form. They look a bit different in the Northern hemisphere, and form in slightly different circumstances, and there, they may form at very low temperatures in salty solutions, possibly too cold for life.
But in the Southern hemisphere, close up they look like this.
The preferred model here uses a solid state greenhouse. Clear ice can act to trap infrared to warm up a layer below the surface.
On the Earth this happens with blue ice like this
which is especially clear, and lets through enough heat so that a layer of liquid water can form - and the interesting thing is it forms about half a meter below the surface, in Antarctica.
So on Mars, if it forms at that depth, it's insulated from the vacuum of the atmosphere, so it won't boil away. So can have a stable liquid layer of water actually within the ice.
It happens in Antarctica and there isn't much changed except less gravity on Mars, and about half the light levels. The models say it should form there also, so long as Mars has some form of clear ice.
Now whether Mars does have the right type of clear ice for this to work we don't know. Very hard to model large scale processes like that on an alien planet. Or to simulate in in a laboratory.
But if it does, well, it almost certainly has these liquid layers just like Antarctica. More so if there happens to be a rock layer to get warmed up and retain heat at the right depth or pebbles and sand mixed into the ice.
There are other ways the flow like features in Richardson crater could form, but they all so far involve liquid water in some form (the other ones involve rather colder water).
Though not so well known as some of the other possible habitats on Mars, in some ways it's one of the most interesting and exciting, I think as interesting as the much better known Warm Seasonal Flows.
In this model the liquid layer starts off millimeters thick, but it is cumulative, stays liquid overnight, and gets thicker and thicker until eventually it can be several cms thick - and it forms about 5 - 10 cms below the surface.
So, will we send any rovers to Mars to look for this?
Well, they'd love to look at the geysers close up, so this would be a natural thing to add in to a mission to observe the geysers. But this is a really big challenge for planetary protection. Because, you are talking about ice here, and liquid water. Can we so thoroughly sterilize our rovers that not a single dormant microbe remains on the rover that could reproduce when it makes contact with liquid water, if there is indeed liquid water there?
Or for that matter, it's also important to look at the possibility of the rover creating liquid regions of its own through its internal heat, and those then getting colonized.
We are developing new and better methods of sterilizing, but I'm not sure if we are up to it yet. Our best sterilized rovers so far are still Viking 1 and 2, and the future rovers planned to date, the ones that will definitely happen, are much less sterilized than that.
But Viking 1 and Viking 2 would arguably not be safe as regards planetary protection if you put them in direct contact with liquid water, because they could have some dormant microbes on them still, possibly. At any rate can't say definitively that they don't.
So I'm not sure what will happen there. So far we only have missions planned to the dry equatorial regions for the next decade of so. After that though, maybe they will find a way to look at the intriguing Martian Geysers and Flow-like Features close up.
There are many other ways now proposed for water to form on Mars, usually small micro-habitats of a few mms or cms thickness, but that as Nilton Renno memorably said, is "like a swimming pool to a microbe".
You can read about this and many other ideas for habitats on Mars here.
Are There Habitats For Life On Mars? - Salty Seeps, Clear Ice Greenhouses, Ice Fumaroles, Dune Bioreactors,...
That's because the air is so thin. You may know that mountain climbers when they climb Mount Everest can't make a really hot cup of tea because the water boils at a lower temperature at altitude. So it's the same on Mars - but more so. With the atmosphere a fiftieth of the pressure you get at the summit of Everest, then the water over much of the surface boils as soon as it melts.
Indeed leave a chunk of ice anywhere in the equatorial region of Mars, even sheltered from the sun and well below zero and eventually it will just evaporate away like dry ice, because the air is so thin. It does it more slowly than dry ice but does eventually, over a period of a few days
Here are a couple of photos taken by the Phoenix lander which shows this phenomenon on Mars.
As you see, there is rather less of that white stuff in the right hand image than the left hand one - in a scrape it dug. Though they couldn't analyse it, this was regarded as proof that the white stuff was water, and not dry ice (dry ice would have evaporated much more quickly) or, say, salt (which wouldn't evaporate). See Proof! Water Ice Found on Mars
So - rather than go liquid, ice on the edge of the polar cap would just sublimate like that into the atmosphere in summer.
So, that's the "No".
But now for the "Yes".
Flow like features
This may be the "smoking gun" of liquid water in the polar regions. The only explanation for them so far involve water in some form or another. Interestingly also, some of the ideas involve pure liquid water, with not much salt in it, at a little above 0C - it might be the warmest liquid water close to the Mars surface, surprising, with it being in the polar regions.
(most of the liquid water suggested for Mars habitats is salty so a little colder than this).
They form in the debris of the Martian Geysers which erupt earlier in the spring.
Artist's impression of the Martian Geysers which have been detected from orbit by the dark marks they leave.
Not observed directly, just indirectly by the dark marks they form on the surface but the evidence is reasonably conclusive that these geysers do exist on Mars, but they are dry ice geysers, not liquid water ones.
But after they erupt, and spread a lot of debris over the surface in dark patches - those patches then gradually extend with these flow like features.
This is very hard to understand as anything else except liquid water in some form. They look a bit different in the Northern hemisphere, and form in slightly different circumstances, and there, they may form at very low temperatures in salty solutions, possibly too cold for life.
But in the Southern hemisphere, close up they look like this.
The preferred model here uses a solid state greenhouse. Clear ice can act to trap infrared to warm up a layer below the surface.
On the Earth this happens with blue ice like this
which is especially clear, and lets through enough heat so that a layer of liquid water can form - and the interesting thing is it forms about half a meter below the surface, in Antarctica.
So on Mars, if it forms at that depth, it's insulated from the vacuum of the atmosphere, so it won't boil away. So can have a stable liquid layer of water actually within the ice.
It happens in Antarctica and there isn't much changed except less gravity on Mars, and about half the light levels. The models say it should form there also, so long as Mars has some form of clear ice.
Now whether Mars does have the right type of clear ice for this to work we don't know. Very hard to model large scale processes like that on an alien planet. Or to simulate in in a laboratory.
But if it does, well, it almost certainly has these liquid layers just like Antarctica. More so if there happens to be a rock layer to get warmed up and retain heat at the right depth or pebbles and sand mixed into the ice.
There are other ways the flow like features in Richardson crater could form, but they all so far involve liquid water in some form (the other ones involve rather colder water).
Though not so well known as some of the other possible habitats on Mars, in some ways it's one of the most interesting and exciting, I think as interesting as the much better known Warm Seasonal Flows.
In this model the liquid layer starts off millimeters thick, but it is cumulative, stays liquid overnight, and gets thicker and thicker until eventually it can be several cms thick - and it forms about 5 - 10 cms below the surface.
So, will we send any rovers to Mars to look for this?
Well, they'd love to look at the geysers close up, so this would be a natural thing to add in to a mission to observe the geysers. But this is a really big challenge for planetary protection. Because, you are talking about ice here, and liquid water. Can we so thoroughly sterilize our rovers that not a single dormant microbe remains on the rover that could reproduce when it makes contact with liquid water, if there is indeed liquid water there?
Or for that matter, it's also important to look at the possibility of the rover creating liquid regions of its own through its internal heat, and those then getting colonized.
We are developing new and better methods of sterilizing, but I'm not sure if we are up to it yet. Our best sterilized rovers so far are still Viking 1 and 2, and the future rovers planned to date, the ones that will definitely happen, are much less sterilized than that.
But Viking 1 and Viking 2 would arguably not be safe as regards planetary protection if you put them in direct contact with liquid water, because they could have some dormant microbes on them still, possibly. At any rate can't say definitively that they don't.
So I'm not sure what will happen there. So far we only have missions planned to the dry equatorial regions for the next decade of so. After that though, maybe they will find a way to look at the intriguing Martian Geysers and Flow-like Features close up.
There are many other ways now proposed for water to form on Mars, usually small micro-habitats of a few mms or cms thickness, but that as Nilton Renno memorably said, is "like a swimming pool to a microbe".
You can read about this and many other ideas for habitats on Mars here.
Are There Habitats For Life On Mars? - Salty Seeps, Clear Ice Greenhouses, Ice Fumaroles, Dune Bioreactors,...
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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