Actually, slight correction on some of the other answers here. Yes it's true that ice sublimes directly to water vapour over much of Mars, because of the low atmospheric pressure (which reduces boiling point of water but has no effect on melting point of ice). But not everywhere on Mars.
There are also many areas where it can be liquid briefly on the surface. It can form a liquid over about 30% of the surface, and at the depths of the Hellas basin the boiling point of water is 10 °C
Making a Splash on Mars
So - if you had a bucket of water at the bottom of the Hellas basin, at just above 0 °C it wouldn't boil away instantly. But it would be within ten degrees of its boiling point and would evaporate quickly.
Indeed in the equatorial regions even ice well below 0 °C will sublime into the atmosphere over time periods of millions of years. So the soil there is probably completely dry to some depth (lower layers of ice gradually moving up to the surface as the top layers dry out). That's why Curiosity doesn't find any ice, even though it's exploring an area that has similar average temperatures to Antarctica.
Still - it's feasible to have liquid water there briefly. The warm seasonal flows in the Valles Marineres also occur in places where the atmospheric pressure is a bit higher, so that water can be stable briefly. The water is probably salty also which helps.
You can also get water forming on the interface between salt and ice. And there may be liquid water also, as much as a few tens of centimeters in thickness, below the surface of the ice in polar regions. Because of the solid state greenhouse effect (transparent ice acts like a greenhouse trapping heat) then when the sun heats up clear water ice, then the surface stays frozen while a layer some way below the surface melts (especially also if there is dust or a layer of rock at just the right depth to get warmed up by the sun).
The poles have ice as well as dry ice. Indeed are mainly ice, with the dry ice as a thin layer of a meter over the northern polar ice cap - and about 8 meters over the Southern cap (which is at a higher altitude).
If the water is trapped under a layer of ice like that, then it doesn't matter that the atmosphere is a near vacuum.
Could also be water in caves. And also you can get deliquescing salts, salts that take up water from the atmosphere. This can be a layer perhaps a cm or so thick, of very salty water. It could be extremely cold - temperature and depth at which it forms depends on the mix of salts.
Curiosity found evidence for a layer like that below the soil of the sand dunes it drives over.
On Mars, Liquid Water Appears at Night, Study Suggests
But it's thought to be too cold for Earth life at least (of course Mars life could be different).
Depends on the composition of the salts though. And in some areas of Mars perhaps the right combinations of salts occur to get layers like that that are liquid at temperatures warm enough for Earth life (down to about -20 °C).
See also:
Page on science20.com
Habitability Of Mars - Salty Seeps, Liquid Layers In Polar Ice, Ice Fumeroles, Sand Dune Bioreactors. ...
There are also many areas where it can be liquid briefly on the surface. It can form a liquid over about 30% of the surface, and at the depths of the Hellas basin the boiling point of water is 10 °C
So - if you had a bucket of water at the bottom of the Hellas basin, at just above 0 °C it wouldn't boil away instantly. But it would be within ten degrees of its boiling point and would evaporate quickly.
Indeed in the equatorial regions even ice well below 0 °C will sublime into the atmosphere over time periods of millions of years. So the soil there is probably completely dry to some depth (lower layers of ice gradually moving up to the surface as the top layers dry out). That's why Curiosity doesn't find any ice, even though it's exploring an area that has similar average temperatures to Antarctica.
Still - it's feasible to have liquid water there briefly. The warm seasonal flows in the Valles Marineres also occur in places where the atmospheric pressure is a bit higher, so that water can be stable briefly. The water is probably salty also which helps.
You can also get water forming on the interface between salt and ice. And there may be liquid water also, as much as a few tens of centimeters in thickness, below the surface of the ice in polar regions. Because of the solid state greenhouse effect (transparent ice acts like a greenhouse trapping heat) then when the sun heats up clear water ice, then the surface stays frozen while a layer some way below the surface melts (especially also if there is dust or a layer of rock at just the right depth to get warmed up by the sun).
The poles have ice as well as dry ice. Indeed are mainly ice, with the dry ice as a thin layer of a meter over the northern polar ice cap - and about 8 meters over the Southern cap (which is at a higher altitude).
If the water is trapped under a layer of ice like that, then it doesn't matter that the atmosphere is a near vacuum.
Could also be water in caves. And also you can get deliquescing salts, salts that take up water from the atmosphere. This can be a layer perhaps a cm or so thick, of very salty water. It could be extremely cold - temperature and depth at which it forms depends on the mix of salts.
Curiosity found evidence for a layer like that below the soil of the sand dunes it drives over.
On Mars, Liquid Water Appears at Night, Study Suggests
But it's thought to be too cold for Earth life at least (of course Mars life could be different).
Depends on the composition of the salts though. And in some areas of Mars perhaps the right combinations of salts occur to get layers like that that are liquid at temperatures warm enough for Earth life (down to about -20 °C).
See also:
Page on science20.com
Habitability Of Mars - Salty Seeps, Liquid Layers In Polar Ice, Ice Fumeroles, Sand 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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