As a planetary geologist (and co-author of the Nature article in question), we suspect water as the fluid involved partly because of the large amount of water in the polar ice caps, the small amount of water in the atmosphere, and the general abundance of water in the solar system and universe (hydrogen is the most common element, and oxygen is up there too). Plus, the climate conditions on Mars are pretty close to allowing liquid water, so it is reasonable to suspect it could have been liquid under past climates.
As one of the authors of the Nature article, I'd like to respond and say that it is not about liquid water, but rather about water ice. The ice collects in the dusty surface during certain climate conditions, then sublimates (solid->vapor) under warmer conditions.
However, there are many situations that others have touched on that allow liquid water to exist on the surface of Mars under current conditions, at least temporarily. The phase diagram only tells us that it is not thermodynamically stable, not whether it may exist unstably (i.e. boiling away). This is a kinetic problem. Imagine that water exists in liquid form underneath the surface (i.e. the added pressure of the rock above moves you into a stable zone in the phase diagram). Then if some of this water is moved to the surface , it will take some time for it to freeze or evaporate.
Again, though, this isn't the case for our terrain that we reported on in Nature.
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As a planetary geologist (and co-author of the Nature article in question), we suspect water as the fluid involved partly because of the large amount of water in the polar ice caps, the small amount of water in the atmosphere, and the general abundance of water in the solar system and universe (hydrogen is the most common element, and oxygen is up there too). Plus, the climate conditions on Mars are pretty close to allowing liquid water, so it is reasonable to suspect it could have been liquid under past climates.
As one of the authors of the Nature article, I'd like to respond and say that it is not about liquid water, but rather about water ice. The ice collects in the dusty surface during certain climate conditions, then sublimates (solid->vapor) under warmer conditions. However, there are many situations that others have touched on that allow liquid water to exist on the surface of Mars under current conditions, at least temporarily. The phase diagram only tells us that it is not thermodynamically stable, not whether it may exist unstably (i.e. boiling away). This is a kinetic problem. Imagine that water exists in liquid form underneath the surface (i.e. the added pressure of the rock above moves you into a stable zone in the phase diagram). Then if some of this water is moved to the surface , it will take some time for it to freeze or evaporate. Again, though, this isn't the case for our terrain that we reported on in Nature.