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It says 1.5 km down. I'd expect there to be significant pressure down there. Enough to make it liquid at -10C? I suspect so, but I'm not an expert on the pressure gradient on Mars...

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Mars only has a surface gravity of about 3.71m/s^2 compared to Earth's 9.81m/s^2. This means that 1.5km of ice will just create something like 0.38 times the pressure the same column of ice of the same density would create here on Earth. If this ice were densely packed, the mass of a 1m^2 column of ice that high would be a bit less than 1400 tons. This creates a pressure of 5.1MPa or around 51 bar.

Now if I could get my hand on a phase diagram for water on which I could find that point (263K, 5.1MPa) I could tell you how close pure ice is to melting under these conditions.

The melting point does not really budge much for standard water.

https://upload.wikimedia.org/wikipedia/commons/0/08/Phase_di...

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