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Beautiful! I guess it's mostly CO2

Edit: holy shit someone else commented that it's water ice. Amazing. I didn't know there was actual water on the surface!

Wouldn't it sublime away within months or weeks in the low-pressure atmosphere?

It won't sublimate if it is always in the shade/in a cold region. See https://en.m.wikipedia.org/wiki/File:Phase_diagram_of_water..... At martian surface pressures (close to 0) it needs to be warmer than ~ -60C to sublimate. Interestingly that's roughly the average temperature measured by rovers. https://en.m.wikipedia.org/wiki/Climate_of_Mars#Temperature

TIL as well. That's pretty cool.