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Only the partial pressure of H2O should matter. So of course that's very low, but it's very low at subzero earth atmospheric temperatures as well. The most I can say here is that I'm surprised by the presumption that this must be water ice, and I don't understand it. Also note, the sublimated ice chunks were evidently in the shade, as well, so I don't understand why their exposure to the atmosphere should have caused the rapid sublimation of these largish chunks of presumed water ice. Out at Jovian and Saturnian distances, ice is a mineral, functionally equivalent to rocks on earth, even on the surface of low atmosphere planetoids with exposure to sunlight.

Well, it's a lot colder out there, and processes such as sublimation tend to have an exponential dependence on temperature. Here's a very pertinent paper, SUBLIMATION OF WATER FROM THE NORTH POLAR CAP ON MARS. They mention sublimation rates of 5cm per year from south-facing cliff scarps. This happens over a summer season, but that's still, what? 200 days? Say 50 times 4 days, so we come up with 0.1 mm for a 4 day period with this figure ... so I don't know.

...so we come up with 0.1 mm for a 4 day period with this figure ... so I don't know.

Oops! 1.0 mm in 4 days. A lot closer but still a stretch, especially with the shaded location.

A lower pressure atmosphere increases the rate of sublimation. It happens more readily, right here on Earth, at higher altitudes.

http://www.lpi.usra.edu/meetings/polar2006/pdf/8031.pdf

Low temp, low pressure, the less chance of finding liquid water. Solid H₂O will sublimate more readily on Mars due to low pressure. I can think of a number of ways in which that ice on Mars not only formed initially, but is now sublimating.

Other bodies in the solar system with probable H₂O ice also have other conditions that vary greatly from those on Mars.