Another (?same?) Mars Ice question

[dpackage]

Maybe this has already been asked, but why do we know that whatever is sublimating on Mars is water ice? Couldn't it also be solid CO2? (which on earth is also stark white)

[iamlucky13]

CO2 doesn't freeze at the temperatures currently experienced near the poles of Mars. It could be CO2 permafrost, but presumably it's possible to estimate the sublimation rates based on the photos and determine if the sublimation is slow enough to be CO2.

[henk21cm]

Maybe this has already been asked, but why do we know that whatever is sublimating on Mars is water ice? Couldn't it also be solid CO2? (which on earth is also stark white)

A thread on this topic was running, you might find more detailed answers and background information in that thread.

As iamlucky13 suggests, it could be permafrost, although i doubt that. Permafrost lies usually rather deep under the surface, of the order of a meter. If you look at the trench dug by the scoop (for mysterious reasons named after an extinct bird "Dodo"), it is rather shallow, JPL says 7 to 8 cm. At that depth the temperature is subjected to the diurnal temperature fluctuations. At a depth of a 2 meters just the yearly variation is visible, the diurnal fluctuations are completely dampened out. These are necessary conditions for the existence of permafrost for a period of more than a few years.

Yet another condition must be met: the average surface temperature (over a year) must below the freezing point of the liquid or gas, for CO2 150K at the current atmospheric pressure. IMHO, the latter condition can not be met.