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There's exposed water ice on Mars, like in craters like this

At the same time, Mars has dust storm, covering the solar panels of rovers, and occasionally covering the entire planet

So why is this ice not covered in dust?

I understand that dry carbon dioxide ice can maintain a clean surface, as it's seasonally sublimated into the atmosphere and re-deposited, but how come water ice (at least some of it) is also left clean?

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  • $\begingroup$ Because water ice also sublimates? Water follows a similar seasonal cycle on Mars as $CO_2$. We can for example, follow the water ice ring that forms around the polar caps. $\endgroup$
    – AtmosphericPrisonEscape
    Commented Sep 22, 2019 at 21:40
  • $\begingroup$ @AtmosphericPrisonEscape Perhaps you mean " because water also freezes out of the Martian atmosphere" ? $\endgroup$
    – Carl Witthoft
    Commented Sep 23, 2019 at 18:11
  • $\begingroup$ who says it isn't, at times? On earth, ice gets covered in dust, and then the wind picks up and the dust gets blown away. No reason to believe the same isn't happening on Mars. $\endgroup$
    – jwenting
    Commented Sep 24, 2019 at 7:53
  • $\begingroup$ Frost at the Viking two landing site (1976): planetary.org/multimedia/space-images/mars/… $\endgroup$
    – Wayfaring Stranger
    Commented Sep 26, 2019 at 17:00
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    $\begingroup$ Check recently asked in Space SE: space.stackexchange.com/questions/64681/…. It is essentially same question but there is a good answer. $\endgroup$
    – Nilay Ghosh
    Commented Nov 6, 2023 at 16:09

2 Answers 2

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A lot of the water ice on mars is covered with dust. NASA's Phoenix mars lander only had to scrape off some of it to find the layer of ice underneath as seen here: https://www.nasa.gov/mission_pages/phoenix/images/press/sol_020_024_change_dodo_v3.html

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    $\begingroup$ I'm not implying that all ice is clean out in the open, I'm asking why the exposed ice is exposed. $\endgroup$
    – SE - stop firing the good guys
    Commented Sep 25, 2019 at 23:52
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    $\begingroup$ The Martian atmosphere is too thin for water to exist in a liquid state so the ice will just freeze directly to a solid and then evaporate back into a gas. Usually this cycle follows the seasons, and if the ice freezes out in the winter and evaporates in the summer faster than it can get coated in dust, it just stays exposed throughout the Martian winter before evaporating. $\endgroup$
    – Kyle
    Commented Sep 25, 2019 at 23:58
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    $\begingroup$ To add, water ice is following the same seasonal sublimation process you outline in your question for CO2, similarly leaving a clean surface. $\endgroup$
    – Kyle
    Commented Sep 26, 2019 at 0:22
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One hypothesis:

at first there is a rapid decrease in albedo, which is then followed by a rapid increase. The decrease could be due to dust contamination after all the CO2 frost sublimation; as temperature continues to rise up, dust could thermally sink into the water–ice and/or fall into cracks between larger grains of water–ice
A conceptual model for explanation of Albedo changes in Martian craters, p.889.

Also, the north polar ice cap is slowly thickening, by 70 microns per year, which might bury the dust.

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