Ice on Mars - Update

(Followup to an earlier article from the end of May.) NASA is now reporting visual evidence of ice on Mars.

I don't know why some in the media chose the pictures they did, but the best ones among the various pictures available in the JPL Phoenix News photo collection are linked here:

• An animation showing a before/after comparison of two photos illustrates the disappearance of many small chunks of material over four days by calling your attention to the changes.
• A side-by-side display of color images of the same trench, with an enlarged inset showing the chunks, shows that these chunks are white in color.

Very striking.

The key phenomenon here is sublimation, where a solid turns directly into a gas without going through a liquid phase. This is what CO₂ (carbon dioxide, which is "dry ice" in its solid form below -78 C or 195 K) does on earth, and also how snow "dries out" and becomes extremely light powder in places that have low humdity yet are well below freezing. The reverse process, deposition, is the way that frost forms on a cold surface without any liquid water being involved.

The important data we need to interpret these observations are given in the weather report for Mars. The atmospheric pressure is 8.5 millibars (0.1 psi) while the temperature varies from -80 C (193 K) at night to -30 C (243 K) during the day. At this low pressure, CO₂ is a gas across this entire temperature range. (That is why the polar "dry ice" cap has vanished in this location during the Martian summer.) If the white material was dry ice, it would rapidly turn to gas between when the trench was dug and when the photo was taken. If it is salt, it would not sublime. The best candidate is water. Water is a solid at these temperatures and would sublimate slowly, just like when clothes are hung out to dry in the winter.

News stories:
The best article is in the New Scientist. The BBC has a good story on this, as does MSNBC (with a good picture choice), while CNN only blogs it with an irrelevant photo of the sampler to illustrate nothing.