This is really cool.*

Below is Astronomy Picture of the Day for 26/9/11.

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These are pits of dry ice near Mars’s South Pole. The gold colour surrounding the pits is very nice but its composition is apparently not known.

Dry ice is not normal water ice but is instead solid carbon dioxide. Unlike most solids, CO₂ does not usually melt but changes directly into a gas. The Astronomy Picture of the Day blurb calls this evaporation but the technical term is sublimation – which describes both the direct solid to gas and the reverse gas to solid processes.

Solid CO₂ can be found in Mars’s polar regions and some of it sublimes in the Martian summer.

Dry ice is most familiar on Earth in that stage effect where you get mist swirling around performers’ feet. The solid sublimes at -56 ^oC and water vapour from the air condenses on the cold gas to form mist.

Carbon dioxide can be made liquid but only at pressures more than 5 times that of Earth’s atmosphere. The Martian atmosphere is of course much, much thinner than that. *And the temperatures can straddle CO₂‘s sublimation point.