Today's Image of Mars shows 'swiss cheese' terrain in the South Polar residual caps on Mars. Just like on Earth, Mars has water ice at its poles, but because Mars is much colder carbon dioxide ice is deposited during the winter and sublimates when the climate gets warmer come Spring. This is similar to water ice snow on Earth which typically appears during the winter season and melts during the Spring.
In this image the carbon dioxide ice has already begun sublimating, but only in patches, thus the 'swiss cheese' like terrain. The reason why the exposed terrain is so flat is likely due to a layer of water ice that exists under the carbon dioxide ice. Because water ice has a higher freezing point than carbon dioxide ice it remains solid when the temperatures increase in the spring.
Today is Christmas for those that take part, so here is to hoping that your Christmas is as white as this image on Mars!
Clicking on this image will take you to the original, captioned HiRISE image.
If you like the HiRISE images showcased in the Mars Photo of the Day posts I encourage you to enter for a free 2012 HiRISE Calendar
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