Mars Probe Reveals Dry Ice, Pointing to a Wetter Past - TIME

"We knew there was some CO2 at the pole," says Roger Phillips of the Southwest Research Institute in Boulder, Colo., lead author of a report on the discovery in the current issue of Science, "but there's about 30 times more than we thought." (See pictures of the Martian landscape.)
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On Earth, the same sort of wobbles help trigger the ends of ice ages, as warmer polar summers melt glaciers and liberate CO2 stored in the oceans. The CO2 acts as a greenhouse gas, warming the planet even more, which liberates even more CO2 in a feedback loop that brings the planet into a relatively balmy interglacial period. (Climate skeptics love to point to these cycles as proof that climate change is natural — which is true, but irrelevant to what's happening today.)
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Since the CO2 cycles in and out of the atmosphere on a 100,000-year timescale and space probes have only been visiting Mars for a couple of decades, this is all theoretical in one sense. But it's hard to imagine how it could be wrong. The changes in Mars' orientation are inevitable, given the laws of celestial mechanics, and it's easy to calculate how much extra heat the poles would get at times of maximum tilt. Plug it all into a computer model of the climate, and the result is pretty straightforward.