The Martian Southern Polar Cap

From Bad Astronomy Blog

This is a composite image of the south polar cap of Mars by Riding with Robots creator Bill Dunford using images taken by the European Space Agency's Mars Express Orbiter. Most of the white mass is frozen water permafrost covered in a few metres of frozen carbon dioxide, which melts and refreezes during the long martian seasons. The melting of this few metres of carbon dioxide during the summer is enough to noticeably thicken the atmosphere of that hemisphere.