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The HiRISE camera on the Mars Reconnaissance Orbiter snapped this series of pictures of sand dunes in the north polar region of Mars. The progression of false color pictures begins in early spring (panel a) when the ground is covered by a seasonal layer of carbon dioxide ice (dry ice) about 2 feet thick. As spring progresses the ice cracks (panel b), releasing dark sand from the dune below. When pressurized gas trapped below the ice layer is released it carries along sand and dust to the top of the ice layer where it is dropped in fan-shaped deposits downhill and downwind (panels c and d). The final panel shows more and more of the dark dunes as the overlying layer of seasonal ice evaporates back into the atmosphere.
Image credit: NASA/JPL-Caltech/Univ. of Arizona