The polar ice cap is higher than the surrounding land, and has a cliff around the edge. In spring, some of the ice melts and creates weak spots in the walls. Avalanches of dust, rock, and ice race down the cliffs and smash into the valley floor below, kicking up large dust clouds. The Mars Reconnaissance Orbiter caught this one in action! This whole scene is about two kilometers wide.
Credit: NASA/JPL/University of Arizona