Crescent-shaped barchan dunes are common on both Earth (top image) and Mars (bottom image). They form in areas where the wind comes predominantly from one direction. As the wind blows, it deposits sand on the gently sloping windward face of the dune. The leeward face of the dune is steeper; its shape is set by the sand’s angle of repose–essentially the steepest angle the sand can maintain without an avalanche. Barchan dunes are very mobile, moving between one and a hundred meters per year. They have also been seen moving through one another or moving along in formation. (Image credits: Google Earth, NASA/JPL/University of Arizona)