The Dotted Dunes of Mars. As spring dawns on the Northern Hemisphere of Mars, dunes of sand near the poles begin to defrost. Thinner regions of ice typically thaw first revealing sand whose darkness soaks in sunlight and accelerates the thaw. The process might involve sandy jets exploding through the thinning ice. By summer, the spots expand to encompass the entire dunes that are then completely thawed and dark. The carbon dioxide and water ice sublime in the thin atmosphere directly to gas.