What causes the black dots on dunes on Mars? Here is the explanation of this great Mars photo. As spring dawned on the Northern Hemisphere of Mars in 2004, dunes of sand near the poles begin to defrost. Thinner regions of ice thaw first revealing sand whose darkness soaks in sunlight and accelerates the thaw. The thaw process may involve sandy jets exploding through the thinning ice. By summer, the spots expanded to encompass the entire dunes that were then completely thawed and dark. The carbon dioxide and water ice actually sublime in the thin atmosphere directly to gas. Taken in July, the above image shows a field of spotted polar dunes spanning about 3 kilometers near the Martian North Pole.