The Dotted Dunes of Mars . ... Credit Copyright: Malin Space Science Systems , MOC , MGS , JPL , NASA Explanation: What causes the black dots on dunes on Mars? As spring dawns on the Northern Hemisphere of Mars, dunes of sand near the poles being to defrost. ... The carbon dioxide and water ice actually sublimes in the thin atmosphere directly to gas. Taken in mid-July, the above image shows a field of spotted polar dunes spanning about 3 kilometers near the Martian North Pole. ...