Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2004 August 31
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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 begin to defrost. Thinner regions of ice typically thaw first revealing sand whose darkness soaks in sunlight and accelerates the thaw. By summer, the spots will have expanded to encompass the entire dunes that will then be completely thawed and dark. The carbon dioxide and water ice actually sublime 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. Meanwhile, in the Southern Hemisphere of Mars, the Earth-sent robot Martian rovers will try to survive through Martian winter, which peaks in mid-September.

Tomorrow's picture: inner neptune

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