ESP_025126_2640_1.0xA piece of Mars: Mars can be a strange place. This is actually a sand dune on Mars not far from the north pole. Here it’s imaged in the springtime when the dunes are still covered in  bright CO2 frost, which is in turn overlain by yellowish dust that has fallen out of the atmosphere. The dark patch is a spot where the sunlight has penetrated the ice cover enough to allow some defrosting to begin — the dark line in the middle is close to the true color of ice-free sand. (HiRISE ESP_025126_2640, NASA/JPL/Univ. of Arizona)

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