A Piece of Mars: This is the crest of one of the largest dunes on Mars (0.5×0.5 km or 0.31×0.31 mi). The wind mostly blows from the right, slowly pushing sand up the windward slope. But frost accumulates on (and probably in) the sand during winter, and sometimes it gets too heavy and slides down the steepest slope (toward the left), carving out big gullies in the sand. And then the wind blows some more, trying to erase the gullies by 1) making ripples, 2) burying the gullies (the featureless blue patches are grainfall, which is a fancy term for sand that fell as airfall), and 3) forming dust devils that leave faint but wide tracks. Who wins this fight, wind or ice? Neither: gravity wins (it usually does). (HiRISE ESP_020876_1330, NASA/JPL/Univ. of Arizona)

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