A piece of Mars: As the wind blows sand over terrain, the grains deepen grooves in weak materials, enhancing the topography in the direction of the strongest wind. Here, over eons, sand marching from right to left has formed dunes (oriented perpendicular to the wind) and elongated grooves downwind of low hills. (HiRISE ESP_030582_1850, NASA/JPL/University of Arizona)

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