A Piece of Mars: Sand pours in from the top of this 1.95×1.95 km (1.21×1.21 mi) scene. The sand piles up and up (here ~115 m or 377 ft high), but ahead (at the bottom) is a mountain poking up. Like water diverting around a rock in a stream, the mountain affects the air flow just upwind of it, causing the sand to move around it. The steep dune slope is a slip face, caused by oversteepened sand avalanching. If you look closely, you’ll see some of those narrow avalanches near the bottom of the slip face (those at the top have been covered by ripples and falling sand). (HiRISE ESP_049045_1760, NASA/JPL/Univ. of Arizona)

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