Wind shadow

A Piece of Mars: There’s a dune field migrating past a 230 m (755 ft) diameter crater, creating a 1.6 km (1 mi) long “shadow” that’s empty of dunes. Why? The rim of the crater pokes up just enough to affect the wind, like pebbles in a stream. Either the sand is diverted around the crater, or the rim produces turbulence that increases erosion (or possibly both at different times). I like the dunes that are disrupted as they migrate into the crater. (HiRISE ESP_037948_1645, NASA/JPL/Univ. of Arizona)