Bearded hills

A Piece of Mars: Bright hills appear to be bearded (or perhaps mustached?). What’s going on? Dark sand has blown over some yellow-crested hills and settled on the downwind side, where the hill blocks enough wind that it can no longer move sand, and it all collects there in rippled drifts. (scene is 386×290 m, ESP_034084_1655, NASA/JPL/Univ. of Arizona)