Bright and dark sand on a dune

A piece of Mars: A lovely little dune is right at the edge of this image, trying its best to migrate into the center of the frame. The bright sickle-shaped part of the dune is its slip face, where sand avalanches down as the wind continually forces the dune to inch forward. The contrast between this bright side and the dark, rippled, windward side, indicates that both bright and dark sand are moving in this scene. (HiRISE ESP_029793_1365, Image credit: NASA/JPL/University of Arizona)