Sand sheets and ripples

A piece of Mars: Not all sand piles up into big dunes. If the grains are the wrong grain size (too fine or too coarse), then it might just form sand sheets with ripples on top, like it does here, slowly migrating from top left to bottom right. There are two different kinds of sand here: the ripples seem more grayish and the underlying sand sheet seems more brownish. Grains of different sizes and densities respond to the wind in varying ways, so that they form different features on the surface. (HiRISE ESP_041977_1515, NASA/JPL/Univ. of Arizona)