Where does the windblown stuff come from?

A Piece of Mars: How far do windblown materials move on Mars? This scene (0.9×1.2 km, 0.56×0.75 mi) shows a bright layer of bedrock (top right) that is eroding, exposing a darker, bluish rock (bottom left). Ripples 5-20 m wide have slowly moved towards the lower right, with some migrating into the darker terrain. Those near the interface show that they’re made of stuff from the brighter terrain, as they are still brighter than the dark, bluish bedrock. But those at the bottom are much more blue. This means that this type of ripple incorporates material from nearby rocks: unlike other kinds of windblown material, they don’t travel far from their source. (HiRISE, ESP_017262_1560, NASA/JPL/Univ. of Arizona)