Different day, different sand

A piece of Mars: Creamy dunes (or maybe they’re very large ripples) wind their way northward between mesas. More recently darker, more mobile sand (bluish here) has piled up against the western slopes of the mesas. We don’t know why the darker sand doesn’t form into dunes like the lighter material – perhaps there simply isn’t enough of it in this spot. (ESP_031993_2115, NASA/JPL/University of Arizona)