A piece of Mars: A single dune sits on the surface of Mars, not too far from the north pole. It’s early spring, but this far north the dune is still covered in white CO2 frost (as well as a thin yellow layer of airfall dust). But the sun has done some work already: the dark spots are areas where enough frost is gone from the warm sun’s rays, revealing the lovely black sand beneath. Before long the frost will be gone and the dark dune will be fully released from its wintery blanket. (HiRISE ESP_033729_2565, NASA/JPL/Univ. of Arizona)
Update: See my post on July 22, 2014 showing a summertime version of this same dune.