#barchan

The dune appears white because it is early spring in this portion of the northern hemisphere on Mars and the dune is mostly covered by CO2 frost. However, you can see active processes caught in this snapshot; some areas of the dune are darkening where the sun has begun to warm it, causing the CO2 to sublimate. This process takes place every year in Mars’s atmosphere; CO2 migrates from one polar cap to the other as the seasons change. The dune’s crescent shape is sculpted by the winds; they generally blow from the top left to the lower right in this shot. The dune itself is made up of dark sand, probably ground up by the actions of the polar ice caps. Much of northern Mars is covered by sand dunes, although they’re not always as isolated as this one. They appear dark because they’re probably made up of basalt; that’s the same lava type found in most of Mars’s crust, in this case ground up into finer particles by the ice. -JBB Image credit: NASA/JPL/Univ. of Arizona http://cosmicdiary.org/lfenton/2013/12/27/wintery-dune