This HiRISE image shows sand dunes near the eastern part of North Polar erg.
The dunes imaged here are similar to barchan dunes, commonly found in desert regions on Earth. Barchan dunes are generally crescent-shaped with a steep slip face bordered by horns oriented in the downwind direction. Barchan dunes form by winds blowing mainly in one direction and thus are good indicators of the dominant wind direction when the dunes formed.
The dunes and surrounding surface appear bright because they are covered with seasonal frost left over from the Northern hemisphere winter. Sunlight is now falling on the North Polar region, and carbon dioxide frost that accumulated during winter is sublimating (going directly from solid to gas) and the surface beneath the frost is being revealed.
Composed primarily of basaltic sand, the dunes will appear dark during the Northern hemisphere summer. The dark spots are areas where some of this frost has begun to sublime away, and/or where wind has exposed the underlying dark sand.
Written by: Maria Banks (11 January 2012)
More info and image formats at http://hirise.lpl.arizona.edu/ESP_025118_2570
Image: NASA/JPL/University of Arizona