Mars has a vast sea of sand dunes in the high latitude region encircling its North polar cap, known as the North Polar erg. These dunes are made up of basalt and gypsum sand grains.
In some regions of the North Polar erg where the sand supply is limited they take on an elongated crescent shape. The icy ground that the dunes are on top of has irregular polygonal patterns. In other areas with an abundant supply of sand the dunes are continuous (see also PSP_007494_2580 ).
The entire North polar erg is covered in the winter with a seasonal polar cap composed of carbon dioxide (dry ice). In the spring time this seasonal polar cap evaporates.
Written by: Candy Hansen (22 July 2009)
More info and image formats at http://hirise.lpl.arizona.edu/PSP_009324_2650
Image: NASA/JPL/University of Arizona