Today's Image of Mars shows the Martian equivalent of terrestrial barchan dunes, which are usually crescent shaped and formed by winds blowing in one uniform direction. These dunes are located in the North Polar region of Mars.
Dunes like these are useful in determining the direction of the wind in this region of Mars because the side facing the wind becomes more eroded, resulting in a sheer, less rounded face. The wind also creates horn-like features oriented in the direction the wind predominantly blows.
In this image the carbon dioxide ice on the dunes is just beginning to sublimate, revealing the dark basaltic sand underneath. The north polar Martian winter is just ending so most of the ground is still covered in the carbon dioxide ice, but by the time summer arrives it will have fully thawed, leaving the ground as dark as the patches in this image.
Clicking on this image will take you to the original, captioned HiRISE image.