These dark features on Mars are called barchan sand dunes. They're in the north polar region of the planet.
Barchan dunes are simple, rounded forms with two horns that extend downwind.
Differences in local wind patterns may result in one horn being extended farther
than the other, as is the case for several dunes in this image, according to
scientists at Malin Space Science Systems.
The picture, released this week, also shows several barchans may merge to form
a long dune ridge. The horns and attendant slip faces on these dunes indicate
wind transport of sand from the upper left toward the lower right.
The image was captured by NASA's Mars Global Surveyor. It shows a region near 77.6°N, 103.6°W. The picture covers an area about 1.9 miles (3 kilometers) wide. Sunlight illuminates the scene from the lower left.
