This observation shows linear dunes in the north polar region of Mars. Linear or longitudinal sand dunes are elongated, sharp crested ridges that are typically separated by a sand-free surrounding surface.
These features form from bi-directional winds and extend parallel to the net wind direction. In this case, the net wind direction appears to be from the west-southwest. Linear sand dunes are found in many different locations on Earth and commonly occur in long parallel chains with regular spacing.
Superimposed on the surface of the linear dunes are smaller secondary dunes or ripples. This is commonly observed on terrestrial dunes of this size as well. Polygons formed by networks of cracks cover the substrate between the linear dunes and may indicate that ice-rich permafrost (permanently frozen ground) is present or has been present geologically recently in this location.
Written by: Maria Banks (22 October 2008)
More info and image formats at http://hirise.lpl.arizona.edu/PSP_009739_2580
Image: NASA/JPL/University of Arizona