Russell Crater has a large field of dunes, including a mega-dune over 500 meters high.
This image was acquired in winter on Mars, when the dunes are covered with carbon dioxide ice (dry ice). The Sun has just come up, and is just 3 degrees above the horizon. At this low light level small ridges cast long shadows and the sides of channels facing the Sun are bright. This lighting emphasizes the troughs on the dunes and even small ripples can be detected.
Also apparent are "sublimation spots" that are caused by the evaporation of the dry ice. They will fade when the ice is gone.
Photo credit: NASA/JPL/University of Arizona