Today's Image of Mars shows part of a dune field found within Proctor Crater. Proctor Crater is approximately 150km in diameter and lies in the Southern Highlands of Mars.
The dark dunes seen in this image are made up of basaltic sand which has collected at the bottom of Proctor Crater. On top of these large dunes lie smaller secondary dunes consistent with terrestrial dunes of similar size.
In this image you can see that between the large, dark dunes lie smaller, brighter formations. It's been suggested that these are smaller dunes or granule ripples composed of material that is different from the larger dunes. The large, dark dunes overly the the smaller, brighter features, implying that they are a more recent addition to the crater.
Both the large and small features were formed through wind erosion. We can see in this image that the orientation of the smaller features was affected by the placement of the larger dunes.This is because wind must flow around the larger dunes.
HiRISE continues monitoring Proctor Crater to determine whether the dunes are still active today. Clicking on this image will take you to a similar, captioned HiRISE image of dunes in Proctor Crater.