The dark sand dunes here are eroded in many places, forming gullies. Some of these gullies were not present in prior images of this spot, or have grown in size.
Monitoring of dune gullies by HiRISE has shown us when the gullies are active: in the winter or early spring! This was a surprise, because the gullies look like ones on Earth that are formed by flowing water or wet debris.
It is far too cold for water to be liquid in the Martian winter, but there is carbon dioxide frost (dry ice) on the ground, lasting through the spring at high latitudes. This frost may serve to reduce grain-to-grain friction, allowing the sand to flow on the steep slopes.
Written by: Alfred McEwen (12 January 2011)
More info and image formats at http://hirise.lpl.arizona.edu/ESP_020797_1280
Image: NASA/JPL/University of Arizona