Every winter a seasonal polar cap of carbon dioxide ice (dry ice) covers this region. In the spring dramatic bright and dark fans emerge as the dry ice sublimates (changes phase directly from solid to gas).
This region is also host to surface features with radially-organized channels carved in the surface by seasonal erosion by escaping carbon dioxide gas, known colloquially as “spiders.” Rapid changes begin to occur as soon as the sun rises over this terrain as carbon dioxide gas jets escape from under the seasonal ice layer and deposit dark fans of surface material on top of the seasonal ice.
Gradually through the spring the dark fine material sinks into the ice leaving behind bright fans of cleaned ice where the dark fans were.
Written by: Candy Hansen (23 March 2011)
More info and image formats at http://hirise.lpl.arizona.edu/ESP_021522_0930
Image: NASA/JPL/University of Arizona