Dark sand dunes at high Northern latitudes on Mars are covered seasonally by a layer of condensed carbon dioxide (dry ice), visible in this image.
When the sun rises in the spring the ice begins to sublimate. Gas flow from the bottom of the ice layer propels sand from the dunes out through cracks to the top of the ice and down the dune slipface forming the dark streaks seen in this image.
Written by: Candy Hansen (3 February 2011)
More info and image formats at http://hirise.lpl.arizona.edu/ESP_016190_2635
Image: NASA/JPL/University of Arizona