At high latitudes every winter carbon dioxide condenses from Mars’ atmosphere onto the surface forming a seasonal polar cap. In the spring, the Sun shines through this semi-translucent layer of dry ice and heats the ground below.
The ice sublimates (goes directly from ice to gas) on the underside of the seasonal ice layer and the gas is trapped. When the pressure is high enough the ice cracks and ruptures allowing the gas to escape. When the conditions are optimal this gas may condense locally near the source, forming a bright fan.
The dark fans are fine bits of surface material that get carried along by the escaping gas up to above the surface ice. Fine particles are also carried downwind and deposited in dark fans on top of the ice, where they may slowly sink into the ice. The rows of dark fans outline the original crack in the ice that allowed the gas to escape.
Written by: Candy Hansen (audio by Tre Gibbs) (24 January 2013)
More info and image formats at http://hirise.lpl.arizona.edu/ESP_029577_0925
Image: NASA/JPL/University of Arizona