The south polar region of Mars contains numerous features dubbed “spiders.” These are actually narrow shallow channels carved in the ground, probably by gas escaping as the seasonal dry ice cap sublimates (evaporates directly from solid ice to gas).
In the spring they are the source of fans of dust. In the summer, the dark fans are only barely discernible from the surface material, because they are the surface material. Dust gets blown to the top ofand deposited on the seasonal layer of ice as the seasonal cap sublimates.
Although the fans fade away in the summer, the spider channels remain clearly visible (2254 x 1522; 3 MB). Small patches of bright seasonal ice still remain in a few places. Written by: Candy Hansen (6 September 2007)
More info and image formats at http://hirise.lpl.arizona.edu/PSP_004748_0945
Image: NASA/JPL/University of Arizona