This image was taken on the first day of spring in the Southern hemisphere. Like the Earth, Mars is tilted on its axis, and the sun crosses the equator twice each year. On Earth we call this the equinox.
In the winter a seasonal polar cap composed of dry ice covers the polar regions. Although this is just the first day of spring and the sun is very low in the sky, activity associated with the sublimation (going directly from solid to gas) of the polar cap is already underway. A layer of ice coats this entire scene, and fans of dark material are deposited on top of the ice. This dark material is carried to the top of the ice by escaping gas flowing through the spidery channels carved in the surface.
In the summer, when the ice is gone, the fans are no longer visible.
Written by: Candy Hansen (audio by Tre Gibbs) (7 November 2012)
This is a stereo pair with ESP_028957_0930 .
More info and image formats at http://hirise.lpl.arizona.edu/ESP_028956_0930
Image: NASA/JPL/University of Arizona