The Answer is Blowing in the Wind
Every winter Mars' polar region is covered with a layer of seasonal carbon dioxide ice (dry ice). In the spring jets of gas carry dust from the ground up through openings in the ice. The dust gets carried downwind by the prevailing wind and falls on top of the seasonal ice layer in a fan-shaped deposit. Many jets appear to be active at the same time since numerous fans are all deposited in the same direction.
This image from the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter appears to show several times at which jets were active. At the top of this image the fans are oriented in one direction while at the bottom they are going in a different direction. This suggests that as the ice layer thins, a set of gas jets becomes active, they die down, then further away another set starts up at a later time with a different prevailing wind direction.
This is a reduced-resolution image from the HiRISE Observation observation catalogued as ESP_011934_0945 http://hirise.lpl.arizona.edu/ESP_011934_0945
, taken on Feb. 11, 2009. The observation is centered at 85.4 degrees south latitude, 104.0 degrees east longitude.
The image was taken at a local Mars time of 6:12 p.m.and the scene is illuminated from the west with a solar incidence angle of 79 degrees, thus the sun was about 11 degrees above the horizon. At a solar longitude of 207.9 degrees, the season on Mars is northern autumn.
NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Mars Reconnaissance Orbiter for NASA's Science Mission Directorate, Washington. Lockheed Martin Space Systems, Denver, is the prime contractor for the project and built the spacecraft. The High Resolution Imaging Science Experiment is operated by the University of Arizona, Tucson, and the instrument was built by Ball Aerospace & Technologies Corp., Boulder, Colo.
Image Credit: NASA/JPL-Caltech/University of Arizona
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