This caption is part of a December 2007 AGU presentation "Spring at the South Pole of Mars."

At the very beginning of spring in the southern hemisphere on Mars the ground is covered with a seasonal layer of carbon dioxide ice. In this image there are two lanes of undisturbed ice bordered by two lanes peppered with fans of dark dust.

When we zoom in to the subimage, the fans are seen to be pointed in the same direction, dust carried along by the prevailing wind. The fans seem to emanate from spider-like features.

The second subimage zooms in to full HiRISE resolution to reveal the nature of the "spiders." The arms are channels carved in the surface, blanketed by the seasonl carbon dioxide ice. The seasonal ice, warmed from below, evaporates and the gas is carried along the channels. Wherever a weak spot is found the gas vents to the top of the seasonal ice, carrying along dust from below.

The anaglyph of this spider shows that these channels are deep, deepening and widening as they converge. Spiders like this are often draped over the local topography and often channels get larger as they go uphill. This is consistent with a gas eroding the channels.

A different channel morphology is apparent in the lanes not showing fans. In these regions the channels are dense, more like lace, and are not radially organized. The third subimage shows an example of "lace."

For information about NASA and agency programs on the Web, visit: http://www.nasa.gov. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, Calif., manages the Mars Reconnaissance Orbiter for NASA's Science Mission Directorate, Washington. Lockheed Martin Space Systems is the prime contractor for the project and built the spacecraft. The HiRISE camera was built by Ball Aerospace and Technology Corporation and is operated by the University of Arizona. The image data were processed using the U.S. Geological Survey’s ISIS3 software.