As on Earth, the seasonal frost caps of Mars grow and recede each year. But seasonal frost on Mars is composed of carbon dioxide ice (also known as dry ice), not water ice as on our planet.

Near the south pole of Mars, the seasonal carbon dioxide frost never completely disappears, leaving a residual ice cap of carbon dioxide ice throughout the summer. This HiRISE image shows part of the south polar residual cap, with many shallow pits dubbed "Swiss cheese" terrain. Because the sun is always low in the sky at this latitude, the steep walls of the pits receive more solar energy than the high-standing, flat areas between the pits. This causes the walls of the pits to retreat several meters per year as sunlight causes the carbon dioxide ice to evaporate directly to gas, a process called "sublimation."

In some depressions, ridges or blocks of material a couple of meters (several feet) across are visible at the base of the depression walls, likely fallen from the walls during the sublimation and retreat process. At this rate, the layer of carbon dioxide ice could completely disappear in about 100 years if not replenished.


Written by: Ken Herkenhoff

All of the images produced by HiRISE and accessible on this site are within the public domain: there are no restrictions on their usage by anyone in the public, including news or science organizations. We do ask for a credit line where possible: Image: NASA/JPL/University of Arizona

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.