Extended mission operations for the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) has provided thousands of opportunities to image sites previously seen by the camera. Often, these are chances to see if anything on the planet has changed. The most surprising changes were documented starting in August 2001, when the south polar cap emerged from winter darkness.

Photo: NASA/JPL/Malin Space Science Systems

In 1999 MOC found that the south polar cap exhibits an array of bizarre layers, arcuate scarps, and "swiss cheese" holes and pits. How these formed was unknown. Once MOC began to re-image theseareas in 2001, however, the team discovered that the polar scarps had changed. They had retreated approximately 3 meters (about 3 yards) in less than one Mars year (a Mars year is 687 Earth days long). In some places, small buttes completely disappeared (e.g., see arrow).

In December 2001, MOC scientists reported that such rapid change could only have occurred if the south polar cap is composed mainly of frozen carbon dioxide. The image on the left, above, was taken on November 28, 1999. The picture on the right was obtained nearly 1 Mars year later on October 9, 2001. Both images are illuminated from the upper right and each covers an area 2 km (1.2 mi.) wide by 6.9 km (4.3 mi.) long.

Since the initial discovery of scarp retreat in the south polar cap in August 2001, MOC Extended Mission operations have included observation of many changes that occurred since 1999, and acquisition of new data to see how the cap changes from Spring in late 2001 through Summer in early 2002. Additional images have been obtained to help document changes when the polar cap returns to Spring in 2003.

Malin Space Science Systems and the California Institute of Technology built the MOC using spare hardware from the Mars Observer mission. MSSS operates the camera from its facilities in San Diego, CA. The Jet Propulsion Laboratory's Mars Surveyor Operations Project operates the Mars Global Surveyor spacecraft with its industrial partner, Lockheed Martin Astronautics, from facilities in Pasadena, CA and Denver, CO.