Small view (340 KBytes)
Large view(1.2 MBytes)

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. 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 these
areas in 2001, however, the team discovered that the polar
scarps had changed. They had retreated approximately 3 meters
(about 3 yards) in less than 1 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,tleAwas 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.