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The south polar residual cap of Mars is composed of layered,
frozen carbon dioxide. In 1999, the Mars Global Surveyor (MGS)
Mars Orbiter Camera (MOC) showed that the carbon dioxide layers
have been eroded to form a variety of circular pits, arcuate
scarps, troughs, buttes, and mesas. In 2001, MOC images designed
to provide repeated views of the areas imaged in 1999–with the
hope of creating stereo (3-D) images, so that the height of
scarps and depth of pits could be measured–showed that the
scarps had retreated, pits enlarged, and buttes and mesas
shrank. Only carbon dioxide is volatile enough in the
martian environment to have caused such dramatic changes–the
scarps were seen to retreat at an average rate of 3 meters
(about 3 yards) per Mars year. Most of the scarp retreat
occurs during the southern summer season; in some areas
the scarps move as much as 8 meters, in others, only 1 meter
per Mars year.
Three Mars years have now elapsed since MOC first surveyed
the south polar cap in 1999. Over the past several months,
MGS MOC has been re-imaging areas that were seen in 1999,
2001, and 2003, to develop a detailed look at how the
landscape has been changing. This animated GIF
provides an example of the dramatic changes that have
occurred during the past three martian years. The first
image, a sub-frame of M09-05244, was acquired on 21 November 1999.
The second image, a sub-frame of S06-00973, was obtained
on 11 May 2005. The animation shows the changes that have
occurred between 1999 and 2005. Each summer, the cap has
lost more carbon dioxide. This may mean that the carbon
dioxide content of the martian atmosphere has been
increasing, bit by very tiny little bit, each of the
years that MGS has been orbiting the red planet. These
observations also imply that there was once a time, in
the not-too-distant past (because there are no impact
craters on the polar cap), when the atmosphere was somewhat
thinner and colder, to permit the layers of carbon dioxide
to form in the first place. Just as Earth’s environment
is very different today than it was just 11,000 or so
years ago, the martian environment has also been changing
on a similar time scale.
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