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The north polar cap of Mars is the only place on the surface of the
planet that is known to have water. Of course, the water there
is frozen. Unfortunately, the martian north polar cap has been a
difficult place for the Mars Global Surveyor (MGS) Mars Orbiter Camera
(MOC) to view. Each winter, the pole spends approximately 6 months in
darkness. Each spring, everything is covered with frost. In summer and
through autumn, the cap is often obscured by clouds–sometimes clouds
of dust from raging dust storms, and sometimes clouds of water ice
crystals. However, a period of excellent viewing conditions occurred
early in the MGS Extended Mission (from February through April
2001). This image, taken by MOC in April 2001, shows the layers
comprising the north polar cap exposed in an arcuate scarp that occurs
at one end of Chasma Boreale. MOC images acquired in 1999 showed that
the polar cap has two types of layers: there is a stack of
light-toned, nearly uniformly-bedded layers at the top, and a stack of
darker-toned beds that form shelves and benches at the bottom. The
darker, lower beds are older. Dozens of MOC images were targeted
during the clear-atmosphere period in 2001 to test the MOC team’s
hypotheses about the polar cap layers and these images have helped in
documenting the nature of these layers. The lower, dark layers of the
polar cap appear to include considerable amounts of sand, while the
upper layers lack sand and instead may be a mixture of ice and
dust. The lower layers appear to contribute sand to the dune fields
that surround the polar cap, though no dunes are present in the image
shown here. This image is illuminated from the lower right and covers
an area 14.5 km (9 mi.) across. The scarp slopes toward the bottom of
the scene.
