The Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) is used by the MOC science team as a tool
to test hypotheses about the geology, geomorphology, and meteorology of Mars. In 1999, MOC images
revealed that the layers of the martian north polar cap are divided into two distinct units: an upper,
light-toned sequence of layers, and a lower, darker-toned suite of layers. The team suspected that the lower
unit, because of its dark tone and apparent association with nearby dune fields, might be a source of
windblown sand. However, most of the 1999 images were of very low contrast because the frequent dust
storms in the region made the atmosphere extremely hazy. Very few images of the north polar cap were
obtained in 2000 because it was first hidden during the long winter's night, then coated by springtime frost.
By early 2001, the north polar cap was in summer and the MOC team set out to test the idea that sand is
eroding out of the lower unit.
This picture, obtained in February 2001, shows streamers of dark sand coming from outcrops of the lower,
dark-toned unit. The streamers join a nearby dune field less than a few kilometers (less than a mile) away.
Erosion of the lower layered unit liberates sand that was long ago deposited in these layers. The upper unit,
by contrast, contains almost no sand. Wind erosion of the lower unit leads to creation of steep scarps as the
sand is removed and the upper unit is undermined. The sand moves downwind (in this case, toward the
bottom left of the image) and creates dunes.
The new views of the martian north polar cap obtained in 1999 and 2001 suggest that it may not contain as
much water ice as previously believed. Indeed, the amount of ice may be as little as half of what was once
thought. The picture shown here is 3 km (1.9 mi) wide and illuminated from the lower left.
Another picture showing the upper and lower layered sequences of the north polar cap can be seen by
clicking here: "MOC Extended Mission View of North Polar Layers," 11 February 2002.
Photo Credit: NASA/JPL/Malin Space Science Systems
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