Mars is the only other planet on which we have identified wind-shaped
(_eolian_) features. Although its surface atmospheric pressure is only
about one-hundredth that of Earth, global circulation patterns on Mars
have formed a circumpolar sand sea of more than five million square
kilometers, an area greater than the Empty Quarter of Saudi Arabia, the
largest sand sea on our planet. Martian sand seas consist predominantly
of crescent-shaped dunes on plains near the perennial ice cap of the
north polar area. Smaller dune fields occupy the floors of many large
craters in the polar regions.
[Illustration: This Viking spacecraft image of Mars shows alternating
layers of ice and windblown dust near the north polar cap. Annual and
other periodic climatic changes due to orbit fluctuations may occur on
Mars (courtesy of USGS Image Processing Facility, Flagstaff, Arizona).]
[Illustration: One of the first images taken at the Viking 2 landing
site on Mars shows the pink sky over Utopia and the desert pavement on
the ground (courtesy of NASA).]
[Illustration: Some of the crescent-shaped dunes in this Viking image of
Mars are more than a kilometer wide. The dark material that streaks from
the horn-shaped features may be dust recently blown from the dunes
(courtesy of NASA).]
[Illustration: Natural Bridge, Arches National Monument, Utah
(photograph by Peter Kresan).]