This detailed astronaut photograph, taken from low earth orbit,
shows classic large and small sand masses of the central Sahara Desert,
where wind is a more powerful land-shaping agent than water. "Draa"
dunes (from the Arabic for "arm") are very large masses of sand, and
they appear here as the broad network of yellow-orange sand masses,
with smooth-floored, almost sand-free basins between them. These sand
masses lie in the western part of Libya's vast Marzuq Sand Sea
(centered at 24.5 degrees north, 12 degrees east). Geologists think
that the draa of the Marzuq were probably formed by winds different
from the prevailing north-northeast winds of today.
Numerous smaller dunes have developed on the backs of the draa.
Three distinct dune types are visible: longitudinal dunes, which are
more or less parallel with the north winds; transverse dunes, which are
usually more curved and formed at right angles to the wind; and star
dunes, in which several linear arms converge towards a single peak.
The upwind sides of the sand masses appear smoother than the
downwind side. Wind is moving sand grains almost all the time. This
means that the draa and the dunes are all moving as sand is added on
the upwind side and blown off the downwind side. Small sand masses move
much faster than large sand masses. The draa are almost stationary, but
the smaller dunes move relatively quickly across their backs. When the
smaller dunes reach the downwind side of the draa, they are
obliterated; their sand is blown across the basins as individual grains.
Astronaut photograph ISS018-E-14770
was acquired on December 20, 2008, with a Nikon D2Xs digital camera
fitted with a 400 mm lens, and is provided by the ISS Crew Earth
Observations experiment and Image Science & Analysis Laboratory,
Johnson Space Center. The image was taken by the Expedition 18 crew. The image in this article has been cropped and enhanced to improve contrast. Lens artifacts have been removed. The International Space Station Program
supports the laboratory to help astronauts take pictures of Earth that
will be of the greatest value to scientists and the public, and to make
those images freely available on the Internet. Additional images taken
by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Caption by M. Justin Wilkinson, NASA-JSC.