NASA: Egypt - Grat Sand Sea - Linear Dunes - Egipto . Gran Mar de Arena - Dunas lineares
Posted by Ricardo Marcenaro | Posted in NASA: Egypt - Grat Sand Sea - Linear Dunes - Egipto . Gran Mar de Arena - Dunas lineares | Posted on 20:08
acquired May 11, 2012
download large image (415 KB, JPEG, 1440x960)
In southwestern Egypt, deep in the
Sahara Desert, the wind dominates the shape of the landscape, as it has
done for the past several thousand years. Winds blowing from the north
have fashioned sands into large dunes, aligned parallel with these
winds.
The so-called linear dunes—shown here in the Great Sand Sea of
southwest Egypt—are easily spotted from space, and local maps show that
they rise 20 to 30 meters above the surrounding plains. The distance
between dunes is interestingly regular, at 1.5 to 2.5 kilometers,
suggesting some equilibrium exists between the wind strength and the
sand supply. It is possible that the linear dunes are a reflection of
earlier times, when winds were stronger or sand more plentiful.
The dark rock outcrops at image lower left stand above the surface by
as much as 150 meters. North winds have been deflected around this high
zone, and smaller secondary linear dunes appear along the left side of
the image, aligned with local winds that become ever more northeasterly
as they approach the outcrops. A dune-free zone on the protected
downwind (south-southeast) side of the outcrop gives a sense of the sand
movement.
At first glance, the large linear dunes appear to be the major
landform in the image; however, a complex pattern of even smaller dunes
can be seen on top of the largest dunes (inset). It is not uncommon to
observe multiple dune forms in large sand seas, as in the Marzuq Sand Sea, the Ar Rub' Al Khali, and the Tenere Desert.
The sand in many dune fields usually derives from some larger river
not very distant upwind; often it comes from a dry river bed that gets
exposed to wind during dry seasons, or from a low-flow river that
changed due to a more arid regional climate. Inland dune fields thus lie
downwind of the source river. West of the dunes shown in this
photograph, a large, unnamed river once flowed to the Mediterranean Sea
and dumped its sand load 300 kilometers northwest of the area shown. It
is likely that this river—evidence of which is now almost completely
obliterated—was the source of the sand in the linear dunes.
Astronaut photograph ISS031-E-30783
was acquired on May 11, 2012, with a Nikon D2Xs digital camera using a
180 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 31 crew. It has been cropped and enhanced to improve contrast, and lens artifacts have been removed. The International Space Station Program supports the laboratory as part of the ISS National Lab
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, Jacobs/ESCG at NASA-JSC.
- Instrument:
- ISS - Digital Camera
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