ESA : Desert bullseye

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A giant, geological wonder in the Sahara Desert of Mauritania is pictured in this satellite image.

The 40 km-diameter circular Richat structure is one of the geological features that is easier to observe from space than from down on the ground, and has been a familiar landmark to astronauts since the earliest missions.

Once thought to be the result of a meteor impact, researchers now believe it was caused by a large dome of molten rock uplifting and, once at the surface, being shaped by wind and water into what we see today. Concentric bands of resistant quartzite rocks form ridges, with valleys of less-resistant rock between them.

The dark area on the left is part of the Adrar plateau of sedimentary rock standing some 200 m above the surrounding desert sands. A large area covered by sand dunes – called an erg – can be seen in the lower-right part of the image, and sand is encroaching into the structure’s southern side.

Zooming in on the southern side of the bullseye, we can see individual trees and bushes as tiny dots. These follow a river-like structure that appears to have been dry when this image was acquired, a few weeks after the rainy season. Some areas to the south and east of the Richat appear to be covered with temporary lakes, which are dry for most of the year.

This image, also featured on the Earth from Space video programme, was acquired on 23 November 2010 by the Advanced Visible and Near Infrared Radiometer on Japan’s ALOS satellite.

ESA
Guillermo Gonzalo Sánchez Achutegui
ayabaca@gmail.com
ayabaca@hotmail.com
ayabaca@yahoo.com
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NASA : Cloud Bands Over the Western Sahara Desert, Mauritania

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Cloud Bands Over the Western Sahara Desert, Mauritania

This photograph of cloud bands over southern Mauritania was taken from the International Space Station with an oblique angle such that the cloud shadows are a prominent part of the view. Beneath the clouds, the plateau of dark sedimentary rocks appears as a ragged, near-vertical escarpment (image top right). Isolated remnants of the plateau appear as dark mesas (flat-topped hills) across the top and near the center of the image. The escarpment is about 250 meters high, with a field of orange-colored dunes at the base (image upper right).

Prevailing winds in this part of the Sahara Desert blow from the northeast. (Note that north is to the right.) The wavy dunes are aligned transverse (roughly right angles) to these winds. The sand that makes the dunes is blown in from a zone immediately upwind (just out of the bottom of the image), where dry river beds and dry lakes provide large quantities of mobile sand. This pattern is typical in the western Sahara Desert, where plateau surfaces are mostly dune free and dune fields are located in the surrounding lowlands. Larger rivers deposit sandy sediment on the few occasions when they flow, sometimes only once in decades.

Astronaut photograph ISS038-E-26862 was acquired on Jan. 8, 2014, with a Nikon D3S digital camera using a 180 millimeter 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 38 crew. It has been cropped and enhanced to improve contrast, and lens artifacts have been removed.

> View annotated image

Image Credit: NASA
Caption: M. Justin Wilkinson, Jacobs at NASA-JSC

NASA
Guillermo Gonzalo Sánchez Achutegui
ayabaca@gmail.com
ayabaca@hotmail.com
ayabaca@yahoo.com
Inscríbete en el Foro del blog y participa : A Vuelo De Un Quinde - El Foro!