The desert creates gorgeous structures, and the Eye of the Sahara is no exception. The stunning structure of bare rock peeks out from a sea of sand, forming a beautiful landmark for astronauts overhead. While circular usually means impacts or eruptions, the eye emerged from differential erosion.
Top Image: The Richat Structure in Mauritania as seen from the International Space Station. Credit: NASA/Roscosmos/Oleg Artemyev Hermanovitch
The massive 40-kilometer diameter circular structure in the Sahara Desert of Mauritania is also known as the Richat Structure or Guell al-Richat.
Contextual view of the Eye of the Sahara (lower right) within the Sahara desert (top and left). Image credit: ESA
Northwest of the eye is with Kediet ej Jill Mountain, Mauritania's highest peak (nearly 1000 meters). The mountain has a large concentration of magnetite, giving it a blueish tinge and totally messing up magnetic surveys and navigation nearby. The western Sahara fills the north.
Landsat satellite image draped over a digital elevation model with 6x vertical exaggeration and enhanced colour. Image credit: NASA/JPL/NIMA
The entire plateau of sedimentary rocks are 200 meters above the desert, and 485 meters above sea level.Prevailing winds from the northeast (lower right) funnel around the bedrock cliffs, sweeping sand clear and exposing a sand-barren streak downwind.
In the Landsat image (above), colour is enhanced by blending visible and infrared light to emphasize particular geologic features. Bedrock is brown, sand yellow and white, vegetation in the drainage channels are green, and salty sediments or evaporates are blue-whites. The Aster image (left) is similarly enhanced, with bare rock in green-grey.
Colour-enhanced view of the structure composed of visible and infrared light. Image credit: NASA/GSFC/MITI/ERSDAC/JAROS
Along with being downright striking, its exact origin of the Eye of the Sahara was originally subject to lively debate. Circular usually translates into impact crater, but this structure is missing shattered glass and shocked quartz that marks a high-energy impact. Later theories that it was some form of volcanic crater were equally as unsupported, lacking any of the defining characteristic of volcanoes.
The Adrar plateau of exposed sedimentary rocks, surrounded by an erg, a sea of sand dunes. For scale, the tiny dots on the south side of the bullseye are trees and bushes. Image credit: ESA/JAXA
Instead, the reigning theory is that this is the result of differential erosion of a geologic dome, a circular anticline revealed by slowly shedding layers under the erosive power of wind-blown sand. Resistant beds of Paleozoic quartzites mark the rings as high ridges with less resistive beds eroding into valleys. Now the whole structure exposed as a flat cross-sectional bulls-eye of underlaying strata peeking out. You can learn more about its formation in this video from the European Space Agency.