Astronauts love photographing the Richat Structure, or Eye of the Sahara, from space. But what is this strange bull’s-eye formation?

What is the strange bull’s eye in the Sahara, oft photographed from space?

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Published: Monday, 15 July 2024 at 08:33 AM

Lying in the middle of the Sahara desert is the Richat Structure, a gigantic circular structure some 40km in diameter that can be seen from space, and which resembles a bull’s eye or target.

The Richat Structure, or the ‘Eye of the Sahara’, as it’s sometimes known, has become a favourite landmark for astronauts photographing Earth from space on board the International Space Station.

Photo of the Richat Structure, or the Eye of the Sahara, in Mauritania, captured from the International Space Station on 10 July 2020. Click image to expand. Credit: NASA

Discovery and analysis

The Richat Structure is located near the town of Ouadane in the Adrar Region of Mauritania.

Locals have known about it for centuries, but no western geologist visited the region until the 1930s.

At that time, the Richat Structure was believed to be an impact crater.

That’s understandable: it looks like one, with its raised outer lip and sunken centre.

And there are three other “crateriform or circular irregularities” (in the words of French naturalist Theodore Monod, who led an expedition to the region in 1952) nearby that were formed as a result of an asteroid or meteor impact.

Further studies revealed the Eye of the Sahara isn’t an impact structure, but has resulted from a combination of different geological, volcanic and meteorological processes.

Photo of the Richat Structure, or the Eye of the Sahara, in Mauritania, captured from the International Space Station by astronaut Nick Hague in Septemebr 2019. Credit: NASA
Photo of the Richat Structure, or the Eye of the Sahara, in Mauritania, captured from the International Space Station by astronaut Nick Hague in Septemebr 2019. Click image to expand. Credit: NASA

How the Richat Structure formed

Stand in the Sahara – in fact, stand most places on Earth – and beneath your feet are layers (strata) of different kinds of sedimentary rock.

In the case of the Richat Structure, volcanic magma has at some point in the distant past intruded into these layers, causing them to bulge upwards and form a dome shape.

This dome became subject to erosion by wind and, as this is the Sahara, vast quantities of sand carried by that wind.

The domed shape exposes different layers of sedimentary rock in different places, and some rocks erode more quickly than others.

Image of the Richat Structure captured on 23 November 2010 by the Advanced Visible and Near Infrared Radiometer on Japan’s ALOS satellite. Credit: ESA/JAXA
Image of the Richat Structure captured on 23 November 2010 by the Advanced Visible and Near Infrared Radiometer on Japan’s ALOS satellite. Click image to expand. Credit: ESA/JAXA

This transformed the dome into a series of circular ridges of different heights that look, from above, like a bull’s eye or target.

Or, as discussed, an impact crater!

But the Richat Structure isn’t an impact crater: it’s a perfectly natural geological feature that just happens to look cool in pictures taken from the International Space Station.

Something else the Richat Structure isn’t, incidentally, is the so-called Lost City of Atlantis, no matter what an assortment of YouTubers and Redditors would have you believe.

But we’ll leave that one for the geologists to explain!