Richat Structure, Mauritania
Viewed from space, the Richat Structure forms a conspicuous 50-kilometer-wide bull’s-eye on the Maur Adrar Desert in the African country of Mauritania. Described by some as looking like an outsized fossil, the feature has become a landmark for astronauts. Although it resembles an impact crater, the structure formed when a volcanic dome hardened and gradually eroded, exposing the onion-like layers of rock. In this 2001 Landsat 7 image, desert sands appear white and pale yellow at the top left and lower right corners of the scene. Less sandy, rocky areas are green, and volcanic rocks are blue.