The ring structure shown in this image is located in Ouadane in Mauritania. It has a diameter of about 45 kilometres.
Astronauts have had their attention drawn to this structure ever since the beginning of spaceflight. Since it is easily identified from space it has served astronauts and cosmonauts on their orbital missions as an unmistakable landmark. The ring consists of limestones, dolomites and brecchias from the late Proterozoic to Ordovician eras (aged about 0.6 to 0.5 billion years) that were centrally uplifted and subsequently eroded. The question as to the structure’s origin, which is widely thought to have been originally a meteorite crater, has not been finally answered.
Analysis of the magnatites which prevail in the centre of the formation suggest that the structure is the expression of a cretaceous alkaline complex. Although the exposed layers have formed a shallow ridge which, in many parts, is no more than a few metres high, the structure can be excellently identified and mapped in a radar image thanks to its surface properties.
Credit: DLR; date: July 8, 2007, 18:53 UTC; original resolution: 16 metres (reduced image); mode: ScanSAR Mode; polarisation: VV.