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.In this type of geologic structure, rocks exposed in the center of the "bullseye" are older than rocks forming the outer rings. This structure measures 45 kilometers (28 miles) across and is made up of igneous and sedimentary rocks. There are several faults visible (lower left) where strata that were once continuous have been shifted apart.