Meridiani Planum is an extended plain on Mars centered at 357.5°E/0.2°N in the western part of an Albedo feature called Sinus Meridiani. Meridiani Planum was the landing site of the Mars Exploration Rover Opportunity (MER) in 2004. In Meridiani Planum cross-bedded sedimentary rocks are exposed that contain crystalline hematite (Fe2O3) and sulfate-rich Minerals such as jarosite. The presence of the latter mineral indicates an acid and saline environment and a soil saturated with liquid water during a considerable period of time. Airy-0, a small crater of 0.5 km in diameter at 0°E/5.1°S, defines the prime meridian of Mars. It is located within the larger (40 km) crater Airy in southern Meridiani Planum.