Mare Australe

50.0°S, 93.0°E

Mare Australe, the ‘Southern Sea’, lies on the southeastern limb of the Moon. Because of limb foreshortening, details are difficult to observe and a large portion lies on the lunar farside. It is a large impact basin. Lunar Orbiter images show an almost perfectly circular structure with a diameter of over 600 km and an area of roughly 320 000 km2.

Images from lunar probes show, in addition, over 200 craters, with a wide range of sizes. Mare Australe is probably one of the oldest basins and was subsequently only partially flooded with mantle lavas. So there are no traces of the basin's rims, and neither can ejecta from craters that were created later be detected. As a result, the diameter is very difficult to determine, with figures, depending on their source, varying between 500 and 900 kilometres. The Australe basin was probably created before the Late Heavy Bombardment, and the walls of the basin were completely destroyed by subsequent impacts. That would also explain the atypical number of large craters for a mare that occurs in this region.

Hanno 56.3°S, 71.0°E

Pontécoulant 58.7°S, 66.0°E

Oken 43.7°S, 75.0°E

Under good libration conditions, the craters Hanno (55 km), Pontécoulant (90 km) and Oken (70 km) may be readily located. All three stand out because of their dark crater floors (darker than the mare lava).