Mare Tranquillitatis is a lunar mare that sits within the Tranquillitatis basin on the Moon. It is the first location on another celestial body to be visited by humans.
The Sea of Tranquility of the Moon
A view of the Apollo 11 landing site at center, facing west, with Maskelyne crater in right foreground
The Moon with Mare Tranquillitatis highlighted and the first crewed lunar landing marked.
Buzz Aldrin salutes the U.S. flag on Mare Tranquillitatis during Apollo 11 in 1969.
The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by lava flowing into ancient impact basins. They were dubbed maria by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich composition, and hence appear dark to the naked eye. The maria cover about 16% of the lunar surface, mostly on the side visible from Earth. The few maria on the far side are much smaller, residing mostly in very large craters. The traditional nomenclature for the Moon also includes one oceanus (ocean), as well as features with the names lacus ('lake'), palus ('marsh'), and sinus ('bay'). The last three are smaller than maria, but have the same nature and characteristics.
Ancient rift valleys – rectangular structure (visible – topography – GRAIL gravity gradients) (1 October 2014)
Ancient rift valleys – context.
Ancient rift valleys – closeup (artist's concept).
Irregular mare patch – evidence of young lunar volcanism (12 October 2014)