Mare Tranquillitatis

Mare Tranquillitatis /træŋˌkwɪlɪˈteɪtɪs/ (Latin for Sea of Tranquillity or Sea of Tranquility) is a lunar mare that sits within the Tranquillitatis basin on the Moon. It contains Tranquility Base, the first location on another celestial body to be visited by humans.

The mare material within the basin consists of basalt formed in the intermediate to young age group of the Upper Imbrian epoch. The surrounding mountains are thought to be of the Lower Imbrian epoch, but the actual basin is probably Pre-Nectarian. The basin has irregular margins and lacks a defined multiple-ringed structure. The irregular topography in and near this basin results from the intersection of the Tranquillitatis, Nectaris, Crisium, Fecunditatis, and Serenitatis basins with two throughgoing rings of the Procellarum basin. Palus Somni, on the northeastern rim of the mare, is filled with the basalt that spilled over from Tranquillitatis.

This mare has a slight bluish tint relative to the rest of the Moon and stands out quite well when color is processed and extracted from multiple photographs. The color is likely due to higher metal content in the basaltic soil or rocks.

Unlike many other maria, there is no mass concentration (mascon), or gravitational high, in the center of Mare Tranquillitatis. Mascons were identified in the center of other maria (such as Serenitatis or Imbrium) from Doppler tracking of the five Lunar Orbiter spacecraft in 1968. The gravity field was mapped at higher resolution with later orbiters such as Lunar Prospector and GRAIL, which unveiled an irregular pattern.

Mare Tranquillitatis was named in 1651 by astronomers Francesco Grimaldi and Giovanni Battista Riccioli in their lunar map Almagestum novum.

Michael van Langren, in his Lumina Austriaca Philippica of 1645, used the name "Mare Belgicum".

On February 20, 1965, the Ranger 8 spacecraft was deliberately crashed into the Mare Tranquillitatis at 2°38′16″N 24°47′17″E / 2.6377°N 24.7881°E / 2.6377; 24.7881 (Ranger 8 crash site) after successfully transmitting 7,137 close-range photographs of the Moon in the final 23 minutes of its mission.

Surveyor 5 landed in Mare Tranquillitatis on September 11, 1967, after transmitting 19,118 images of the Moon, and was the fifth lunar lander of the uncrewed Surveyor program.