Mare Tranquillitatis (
Latin for
Sea of Tranquility or
Sea of Tranquillity (see
spelling differences)) is a
lunar mare that sits within the Tranquillitatis basin on the
Moon. 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.