Mare Tranquillitatis sits within the Tranqillitatis basin. The
mare material within the basin consists of basalt 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. Theirregualr 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. Mare Serenitatis is seen in the upper
left of the photo below. Palus Somn, on the northeastern
rim of the mare, is filled with the basalt that spilled over
from Tranquillitatis. This mare also served as the landing
site for Apollo 11, the first manned landing on the moon.
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