Geological Chronicle
Mare Nectaris is a small lunar mare on the near side of the Moon, covering 84,000 square kilometers as a volcanic lava plain noticeably darker than the rest of the lunar surface. It lies south of Mare Tranquillitatis and southwest of Mare Fecunditatis. Montes Pyrenaeus borders the mare to the east, and Sinus Asperitatis fuses to its northwestern edge. The plain sits in the central part of an 860-km-diameter impact basin formed 3.8 to 3.9 billion years ago, an event that marks the beginning of the Nectarian period of the lunar geologic timescale. In the southwest part of the basin, the best-preserved section of the basin rim is called Rupes Altai.
Lava filling of Mare Nectaris is younger than the basin itself, reaching an approximate depth of 1000 m. The mare material is mainly of the Nectarian period and the Lower Imbrian epoch, with mare material of the Upper Imbrian epoch. Subsidence later opened a few arcuate grabens on the western margin of the mare.
Several large craters border the mare, including Fracastorius at 124 km, which fuses with the southern coast of the sea, and Daguerre, a ghost crater almost entirely covered with lava in the northern part. A prominent trio of 100-km craters, Theophilus, Cyrillus, and Catharina, sits near the northwestern coast. Doppler tracking of five Lunar Orbiter spacecraft in 1968 identified a mass concentration (mascon) in the center of the mare, which later orbiters such as Lunar Prospector and GRAIL confirmed and mapped at higher resolution.