Geological Chronicle
Mare Serenitatis is a circular impact basin spanning 674 kilometers across the northeastern quadrant of the lunar near side. The basin formed during the Nectarian period, roughly 3.87 billion years ago, when a large asteroid struck the early lunar crust. Mountain ranges define several segments of the basin perimeter, including the Montes Haemus along the southwestern edge and the Montes Caucasus to the northwest. The northeastern rim is broken by the 95-kilometer crater Posidonius, where ancient volcanic lavas breached and flooded the crater floor.
Volcanic activity flooded the Serenitatis basin in distinct stages. An older sequence of titanium-rich basaltic lava lines the outer margin, creating a dark border that contrasts with the lighter, younger basalts filling the central plain. As dense lava accumulated up to two kilometers thick, the weight caused the basin center to subside. This subsidence generated compressive stresses that buckled the cooling rock, creating prominent ridges such as Dorsum Nicol and Dorsa Lister, along with concentric graben and fracture systems along the basin perimeter.
Serenitatis contains one of the Moon's strongest mass concentrations (mascons), a localized gravitational high caused by the thick plug of dense basalt and uplifted mantle rock beneath the basin center. NASA scientists first identified the mascon in 1968 through Doppler tracking of the Lunar Orbiter 5 spacecraft, which experienced orbital acceleration whenever it passed over the sea. Subsequent gravity mapping by the Lunar Prospector and GRAIL missions confirmed the compact, high-density structure of the Serenitatis mascon.
The margins of Mare Serenitatis provided landing sites for two major surface exploration missions. In December 1972, Apollo 17 landed in the narrow Taurus-Littrow valley on the southeastern rim. Astronauts Eugene Cernan and Harrison Schmitt collected 110.5 kilograms of lunar rock and soil, including orange pyroclastic glass beads produced by ancient explosive volcanic fountains. In January 1973, the Soviet Union's Luna 21 lander deployed the Lunokhod 2 rover inside Le Monnier crater on the eastern rim, traversing 39 kilometers across the mare floor to measure local magnetic fields and soil mechanics.