The Lunar Magma Ocean (LMO) is the layer of molten rock that is theorized to have been present on the surface of the Moon. The LMO was likely present on the Moon from the time of the Moon's formation (about 4.5 or 4.4 billion years ago) to tens or hundreds of millions of years after that time. The LMO was a thermodynamic consequence of the Moon's relatively rapid formation in the aftermath of a giant impact between the proto-Earth and another planetary body. As the Moon accreted from the debris from the giant impact, gravitational potential energy was converted to thermal energy. Due to the rapid accretion of the Moon (in about a month to a year), thermal energy was trapped since it did not have sufficient time to thermally radiate away energy through the lunar surface. The subsequent thermochemical evolution of the LMO explains the Moon's largely anorthositic crust, europium anomaly, and KREEP material.
The LMO was initially proposed by two groups in 1970 after they analyzed anorthositic rock fragments found in the Apollo 11 sample collection. Wood et al. used fragments of bulk sample 10085 for their analyses. Ferroan anorthosite (FAN) rocks found during the Apollo program are composed primarily (over 90%) of the mineral plagioclase. More specifically, FAN rocks found on the Moon consist of the calcium (Ca) end-member of plagioclase (i.e., anorthite). This suggests that at least upper layers of the Moon were molten in the past due to the purity of lunar anorthosites and the fact that anorthite generally has a high crystallization temperature.
