Origin of the Moon

Theories for the origin of the Moon have been debated for more than a century. The classic theories claim (1) that the Moon formed as a separate planet and was captured by the Earth, (2) that the Moon was originally part of the Earth and that the Earth broke into two parts, and (3) that the Earth and moon formed as a binary planet. None of these theories has been able to satisfy all the major constraints on lunar origin, which include the large prograde angular momentum of the Earth–Moon system relative to the other planets and the Moon’s depletion in volatile elements and iron compared with the cosmic (chondritic) abundances. Another theory proposes that the Moon formed by accreting from a disc of ejecta orbiting the Earth after the impact of a Mars-size body with the Earth. The giant impact origin of the Moon has gained widespread support because it does not violate any of the major observational constraints on lunar origin. One of the major consequences of the giant impact hypothesis of lunar origin is a hot, partially molten (or perhaps completely molten) Moon upon accretion from the circumterrestrial ejecta disk.