Origin of the Moon

The origin of the Moon is the process by which Earth’s natural satellite formed. The leading explanation is that debris from a giant impact between early Earth and a Mars-sized body coalesced into the Moon.

Giant-impact hypothesis: The leading model in which a large collision with early Earth ejected material that formed the Moon. This is the principal explanation for the Moon’s formation.
Isotopic signature: The characteristic proportions of an element’s isotopes in a sample or body. Isotopic comparisons reveal how closely lunar material resembles Earth’s.
Fission hypothesis: A proposal that the Moon separated from rapidly rotating early Earth. It links lunar formation to Earth’s rotation rather than a giant collision.
Lunar isotopic similarity: The close match between Earth and lunar isotope ratios for several elements. Explaining this similarity remains a central constraint on the impactor’s origin and mixing.
Theia: The hypothetical protoplanet whose collision with early Earth may have produced the Moon-forming debris. The impactor’s size, composition, and orbit shape models of lunar formation.
Oxygen isotope ratio: The relative abundance of oxygen isotopes used to compare the origins of planetary materials. The near match between Earth and lunar oxygen isotopes constrains the impactor and mixing.
Capture hypothesis: A proposal that the Moon formed elsewhere and was later captured by Earth’s gravity. It offers a distinct origin but must explain the Moon’s compositional similarity to Earth.
Impact mixing: The exchange and homogenization of material between colliding bodies and their debris after impact. The degree of mixing determines whether Earth-like lunar isotopes arise naturally.
Lunar magma ocean: A deep layer of molten rock thought to have covered the Moon early in its history. A hot debris disk could form a molten Moon whose cooling left distinctive crustal evidence.
Lunar samples: Rock and soil specimens collected from the Moon, including samples returned by Apollo and robotic missions. Laboratory measurements of returned material anchor claims about lunar composition and age.

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