The overwhelming consensus view among scientists today is that the moon formed following the impact of a Mars sized planet (Theia) on the proto-Earth during the early stages of the formation of the solar system. A huge volume of material was thrown into space, much of it at such speed it was able to escape the gravitational attraction of the Earth. However, some remained in orbit as a debris disc that rapidly (perhaps in less than one hundred years) condensed and accreted to form the moon.
Any hypothesis for lunar formation has to account for several features of the moon:
• A comparatively large size relative to its parent. (The moon is ~1% of the mass of the Earth.)
• The large angular momentum of the Earth-Moon system
• Low lunar density, implying a major depletion in iron
• A lunar orbit once much closer to the Earth and inclined at 10° to the ecliptic
Data from the Apollo samples added further complications: the lunar composition bore striking similarities to terrestrial mantle material, but also striking differences. For example, the oxygen isotope ratios of mantle and lunar rocks differed from chondrites, the posited source for each, by similar amounts; however the moon was clearly seriously depleted in volatiles.
Three hypotheses were considered as plausible:
• Capture of a body formed elsewhere in the system
• Fission as a consequence of very high rotational speed of the proto-Earth
• Co-formation alongside the Earth
Each of these three hypotheses had serious problems. Consequently, two independent groups developed an alternative impact hypothesis: Hartmann and Davis in 1975 and Cameron and Ward in 1976. This idea languished until all four origin hypotheses were examined in detail in a 1984 conference in Hawaii, from which the impact hypothesis emerged as the clear winner.
Subsequent advances in computer power, especially using smooth particle hydrodynamics (SPH), have enabled detailed simulations that have refined the hypothesis and removed most of the contradictions, or remaining questions. In parallel with this, work on lunar geochemistry, especially isotope ratios, and the character and formation of the magma ocean, has complimented the dynamical findings. As a consequence, the impact origin of the moon is now generally accepted and work focuses on resolving any remaining inconsistencies.
Selected Bibliography:
Cameron, A.G.W. & Ward, W.R. The origin of the Moon. Lunar Sci.7: 120-122 (1976).
Canup,R.M. & Asphaug,E. Origin of the Moon in a giant impact near the end of the Earth's formation Nature 412: 708-712 (2001)
Canup R.M. Dynamics of Lunar Formation Annu.Rev.Astron.Astrophys.42: 441–75 (2004)
Hartmann, W.K. & Davis, D.R. Satellite-sized planetesimals and lunar origin. Icarus24: 504-515 (1975)
Hartmann,W.K.,Phillips R.J.,Taylor G.J.,eds. Origin of the Moon. Houston: LunarPlanet.Inst.781pp (1986)
Warren The Magma Ocean Concept and Lunar Evolution Ann. Rev. Earth Planet. Sci. 1985. 13: 201-40
Note: Evo's NASA link is excellent.