Giant impact hypothesis

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Giant impact hypothesis

Post  meodingu on Tue Oct 19, 2010 12:03 am

Giant impact hypothesis

Several mechanisms have been proposed for the Moon's formation 4.527 ± 0.010 billion years ago,[nb 5] some 30–50 million years after the origin of the Solar System.[11] These include the fission of the Moon from the Earth's crust through centrifugal forces,[12] which would require too great an initial spin of the Earth,[13] the gravitational capture of a pre-formed Moon,[14] which would require an unfeasibly extended atmosphere of the Earth to dissipate the energy of the passing Moon,[13] and the co-formation of the Earth and the Moon together in the primordial accretion disk, which does not explain the depletion of metallic iron in the Moon.[13] These hypotheses also cannot account for the high angular momentum of the Earth–Moon system.[15]

The prevailing hypothesis today is that the Earth–Moon system formed as a result of a giant impact: a Mars-sized body hit the nearly formed proto-Earth, blasting material into orbit around the proto-Earth, which accreted to form the Moon.[16] Giant impacts are thought to have been common in the early Solar System. Computer simulations modelling a giant impact are consistent with measurements of the angular momentum of the Earth–Moon system, and the small size of the lunar core; they also show that most of the Moon came from the impactor, not from the proto-Earth.[17] However, meteorites show that other inner Solar System bodies such as Mars and Vesta have very different oxygen and tungsten isotopic compositions to the Earth, while the Earth and Moon have near-identical isotopic compositions. Post-impact mixing of the vaporized material between the forming Earth and Moon could have equalized their isotopic compositions,[18] although this is debated.[19]

The large amount of energy released in the giant impact event and the subsequent reaccretion of material in Earth orbit would have melted the outer shell of the Earth, forming a magma ocean.[20][21] The newly formed Moon would also have had its own lunar magma ocean; estimates for its depth range from about 500 km to the entire radius of the Moon.[20]

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