Several mechanisms have been historically proposed for the Moon’s formation 4.527 ± 0.010 billion years ago, some 30–50 million years after the origin of the Solar System. These include the fission of the Moon from the Earth’s crust throughcentrifugal forces,which would require too great an initial spin of the Earth, the gravitational capture of a pre-formed Moon,which would require an unfeasibly extended atmosphere of the Earth to dissipate the energy of the passing Moon, 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. These hypotheses also cannot account for the high angular momentum of the Earth–Moon system.
The prevailing hypothesis today is that the Earth–Moon system formed as a result of a giant impact: a Mars-sized body hit the proto-Earth, blasting material into orbit around the proto-Earth, which accreted to form the Moon. 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, as well as the small size of the lunar core.Unresolved questions are the relative sizes of the proto-Earth and impactor,and how much material from these two bodies formed the Moon. However, recent oxygen isotope composition analysis of the Moon shows its oxygen isotope composition is more similar to the Earth’s than this hypothesis would suggest.
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.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.
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发表于 2022-11-3 10:09:00
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