The Origin of the Moon

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Source: Alex N. Halliday, Science: Vol. 338 no. 6110 pp. 1040-1041 DOI: 10.1126/science.1229954
Excerpt: Any successful model of lunar origin has to explain the Moon’s angular momentum and its slightly lower density (than Earth). …The Moon formed relatively late given its size … more than 30 million years after the start of the solar system, whereas most objects this size are predicted to have formed in the first few hundred thousand years. The oldest rocks appear to have formed from a magma ocean, implying an intensely energetic fiery start…. The oxygen isotopic composition of the Moon is identical to that of Earth to within 5 parts per million, whereas that of nearly all asteroidal and planetary objects are different…. The Giant Impact Theory is the … widely accepted current explanation for the Moon’s late, molten start as a low-density object that now contributes most of the angular momentum in the Earth-Moon system. … an Earth when it was about 85 to 90% formed and another planet, sometimes called Theia, that was about 10 to 15% of the mass of Earth or roughly the size of Mars …catastrophic glancing blow … most of the material from Theia was added to Earth, with a small fraction of silicate-rich material left as a disk from which the Moon accreted. Seewww.sciencemag.org/content/338/6110/1040/F1.large.jpg for illustration about 3 types of models/simulations: Standard” impactor (10% of Earth’s final mass), “Small” impactor (2.5% of Earth’s final mass, and “Large” impactor (45% of Earth’s final mass)….

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