Giant impact hypothesis

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The giant impact hypothesis proposes that the Moon was created out of the debris left over from a collision between the young Earth and a Mars-sized body. This is the favored[1] scientific hypothesis for the formation of the Moon. Evidence for this hypothesis includes Moon samples which indicate the surface of the Moon was once molten, the Moon's apparently relatively small iron core and a lower density than the Earth, and evidence of similar collisions in other star systems (which result in debris disks). The colliding body is sometimes called Theia (or Euryphaessa) for the mythical Greek Titan who was the mother of Selene, the goddess of the moon.[2][3]

There remain several unanswered issues surrounding this hypothesis. Lunar oxygen isotopic ratios are essentially identical to Earth's, with no evidence of a contribution from another solar body.[4] Also, lunar samples do not have expected ratios of volatile elements, iron oxide, or siderophilic elements, and there is no evidence to suggest that the Earth ever had the magma ocean implied by this hypothesis.



In 1898, George Howard Darwin made an early suggestion that the Earth and Moon had once been one body. Darwin's hypothesis was that a molten Moon had been spun from the Earth because of centrifugal forces, and this became the dominant academic explanation.[5] Using Newtonian mechanics, he calculated that the Moon had actually orbited much closer in the past and was drifting away from the Earth. This drifting was later confirmed by American and Soviet experiments using laser ranging targets placed on the Moon.

However, Darwin's calculations could not resolve the mechanics required to trace the Moon backwards to the surface of the Earth. In 1946, Reginald Aldworth Daly of Harvard University challenged Darwin's explanation, adjusting it to postulate that the creation of the Moon was caused by an impact rather than centrifugal forces.[6] Little attention was paid to Professor Daly's challenge until a conference on satellites in 1974 where it was reintroduced. It was then republished in Icarus in 1975 by Drs. William K. Hartmann and Donald R. Davis. Their models suggested that, at the end of the planet formation period, several satellite-sized bodies had formed that could collide with the planets or be captured. They proposed that one of these objects may have collided with the Earth, ejecting refractory, volatile-poor dust that could coalesce to form the Moon. This collision could help explain the unique geological properties of the Moon.[7]

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