Giant impact hypothesis

Origins
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]

A similar approach was taken by Alfred G. W. Cameron and William Ward, who suggested that the Moon was formed by the tangential impact of a body the size of Mars. The outer silicates of the colliding body would mostly be vaporized, whereas a metallic core would not. Hence, most of the collisional material sent into orbit would consist of silicates, leaving the coalescing Moon deficient in iron. The more volatile materials that were emitted during the collision would probably escape the Solar System, whereas silicates would tend to coalesce.[8]