Evidence
Indirect evidence for the giant impact scenario comes from rocks collected during the Apollo Moon landings, which show oxygen isotope ratios identical to those of Earth, which can be explained if the Earth–Moon system experienced turbulent mixing in the aftermath of the giant impact, as discussed above. In addition, the highly anorthositic composition of the lunar crust, as well as the existence of KREEP-rich samples, gave rise to the idea that a large portion of the Moon once was molten, and a giant impact scenario could easily have supplied the energy needed to form such a magma ocean. Several lines of evidence show that if the Moon has an iron-rich core, it must be a small one. In particular, the mean density, moment of inertia, rotational signature, and magnetic induction response all suggest that the radius of the core is less than about 25% the radius of the Moon, in contrast to about 50% for most of the other terrestrial bodies. Impact conditions can be found that give rise to a Moon that formed mostly from the mantles of the Earth and impactor, with the core of the impactor accreting to the Earth, and which satisfy the angular momentum constraints of the Earth–Moon system.
Warm silica-rich dust and abundant SiO gas, products of high velocity (> 10 km/s) impacts between rocky bodies, have been detected around the nearby (29 pc distant) young (~12 My old) Beta Pic Moving Group star HD172555 by the Spitzer Space Telescope. A belt of warm dust in a zone between 0.25AU and 2AU from the young star HD 23514 in the Pleiades cluster appears similar to the predicted results of Theia's collision with the embryonic Earth, and has been interpreted as the result of planet-sized objects colliding with each other. This is similar to another belt of warm dust detected around the star, BD +20°307 (HIP 8920, SAO 75016).
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