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Concept
Giant-impact hypothesis
Summary
The giant-impact hypothesis, sometimes called the Big Splash, or the Theia Impact, suggests that the Moon was formed from the ejecta of a collision between the early Earth and a Mars-sized planet, approximately 4.5 billion years ago in the Hadean eon (about 20 to 100 million years after the Solar System coalesced). The colliding body is sometimes called Theia, named after the mythical Greek Titan who was the mother of Selene, the goddess of the Moon. Analysis of lunar rocks published in a 2016 report suggests that the impact might have been a direct hit, causing a fragmentation and thorough mixing of both parent bodies.
The giant-impact hypothesis is currently the favored scientific hypothesis for the formation of the Moon. Supporting evidence includes:
Earth's spin and the Moon's orbit have similar orientations.
The Earth–Moon system contains an anomalously high angular momentum, meaning the momentum contained in Earth's rotation, the Moon's rotation and the Moon revolving around Earth is significantly higher than the other terrestrial planets. A giant impact might have supplied this excess momentum.
Moon samples indicate that the Moon was once molten down to a substantial, but unknown, depth. This might have required more energy than predicted to be available from the accretion of a body of the Moon's size. An extremely energetic process, such as a giant impact, could provide this energy.
The Moon has a relatively small iron core, which gives the Moon a lower density than Earth. Computer models of a giant impact of a Mars-sized body with Earth indicate the impactor's core would likely penetrate Earth and fuse with its own core. This would leave the Moon, which was formed from the ejectae that were not fused with proto-Earth, with less remaining metallic iron than other planetary bodies.
The Moon is depleted in volatile elements compared to Earth. Vaporizing at comparably lower temperatures, they could be lost in a high-energy event, with the Moon's smaller gravity unable to recapture them while Earth did.
Official source
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