Giant-impact hypothesis

The giant-impact hypothesis, sometimes called the Theia Impact, is an astrogeology hypothesis for the formation of the Moon first proposed in 1946 by Canadian geologist Reginald Daly. The hypothesis suggests that the Early Earth collided with a Mars-sized protoplanet of the same orbit approximately 4.5 billion years ago in the early Hadean eon (about 20 to 100 million years after the Solar System coalesced), and the ejecta of the impact event later accreted to form the Moon.^[1] The impactor planet is sometimes called Theia, named after the mythical Greek Titan who was the mother of Selene, the goddess of the Moon.^[2]

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.^[3]

The giant-impact hypothesis is currently the favored hypothesis for lunar formation among astronomers.^[4] Evidence that supports this hypothesis includes:

• The Moon's orbit has a similar orientation to Earth's rotation,^[5] both of which are at a similar angle to the ecliptic plane of the Solar System.
• The stable isotope ratios of lunar and terrestrial rock are identical, implying a common origin.^[6]
• 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 to a substantial, but unknown, depth. This might have required much more energy than predicted to be available from the accretion of a celestial body of the Moon's size and mass. An extremely energetic process, such as a giant impact, could provide this energy.
• The Moon has a relatively small iron core, which gives it a much 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 deep into Earth and fuse with its own core. This would leave the Moon, which was formed from the ejecta of lighter crust and mantle fragments that went beyond the Roche limit and were not pulled back by gravity to re-fuse with 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.
• There is evidence in other star systems of similar collisions, resulting in debris discs.
• Giant collisions are consistent with the leading theory of the formation of the Solar System.

However, several questions remain concerning the best current models of the giant-impact hypothesis.^[7] The energy of such a giant impact is predicted to have heated Earth to produce a global magma ocean, and evidence of the resultant planetary differentiation of the heavier material sinking into Earth's mantle has been documented.^[8] However, there is no self-consistent model that starts with the giant-impact event and follows the evolution of the debris into a single moon.