Giant-impact hypothesis

[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.

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.

[10] Little attention was paid to Professor Daly's challenge until a conference on satellites in 1974, during which the idea was reintroduced and later published and discussed in Icarus in 1975 by William K. Hartmann and Donald R. Davis.

Before the conference, there were partisans of the three "traditional" theories, plus a few people who were starting to take the giant impact seriously, and there was a huge apathetic middle who didn't think the debate would ever be resolved.

[13]The name of the hypothesised protoplanet is derived from the mythical Greek titan Theia /ˈθiːə/, who gave birth to the Moon goddess Selene.

[2][14] According to modern theories of planet formation, Theia was part of a population of Mars-sized bodies that existed in the Solar System 4.5 billion years ago.

[17] However, oxygen isotope abundance in lunar rock suggests "vigorous mixing" of Theia and Earth, indicating a steep impact angle.

The inner disk slowly and viscously spread back out to Earth's Roche limit, pushing along outer bodies via resonant interactions.

Estimates based on computer simulations of such an event suggest that some twenty percent of the original mass of Theia would have ended up as an orbiting ring of debris around Earth, and about half of this matter coalesced into the Moon.

[23] Above a high resolution threshold for simulations, a study published in 2022 finds that giant impacts can immediately place a satellite with similar mass and iron content to the Moon into orbit far outside Earth's Roche limit.

Even satellites that initially pass within the Roche limit can reliably and predictably survive, by being partially stripped and then torqued onto wider, stable orbits.

In 2007, researchers from the California Institute of Technology showed that the likelihood of Theia having an identical isotopic signature as Earth was very small (less than 1 percent).

Further computer modelling determined that the observed result could be obtained by having the pre-Earth body spinning very rapidly, so much so that it formed a new celestial object which was given the name 'synestia'.

Further modelling of this transient structure has shown that the primary body spinning as a doughnut-shaped object (the synestia) existed for about a century (a very short time)[citation needed] before it cooled down and gave birth to Earth and the Moon.

[31][32] Indirect evidence for the giant impact scenario comes from rocks collected during the Apollo Moon landings, which show oxygen isotope ratios nearly identical to those of Earth.

The highly anorthositic composition of the lunar crust, as well as the existence of KREEP-rich samples, suggest 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.

Comparison of the zinc isotopic composition of lunar samples with that of Earth and Mars rocks provides further evidence for the impact hypothesis.

[39] Warm silica-rich dust and abundant SiO gas, products of high velocity impacts – over 10 km/s (6.2 mi/s) – between rocky bodies, have been detected by the Spitzer Space Telescope around the nearby (29 pc distant) young (~12 My old) star HD 172555 in the Beta Pictoris moving group.

[55] Simulations of the chaotic period of terrestrial planet formation suggest that impacts like those hypothesised to have formed the Moon were common.

[56] In 2004, Princeton University mathematician Edward Belbruno and astrophysicist J. Richard Gott III proposed that Theia coalesced at the L4 or L5 Lagrangian point relative to Earth (in about the same orbit and about 60° ahead or behind),[57][58] similar to a trojan asteroid.

[57] In this scenario, gravitational perturbations by planetesimals caused Theia to depart from its stable Lagrangian location, and subsequent interactions with proto-Earth led to a collision between the two bodies.

[59] A 2014 comparison of computer simulations with elemental abundance measurements in Earth's mantle indicated that the collision occurred approximately 95 My after the formation of the Solar System.

[62] This collision, simulations have supported, would have been at a low enough velocity so as not to form a crater; instead, the material from the smaller body would have spread out across the Moon (in what would become its far side), adding a thick layer of highlands crust.

[66] The asteroid impact in this scenario would have created a magma ocean on Earth and the proto-Moon with both bodies sharing a common plasma metal vapor atmosphere.

The shared metal vapor bridge would have allowed material from Earth and the proto-Moon to exchange and equilibrate into a more common composition.