The origin of the Moon is usually explained by a Mars-sized body, known as Theia, striking Earth, creating a debris ring that eventually collected into a single natural satellite, the Moon, but there are a number of variations on this giant-impact hypothesis, as well as alternative explanations, and research continues into how the Moon came to be formed. Other proposed scenarios include captured body, fission, formed together (accretion, synestia), planetesimal collisions (formed from asteroid-like bodies), and collision theories. The standard giant-impact hypothesis suggests that a Mars-sized body called Theia impacted the proto-Earth, creating a large debris ring around Earth, which then accreted to form the Moon. The Moon's oxygen isotopic ratios seem to be essentially identical to Earth's. Oxygen isotopic ratios, which may be measured very precisely, yield a unique and distinct signature for each Solar System body. If Theia had been a separate protoplanet, it probably would have had a different oxygen isotopic signature than proto-Earth, as would the ejected mixed material. Also, the Moon's titanium isotope ratio (50Ti/47Ti) appears so close to the Earth's (within 4 parts per million) that little if any of the colliding body's mass could have been part of The Moon.
Formation
Some theories assume that early in the formation of the Solar System, 4.425 billion years ago, the Earth was largely made of rock and lava, and had no large moons. Theia, an early protoplanet the size of Mars, hit Earth in such a way that it ejected a considerable amount of material away from Earth. Some of this material escaped into space, but the rest consolidated into a single spherical body in orbit about Earth, creating the Moon. The hypothesis involves a collision between a proto-Earth that was about 90% of the diameter of present Earth, and another body that had half the diameter of the earth and about a tenth of its mass, which would make it about the size of Mars. The latter has sometimes been referred to as Theia, the name of the mother of Selene, the Moon goddess in Greek mythology. This size ratio is needed in order for the resulting system to have sufficient angular momentum to match the current orbital configuration. Such an impact would have put enough material into orbit around Earth to have eventually accumulated to form the Moon. Computer simulations show a need for a glancing blow, which causes a portion of the collider to form a long arm of material that then shears off. The asymmetrical shape of the Earth following the collision then causes this material to settle into an orbit around the main mass. The energy involved in this collision is impressive: possibly trillions of tonnes of material would have been vaporized and melted. In parts of the Earth, the temperature would have risen to 10,000 °C (18,000 °F). The Moon's relatively small iron core (compared to other rocky planets and moons in the Solar System) is explained by Theia's core mostly merging into that of Earth. The lack of volatiles in the lunar samples is also explained in part by the energy of the collision. The energy liberated during the reaccretion of material in orbit around Earth would have been sufficient to melt a large portion of the Moon, leading to the generation of a magma ocean. The newly formed Moon orbited at about one-tenth the distance that it does today, and spiraled outward because of tidal friction transferring angular momentum from the rotations of both bodies to the Moon's orbital motion. Along the way, the Moon's rotation became tidally locked to Earth, so that one side of the Moon continuously faces toward Earth. In this scenario, the Moon would have collided with and incorporated any small preexisting satellites of Earth, which would have shared the Earth's composition, including isotopic abundances. The geology of the Moon has since been more independent of the Earth. A 2012 study on the depletion of zinc isotopes on the Moon found evidence for volatile depletion consistent with the giant-impact origin for Earth and the Moon. In 2013, a study was released that indicated that water in lunar magma is indistinguishable from that in carbonaceous chondrites and nearly the same as that of Earth in isotopic composition.
Derivatives of the hypothesis Although the giant-impact hypothesis explains many aspects of the Earth–Moon system, there are still a few unresolved problems, such as the Moon's volatile elements not being as depleted as expected from such an energetic impact. Another issue is lunar and Earth isotope comparisons. In 2001, the most precise measurement yet of the isotopic signatures of Moon rocks was published. Surprisingly, the Apollo lunar samples carried an isotopic signature identical to Earth rocks, but different from other Solar System bodies. Because most of the material that went into orbit to form the Moon was thought to come from Theia, this observation was unexpected. In 2007, researchers from Caltech showed that the likelihood of Theia having an identical isotopic signature as the Earth is very small (less than 1 percent chance). Published in 2012, an analysis of titanium isotopes in Apollo lunar samples showed that the Moon has the same composition as Earth, which conflicts with the Moon forming far from Earth's orbit.
Merger of two planets To help resolve these problems, a hypothesis published in 2012 posits that two bodies—each five times the size of Mars—collided, then recollided, forming a large disc of mixed debris that eventually formed Earth and the Moon.
Immediate origin of the Moon as a post-impact satellite
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