Origin of the Moon

" Some theories have been stated that presume the proto-Earth had no large moons early in the formation of the Solar System, 4.425 billion years ago, Earth being basically rock and lava. 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 proportion of these ejecta escaped into space, but the rest consolidated into a single spherical body in orbit about Earth, creating the Moon. The hypothesis requires a collision between a proto-Earth about 90% of the diameter of present Earth, and another body the diameter of Mars (half of the terrestrial diameter and a tenth of its mass). 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 . 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 continually faces toward Earth. Also, 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. 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. Multiple impacts In 2004, Russian astrophysicist Nikolai Gorkavyi proposed a novel model titled the multiple large asteroid impacts model, which found support from a notable group of Russian astronomers in 2013 and later, in 2017, by planetary researchers at Weizmann Institute of Science in Rehovot, Israel. In general terms, the main idea of the model suggests that the Moon was formed as a result of a violent rain of large asteroids (1–100 km) that repeatedly hammered the fledgling Earth over millions of years. Such a series of smaller impacts, which were probably more common in the early Solar System, could blast enough rocky Earth debris into orbit to form a protosatellite disk which later forms into a small moonlet. ==Other hypotheses==

Capture This hypothesis states that the Moon was captured by the Earth. However, this hypothesis does not adequately explain the essentially identical oxygen isotope ratios of the two bodies. This was first proposed by George Darwin (son of the famous biologist Charles Darwin) in 1879 and retained some popularity until Apollo. It was proposed that the Pacific Ocean represented the scar of this event. or even a black hole. The problem with this hypothesis is that it does not explain the angular momentum of the Earth-Moon system or why the Moon has a relatively small iron core compared to the Earth (25% of its radius compared to 50% for the Earth). This natural nuclear fission reactor has been observed on Earth at a much smaller scale. The fission hypothesis can adequately explain the similarities and differences in the elemental and isotopic compositions of the Earth and the Moon. ==Additional theories and studies==

around 2012 2011 In 2011, it was theorized that a second moon existed 4.5 billion years ago, and later had an impact with The Moon, as a part of the accretion process in the formation of The Moon. 2013 One hypothesis, presented only as a possibility, was that Earth captured The Moon from Venus. 2017 Uranium–lead dating of Apollo 14 zircon fragments shows the age of The Moon to be around 4.53 billion years. 2020 A team of researchers of the Miniature Radio Frequency (Mini-RF) instrument on NASA's Lunar Reconnaissance Orbiter (LRO) spacecraft concluded that the Moon's subsurface may be richer in metals, like iron and titanium, than scientists had believed. In July 2020 scientists report that the Moon formed 4.425 ±0.025 bya, about 85 million years later than thought, and that it hosted an ocean of magma for substantially longer than previously thought (for ~200 million years). 2023 On 1 November 2023, scientists reported that, according to computer simulations, remnants of a protoplanet, named Theia, could be inside Earth, left over from a collision with Earth in ancient times, and afterwards becoming The Moon. ==See also==