Origin of the Moon
how the Earth's moon was formed
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.
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This entry incorporates text from “Origin of the Moon” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.