The origin of the Moon is usually explained by a Mars-sized body striking the 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.[1][2] Other proposed scenarios include captured body, fission, formed together (condensation theory, synestia), planetesimal collisions (formed from asteroid-like bodies), and collision theories.[3]
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. This collision also resulted in the 23.5° tilted axis of the Earth, thus causing the seasons.[1][irrelevant citation] The Moon's oxygen isotopic ratios seem to be essentially identical to Earth's.[4] Oxygen isotopic ratios, which may be measured very precisely, yield a unique and distinct signature for each Solar System body.[5] 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.[6] 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.[7]
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