Origin of the Moon

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 likely have been part of the Moon.^[7]