The most widely accepted scientific explanation for how the Moon formed is the giant-impact hypothesis. According to this theory, about 4.5 billion years ago, a Mars-sized protoplanet called Theia collided with the early Earth (proto-Earth). The immense collision caused both bodies to melt and eject a large amount of debris into orbit around Earth. This debris eventually coalesced to form the Moon
. Key points supporting this theory include:
- The chemical and isotopic compositions of Moon rocks brought back by the Apollo missions are very similar to those of Earth's mantle, suggesting a linked origin rather than the Moon being a captured object or having formed independently
- The Moon formed from molten material and was once covered by a magma ocean, consistent with formation from high-energy impact debris
- The impact likely tilted Earth's rotational axis to about 23.5°, which is responsible for Earth's seasons
- Simulations suggest the Moon could have formed rapidly from the debris disk created by the impact, possibly within hours to years after the collision
Other older theories, such as co-formation with Earth from the primordial solar nebula or capture of a passing body, are less favored because they do not explain the Moon's composition and orbital characteristics as well as the giant-impact model
. In summary, the Moon formed when a Mars-sized body collided with the young Earth, and the resulting debris from this giant impact accreted into the Moon we see today
