giant-impact hypothesis

The giant-impact hypothesis is the leading scientific theory for the Moon's formation. It proposes that about 4.6 billion years ago, a Mars-sized protoplanet named Theia collided with the early Earth. This colossal impact ejected a massive cloud of debris, primarily composed of mantle material from both bodies, into orbit around Earth. This debris then coalesced to form the Moon.1

This theory is favored because it explains key observations, such as the Moon's relatively low density and its similar isotopic composition to Earth's mantle, while also accounting for the Moon's depletion in iron and volatiles. Earlier theories, like coaccretion, fission, and capture, failed to adequately explain these characteristics of the Earth-Moon system. The giant-impact hypothesis provides a robust framework for understanding our Moon's origin.12