Giant-impact hypothesis

The hypothesis that the Moon formed from debris ejected when a Mars-sized body collided with the early Earth. It explains the Moon’s origin through a catastrophic impact during the Solar System’s formation.

Theia: The proposed planetary body, roughly Mars-sized, whose collision with early Earth produced debris that formed the Moon. Theia is the name commonly given to the impactor in the hypothesis.
Accretion (astrophysics): The accumulation of matter under gravity into larger bodies, including planets and moons. The Moon’s assembly from impact debris is a form of accretion.
Lunar iron depletion: The Moon’s relative scarcity of iron compared with Earth, especially its small metallic core. A collision can leave much of Earth’s iron-rich core out of the orbiting debris.
Fission hypothesis: The proposal that the Moon split from a rapidly rotating early Earth. It also links lunar material to Earth but requires extreme terrestrial rotation.
High-angular-momentum impact hypothesis: A Moon-formation model in which a rapidly rotating post-impact Earth and disk later lose angular momentum through solar tides. It addresses the system’s angular momentum while permitting Earth-like lunar composition.
Lunar accretion: The growth of the Moon as orbiting debris collided and accumulated into a body. It describes how impact debris could assemble into the Moon.
Terrestrial planet: A rocky planet composed chiefly of silicate minerals and metal, such as Earth, Mars, Venus, or Mercury. The hypothesis concerns a collision between two growing rocky planetary bodies.
Lunar volatile depletion: The Moon’s relative scarcity of easily vaporized elements compared with Earth and many meteorites. The impact’s intense heating offers a possible explanation for volatile loss.
Capture theory (Moon): The proposal that the Moon formed elsewhere and was later gravitationally captured by Earth. Capture struggles to explain the Moon’s orbit and Earth-like isotopic composition.
Equilibrium condensation: A process in which vapor and liquid or solid phases exchange material until chemical equilibrium is approached. Equilibration between impact vapor and Earth may help explain shared isotope signatures.

Linked from 29 pages