Isotope dating of lunar samples suggests the Moon formed around 50 million years after the origin of the Solar System.[33][34] Historically, several formation mechanisms have been proposed,[35] but none satisfactorily explained the features of the Earth–Moon system. A fission of the Moon from Earth’s crust through centrifugal force[36] would require too great an initial rotation rate of Earth.[37] Gravitational capture of a pre-formed Moon[38] depends on an unfeasibly extended atmosphere of Earth to dissipate the energy of the passing Moon.[37] A co-formation of Earth and the Moon together in the primordial accretion disk does not explain the depletion of metals in the Moon.[37] None of these hypotheses can account for the high angular momentum of the Earth–Moon system.[39]

File:Evolution of the Moon.ogv
The evolution of the Moon and a tour of the Moon
The prevailing theory is that the Earth–Moon system formed after a giant impact of a Mars-sized body (named Theia) with the proto-Earth. The impact blasted material into Earth’s orbit and then the material accreted and formed the Moon[40][41] just beyond the Earth’s Roche limit of ~2.56 R⊕.[42] This theory best explains the evidence.

Giant impacts are thought to have been common in the early Solar System. Computer simulations of giant impacts have produced results that are consistent with the mass of the lunar core and the angular momentum of the Earth–Moon system. These simulations also show that most of the Moon derived from the impactor, rather than the proto-Earth.[43] However, more recent simulations suggest a larger fraction of the Moon derived from the proto-Earth.[44][45][46][47] Other bodies of the inner Solar System such as Mars and Vesta have, according to meteorites from them, very different oxygen and tungsten isotopic compositions compared to Earth. However, Earth and the Moon have nearly identical isotopic compositions. The isotopic equalization of the Earth-Moon system might be explained by the post-impact mixing of the vaporized material that formed the two,[48] although this is debated.[49]

The impact released a lot of energy and then the released material re-accreted into the Earth–Moon system. This would have melted the outer shell of Earth, and thus formed a magma ocean.[50][51] Similarly, the newly formed Moon would also have been affected and had its own lunar magma ocean; its depth is estimated from about 500 km (300 miles) to 1,737 km (1,079 miles).[50]