Giant impact hypothesis

The giant impact hypothesis proposes that the Moon was created out of the debris left over from a collision between the young Earth and a Mars-sized body. This is the favored[1] scientific hypothesis for the formation of the Moon. Evidence for this hypothesis includes Moon samples which indicate the surface of the Moon was once molten, the Moon's apparently relatively small iron core and a lower density than the Earth, and evidence of similar collisions in other star systems (which result in debris disks). The colliding body is sometimes called Theia (or Orpheus) for the mythical Greek Titan who was the mother of Selene, the goddess of the moon.[2][3]

There remain several unanswered issues surrounding this hypothesis. Lunar oxygen isotopic ratios are essentially identical to Earth's, with no evidence of a contribution from another solar body.[4] Also, lunar samples do not have expected ratios of volatile elements, iron oxide, or siderophilic elements, and there is no evidence to suggest that the Earth ever had the magma ocean implied by this hypothesis.