Giant-impact hypothesis

The giant-impact hypothesis, sometimes called the Big Splash, or the Theia Impact suggests that the Moon formed out of the debris left over from a collision between Earth and an astronomical body the size of Mars, approximately 4.5 billion years ago, in the Hadean eon; about 20 to 100 million years after the Solar System coalesced.[1] The colliding body is sometimes called Theia, from the name of the mythical Greek Titan who was the mother of Selene, the goddess of the Moon.[2] Analysis of lunar rocks, published in a 2016 report, suggests that the impact may have been a direct hit, causing a thorough mixing of both parent bodies.[3]

The giant-impact hypothesis is currently the favored scientific hypothesis for the formation of the Moon.[4] Supporting evidence includes:

  • Earth’s spin and the Moon’s orbit have similar orientations.[5]
  • Moon samples indicate that the Moon’s surface was once molten.
  • The Moon has a relatively small iron core.
  • The Moon has a lower density than Earth.
  • There is evidence in other star systems of similar collisions, resulting in debris disks.
  • Giant collisions are consistent with the leading theories of the formation of the Solar System.
  • The stable-isotope ratios of lunar and terrestrial rock are identical, implying a common origin.[6]

However, there remain several questions concerning the best current models of the giant-impact hypothesis.[7] The energy of such a giant impact is predicted to have heated Earth to produce a global magma ocean, and evidence of the resultant planetary differentiation of the heavier material sinking into Earth’s mantle has been documented.[8] However, as of 2015 there is no self-consistent model that starts with the giant-impact event and follows the evolution of the debris into a single moon. Other remaining questions include when the Moon lost its share of volatile elements and why Venus—which experienced giant impacts during its formation—does not host a similar moon.