The giant impact hypothesis states that the Moon was formed out of the debris left over from a collision between the Earth and a body the size of Mars, approximately 4.5 Gya (four and a half billion years ago). The colliding body is sometimes called Theia, for the mythical Greek Titan who was the mother of Selene, the goddess of the Moon.
The giant impact hypothesis is the currently-favoured scientific hypothesis for the formation of the Moon. Supporting evidence includes: the identical direction of the Earth's spin and the Moon's orbit, Moon samples that indicate the surface of the Moon was once molten, the Moon's relatively small iron core, lower density compared to the Earth, evidence of similar collisions in other star systems (that result in debris disks), and that giant collisions are consistent with the leading theories of the formation of the solar system.
There remain several questions concerning the best current models of the giant impact hypothesis, however. The energy of such a giant impact is predicted to heat Earth to produce a global 'ocean' of magma; yet there is no evidence of the resultant planetary differentiation of the heavier material sinking into Earth's mantle. At present, 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 did the Moon lose its share of volatile elements; and why Venus, which also experienced giant impacts during its formation, does not host a similar moon.
Read more about Giant Impact Hypothesis: History of Model, Theia, Basic Model of Impact, Energetic Aftermath, Evidence, Difficulties, Possible Origin of Theia, Modified Hypothesis, Alternative Hypotheses
Famous quotes containing the words giant, impact and/or hypothesis:
“So in your discussions of the nuclear freeze proposals, I urge you to beware the temptation of pridethe temptation blithely to declare yourselves above it all and label both sides equally at fault, to ignore the facts of history and the aggressive impulses of an evil empire, to simply call the arms race a giant misunderstanding and thereby remove yourself from the struggle between right and wrong, good and evil.”
—Ronald Reagan (b. 1911)
“Conquest is the missionary of valour, and the hard impact of military virtues beats meanness out of the world.”
—Walter Bagehot (18261877)
“On pragmatistic principles, if the hypothesis of God works satisfactorily in the widest sense of the word, it is true.”
—William James (18421910)