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.