Various theories have been proposed to explain how the Moon formed. The most widely accepted scenario begins shortly after the Earth formed about 4.6 billion years ago.
At this time, an object about the size of Mars struck the Earth. This early planet, which has been named Theia, was partially absorbed into the Earth, but a large amount of debris was also sprayed out into space. Gravity pulled the debris into orbit around our planet and, as the numerous fragments collided, they began to clump together. The Moon was formed as these clumps grew larger and larger.
Rock samples gathered by the astronauts gave us a better understanding of the Moon and provided evidence used to support the so-called "giant impact hypothesis" described above.
Image: Artwork showing the massive collision between the Earth and another planet that may have formed the Moon (credit: Richard Bizley/SPL)
A possible explanation of the Moon's origin emerges.
In the early 1970s, Dr William K Hartmann proposed that the early Earth collided with a planet the size of Mars, sending debris into the battered Earth's orbit. He theorised that the Moon formed from this debris. This clip refers to the Moon as being dry. In recent years new studies have resulted in conflicting conclusions about the Moon's water content.
The early lunar landings answer questions about the Moon's origins.
The Moon rocks that the Apollo astronauts brought back answered some questions about the Moon, but raised others. Scientists were able to rule out extraterrestrial lunar origins but, at the time of the early Apollo missions, a clear explanation was still elusive. This clip refers to the Moon as being dry. In recent years new studies have resulted in conflicting conclusions about the Moon's water content.
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. 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. Analysis of lunar rocks, published in 2016, suggests that the impact may have been a direct hit, causing a thorough mixing of both parent bodies.
The giant-impact hypothesis is currently the favoured scientific hypothesis for the formation of the Moon. Supporting evidence includes:
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 have heated Earth to produce a global "ocean" of magma, and evidence of the resultant planetary differentiation of the heavier material sinking into Earth's mantle has been documented. However, as of 2015[update] 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.