The Moon forms

Artwork showing the impact that may have formed the Moon

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)

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Artwork showing the impact that may have formed the Moon The Moon forms

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The Moon forms

The giant impact hypothesis, sometimes called the Big Splash, states that the Moon was formed out of the debris left over from an indirect collision between the Earth and an astronomical body the size of Mars, approximately 4.5 billion years ago, in the Hadean eon; about 20–100 million years after the solar system coalesced. 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 currently the favoured scientific hypothesis for the formation of the Moon. Supporting evidence includes the Earth's spin and Moon's orbit having similar orientations, Moon samples indicating 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. Finally, the stable isotope ratios of lunar and terrestrial rock are identical, implying a common origin.

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; 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 the Moon lost its share of volatile elements and why Venus, which also experienced giant impacts during its formation, does not host a similar moon.

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