Moon formed by a head-on collision

The Moon is the product of a violent, head-on collision between Earth and a planetary embryo called Theia that occurred about 4.5 billion years ago, according to a new study.

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The team behind the study: Paul Warren, Edward Young (holding a sample of a rock from the Moon) and Issaku Kohl.
Credit: Christelle Snow/UCLA

It is already a well-regarded theory that the Moon formed from a collision between Earth and a growing planetary 'embryo' called Theia about 100 million years after Earth formed, but many believe the two bodies crashed at a 45° angle. New evidence presented by UCLA geochemists suggests that a head-on collision was more likely.

The team behind the study looked at seven rocks brought back by the Apollo 12, 15 and 17 missions, as well as six volcanic rocks from Earth’s mantle.

The scientists were interested in the chemical signature of the rocks’ oxygen atoms, in order to get an idea of the exact nature of the collision.

Earth’s oxygen is described as O-16 because each atom contains eight protons and eight neutrons. A team of German scientists reported in 2014 that the Moon has its own unique ratio, but this latest study suggests the Moon and Earth in fact have the same signature.

“We don’t see any difference between Earth’s and the Moon’s oxygen isotopes; they’re indistinguishable,” says Edward Young, lead author of the new study and a UCLA professor of geochemistry and cosmochemistry.

This led the team to conclude that Earth and Theia had collided head-on, as a side-on collision would have created a body primarily made of Theia. The discovery that the Moon and Earth have rocks with the same signature of oxygen atoms, however, makes a head-on crash more likely.

“Theia was thoroughly mixed into both the Earth and the moon, and evenly dispersed between them,” Young says. “This explains why we don’t see a different signature of Theia in the moon versus the Earth.”


Front image: Artist's conception of the collision between Earth and Theia, thought to have been the driving force behind the formation of the Moon
Copyright: William K. Hartmann
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