The Moon Was Formed From A Head-On Collision

New research from UCLA lends credence to the theory that the moon was formed from a violent, head on collision with a “planetary embryo” known as Theia.

Other theories suggest the moon may have formed from a side-swipe collision, such as the one simulated in this YouTube video. So which one is correct?


Graphic of Theia colliding into a young Earth. Image credit: NASA

Scientists analyzed seven rocks recovered by Apollo missions, as well as six volcanic samples taken from Hawaii and one from Arizona. They used chemical signatures left in the oxygen atoms of rocks, (a rocks volume is 90% oxygen) to identify a distinctive fingerprint. Earths oxygen is almost entirely O-16 because each atom contains eight protons and eight neutrons. Other bodies in our solar system have their own unique oxygen fingerprint.

“We don’t see any difference between the Earth’s and the moon’s oxygen isotopes; they’re indistinguishable,” said Edward Young, a professor of geochemistry at UCLA and lead-author of a study published in the journal Science.

Discovering the same Earth oxygen signatures in moon rocks is an important verification for the head on collision theory. Had the Moon formed from a side swipe collision, the majority of the Moon would have been made of Theia – and presumably have different oxygen isotopes than Earth.

The results go against a 2014 study where German scientists said the Moon had its own unique set of oxygen isotopes, different from Earths.

The team used UCLA’s new state-of-the-art mass spectrometer to verify measurements.

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

Interestingly, scientists think that if Theia had not collided with Earth it may have went on to become a planet. Young and his colleagues suggest the planet was about the same size as Earth, while others have suggested it’s smaller – approximately the size of Mars.

The team believes the collision happened about 100 million years after the Earth formed, approximately 4.5 billion years ago.

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