Was the Moon born in a head-on collision?

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A close-up of lunar rock from the Apollo 17 mission. Its oxygen ‘fingerprint’ matches that of Earth rocks. – Paul Warren/UCLA

As births go, they don’t come much more violent than the Moon's.

Some 4.5 billion years ago, the young Earth collided with a developing planet, Theia. But instead of dealing each other a glancing blow, chemical analysis shows the collision was head-on, disintegrating Theia and part of Earth into a hot swirling disk of water and dust surrounding what was left of Earth.

This mix eventually clumped together to become the Moon, a new study suggests.

The key to the findings is the unique oxygen “fingerprint” that is found in all the planets, moons, comets and asteroids in our Solar System, including the Earth and the Moon.

More than 99.9% of Earth’s oxygen is “normal”, with each atom containing eight protons and eight neutrons. But there are small quantities of slightly heavier oxygen – molecules with an extra neutron jammed in.

While this "fingerprint" is a reliable identifier, it has traditionally been very hard to detect. So the University of California Los Angeles-led team used new, super-sensitive equipment to analyse seven lunar rocks brought back by the Apollo 12, 15 and 17 missions, as well as a lunar meteorite, and compared them to six volcanic rocks from the Earth’s mantle. They found their heavy oxygen levels to be almost identical – within five parts per million.

To have that level of similarity, the planetary objects must have crashed into each other straight on. A side blow couldn’t account for that degree of mixing.

“Theia was thoroughly mixed into both the Earth and the Moon, and evenly dispersed between them,” says study lead author and UCLA geochemist Edward Young, whose work is published in Science.

“This explains why we don’t see a different signature of Theia in the Moon versus the Earth.”

Had Theia side-swiped Earth instead, and become hooked into Earth’s orbit, most of the Moon would be made from Theia. It should, then, have a completely different oxygen fingerprint to Earth.

But there is still one enduring question – just how big was Theia?

Some, including Young, believe it was around the same size as Earth. Others think it was smaller, around the size of Mars. But had it not been wiped out in the Earth smash, Young says, it would probably have grown to become a planet.

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