Scientists have a pretty good understanding of how the moon formed. Billions of years ago, the early Earth collided with a forming planet called Theia. But, the exact mechanics of this catastrophic collision are still being theorized.

Previous theories suggest Earth and Theia collided in a side-swipe motion. This video below perfectly illustrates that type of collision.

New evidence points to a much more destructive head-on collision. That’s according to new research conducted by UCLA geochemists.

Earth and Theia collide

Ok, scientists agree a collision between these two bodies took place billions of years ago. But, how do you tell the exact type of collision? First, you need two types of rocks. A set of volcanic rocks from Earth (five from Hawaii and one from Arizona) and a set from the Moon. The seven moon rocks researchers analyzed were brought back to Earth from the Moon by the Apollo 12, 15 and 17 missions.

Young holding moon rock

Paul Warren, Edward Young (holding moon rock) and Issaku Kohl. Credit: Christelle Snow/UCLA

Armed with these two sets of rocks, researchers carefully examined the chemical signature in the rocks’ oxygen atoms. Nearly all of Earth’s oxygen is O-16. What is O-16? It’s named that because each oxygen atom has eight protons and eight neutrons. But, not all of Earth’s oxygen is O-16. There are small bits of O-17 (an extra neutron) and O-18 (two extra neutrons).

O-16 and O-17 are the important ones to look at. Every planetary body in our solar system has a unique ratio of O-17 to O-16.

Edward Young, lead author of the new study, explained what they found. “We don’t see any difference between the Earth’s and the moon’s oxygen isotopes; they’re indistinguishable.”

Remember, each planet has a unique ratio of O-17 to O-16. Because this ratio on Earth and the Moon is “indistinguishable,” it suggests Earth and Theia smashed head-on. If Earth and Theia did hit each other in a side-swipe collision (like the video above), the Moon should have different oxygen isotope ratios. The moon would be made up of materials from Theia, with a little bit of Earth mixed in. But, it doesn’t.

Apollo 17 moon rock

Moon rock from Apollo 17. Credit: Paul Warren

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

What we know about Theia

Obviously, it didn’t survive the collision. A smaller chunk of it would go on to become the moon, and its material evenly distributed between the pair according to Young.

But, what if Theia didn’t collide with Earth? It would most likely have become a planet according to Young. And it may have even rivaled the size of Earth. But, most scientists put the ‘what-if’ planet in Mars’ weight class.

What’s next for the Earth-moon system?

The latest research now waits for modeling to catch up. Previous models focused on a glancing blow. But, how did the Earth and moon get in their present orbits after a head-on collision. That’s just one of many questions scientists will continue to tackle about how the Earth-moon system became what it is today.


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