Creating the Moon in a Head-on Collision

The Moon’s Catastrophic Birth

Two worlds collide to make the Moon
The extremely similar chemical composition of rocks on the Earth and moon helped scientists determine that a head-on collision, not a glancing blow, took place between Earth and Theia. Copyright William K. Hartmann

The more we learn about the early solar system, the more chaotic those times seem. It was a busy time. Newborn planets were jostling around, some of them were migrating outwards, and others — such as Earth — were ground zero for ongoing collisions and impacts. One of those collisions formed the Moon. For a long time, planetary scientists cited the glancing blow by a planetary embryo called Theia as the event that made the Moon. But, it turns out that the collision was more of a head-on smack-up than they suspected. Instead of sideswiping Earth at a 45-degree angle, Theia hit Earth squarely in the gizzards.

Analysis of a Head-on Collision

The clues that helped scientists figure out the details of this collision after the fact lies locked away in rocks from both Earth and the Moon. You analyze the chemical compositions of rocks to find out their formation history and subsequent erosion or bombardment. The Apollo missions brought back Moon rocks for just that reason — so that scientists could study their chemical make-up and compare it to Earth’s. In 2014, scientists who did chemical analysis of Moon rocks reported that the Moon’s rocks have their own unique chemical signature — a ratio of oxygen isotopes — that is different from Earth rocks. However, that conclusion is under debate, and more recent studies show there’s very, very little difference between the oxygen isotopes in Moon and Earth rocks.

The scientists who did this work are geochemists and cosmochemists based at the University of California at Los Angeles (UCLA). They used an instrument called a mass spectrometer to analyze the chemical composition of both sets of rocks. That’s when they found that the oxygen signatures in the rocks were the same. And, that tells a different story than one of Theia simply giving Earth a glancing blow in the collisional chaos of the early solar system.

Why is this?

If Earth and Theia had simply “bumped” alongside each other and went their merry way, the oxygen ratios would be slightly different. Instead, this “sameness” argues for head-on encounter where the materials from both worlds were mixed quite thoroughly. That idea is not new — a number of planetary scientists have suggested such a direct crash scenario for several years now.

Here’s more to think about: Theia didn’t survive the collision. Most of its body was mixed in with Earth and the Moon. That’s where the similarity in isotope ratios come in. It’s like mixing batter for a white cake and a chocolate cake together — if they’re mixed well, the batter is going to have the same elements, even if you bake two cakes in two different pans.

Now, the sad thing is (for Theia) that up until the point of impact, Theia was a growing world. It could have been (as the character Terry says in On the Waterfront) a contender. It could have been a planet. Not a big one, maybe something the size of Mars. And, our solar system would have had another rocky planet on its hands. Instead, Theia had that encounter with the newborn Earth and the rest is geologic and lunar history.


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