Science & Technology

There's a Wild New Explanation for How the Moon Formed

For an object that's so near and dear to our planet, the moon certainly has its share of controversy. Theories abound for how the moon came to be: some have said it was captured, fully formed, by our planet's gravity; others say it formed at the same time Earth did. While one hypothesis has risen to the forefront — we'll get to that one in a second — there's a new explanation that might topple its place as the reigning theory.

Crash Into Me

The reigning theory is pretty explosive: 4.5 billion years ago, when the Earth was still young, a Mars-sized object called Theia slammed into the planet, slinging large portions of its mantle out into space. In the aftermath, the gravity of Earth eventually gathered that rocky debris into the planetary body that would become the moon.

There's good evidence to back this up. For one thing, the geological makeup of moon rocks is almost, but not quite, identical to that of rocks on Earth, which makes sense if you're talking about a third object getting in the mix. Since the collision between the Earth and Theia would have vaporized many of the lighter elements, you should also expect the moon to be made of a higher ratio of heavy elements than Earth rocks — and geological analysis shows that to be the case. But there are still questions. The moon's makeup matches the Theia-impact model almost perfectly — but the keyword there is "almost."

In May 2017, Harvard's Simon Lock UC Davis's Sarah Stewart proposed an entirely new type of planetary object called the "synestia," which they described as "a huge, spinning, donut-shaped mass of hot, vaporized rock, formed as planet-sized objects smash into each other." Less than a year later, they're publishing research to suggest that this new object is the missing piece of the moon's puzzle.

Same But Different

Their hypothesis is pretty similar to the impact model, but with some important differences. Instead of a glancing blow with a Mars-sized object breaking off chunks of the Earth, the impact positively vaporized our young planet, turning it into a rapidly spinning cloud of molten and vaporized rock. That's where our lunar neighbor is born. "The Moon forms inside the vaporized Earth at temperatures of four to six thousand degrees Fahrenheit and pressures of tens of atmospheres," Lock explained in a press release.

This new model has some crucial benefits over the old Theia story. For one thing, researchers have debated how big the colliding object had to be and how fast the Earth had to spin to make the conditions right for the moon to form the way it did. A synestia isn't as delicate: it can form with a wide variety of objects moving at a wide variety of speeds. It also lines up with the tiny differences observed in the moon's composition: it's almost identical to the Earth, but it's missing those lighter elements that might have stayed in the larger cloud while vaporizing from the smaller moon that formed out of it.

It should be said, however, that we still don't have definitive proof that synestias are actually a thing. We've never observed one, and since they only last between a few hundred to a few thousand years, spotting one in the future will likely be tough. Still, Lock and Stewart think their model is the closest we've come yet to the true explanation of the moon's origin. All that's left is performing a more detailed analysis of moon rocks and finding proof that synestias exist.

For more answers to deceptively complex science questions, check out "Ask a Science Teacher: 250 Answers to Questions You've Always Had About How Everyday Stuff Really Works" by Larry Scheckel. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

How Did the Moon Form?

Written by Ashley Hamer March 19, 2018

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