How a Violent Collision with Theia Created Earth's Moon

Scientists have taken a closer look at the violent formation of our planet's moon. Within the first 150 million years after our solar system formed, a giant body roughly the size of Mars struck and merged with Earth, blasting a huge cloud of rock and debris into space that eventually coalesced to form the moon. Now, though, this explanation is being called into question.

Although this scenario makes sense when you look at the size of the moon and the physics of its orbit around the Earth, things start to break down when you compared their isotopic compositions; these compositions are the geological equivalent of a DNA fingerprint. More specifically, the Earth and moon are simply too much alike.

If the current theory were to hold, then the moon would carry the isotopic fingerprint of the foreign body, which scientists have nicknamed "Theia." Because Theia came from elsewhere in the solar system, it probably had a much different isotopic fingerprint than early Earth.

Now, researchers have generated a new isotopic fingerprint of the moon that could provide the missing piece of the puzzle. They've zeroed in on an isotope of tungsten present in both the moon and the Earth. This has helped them reconcile the accepted model of the moon's formation with the unexpectedly similar isotopic fingerprints of both bodies.

"The problem is that Earth and the moon are very similar with respect to their isotopic fingerprints, suggesting that they are both ultimately formed form the same material that gathered early in the solar system's history," said Richard Walker, co-author of the new study, in a news release. "This is surprising, because the Mars-sized body that created the moon is expected to have been very different. So the conundrum is that Earth and the moon shouldn't be as similar as they are."

The researchers examined another well-documented phenomenon in the history of the early solar system. Evidence suggests that both the Earth and moon gathered additional material after the main impact, and this new material contained a lot of Tungsten. When comparing rocks from the moon and Earth, the scientists found that the moon has a slightly higher proportion of Tungsten-182.

The small but significant difference in Tungsten isotopic composition between the Earth and moon corresponds to the different amounts of material gathered by Earth and the moon post-impact. This finding supports the idea that the mass of material created by the impact, which later formed the moon, must have mixed together thoroughly before the moon coalesced and cooled. This would explain the similarities.

The findings are published in the journal Nature.

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