Where did the Moon come from and how was it formed? It's a question that scientists have been scratching their heads over for decades.

According to the widely-accepted "giant-impact hypothesis," the Moon was formed billions of years ago when a Mars-sized planet called Theia collided with the Earth, breaking off a large chunk in the process.

Now, researchers from the University of New Mexico say in a new paper published in Nature Geoscience that they believe they've found the remains of Theia buried beneath the lunar surface.

The big drawback to the impact hypothesis — as discussed at length by numerous scientists over many years — is the fact that even though oxygen isotopes in lunar rocks collected during NASA's Apollo missions closely resemble those found on Earth, they're strikingly different from those found on other objects in our Solar System, as explained by Science Alert.

So how could the Moon be largely made up of the remains of Theia, as many models have predicted?

Despite very small odds, Earth and Theia may have originally had similar compositions, but then been mixed up beyond recognition.

In their research, the New Mexico scientists examined oxygen isotopes in a range of different lunar rock types, collected from a variety of altitudes. They found that the deeper the origin of the rock, the heavier the oxygen isotopes, when compared to the ones found on Earth.

"Clearly, Theia's distinct oxygen isotope composition was not completely lost through homogenisation during the giant impact," the researchers wrote in their paper.

That means the oxygen isotopes didn't fully mix during the Theia-Earth impact, leading to their conclusion that Theia could've still originated from outside of the Solar System before impacting with Earth.

"This data suggests that the deep lunar mantle may have experienced the least mixing and is most representative of the impactor Theia," said research scientist and co-author Zach Sharp in a statement.

READ MORE: We May Have Finally Found a Chunk of Theia Buried Deep Inside The Moon [Science Alert]

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