An artist’s rendering shows a planetary collision near the star Vega. The moon may have formed from the debris of such an impact between Earth and a Mars-sized body. Credit: NASA Despite sometimes studying objects literally trillions of miles away, astronomers still have plenty of unanswered questions about the moon, which is basically right there. Even as basic a question as where it came from remains unclear, though a well-explored theory suggests that it arose from a colossal collision deep in Earth’s past. A new study this week finally has some proof to back that up: Researchers have detected rubble from a foreign body in the moon's makeup.

Moon's Origin

The current thinking is that a nascent, proto-Earth was going along its merry way when a Mars-sized object, dubbed Theia, smashed into it. The debris from this crash eventually coalesced into the familiar moon. This is called the giant impact hypothesis. Numerically, the hypothesis makes sense and also explains a lot of our satellite’s features and characteristics. But when experimentalists wanted to prove this, they came up short. Most large bodies in the solar system have different amounts of chemical isotopes; it would follow that Earth and Theia did, too. The moon, as a mixture of the two (likely about 70-90 percent Theia to 10-30 percent proto-Earth), should also have different isotope figures than Earth. Unfortunately, that’s not what the experiments showed: Effectively, the two worlds had identical isotope levels. And that left scientists scratching their heads.

Theia's Fingerprint

Now, a team of German scientists have taken another crack at the problem, applying new, more sensitive analysis techniques to existing lunar samples. (They initially used lunar meteorites, but when those proved too tainted by Earth’s chemical influence to be useful, they went right to the source, analyzing samples brought back from the Apollo 11, 12 and 16 missions.) They found that, in fact, Earth rocks and moon rocks are fundamentally different when it comes to isotopes of oxygen, differing by about 12 ± 3 parts per million. That might not seem like a huge difference, but it’s more than enough. As the authors put it in their paper in this week’sScience:

This unequivocally identifies an isotopic difference between Earth and the Moon, and supports the view that the Moon formed by a giant collision of the proto-Earth with Theia.

Of course, it’s a relief that the prevailing theory for the moon’s formation might actually be right. But the results are also important for what they have to say about the composition of Theia in the first place. They indicate that Theia was likely a rare kind of meteorite known as an enstatite chondrite. Which, in turn, tells us more about the moon we see, so tantalizingly clearly, in our skies.