The early solar system was, for lack of a better term, a chaotic hellscape. Everything we see today, from Mercury on out to the inner Oort Cloud, was a product of a series of collisions that accumulated into moons, asteroids, and planets. And perhaps one of the most violent blows came when a planet roughly the size of Mars smashed into a fledging planet called Earth. At the end of the cataclysmic event, two bodies were left standing: Earth itself, and a fragmentary, molten piece of the two planets that coalesced into the Moon.

But there's been a debate in the planetary science community: Did that early small planet, Theia, side-swipe the Earth, or did it run into it full-on? For a while, all signs seemed to point to Theia landing a glancing shot at Earth, resulting in the orbits and speeds we see today.

But recent analysis of material taken from the Apollo 12, 15, and 17 missions tells a different tale: that of a violent head-on collision, one that left the Earth forever scarred with the fragments of Theia, and that left the Moon with the same ratios of material. The results of this study were published today in Science.

"The collision was so vigorous, so powerful, so rich in energy that it probably mixed the whole system very thoroughly," said Edward Donald Young, lead author of the paper and a professor of Department of Earth, Planetary, and Space Sciences at UCLA.

The proof is in the oxygen isotopes: that is, the Moon share the same oxygen isotopes in the same ratios as Earth, and preliminary results of tungsten isotopes shows about the same thing. In other words, the Earth and the Moon are made of the same materials. In the "glancing blow" model, the Moon would primarily have contained material from Theia with some admixture from Earth.

But analysis of salt-heavy lunar rocks and soil show it to be virtually identical to the floor of the ocean. As you can see from the map below, the Apollo rocks all came from vastly disparate parts of the Moon, but the ratios in each region remain the same.

Wikimedia Commons

The only outlier is a sample taken from the Northern Highlands of the moon, but Young says that owes to being an entirely different rock type. The rock, Young says, is likely to have more in common with the Bushveld Igneous Complex, a deposit of subsurface materials jutting through the crust in South Africa. The geochemical processing in each of those regions is likely to have remained the same, Young says.

"I think this will motivate impact modeling in [the head on collision] direction, so people will be forced to abandon the glancing blow model," Young says. "People will keep looking with better and better analytic precision."

To Young, this is a closing of the book on the glancing blow hypothesis. Now it's time to figure out how Theia and the Earth went from a head-on collission and became the Earth-Moon system we see today. There's a lot to account for, like how the Earth and Moon established their present rotations and orbits. But one of the bigger parts of the mystery is, to Young and his colleagues, solved.

"My view is that the issue of Theia content is more or less settled," Young says. "Now it's in the hands of the modelers."

Photomicrograph of an Apollo 17 sample of lunar highland rock as viewed in cross-polarized transmitted light. Paul Warren / UCLA

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