Desch and Robinson then incorporated the data from the Apollo 15 samples into calculations and modeling. From this, they concluded that Theia was a very large body, comparable in size to the proto-Earth. This means that the impact of Theia on proto-Earth was likely a “merger” of similarly sized bodies.

“There must have been solar nebula gas in the gas of disk and dust from which the planets formed,” Desch said. “And the only way to get substantial solar nebula gas into a planetary body is for the body to accrete a lot of nebula gas into an atmosphere around it, while its surface is all molten rock.”

When this happens, the hydrogen can chemically dissolve into the molten rock, the way oxygen and carbon dioxide dissolve into our water oceans, a process called “ingassing.”

Recent studies have shown that this ingassing happened with proto-Earth, but it didn't change the D/H ratio inside proto-Earth that to the extremes seen in the lunar samples.

“That's because there was already a lot of hydrogen in the proto-Earth with high D/H, accreted in the form of water or clay minerals in the material that built up the Earth,” Desch explained. “So whatever hydrogen proto-Earth ingassed, it was mixed with other hydrogen and didn't result in D/H as low as in those lunar samples.”

What this helps explain is that the light hydrogen must have been in Theia. It also helps define Theia as dry (so that the solar nebula hydrogen didn't mix with other hydrogen) and big (because otherwise it wouldn't accrete a thick atmosphere of nebula gas that could then dissolve into the magma ocean).

“What's significant about this finding, beyond just understanding the moon, is how many implications it has for how planets form,” Desch said. “Our research strongly suggests that the early solar system made a handful of large, Mars- to almost Earth-mass planetary embryos that merged into planets, rather than hundreds of moon-sized embryos.

“It strongly suggests as well that much of the material in the inner solar system was bone-dry, with no water in it.”

The researchers hope that these findings spur new thinking about planet formation. Their constraints on Theia suggest the building blocks of Earth got much larger, much faster, than previously thought. New measurements of D/H in lunar and even terrestrial samples should be made to see if they fit into this new context for planet formation and hydrogen in the moon. Desch and Robinson hope to follow up on this to learn more about how the moon formed and about the origins of Earth’s water.

Karin Valentine

Media Relations & Marketing manager, School of Earth and Space Exploration