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Roughly four and a half billion years ago, the planet Theia slammed into Earth, destroying Theia, melting large fractions of Earth’s mantle, and ejecting a huge debris disk that later formed the moon. Scientists have long wondered what Theia was made of and where it came from. Now they have evidence that it formed very close to home.

The original giant impact model of the moon’s creation, proposed in the 1970s, predicted the moon was made mostly of Theia’s material. This scenario implied there should be differences in the chemical composition of the moon and Earth, but research has found that the two are nearly identical—far more similar than two independent planetary bodies should be. A new study, published today in Science, took a close look at other things Theia gave us beside the moon: additional molybdenum and iron left behind in the collision.

Ancient Earth would have had these heavy elements accumulate in its core but not in the rocky mantle closer to the surface, so any iron present now in Earth’s mantle likely came from Theia and can tell us about that planet’s composition, says study co-author Thorsten Kleine, director of the Max Planck Institute for Solar System Research in Göttingen, Germany.

Kleine and his colleagues analyzed 15 terrestrial rocks and six lunar samples brought to Earth by Apollo missions. They focused first on iron isotopes: variations of the element with different numbers of neutrons. Rocks and planets in the Solar System share nearly identical distributions of these isotopes, but in the last few years, Kleine and some of the new paper’s co-authors discovered that certain very tiny deviations from the standard iron isotope ratio can reveal the sample’s place of origin. “The discovery of iron isotope anomalies is relatively recent, which I guess is why no one has done it for the moon so far,” Kleine says. “These analyses are difficult and the variations are small, so it is not an easy experiment to do.” The team combined the data on iron with isotope distributions of molybdenum and zirconium found in the same samples to reverse engineer Theia’s likely size and composition. The researchers also compared the measurements to those of samples from 20 meteorites that originated in both the inner and outer Solar System to determine Theia’s place of origin.

The new study reveals Theia as a rocky planet with a metallic core that most likely contained five to ten percent the mass of Earth and formed in the inner Solar System, closer to the sun than Earth is. This picture lines up with previous hypotheses for why the bodies were so similar, Kleine says; what we didn’t know was where it formed exactly.

Back in 2020, Kleine and other scientists demonstrated that celestial bodies that formed closer to the sun are richer in heavy elements such as molybdenum. Following this principle, Kleine and the new study’s co-authors estimated that Earth has a bit more molybdenum and zirconium than it should and figured these additional heavy elements must have been brought here by Theia. They combined that data with what they’d learned about the iron.

“The authors make new iron isotope measurements at exceptional levels of precision,” says planetary scientist Sara Russell, leader of the Planetary Materials Group at the Natural History Museum, London, who was not involved in the new study. For her, the study’s implications go beyond just the origins of Theia—they help us understand what eventually shaped the Earth-moon system into a cradle of life. “This careful work and insightful modeling help us better understand our origins,” she adds.

The team hasn’t yet run its proposed scenario through simulations of the giant impact, Kleine says, but he looks forward to running those simulations, as well as analyzing lunar samples to look for other element isotopes.

Russell hopes future sample-return missions can boost this type of analysis. “I find it amazing we are still learning new things about the moon and Earth over 50 years since the Apollo astronauts collected these rocks from the lunar surface,” Russell says. “Collecting samples in space and bringing them to curate on Earth means we can make much more detailed measurements than are possible in space and preserves them for future generations to make their own discoveries.”

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