Lost Planet Theia that Created the Moon Came From the Inner Solar System

The Lost Planet That Created the Moon Came from the Inner Solar System

TO Jacek Krivko edited by Sarah Levin Fraser

About four and a half billion years ago The planet Theia crashed into the Earthdestroying Theia, melting large parts of the Earth's mantle and ejecting a huge disk of debris that later formed the Moon. Scientists have long wondered what Theia is made of and where it came from. Now they have evidence that it formed very close to home.

The original giant impact model for creating the Moon, proposed in the 1970s, predicted that the Moon was composed primarily of Theia material. This scenario implied that there would be differences in the chemical composition of the Moon and Earth, but research has shown that they are almost identical—much more similar than two independent planetary bodies should be. New research published today in Science, Looked closely at the other things Theia gave us besides the Moon: extra molybdenum and iron left over from the impact.

On ancient Earth, these heavy elements accumulated in its core rather than in the rocky mantle closer to the surface, so any iron now present in Earth's mantle likely came from Theia and could 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.

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Kleine and his colleagues analyzed 15 Earth rocks and six lunar samples brought back to Earth by the Apollo missions. They first focused on iron isotopes: variations of the element with different numbers of neutrons. The rocks and planets of the solar system have almost identical distributions of these isotopes, but in the last few years, Klein and some co-authors of the new paper discovered that some very slight deviations from the standard iron isotope ratio can indicate the origin of a sample. “The discovery of iron isotope anomalies is relatively recent, and I think that's why no one has done it on the Moon yet,” Kleine says. “These assays are complex and the variations are small, so it is not easy to carry out the experiment.” The team combined the iron data with the distribution of molybdenum and zirconium isotopes found in the same samples to reconstruct Theia's likely size and composition. The researchers also compared the measurements with samples of 20 meteorites originating from both the inner and outer Solar System to determine Theia's place of origin.

New research shows that Theia is a rocky planet with a metallic core that likely contains between five and ten percent of Earth's mass and formed in the inner solar system, closer to the Sun than Earth. According to Kleine, this pattern is consistent with previous hypotheses about why the bodies were so similar; we didn't know where exactly it was formed.

Back in 2020, Klein and other scientists demonstrated that celestial bodies formed closer to the Sun are richer in heavy elements, such as molybdenum. Following this principle, Klein and co-authors of the new study calculated that Earth has slightly more molybdenum and zirconium than it should have, and concluded that these extra heavy elements must have been brought here by Theia. They combined this data with what they learned about hardware.

“The authors make new measurements of iron isotopes with an exceptional level of precision,” says planetary scientist Sarah Russell, leader of the planetary materials group at the Natural History Museum in London, who was not involved in the new study. In her opinion, the significance of the study goes beyond simply the origin of Theia – it helps us understand what ultimately turned the Earth-Moon system into the cradle of life. “This careful work and in-depth modeling helps us better understand our origins,” she adds.

Klein said the team has not yet implemented the proposed scenario through giant impact simulations, but he is looking forward to running those simulations as well as analyzing lunar samples to look for isotopes of other elements.

Russell is hopeful for the future sample return missions may improve the efficiency of this type of analysis. “I find it amazing that we are still learning new things about the Moon and Earth more than 50 years since the Apollo astronauts collected these rocks from the lunar surface,” Russell says. “Collecting samples in space and bringing them back to Earth means we can take much more detailed measurements than is possible in space and preserve them for future generations to make their own discoveries.”

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