Theia—the Mars-sized impactor that spawned the Moon—most likely formed in the inner Solar System, closer to the Sun than Earth. Scientists combined ultra-precise iron isotope measurements with molybdenum and zirconium data from 15 Earth rocks, six Apollo lunar samples and ~20 meteorites to trace Theia's origin. The results suggest Theia was rocky with a metallic core roughly 5–10% of Earth's mass and delivered extra heavy elements to Earth's mantle. The team will run impact simulations and seek further isotopic evidence from additional lunar samples.
Lost Planet Theia Likely Formed Closer to the Sun, New Isotope Study Suggests
Theia—the Mars-sized impactor that spawned the Moon—most likely formed in the inner Solar System, closer to the Sun than Earth. Scientists combined ultra-precise iron isotope measurements with molybdenum and zirconium data from 15 Earth rocks, six Apollo lunar samples and ~20 meteorites to trace Theia's origin. The results suggest Theia was rocky with a metallic core roughly 5–10% of Earth's mass and delivered extra heavy elements to Earth's mantle. The team will run impact simulations and seek further isotopic evidence from additional lunar samples.
07:28 AM, 11/21/2025Science
About 4.5 billion years ago a Mars-sized body, dubbed Theia, collided with the early Earth, vaporizing itself, melting large portions of Earth's mantle and launching a vast debris disk that later coalesced into the Moon. Scientists have long debated Theia's composition and birthplace; a new isotope study provides strong evidence that Theia formed closer to the Sun than Earth did.
Researchers analyzed iron isotopes together with molybdenum and zirconium signatures in 15 terrestrial rocks and six lunar samples returned by the Apollo missions. They compared those results with measurements from roughly 20 meteorites known to originate in either the inner or outer Solar System. Because heavy elements such as iron and molybdenum preferentially sink into a planet's core during early differentiation, any excess of these elements in Earth's mantle can point to material delivered by an impactor like Theia.
How the team traced Theia
The study used extremely precise iron-isotope measurements to detect minute deviations that act like fingerprints for a sample's origin. The scientists combined those iron-isotope results with molybdenum and zirconium distributions in the same specimens to reconstruct Theia's likely composition and mass. Comparing those fingerprints with meteorites from known regions of the Solar System allowed the team to infer where Theia formed.
The combined data paint Theia as a rocky planet with a metallic core, containing roughly 5–10% of Earth's mass, that most likely originated in the inner Solar System—closer to the Sun than Earth. That origin helps explain why Earth and Moon are so compositionally similar: if Theia itself formed near Earth’s region, the two bodies would share many chemical traits.
“The authors make new iron isotope measurements at exceptional levels of precision,” says planetary scientist Sara Russell of the Natural History Museum in London, who was not involved in the study. “These careful measurements and thoughtful modeling give us fresh insight into the processes that shaped the Earth–Moon system.”
The research team plans to run dynamical simulations of the giant impact under this scenario and to search lunar samples for additional isotopic signatures. Scientists also emphasize the value of future sample-return missions: new, well-documented lunar material would enable even more detailed isotope studies and could help settle remaining questions about the Moon's origins.