The Moon-forming impactor Theia originated from the inner Solar System

The Moon formed from a giant impact of a planetary body, called Theia, with proto-Earth. It is unknown whether Theia formed in the inner or outer Solar System. We measured iron isotopes in lunar samples, terrestrial rocks, and meteorites representing the isotopic reservoirs from which Theia and proto-Earth might have formed. Earth and the Moon have indistinguishable mass-independent iron isotopic compositions; both define one end of the range measured in meteorites. Combining our results with those for other elements, we performed mass balance calculations for Theia and proto-Earth. We found that all of Theia and most of Earth’s other constituent materials originated from the inner Solar System. Our calculations suggest that Theia might have formed closer to the Sun than Earth did.