By tracking the isotopic fingerprints of iron in lunar and terrestrial rocks, researchers trying to understand the origin of the Moon’s mysterious progenitor add evidence to the idea that it came from the inner Solar System. According to the findings, Theia – the Mars-sized planetary body that collided with Earth to form the Moon – was born possibly closer to the Sun than to Earth. The Moon is believed to have formed when Theia collided with early Earth roughly a hundred million years after the formation of the Solar System. Most models of this process suggest that the Moon is mostly composed of materials derived from this ancient impactor. If Theia had a different isotopic makeup from Earth, it would be expected that the Moon would as well. Such isotopic variations can reveal where a planetary body originated in the Solar System, which could provide insight into the origin of Theia. However, analyses of lunar rock show that the Moon and Earth are nearly identical in their isotopic compositions for many elements. Although competing models have attempted to explain this similarity, the absence of clear isotopic differences and uncertainty over which processes caused this have made it challenging to determine where Theia originally formed. Here, Timo Hopp and colleagues conducted new high-precision iron isotope analyses of lunar samples, terrestrial rocks, and meteorites representing the isotopic reservoirs from which Theia and proto-Earth might have formed. According to the analysis, Earth and the Moon have indistinguishable iron isotopic compositions and both fall within that of non-carbonaceous meteorites, which are thought to represent material formed in the inner Solar System. Integrating these results with previous isotopic data for other elements and performing mass balance calculations for Theia and proto-Earth, Hopp et al. conclude that Theia likely originated in the inner Solar System and formed even closer to the Sun than proto-Earth.
