A primordial origin for the compositional similarity between the Earth and the Moon

Most of the properties of the Earth-Moon system can be explained by a collision between a planetary embryo (giant impactor) and the growing Earth late in the accretion process^1-3. Simulations show that most of the material that eventually aggregates to form the Moon originates from the impactor^1,4,5. However, analysis of the terrestrial and lunar isotopic compositions show them to be highly similar^6-11. In contrast, the compositions of other Solar System bodies are significantly different from those of the Earth and Moon^12-14, suggesting that different Solar System bodies have distinct compositions. This challenges the giant impact scenario, because the Moon-forming impactor must then also be thought to have a composition different from that of the proto-Earth. Here we track the feeding zones of growing planets in a suite of simulations of planetary accretion¹⁵, to measure the composition of Moon-forming impactors. We find that different planets formed in the same simulation have distinct compositions, but the compositions of giant impactors are statistically more similar to the planets they impact. A large fraction of planet-impactor pairs have almost identical compositions. Thus, the similarity in composition between the Earth and Moon could be a natural consequence of a late giant impact.