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'. 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 accretion(15), 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.