Dynamical origin of Theia, the last giant impactor on Earth

Abstract

Cosmochemical studies have proposed that Earth accreted roughly 5%–10% of its mass from carbonaceous (CC) material, with a large fraction delivered late via its final impactor, Theia (the Moon-forming impactor). Here, we evaluate this idea using dynamical simulations of terrestrial planet formation, starting from a standard setup with a population of planetary embryos and planetesimals laid out in a ring centered between Venus and Earth’s orbits, and also including a population of CC planetesimals and planetary embryos scattered inward by Jupiter. We find that this scenario can match a large number of constraints, including (i) the terrestrial planets’ masses and orbits; (ii) the CC mass fraction of Earth; (iii) the much lower CC mass fraction of Mars, as long as Mars only accreted CC planetesimals (but no CC embryos); (iv) the timing of the last giant (Moon-forming) impact; and (v) a late accretion phase dominated by non-carbonaceous (NC) bodies. For this scenario to work, the total mass in scattered CC objects must have been $\sim 0.2-0.3M_{\oplus }$, with an embryo-to-planetesimal mass ratio of at least 8, and CC embryos in the $\sim 0.01-0.05M_{\oplus }$ mass range. In that case, our simulations show there are roughly 50-50 odds of Earth’s last giant impactor (Theia) having been a carbonaceous object — either a pure CC embryo or an NC embryo that previously accreted a CC embryo. Our simulations thus provide dynamical validation of cosmochemical studies.