Brief follow-up on recent studies of Theia’s accretion

• We model the accretion of the Solar System’s terrestrial planets.
• We examine feeding zones of planets and their major impactors.
• We find Theia is unlikely to have the same feeding zone as Earth.
• There is a ∼5% chance that Theia is isotopically identical to Earth.
• Venus is also isotopically distinct from Earth.

Kaib and Cowan (Kaib, N.A., Cowan, N.B. [2015]. Icarus 252, 161–174) recently used terrestrial planet formation simulations to conclude that the moon-forming impactor (Theia) had only a ∼5% or less chance of having the same oxygen isotope composition as Earth, while Mastrobuono-Battisti et al. (Mastrobuono-Battisti, A., Perets, H.B., Raymond, S.N. [2015]. Nature 520, 212–215) used seemingly similar simulations and methods to arrive at a higher value of ∼20% or more. Here we derive the results of both papers from a single set of simulations. Compared to Kaib and Cowan (Kaib, N.A., Cowan, N.B. [2015]. Icarus 252, 161–174), the analysis of Mastrobuono-Battisti et al. (Mastrobuono-Battisti, A., Perets, H.B., Raymond, S.N. [2015]. Nature 520, 212–215) systematically yields more massive Theia analogs and imposes flatter Δ17OΔ17O gradients across the original protoplanetary disk. Both of these effects diminish isotopic differences between Earth and Theia analogs. While it is notoriously difficult to produce systems resembling our actual terrestrial planets, the analysis of Kaib and Cowan (Kaib, N.A., Cowan, N.B. [2015]. Icarus 252, 161–174) more often selects and analyzes Earth and Mars analogs at orbital locations near the real planets. Given this, we conclude that the greater isotopic differences between Earth and Theia found in Kaib and Cowan (Kaib, N.A., Cowan, N.B. [2015]. Icarus 252, 161–174) better reflect the predictions of terrestrial planet formation models. Finally, although simulation uncertainties and a terrestrial contribution to Moon formation enhance the fraction of Theia analogs consistent with the canonical giant impact hypothesis, this fraction still remains in the 5–8% range.