Geochemical Constraints on the Size of the Moon‐Forming Giant Impact

Abstract

Abstract Recent models involving the Moon‐forming giant impact hypothesis have managed to reproduce the striking isotopic similarity between the two bodies, albeit using two extreme models: one involves a high‐energy small impactor that makes the Moon out of Earth's proto‐mantle; the other supposes a gigantic collision between two half‐Earths creating the Earth‐Moon system from both bodies. Here we modeled the geochemical influence of the giant impact on Earth's mantle and found that impactors larger than 15% of Earth mass result in mantles always violating the present‐day concentrations of four refractory moderately siderophile trace elements (Ni, Co, Cr, and V). In the aftermath of the impact, our models cannot further discriminate between a fully and a partially molten bulk silicate Earth. Then, the preservation of primordial geochemical reservoirs predating the Moon remains the sole argument against a fully molten mantle after the Moon‐forming impact.