The state of the Earth’s mantle after the giant impact

Abstract:

At the end of the planetary formation stage, the Earth experienced multiple giant impacts. The last giant impact, which is believed to have formed the Moon, could have been so energetic that it would have mixed the Earth’s whole mantle. This view, however, appears to be inconsistent with geochemical studies that suggest that the Earth’s mantle was not mixed by the impact. To reconcile this discrepancy, we determine the state of the Earth’s mantle after the impact by performing giant impact simulations using smoothed particle hydrodynamics (SPH). Three models are investigated: (a) standard model: a Mars-sized impactor hits the proto-Earth, (b) fast-spinning Earth: a small impactor hits a rapidly rotating proto-Earth, and (c) sub-Earths: two half Earth-sized planets collide. We find that the mantle remains unmixed in (a), but it may be mixed in (b) and (c). Therefore, (a) is most consistent with the preservation of the mantle heterogeneity. We also determine that the Earth’s mantle becomes mostly molten by the impact in all of the models. Additionally, our results indicate that the Rankine-Hugoniot equations do not predict entropy gains of the mantle materials by a giant impact very well. This is because the entropy increase of the mantle is caused not only by the primary impact-induced shock, which provides the peak shock pressure, but also by subsequent processes. We further discuss the time evolution of the Earth’s mantle.