A Scaling Relation for Core Heating by Giant Impacts and Implications for Dynamo Onset

Abstract

Accretional heating of Earth's interior during formation is pivotal to its subsequent thermal and chemical evolution. In particular, impact heating of Earth's core is expected, but its amplitude and radial distribution within the core is unknown and could influence the onset of the geodynamo. The uncertainty is due, in part, to the lack of constraints on the temperature of the interior following formation due to the difficulty of preserving a record of such a high energy environment, and the assertion that super-heating during formation would be rapidly lost through magma ocean cooling. Here we systematically investigate core heating due to giant impacts using a Smoothed Particle Hydrodynamics (SPH) code with simulations spanning a range of impact angles, velocities, and masses. From these simulations we derive a scaling relation for core heating that depends on the impact parameters and predicts the radial core temperature profile following the impact. Our findings show that a significant amount of heat is deposited into the core, with a canonical impact scenario resulting in an average core temperature increase of about 3000 K, approximately 500 K higher than that of the overlying mantle. In this case the heat distribution within the core produces a strong thermal stratification. We use a parameterized cooling model to estimate that the core could have cooled to an adiabatic state ∼290 Myr after a canonical impact, which is consistent with the observed time span between the age of the Moon and evidence for an active geodynamo.

Key Points

• A giant impact can significantly heat Earth's core with an amplitude and distribution that depends on the impact conditions

• Giant impacts induce heterogeneous heating within the core, often leading to thermal stratification, inhibiting dynamo onset

• Scaling relations are developed relating impact conditions with the post-collision radial temperature profile of Earth's core

Plain Language Summary

Understanding how the Earth cools down is a pivotal question in the study of our planet's evolutionary history. The initial thermal state of the Earth's core plays a significant role because it determines when the geomagnetic field first emerged. To study this issue, we systematically investigated core heating during giant impacts using a Smoothed Particle Hydrodynamics (SPH) code. Each simulation runs a single giant impact under specific initial conditions. These simulations covered a range of impact angles, velocities, and impactor masses. First, we found that different giant impacts cause significant differences in the Earth's core temperature. Second, the temperature distribution within the core is highly heterogeneous, with high temperatures mainly concentrated at the outermost part of the core. This leads to the natural formation of a thermally stratified structure in the core. Such a stratified structure is very stable and will not undergo convection, which may delay the initiation of the Earth's earliest magnetic field.