Thermal convection in Earth's inner core with phase change at its boundary

TL;DR

This paper investigates the dynamics of Earth's inner core thermal convection considering phase change effects at the boundary, highlighting the influence of viscosity, thermal conductivity, and compositional stratification on convection modes and hemispherical asymmetry.

Contribution

It provides a detailed analysis of how phase change, viscosity, and thermal properties affect inner core convection modes and hemispherical dichotomy, incorporating new insights into compositional stratification effects.

Findings

Translation mode dominates at high viscosity ($ ext{~}3 imes 10^{18}$ Pa·s)

Classical convection patterns emerge at lower viscosity levels

Iron-rich layer formation may promote inner core convection even with high thermal conductivity

Abstract

Inner core translation, with solidification on one hemisphere and melting on the other, provides a promising basis for understanding the hemispherical dichotomy of the inner core, as well as the anomalous stable layer observed at the base of the outer core - the F-layer - which might be sustained by continuous melting of inner core material. In this paper, we study in details the dynamics of inner core thermal convection when dynamically induced melting and freezing of the inner core boundary (ICB) are taken into account. If the inner core is unstably stratified, linear stability analysis and numerical simulations consistently show that the translation mode dominates only if the viscosity $\eta$ is large enough, with a critical viscosity value, of order $3 10^{18}$ Pas, depending on the ability of outer core convection to supply or remove the latent heat of melting or solidification. If $\eta$ is smaller, the dynamical effect of melting and freezing is small. Convection takes a more classical form, with a one-cell axisymmetric mode at the onset and chaotic plume convection at large Rayleigh number. [...] Thermal convection requires that a superadiabatic temperature profile is maintained in the inner core, which depends on a competition between extraction of the inner core internal heat by conduction and cooling at the ICB. Inner core thermal convection appears very likely with the low thermal conductivity value proposed by Stacey & Davis (2007), but nearly impossible with the much higher thermal conductivity recently put forward. We argue however that the formation of an iron-rich layer above the ICB may have a positive feedback on inner core convection: it implies that the inner core crystallized from an increasingly iron-rich liquid, resulting in an unstable compositional stratification which could drive inner core convection, perhaps even if the inner core is subadiabatic.