Fingering convection in the stably-stratified layers of planetary cores

TL;DR

This study investigates fingering convection in stably-stratified planetary core layers, revealing how rotation and stratification influence flow structures, with implications for planetary magnetic field generation.

Contribution

It models fingering convection in planetary cores using hydrodynamical simulations, highlighting the effects of rotation, stratification, and diffusivity differences on flow patterns.

Findings

Fingering convection forms columnar flows aligned with the rotation axis at low Rayleigh numbers.

At higher Rayleigh numbers, flows become thin, sheet-like, and elongated meridionally.

Radial flows remain laminar with low Reynolds numbers, and zonal flows can dominate at high Rayleigh numbers.

Abstract

Stably-stratified layers may be present at the top of the electrically-conducting fluid layers of many planets either because the temperature gradient is locally subadiabatic or because a stable composition gradient is maintained by the segregation of chemical elements. Here we study the double-diffusive processes taking place in such a stable layer, considering the case of Mercury's core where the temperature gradient is stable but the composition gradient is unstable over a 800km-thick layer. The large difference in the molecular diffusivities leads to the development of buoyancy-driven instabilities that drive radial flows known as fingering convection. We model fingering convection using hydrodynamical simulations in a rotating spherical shell and varying the rotation rate and the stratification strength. For small Rayleigh numbers (i.e. weak background temperature and composition gradients), fingering convection takes the form of columnar flows aligned with the rotation axis and with an azimuthal size comparable with the layer thickness. For larger Rayleigh numbers, the flows retain a columnar structure but the azimuthal size is drastically reduced leading to thin sheet-like structures that are elongated in the meridional direction. The azimuthal length decreases when the thermal stratification increases, following closely the scaling law expected from the linear non-rotating planar theory (Stern, 1960). We find that the radial flows always remain laminar with local Reynolds number of order 1-10. Equatorially-symmetric zonal flows form due to latitudinal variations of the axisymmetric composition. The zonal velocity exceeds the non-axisymmetric velocities at the largest Rayleigh numbers. We discuss plausible implications for planetary magnetic fields.