Layer formation in a stably-stratified fluid cooled from above. Towards an analog for Jupiter and other gas giants

TL;DR

This study uses 2D simulations to explore how convective layers form in stably-stratified fluids under cooling, providing insights relevant to gas giant planet evolution and challenging existing 1D models.

Contribution

It demonstrates that multiple convective layers form at high Prandtl numbers due to boundary layer instabilities, while at low Pr, layers do not form, affecting long-term stratification.

Findings

Multiple layers form at high Prandtl numbers due to boundary layer instability.

Low Prandtl number suppresses layer formation due to smaller temperature gradients.

Long-term, the entire fluid becomes well-mixed, contradicting 1D model predictions.

Abstract

In 1D evolution models of gas giant planets, an outer convection zone advances into the interior as the surface cools, and multiple convective layers form beneath that convective front. To study layer formation below an outer convection zone in a similar scenario, we investigate the evolution of a stably-stratified fluid with a linear composition gradient that is constantly being cooled from above. We use the Boussinesq approximation in a series of 2D simulations at low and high Prandtl numbers ($\mathrm{Pr} = 0.5$ and 7), initialized with constant temperature everywhere, and cooled at different rates. We find that multiple convective layers form at $\mathrm{Pr} = 7$, {as the result of an instability in the} diffusive thermal boundary layer below the outer convection zone. At low Pr, layers do not form because the temperature gradient within the boundary layer is much smaller than at large Pr and, consequently, is not large enough to overcome the stabilizing effect of the composition gradient. For the stratification used in this study, on the long-term the composition gradient is an ineffective barrier against the propagation of the outer convection zone and the entire fluid becomes fully-mixed, whether layers form or not. Our results challenge 1D evolutionary models of gas giant planets, which predict that layers are long-lived and that the outer convective envelope stops advancing inwards. We discuss what is needed for future work to build more realistic models.