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

Abstract

It is theorized that in gas giants, 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 (Pr = 0.5 and 7), initialized with different temperature stratifications, and cooled at different rates. We find that simulations initialized with an isothermal temperature profile form multiple convective layers at Pr = 7. These layers result from an instability of a diffusive thermal boundary layer below the outer convection zone. At low Pr, layers do not form. Doublediffusive instabilities drive the fluid below the outer convection zone into a state of turbulent diffusion rather than layered convection. Changing the initial distribution of temperature to decrease linearly with depth results in lower values of the inverse density ratio R −1 ≡ S z /T z (given the normalization in this work), and consequently, the spontaneous formation of multiple convective layers at low Pr. For the stratifications 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.

Compositional Turbulence and Layering in the Gaseous Envelopes of Forming Planets

“…the fast runaway gas accretion stage) and become permanent features of the final planetary envelopes thus formed. Compositional layers have been proposed to better explain the structure and evolution of Solar System and extrasolar gaseous planets (Leconte & Chabrier 2012 but their origin remains unclear (see also Fuentes et al 2022).…”