Idealised 3D simulations of diabatically-forced Ledoux convection. Application to the atmosphere of hot rocky exoplanets

TL;DR

This study uses idealized 3D simulations to explore how diabatic processes influence convection and temperature gradients in hot rocky exoplanet atmospheres, revealing potential spectral signatures and bifurcation phenomena.

Contribution

It demonstrates that diabatic forcing can reduce temperature gradients in convective atmospheres, a novel insight with implications for exoplanet spectral analysis.

Findings

Reduced temperature gradient persists over time and resolution.

Fast chemical forcing causes bifurcation in temperature profiles.

Potential for spectral reddening analogous to brown dwarfs.

Abstract

We investigate the impact on convective numerical simulations of thermo-compositional diabatic processes. We focus our study on simulations with a stabilizing temperature gradient and a destabilizing mean-molecular weight gradient. We aim to establish the possibility for a reduced temperature-gradient in such setups. A suite of 3D simulations were conducted using a numerical hydrodynamic code. We used as a simplified test case, a sample region of the secondary atmosphere of a hot rocky exoplanet within which the chemical transition CO+O $\leftrightarrow$ CO$_{2}$ could occur. Newtonian cooling and a chemical source term was used to maintain a negative mean molecular weight gradient. Our results demonstrate that this setup can reduce the temperature gradient, a result which does not converge away with resolution or over time. We also show that the presence of the reduced temperature gradient is a function of the forcing timescales. The above transition leads to a bifurcation of the temperature profile when the chemical forcing is fast, reminiscent of the bifurcation seen in the boiling crisis for steam/liquid convection. With the reduced temperature gradient in these idealized setups, there exists the possibility for an analogy of the reddening (currently observed in the spectra of brown dwarfs) in the spectra of rocky exoplanet atmospheres. Detailed 1D modelling is needed, in order to characterize the equilibrium thermal and compositional gradients, the timescales, and the impact of a realistic equation of state, in order to assess if the regime identified here will develop in realistic situations. This possibility cannot, however, be excluded a priori. This prediction is new for terrestrial atmospheres and represents strong motivation for the use of diabatic models when analysing atmospheric spectra of rocky exoplanets that will be observed with e.g. the James Webb Space Telescope.