Signature of Vertical Mixing in Hydrogen-dominated Exoplanet Atmospheres

TL;DR

This paper investigates how vertical mixing affects the spectra of hydrogen-dominated exoplanet atmospheres, exploring parameter space and assessing retrieval models' ability to constrain mixing strength using JWST simulations.

Contribution

It introduces a fast disequilibrium model with vertical mixing, coupled with retrieval techniques, to better understand and constrain atmospheric mixing in exoplanets.

Findings

Vertical mixing significantly impacts spectra in a narrow parameter space.

Retrieval models can constrain eddy diffusion coefficient in certain conditions.

NH3 abundance constrains internal temperature for T_equi > 1400 K.

Abstract

Vertical mixing is a crucial disequilibrium process in exoplanet atmospheres, significantly impacting chemical abundance and observed spectra. While current state-of-the-art observations have detected its signatures, the effect of vertical mixing on atmospheric spectra varies widely based on planetary parameters. In this study, we explore the influence of disequilibrium chemistry across a parameter space that includes eddy diffusion, surface gravity, internal and equilibrium temperature, and metallicity. We also assess the effectiveness of retrieval models in constraining the eddy diffusion coefficient. By running numerous 1D chemical kinetics models, we investigate the impact of vertical mixing on the transmission spectrum. We also built a custom fast-forward disequilibrium model, which includes vertical mixing using the quenching approximation and calculates the model abundance orders of magnitude faster than the chemical kinetics model. We coupled this forward model with an open source atmospheric retrieval code and used it on the JWST simulated output data of our chemical kinetics model and retrieved eddy diffusion coefficient, internal temperature and atmospheric metallicity. We find that there is a narrow region in the parameters space in which vertical mixing has a large effect on the atmospheric transmission spectrum. In this region of the parameter space, the retrieval model can put high constraints on the transport strength and provide optimal exoplanets to study vertical mixing. Also, the NH3 abundance can be used to constrain the internal temperature for equilibrium temperature T_equi > 1400 K.