The mutual influence of disequilibrium composition and temperature in exoplanet atmospheres

TL;DR

This paper presents a self-consistent 1D model of exoplanet atmospheres that couples temperature evolution with disequilibrium chemistry, revealing moderate temperature impacts for gas giants and significant effects in certain secondary atmospheres.

Contribution

The study introduces a comprehensive 1D atmospheric model with an extensive reaction network, applied to multiple exoplanets, to analyze the interplay between disequilibrium chemistry and temperature.

Findings

Disequilibrium chemistry causes small temperature corrections (~100 K) in irradiated gas giants.

Significant temperature effects occur in reducing and oxidizing secondary atmospheres.

TiO depletion by UV photons can impact temperature but remains uncertain due to photodestruction knowledge gaps.

Abstract

We have developed a 1D planetary atmosphere model that solves in a self-consistent manner the evolution of temperature and disequilibrium chemistry in the vertical direction. Thermochemical kinetics is based on a reaction network built from scratch that includes 164 gaseous species composed of H, C, N, O, S, Si, P, Ti, He, and Ar, connected by 2352 forward reactions. The model is applied to the well-known gas giant exoplanets WASP-33b, HD209458b, HD189733b, GJ436b, and GJ1214b, and to secondary atmospheres that exoplanets characterized in the future may plausibly have. For irradiated gas giants with solar or supersolar metallicity, the corrections to the temperature due to disequilibrium chemistry are relatively small, on the order of 100 K at most, in agreement with previous studies. Although the atmospheric composition of some of these planets deviates significantly from chemical equilibrium, the impact on the temperature is moderate because the abundances of the main atmospheric species that provide opacity, such as H2O, CO2, CO, and/or CH4, are not seriously modified by disequilibrium chemistry. An impact on the temperature greater than 100 K appears in hot Jupiters due to TiO, which is predicted to be seriously depleted by UV photons in the upper layers. However, the extent of this depletion, and thus of its impact on the temperature, is uncertain due to the lack of knowledge about TiO photodestruction. In secondary atmospheres, the impact of disequilibrium chemistry on the temperature depends on the composition. In atmospheres dominated by H2O and/or CO2 the temperature is not affected to an important extent. However, reducing atmospheres dominated by CH4 and oxidizing atmospheres dominated by O2 see their temperature being seriously affected due to the important processing of the atmospheric composition induced by disequilibrium chemistry.