High Temperature Photochemistry in the Atmosphere of HD189733b

TL;DR

This paper investigates the interplay of thermochemical equilibrium and photochemistry in the atmospheres of hot Jupiters, focusing on HD189733b, revealing how photochemical processes can alter atmospheric compositions from equilibrium predictions.

Contribution

It identifies key chemical pathways affecting atmospheric species and demonstrates the impact of photochemistry on hot Jupiter atmospheres, highlighting the need to consider both processes.

Findings

Photochemistry can significantly alter atmospheric species abundances.

Thermochemical and photochemical processes compete in the 1-10 bar range.

Caution is needed when assuming thermochemical equilibrium in exoplanet atmospheres.

Abstract

Recent infrared spectroscopy of hot exoplanets is beginning to reveal their atmospheric composition. Deep with in the planetary atmosphere, the composition is controlled by thermochemical equilibrium. Photochemistry becomes important higher in the atmosphere, at levels above ~1 bar. These two chemistries compete between ~1-10 bars in hot Jupiter-like atmospheres, depending on the strength of the eddy mixing and temperature. HD189733b provides an excellent laboratory in which to study the consequences of chemistry of hot atmospheres. The recent spectra of HD189733b and HD209458b contain signatures of CH4, CO2, CO and H2O. Here we identify the primary chemical pathways that govern the abundances of CH4, CO2, CO and H2O in the cases of thermochemical equilibrium chemistry, photochemistry, and their combination. Our results suggest that the abundance of these species can be photochemically enhanced above or below the thermochemical equilibrium value, so some caution must be taken when assuming that an atmosphere is in strict thermochemical equilibrium.