High-temperature photochemistry in the atmosphere of HD 189733b

Recent infrared spectroscopy of hot exoplanets is beginning to reveal their atmospheric composition. Deep within 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 and 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 contain signatures of CH₄, CO ₂, 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 disequilibrium mechanisms can significantly enhance the abundances of these species above their thermochemical equilibrium value, so some caution must be taken when assuming that an atmosphere is in strict thermochemical equilibrium.