HIGH-TEMPERATURE PHOTOCHEMISTRY IN THE ATMOSPHERE OF HD 189733b

Abstract

Recent infrared spectroscopy of hot exoplanets is beginning to reveal their atmospheric composition. Deep within
\nthe planetary atmosphere, the composition is controlled by thermochemical equilibrium. Photochemistry becomes
\nimportant higher in the atmosphere, at levels above ~1 bar. These two chemistries compete between ~1 and 10 bars
\nin hot-Jupiter-like atmospheres, depending on the strength of the eddy mixing and temperature. HD 189733b
\nprovides an excellent laboratory in which to study the consequences of chemistry of hot atmospheres. The
\nrecent spectra of HD 189733b contain signatures of CH_4, CO_2, CO, and H_2O. Here we identify the primary
\nchemical pathways that govern the abundances of CH_4, CO_2, CO, and H_2O in the cases of thermochemical
\nequilibrium chemistry, photochemistry, and their combination. Our results suggest that the disequilibrium
\nmechanisms can significantly enhance the abundances of these species above their thermochemical equilibrium
\nvalue, so some caution must be taken when assuming that an atmosphere is in strict thermochemical equilibrium.