Disequilibrium carbon, oxygen, and nitrogen chemistry in the atmospheres of HD189733b and HD209458b

We have developed a one-dimensional photochemical and thermochemical kinetics and diffusion model to study the effects of disequilibrium chemistry on the atmospheric composition of "hot-Jupiter" exoplanets. Here we investigate the coupled chemistry of neutral carbon, hydrogen, oxygen, and nitrogen species on HD189733b and HD209458b and we compare the model results with existing transit and eclipse observations. We find that the vertical profiles of molecular constituents are significantly affected by transport-induced quenching and photochemistry, particularly on the cooler HD189733b; however, the warmer stratospheric temperatures on HD209458b help maintain thermochemical equilibrium and reduce the effects of disequilibrium chemistry. For both planets, the methane and ammonia mole fractions are found to be enhanced over their equilibrium values at pressures of a few bar to less than an mbar due to transport-induced quenching, but CH₄ and NH ₃ are photochemically removed at higher altitudes. Disequilibrium chemistry also enhances atomic species, unsaturated hydrocarbons (particularly C₂H₂), some nitriles (particularly HCN), and radicals like OH, CH₃, and NH₂. In contrast, CO, H₂O, N ₂, and CO₂ more closely follow their equilibrium profiles, except at pressures ≲1μbar, where CO, H₂O, and N₂ are photochemically destroyed and CO₂ is produced before its eventual high-altitude destruction. The enhanced abundances of CH₄, NH₃, and HCN are expected to affect the spectral signatures and thermal profiles of HD189733b and other relatively cool, transiting exoplanets. We examine the sensitivity of our results to the assumed temperature structure and eddy diffusion coefficients and discuss further observational consequences of these models.