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We have developed 1-D photochemical and thermochemical kinetics and diffusionmodels for the transiting exoplanets HD 189733b and HD 209458b to study theeffects of disequilibrium chemistry on the atmospheric composition of "hotJupiters." Here we investigate the coupled chemistry of neutral carbon,hydrogen, oxygen, and nitrogen species, and we compare the model results withexisting transit and eclipse observations. We find that the vertical profilesof molecular constituents are significantly affected by transport-inducedquenching and photochemistry, particularly on cooler HD 189733b; however, thewarmer stratospheric temperatures on HD 209458b can help maintainthermochemical equilibrium and reduce the effects of disequilibrium chemistry.For both planets, the methane and ammonia mole fractions are found to beenhanced over their equilibrium values at pressures of a few bar to less than ambar due to transport-induced quenching, but CH4 and NH3 are photochemicallyremoved at higher altitudes. Atomic species, unsaturated hydrocarbons(particularly C2H2), some nitriles (particularly HCN), and radicals like OH,CH3, and NH2 are enhanced overequilibrium predictions because of quenching andphotochemistry. In contrast, CO, H2O, N2, and CO2 more closely follow theirequilibrium profiles, except at pressures < 1 microbar, where CO, H2O, and N2are photochemically destroyed and CO2 is produced before its eventualhigh-altitude destruction. The enhanced abundances of HCN, CH4, and NH3 inparticular are expected to affect the spectral signatures and thermal profilesHD 189733b and other, relatively cool, close-in transiting exoplanets. Weexamine the sensitivity of our results to the assumed temperature structure andeddy diffusion coefficientss and discuss further observational consequences ofthese models.

Disequilibrium Carbon, Oxygen, and Nitrogen Chemistry in the Atmospheres of HD 189733b and HD 209458b