Dynamics and Disequilibrium Carbon Chemistry in Hot Jupiter Atmospheres, with Application to HD 209458b

Chemical equilibrium considerations suggest that, assuming solar elemental abundances, carbon on HD 209458b is sequestered primarily as carbon monoxide (CO) and methane (CH₄). The relative mole fractions of CO(g) and CH₄(g) in chemical equilibrium are expected to vary greatly according to variations in local temperature and pressure. We show, however, that in the p = 1-1000 mbar range, chemical equilibrium does not hold. To explore disequilibrium effects, we couple the chemical kinetics of CO and CH₄ to a three-dimensional numerical model of HD 209458b's atmospheric circulation. These simulations show that vigorous dynamics caused by uneven heating of this tidally locked planet homogenize the CO and CH₄ concentrations at p < 1 bar, even in the presence of lateral temperature variations of ~500-1000 K. In the 1-1000 mbar pressure range we find that over 98% of the carbon is in CO. This is true even in cool regions where CH₄ is much more stable thermodynamically. Our work shows, furthermore, that planets 300-500 K cooler than HD 209458b can also have abundant CO in their upper layers due to disequilibrium effects. We demonstrate several interesting observational consequences of these results.