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Chemical equilibrium considerations suggest that, assuming solar elementalabundances, carbon on HD 209458b is sequestered primarily as carbon monoxide(CO) and methane (CH4). The relative mole fractions of CO(g) and CH4(g) inchemical equilibrium are expected to vary greatly according to variations inlocal temperature and pressure. We show, however, that in the p = 1--1000 mbarrange, chemical equilibrium does not hold. To explore disequilibrium effects,we couple the chemical kinetics of CO and CH4 to a three-dimensional numericalmodel of HD 209458b's atmospheric circulation. These simulations show thatvigorous dynamics caused by uneven heating of this tidally locked planethomogenize the CO and CH4 concentrations at p < 1 bar, even in the presence oflateral temperature variations of ~500--1000 K. In the 1--1000 mbar pressurerange, we find that over 98% of the carbon is in CO. This is true even in coolregions where CH4 is much more stable thermodynamically. Our work showsfurthermore that planets 300--500 K cooler than HD 209458b can also haveabundant CO in their upper layers due to disequilibrium effects. We demonstrateseveral interesting observational consequences of these results.

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