2006ApJ...649.1048C

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 CH₄to 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 CH₄concentrations 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 CH₄is 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.