  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.
