Thermochemical and photochemical kinetics in cooler hydrogen-dominated extrasolar planets: A methane-poor GJ436b?

We introduce a thermochemical kinetics and photochemical model. We use high-temperature bidirectional reaction rates for important H, C, O, and N reactions (most importantly for CH₄ to CO interconversion), allowing us to attain thermochemical equilibrium, deep in an atmosphere, purely kinetically. This allows the chemical modeling of an entire atmosphere, from deep-atmosphere thermochemical equilibrium to the photochemically dominated regime. We use our model to explore the atmospheric chemistry of cooler (T _eff < 10³K) extrasolar giant planets. In particular, we choose to model the nearby hot-Neptune GJ436b, the only planet in this temperature regime for which spectroscopic measurements and estimates of chemical abundances now exist. Recent Spitzer measurements with retrieval have shown that methane is driven strongly out of equilibrium and is deeply depleted on the day side of GJ436b, whereas quenched carbon monoxide is abundant. This is surprising because GJ436b is cooler than many of the heavily irradiated hot Jovians and thermally favorable for CH₄, and thus requires an efficient mechanism for destroying it. We include realistic estimates of ultraviolet flux from the parent dM star GJ436, to bound the direct photolysis and photosensitized depletion of CH₄. While our models indicate fairly rich disequilibrium conditions are likely in cooler exoplanets over a range of planetary metallicities, we are unable to generate the conditions for substantial CH₄ destruction. One possibility is an anomalous source of abundant H atoms between 0.01 and 1 bars (which attack CH₄), but we cannot as yet identify an efficient means to produce these hot atoms.