Thermochemistry and Photochemistry in Cooler Hydrogen Dominated Extrasolar Planets: The Case of GJ436b
We introduce a new thermochemical kinetics and photochemical model. We use high-temperature bidirectional reaction rates for important H, C, O and N reactions (most importantly for CH$_4$ to CO interconversion), allowing us to attain thermochemical equilibrium, deep in an atmosphere, purely kinetically. This allows ab initio 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 ($T_{eff} < 10^3$ K) 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 {\it Spitzer} measurements with retrieval have shown that methane is driven strongly out of equilibrium and is deeply depleted on the dayside of GJ 436b, whereas quenched carbon monoxide is abundant. This is surprising because GJ 436b is cooler than many of the heavily irradiated hot Jovians and thermally favorable for CH$_4$, and thus requires an efficient mechanism for destroying it. We include realistic estimates of ultraviolet flux from the parent dM star GJ 436, to bound the direct photolysis and photosensitized depletion of CH$_4$. 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 CH$_4$ destruction. One possibility is an anomalous source of abundant H atoms between 0.01-1 bars (which attack CH$_4$), but we cannot as yet identify an efficient means to produce these hot atoms.