2011ApJ...738...72V

We explore CO\rightleftarrows CH_{4} quench kinetics in the atmospheres of substellar objects using updated timescale arguments, as suggested by a thermochemical kinetics and diffusion model that transitions from the thermochemical-equilibrium regime in the deep atmosphere to a quench-chemical regime at higher altitudes. More specifically, we examine CO quench chemistry on the T dwarf Gliese 229B and CH₄ quench chemistry on the hot-Jupiter HD 189733b. We describe a method for correctly calculating reverse rate coefficients for chemical reactions, discuss the predominant pathways for CO\rightleftarrows CH_{4} interconversion as indicated by the model, and demonstrate that a simple timescale approach can be used to accurately describe the behavior of quenched species when updated reaction kinetics and mixing-length-scale assumptions are used. Proper treatment of quench kinetics has important implications for estimates of molecular abundances and/or vertical mixing rates in the atmospheres of substellar objects. Our model results indicate significantly higher K_zz values than previously estimated near the CO quench level on Gliese 229B, whereas current-model-data comparisons using CH₄ permit a wide range of K_zzvalues on HD 189733b. We also use updated reaction kinetics to revise previous estimates of the Jovian water abundance, based upon the observed abundance and chemical behavior of carbon monoxide. The CO chemical/observational constraint, along with Galileo entry probe data, suggests a water abundance of approximately 0.51-2.6 xsolar (for a solar value of H₂O/H₂= 9.61x10^–4) in Jupiter's troposphere, assuming vertical mixing from the deep atmosphere is the only source of tropospheric CO.