A reduced chemical scheme for modelling warm to hot hydrogen-dominated atmospheres

Abstract

Context. Three dimensional models that account for chemistry are useful tools to predict the chemical composition of (exo)planet and brown dwarf atmospheres and interpret observations of future telescopes, such as JWST and ARIEL. Recent Juno observations of the NH3 tropospheric distribution in Jupiter (Bolton et al. 2017) also indicate that 3D chemical modelling may be necessary to constrain the deep composition of the giant planets of the Solar System. However, due to the high computational cost of chemistry calculations, 3D chemical modelling has so far been limited. Aims. Our goal is to develop a reduced chemical scheme from the full chemical scheme of Venot et al. (2012) able to reproduce accurately the vertical profiles of the observable species (H2O, CH4, CO, CO2, NH3, and HCN). This reduced scheme should have a size compatible with three dimensional models and be usable across a large parameter space (e.g. temperature, pressure, elemental abundance). The absence of C2H2 from our reduced chemical scheme prevents its use to study hot C-rich atmospheres. Methods. We use a mechanism-processing utility designed for use with Chemkin-Pro to reduce a full detailed mechanism. ANSYS Chemkin-Pro Reaction Workbench allows the reduction of a reaction mechanism for a given list of target species and a specified level of accuracy. We take a warm giant exoplanet with solar abundances, GJ 436b, as a template to perform the scheme reduction. To assess the validity of our reduced scheme, we take the uncertainties on the reaction rates into account in Monte-Carlo runs with the full scheme, and compare the resulting vertical profiles with the reduced scheme. We explore the range of validity of the reduced scheme even further by applying our new reduced scheme to GJ 436b’s atmosphere with different elemental abundances, to three other exoplanet atmospheres (GJ 1214b, HD 209458b, HD 189733b), a brown dwarf atmosphere (SD 1110), and to the troposphere of two giant planets of the Solar System (Uranus and Neptune). Results. For all cases except one, the abundances predicted by the reduced scheme remain within the error bars of the model with the full scheme. Expectedly, we found important differences that cannot be neglected only for the C-rich hot atmosphere. The reduced chemical scheme allows more rapid runs than the full scheme it derived from (∼30 times faster). Conclusions. We have developed a reduced scheme containing 30 species and 181 reversible reactions. This scheme has a large range of validity and can be used to study all kind of warm atmospheres, except hot C-rich ones, which contains a high amount of C2H2. It can be used in 1D models, for fast computations, but also in 3D models for hot giant (exo)planet and brown dwarf atmospheres.