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 NH 3 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 (H 2 O, CH 4 , CO, CO 2 , NH 3 , 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 C 2 H 2 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 C 2 H 2 . It can be used in 1D models, for fast computations, but also in 3D models for hot giant (exo)planet and brown dwarf atmospheres.

Implications of three-dimensional chemical transport in hot Jupiter atmospheres: Results from a consistently coupled chemistry-radiation-hydrodynamics model

“…For HD 189733b there is a very good agreement for the abundance distributions of CH 4 , H 2 O and CO found using the Cooper & Showman (2006) chemical relaxation scheme (Drummond et al 2018c) or using the coupled chemical kinetics scheme (this work). This implies that in the particular case of HD 189733b the parameterised chemical timescale developed by Cooper & Showman (2006) gives a similar chemical evolution to the reduced chemical network of Venot et al (2019a) for CO. The results and conclusions presented in Drummond et al (2018c) are therefore validated here with the use of a more accurate chemical scheme.…”

“…Since the early work of Liang et al (2003) the trend has been for such networks to grow in size and complexity. However, more recently the focus has shifted to the development of smaller chemical networks (Tsai et al 2017;Venot et al 2019a) that are less computationally demanding, making them feasible for use in a 3D atmosphere model. We use the "reduced" chemical network recently developed by Venot et al (2019a).…”

“…On the other hand, significant differences were found for cooler atmospheres. As we only consider hot Jupiter atmospheres in this study, our use of the Venot et al (2019a) chemical network, based on the original Venot et al (2012) network without updates to the methanol chemistry, will not affect our results.…”

“…at a latitude of θ = 0). We also show the vertical abundance profiles for a latitude of 45 • in Appendix D. We show this particular subset of six molecules (out of a total of 30 species included in the chemical network) since each of these species are included as opacity sources in the model and they are also the six "target species" for which the reduced chemical network was constructed (Venot et al 2019a).…”

“…However, we note that the chemical timescale, for a given pressure and temperature, varies significantly for different molecules ). The chemical timescale for CH 4 that we use in this analysis has been specifically calculated for the Venot et al (2019a) Article number, page 10 of 32 Drummond et al: Chemical transport in hot Jupiter atmospheres 10 -12 10 -11 10 -10 10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 Mole Fraction chemical network. In practice, we calculated the CH 4 chemical timescale for a grid of pressures and temperatures.…”