New chemical scheme for giant planet thermochemistry

O Venot,R Lhoussaine Ben Brahim,P Tremblin,L Brouillard,R Bounaceur,T Cavalié

doi:10.1051/0004-6361/201936697

Abstract

Context.Several chemical networks have been developed to study warm (exo)planetary atmospheres. The kinetics of the reactions related to the methanol chemistry included in these schemes have been questioned.Aims.The goal of this paper is to update the methanol chemistry for such chemical networks based on recent publications in the combustion literature. We also aim to study the consequences of this update on the atmospheric compositions of (exo)planetary atmospheres and brown dwarfs.Methods.We performed an extensive review of combustion experimental studies and revisited the sub-mechanism describing methanol combustion in a scheme published in 2012. The updated scheme involves 108 species linked by a total of 1906 reactions. We then applied our 1D kinetic model with this new scheme to the case studies HD 209458b, HD 189733b, GJ 436b, GJ 1214b, ULAS J1335+11, Uranus, and Neptune; we compared these results with those obtained with the former scheme.Results.The update of the scheme has a negligible impact on the atmospheres of hot Jupiters. However, the atmospheric composition of warm Neptunes and brown dwarfs is modified sufficiently to impact observational spectra in the wavelength range in whichJames WebbSpace Telescope will operate. Concerning Uranus and Neptune, the update of the chemical scheme modifies the abundance of CO and thus impacts the deep oxygen abundance required to reproduce the observational data. For future 3D kinetics models, we also derived a reduced scheme containing 44 species and 582 reactions.Conclusions.Chemical schemes should be regularly updated to maintain a high level of reliability on the results of kinetic models and be able to improve our knowledge of planetary formation.

Highlights

Knowledge of the deep composition of the Solar System giant planets is essential to constrain their formation models (Pollack et al 1996; Boss 1997; Owen et al 1999; Gautier & Hersant 2005)
The atmosphere of hot exoplanets is schematically divided into three parts: (1) the deepest, which is very hot, has a chemical composition governed by thermochemical equilibrium; (2) the middle has a lower temperature and a composition controlled by transport-induced quenching; and (3) the upper layers are subject to photochemistry (Madhusudhan et al 2016, their Fig. 1)
Brown dwarfs are subject to a transition between thermochemical equilibrium and a quenching zone, but photochemistry can be ignored in this case because the object is isolated (e.g., Griffith 2000)

Summary

Introduction

Knowledge of the deep composition of the Solar System giant planets is essential to constrain their formation models (Pollack et al 1996; Boss 1997; Owen et al 1999; Gautier & Hersant 2005). Thermochemical modeling remains a tool that is complementary to remote observations to infer the deep composition of the Solar System giant planets (Lodders & Fegley 1994; Visscher & Fegley 2005; Visscher et al 2010; Cavalié et al 2017). The atmosphere of hot exoplanets is schematically divided into three parts: (1) the deepest, which is very hot, has a chemical composition governed by thermochemical equilibrium; (2) the middle has a lower temperature and a composition controlled by transport-induced quenching; and (3) the upper layers are subject to photochemistry (Madhusudhan et al 2016, their Fig. 1). To interpret observations of brown dwarf and exoplanet atmospheres probing the regions governed by quenching (and eventually influenced by photochemistry for exoplanets), we must evaluate correctly the quenching level and abundances of species at this level.

Objectives

Findings

Discussion

Conclusion