[Paper Review] A new chemical scheme for giant planet thermochemistry. Update of the methanol chemistry and new reduced chemical scheme

Olivia Vénot, T. Cavalié+4 more|arXiv (Cornell University)|Dec 16, 2019

Advanced Chemical Physics Studies77 references12 citations

TL;DR

This paper updates the methanol chemistry in a thermochemical model for giant planets and brown dwarfs using recent combustion literature, significantly improving accuracy for warm Neptunes and brown dwarfs. The revised scheme alters CO and CO₂ abundances, affecting JWST-observable spectra and revising oxygen abundances in Uranus and Neptune, while a reduced 44-species version enables future 3D kinetic modeling.

ABSTRACT

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. The goal of this paper is to update the methanol chemistry for such chemical networks thanks to recent publications in the combustion literature. We aim also at studying the consequences of this update on the atmospheric compositions of (exo)planetary atmospheres and brown dwarfs. We have performed an extensive review of combustion experimental studies and revisited the sub-mechanism describing methanol combustion in the scheme of Venot et al. (2012, A&A 624, A58). The updated scheme involves 108 species linked by a total of 1906 reactions. We have then applied our 1D kinetic model with this new scheme to several case studies (HD 209458b, HD 189733b, GJ 436b, GJ 1214b, ULAS J1335+11, Uranus, Neptune), and compared the results obtained with those obtained with the former scheme. The update of the scheme has a negligible impact on hot Jupiters atmospheres. However, the atmospheric composition of warm Neptunes and brown dwarfs is modified sufficiently to impact observational spectra in the wavelength range JWST 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. Chemical schemes should be regularly updated in order to maintain a high level of reliability on the results of kinetic models and be able to improve our knowledge on planetary formation.

Motivation & Objective

• Address discrepancies in methanol reaction rates between prior models and recent observations, particularly in CO and CH₄ abundances.
• Improve the reliability of thermochemical models for giant planets and brown dwarfs by incorporating updated combustion kinetics.
• Re-evaluate deep oxygen abundances in Uranus and Neptune using a revised chemical network.
• Develop a reduced chemical scheme (44 species, 582 reactions) for application in future 3D kinetic models of planetary atmospheres.
• Enable more accurate interpretation of upcoming JWST and ARIEL observations by refining quenching-level species abundances.

Proposed method

• Revised the methanol sub-network in the Venot et al. (2012) chemical scheme using updated rate constants from combustion literature.
• Validated the new scheme against experimental data from methanol combustion studies, particularly focusing on key reactions like CH₃OH + H ⇌ CH₃ + H₂O.
• Replaced the controversial CH₃OH + H ⇌ CH₃ + H₂O reaction with updated kinetics, removing it from the network.
• Applied a 1D kinetic model with the updated scheme to model atmospheres of HD 209458b, HD 189733b, GJ 436b, GJ 1214b, ULAS J1335+11, Uranus, and Neptune.
• Compared results from the new V20 scheme with the original V12 scheme to quantify changes in quenching levels and species abundances.
• Derived a reduced chemical mechanism (44 species, 582 reactions) via sensitivity and flux analysis, suitable for 3D kinetic modeling.

Experimental results

Research questions

• RQ1 How do updated methanol reaction rates from combustion literature affect the predicted atmospheric composition of giant planets and brown dwarfs?
• RQ2 To what extent does the revised chemical scheme alter the quenching levels and abundances of key species like CO, CO₂, and CH₄ in warm Neptunes and T dwarfs?
• RQ3 What impact does the updated scheme have on the inferred deep oxygen abundance in Uranus and Neptune compared to previous models?
• RQ4 Can the new reduced chemical scheme (44 species, 582 reactions) maintain accuracy while enabling efficient 3D kinetic modeling of planetary atmospheres?
• RQ5 How detectable are the differences between the new and old schemes in future JWST and ARIEL observations?

Key findings

• The updated chemical scheme has a negligible impact on hot Jupiters, with CO₂ abundance changes in HD 189733b resulting in only 50 ppm spectral shifts at 4–5 µm.
• For warm Neptunes and T dwarfs, the updated scheme leads to deeper quenching of CO, CO₂, and H₂O, causing changes detectable by JWST (up to 100 ppm in transmission for warm Neptunes, a factor of 2 in emission for T dwarfs).
• The revised scheme reduces the required O/H ratio for Uranus to less than 45 ⊙ and for Neptune to 250 ⊙, compared to previous estimates, due to higher CO abundances.
• The controversial reaction CH₃OH + H ⇌ CH₃ + H₂O was removed from the network due to inconsistencies with experimental data, improving model reliability.
• The new reduced chemical scheme (44 species, 582 reactions) enables efficient 3D kinetic modeling while preserving key thermochemical behavior.
• The study demonstrates the critical importance of cross-disciplinary collaboration between astrophysicists and combustion scientists for accurate high-temperature atmospheric modeling.