Benchmarking Two Chemical Networks used in General Circulation Models of Hot Jupiters

TL;DR

This study compares two chemical networks used in hot Jupiter GCMs, revealing how numerical criteria and reaction rates influence predicted molecular abundances and emphasizing the need for better reaction data.

Contribution

It provides a benchmark between a reduced and an effective chemical network in hot Jupiter GCMs, highlighting the impact of numerical and chemical rate choices on model predictions.

Findings

Numerical escape criteria cause artificial quenching and overestimate certain molecules.

Disabling the escape criterion improves agreement between networks.

Reaction rate choices significantly affect predicted abundances of key molecules.

Abstract

Chemical kinetics is becoming an increasingly vital component of hot Jupiter general circulation models (GCMs). Here we simulate the hot Jupiter WASP-96b using two chemical networks, a reduced chemical network frequently used in the GCM literature (which we refer to as V19) and a more recent effective network making use of tables of net reactions (MiniCHEM), coupled to the same GCM in order to provide a robust benchmark. We find a numerical escape criterion used by the Unified Model chemical kinetics solver to stop integration for the duration of the chemical timestep, independent of the chemical network, results in artificial quenching, overestimating of HCN, CH$_4$, and NH$_3$ abundances by factors of 1.5 to 3. With this criterion disabled, agreement between the two networks is improved, except for HCN and NH$_3$, where different reaction rates and included species results in lower abundances in the V19 network. While many rates differ between the networks, the lower quenched NH$_3$ abundances in the V19 simulations are, in particular, due to the choice of NH$_2$ + NH$_3$ $\rightarrow$ N$_2$H$_3$ + H$_2$ reaction rate, which is poorly constrained in the literature. This reaction also impacts the quenching of HCN, which is additionally affected by the lack of CH$_2$NH$_2$ in the V19 network. While there are reasons to favour the MiniCHEM HCN and NH$_3$ abundances, ultimately, improved experimental and theoretical determination of reaction rates are needed to address the uncertainties and better characterize the quenching behaviour.