Methane, Carbon Monoxide, and Ammonia in Brown Dwarfs and Self-Luminous Giant Planets
Kevin J. Zahnle, Mark S. Marley

TL;DR
This study uses a kinetics-based 1D atmospheric chemistry model to analyze disequilibrium abundances of molecules like methane, carbon monoxide, and ammonia in brown dwarfs and giant planets, revealing temperature and gravity effects on chemical compositions.
Contribution
It provides a comprehensive survey of disequilibrium chemistry in substellar atmospheres using full kinetics models and derives simple quenching relations for key molecules.
Findings
CO dominates over CH4 in disequilibrium in most cases.
Threshold temperatures for methane to CO transition vary with gravity and mixing.
NH3/N2 ratio is insensitive to mixing, serving as a gravity proxy.
Abstract
We address disequilibrum abundances of some simple molecules in the atmospheres of solar composition brown dwarfs and self-luminous extrasolar giant planets using a kinetics-based 1D atmospheric chemistry model. Our approach is to use the full kinetics model to survey the parameter space with effective temperatures between 500 K and 1100 K. In all of these worlds equilibrium chemistry favors CH4 over CO in the parts of the atmosphere that can be seen from Earth, but in most disequilibrium favors CO. The small surface gravity of a planet strongly discriminates against CH4 when compared to an otherwise comparable brown dwarf. If vertical mixing is like Jupiter's, the transition from methane to CO occurs at 500 K in a planet. Sluggish vertical mixing can raise this to 600 K; but clouds or more vigorous vertical mixing could lower this to 400 K. The comparable thresholds in brown dwarfs are…
Peer Reviews
No public reviews on file for this paper yet. If you reviewed it on a platform where reviews are public (OpenReview, ICLR, NeurIPS, ICML), you can paste yours below so the community can read it here.
Videos
No videos yet. Explain this paper in a talk, walkthrough, or lecture? Add one.
