The CH$_4$ abundance in Jupiter's upper atmosphere

TL;DR

This study models methane radiance at 3.3 μm in Jupiter's upper atmosphere using advanced non-LTE techniques, revealing higher methane levels than previous analyses and constraining atmospheric composition and dynamics.

Contribution

It introduces a comprehensive non-LTE model with recent laboratory data to accurately determine methane abundance in Jupiter's upper atmosphere.

Findings

High CH₄ concentration needed to fit the data

Observations favor lower limit of collisional relaxation rates

Provides constraints on Jupiter's atmospheric composition

Abstract

Hydrocarbon species, and in particular CH$_4$, play a key role in the stratosphere--thermosphere boundary of Jupiter, which occurs around the $\mu$-bar pressure level. Previous analyses of solar occultation, He and Ly-$\alpha$ airglow, and ISO/SWS measurements of the radiance around 3.3 $\mu$m have inferred significantly different methane concentrations. Here we aim to accurately model the CH$_4$ radiance at 3.3 $\mu$m measured by ISO/SWS by using a comprehensive non-local thermodynamic equilibrium model and the most recent collisional rates measured in the laboratory for CH$_4$ to shed new light onto the methane concentration in the upper atmosphere of Jupiter. These emission bands have been shown to present a peak contribution precisely at the $\mu$-bar level, hence directly probing the region of interest. We find that a high CH$_4$ concentration is necessary to explain the data, in contrast with the most recent analyses, and that the observations favour the lower limit of the latest laboratory measurements of the CH$_4$ collisional relaxation rates. Our results provide precise constraints on the composition and dynamics of the lower atmosphere of Jupiter.