Neptune's Atmospheric Composition from AKARI Infrared Spectroscopy

TL;DR

This study analyzes Neptune's infrared spectra from AKARI to determine its atmospheric temperature, hydrocarbon abundances, and CO emission, revealing a stable stratosphere and insights into its chemical composition and dynamics.

Contribution

First detailed analysis of Neptune's infrared spectra from AKARI, providing updated atmospheric temperature and composition profiles, including CO fluorescent emission modeling.

Findings

Stratospheric temperature is quasi-isothermal between 1-1000 μbar.

Methane abundance decreases with altitude, indicating complex atmospheric dynamics.

Detection of CO fluorescent emission suggests external and internal sources.

Abstract

Aims: Disk-averaged infrared spectra of Neptune between 1.8 and 13 $\mu$m, obtained by the AKARI Infrared Camera (IRC) in May 2007, have been analysed to (a) determine the globally-averaged stratospheric temperature structure; (b) derive the abundances of stratospheric hydrocarbons; and (c) detect fluorescent emission from CO at 4.7 $\mu$m. Methods: Mid-infrared spectra were modelled using a line-by-line radiative transfer code to determine the temperature structure between 1-1000 $\mu$bar and the abundances of CH$_4$, CH$_3$D and higher-order hydrocarbons. A full non-LTE radiative model was then used to determine the best fitting CO profile to reproduce the fluorescent emission observed at 4.7 $\mu$m in the NG channel (with a spectral resolution of 135). Results: The globally-averaged stratospheric temperature structure is quasi-isothermal between 1-1000 $\mu$bar, which suggests little variation in global stratospheric conditions since studies by the Infrared Space Observatory a decade earlier. The derived CH$_4$ mole fraction of $(9.0\pm3.0)\times10^{-4}$ at 50 mbar, decreasing to $(0.9\pm0.3)\times10^{-4}$ at 1 $\mu$bar, is larger than that expected if the tropopause at 56 K acts as an efficient cold trap, but consistent with the hypothesis that CH$_4$ leaking through the warm south polar tropopause (62-66 K) is globally redistributed by stratospheric motion. The ratio of D/H in CH$_4$ of $3.0\pm1.0\times10^{-4}$ supports the conclusion that Neptune is enriched in deuterium relative to the other giant planets. We determine a mole fraction of ethane of $(8.5\pm2.1)\times10^{-7}$ at 0.3 mbar, consistent with previous studies, and a mole fraction of ethylene of $5.0_{-2.1}^{+1.8}\times10^{-7}$ at 2.8 $\mu$bar. An emission peak at 4.7 $\mu$m is interpreted as a fluorescent emission of CO, and requires a vertical distribution with both external and internal sources of CO.