Neptune's Atmospheric Composition from AKARI Infrared Spectroscopy

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.