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 (SG1 and SG2 channels of AKARI/IRC), with spectral resolutions of 47 and 34 respectively, were modelled using a line-by-line radiative transfer code to determine the temperature structure between 1-1000 mu bar and the abundances of CH4, CH3D 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 CH4 mole fraction of (9.0 +/- 3.0) x 10(-4) at 50 mbar, decreasing to (0.9 +/- 0.3) x 10(-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 CH4 leaking through the warm south polar tropopause (62-66 K) is globally redistributed by stratospheric motion. The ratio of D/H in CH4 of 3.0 +/- 1.0 x 10(-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 +/- 2.1) x 10(-7) at 0.3 mbar, consistent with previous studies, and a mole fraction of ethylene of 5.0(-2.1)(+ 1.8) x 10(-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. Finally, comparisons to previous L-band studies indicate significant variability of Neptune's flux densities in the 3.5-4.1 mu m range, related to changes in solar energy deposition.