Neptune's atmospheric composition from AKARI infrared spectroscopy

¹ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, 91109, USA
² Atmospheric, Oceanic and Planetary Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK e-mail: fletcher@atm.ox.ac.uk
³ LESIA, Observatoire de Paris, UPMC, Universite Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
⁴ SOFIA Science Center, Deutsches SOFIA Institut, NASA Ames Research Center, Mail Stop 211-3, Moffett Field, CA 94035, USA

Received: 25 September 2009
Accepted: 8 December 2009

Abstract

Aims. Disk-averaged infrared spectra of Neptune between 1.8 and 13 μm, 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 μm.

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 μbar 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 μm in the NG channel (with a spectral resolution of 135).

Results. The globally-averaged stratospheric temperature structure is quasi-isothermal between 1–1000 μbar, 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)× 10^-4 at 50 mbar, decreasing to (0.9 ± 0.3)× 10^-4 at 1 μbar, 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 × 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)× 10^-7 at 0.3 mbar, consistent with previous studies, and a mole fraction of ethylene of 5.0 × 10^-7 at 2.8 μbar. An emission peak at 4.7 μm 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 μm range, related to changes in solar energy deposition.