2009/12/21 by L. N. Fletcher, Leigh N. Fletcher, P. Drossart +7 · 4 citations
Physics and Astronomy · #Astro and Planetary Science #Astronomy and Astrophysical Research #Scientific Research and Discoveries #astro-ph.EP
paper · pdf · doi:10.1051/0004-6361/200913358
published as Astronomy and Astrophysics, 2010 · In press, accepted manuscript
openalex publication_date 2009/12/21 · arxiv created 2010/03/29 · arxiv updated 2010/03/30 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/01
<i>Aims. <i/>Disk-averaged infrared spectra of Neptune between 1.8 and 13 <i>μ<i/>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 <i>μ<i/>m.<i>Methods. <i/>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 <i>μ<i/>bar and the abundances of CH<sub>4<sub/>, CH<sub>3<sub/>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 <i>μ<i/>m in the NG channel (with a spectral resolution of 135).<i>Results. <i/>The globally-averaged stratospheric temperature structure is quasi-isothermal between 1–1000 <i>μ<i/>bar, which suggests little variation in global stratospheric conditions since studies by the Infrared Space Observatory a decade earlier. The derived CH<sub>4<sub/> mole fraction of (9.0 <i>±<i/> 3.0)× 10<sup>-4<sup/> at 50 mbar, decreasing to (0.9 <i>±<i/> 0.3)× 10<sup>-4<sup/> at 1 <i>μ<i/>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<sub>4<sub/> leaking through the warm south polar tropopause (62–66 K) is globally redistributed by stratospheric motion. The ratio of D/H in CH<sub>4<sub/> of 3.0 <i>±<i/> 1.0 × 10<sup>-4<sup/> 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 <i>±<i/> 2.1)× 10<sup>-7<sup/> at 0.3 mbar, consistent with previous studies, and a mole fraction of ethylene of 5.0 × 10<sup>-7<sup/> at 2.8 <i>μ<i/>bar. An emission peak at 4.7 <i>μ<i/>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 <i>L<i/>-band studies indicate significant variability of Neptune's flux densities in the 3.5–4.1 <i>μ<i/>m range, related to changes in solar energy deposition.