Article Dans Une Revue Astronomy & Astrophysics - A&A Année : 2010

Neptune's atmospheric composition from AKARI infrared spectroscopy

Résumé


Aims: Disk-averaged infrared spectra of Neptune between 1.8 and 13 mum, 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 mum.
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 mubar 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 mum in the NG channel (with a spectral resolution of 135).
Results: The globally-averaged stratospheric temperature structure is quasi-isothermal between 1-1000 mubar, 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 mubar, 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 5.0-2.1+1.8 × 10-7 at 2.8 mubar. An emission peak at 4.7 mum 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 mum range, related to changes in solar energy deposition.

Dates et versions

hal-03785881 , version 1 (23-09-2022)

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Leigh N. Fletcher, Pierre Drossart, M. Burgdorf, Glenn S. Orton, Thérèse Encrenaz. Neptune's atmospheric composition from AKARI infrared spectroscopy. Astronomy & Astrophysics - A&A, 2010, 514, pp.17. ⟨10.1051/0004-6361/200913358⟩. ⟨hal-03785881⟩
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