Abstract
<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.
Highlights
Disk-integrated spectra of Neptune between 1.8 and 13 microns were recorded on May 13, 2007, using both the prism and the grism modes of the infrared camera (IRC, Onaka et al 2007) on board ISAS/JAXA’s AKARI infrared astronomy satellite (Murakami et al 2007), which launched on February 21, 2006 (UT)
Neptune’s stratospheric temperature structure and composition have been previously investigated by a number of authors, and we show that the AKARI SG1 and SG2 modules provide an independent verification of many of these results, of the photochemical models of Moses et al (2005)
Disk-averaged spectroscopy is insensitive to spatial variations in stratospheric temperatures across the planet, and it is likely that the compositional and thermal results derived here are weighted towards the high-temperature south polar region
Summary
Disk-integrated spectra of Neptune between 1.8 and 13 microns were recorded on May 13, 2007, using both the prism and the grism modes of the infrared camera (IRC, Onaka et al 2007) on board ISAS/JAXA’s AKARI infrared astronomy satellite (Murakami et al 2007), which launched on February 21, 2006 (UT). The broad wavelength-coverage provided by AKARI permits simultaneous observation of near-IR reflectance of sunlight and mid-IR thermal emission. These data are used to constrain models of Neptune’s atmospheric structure, composition and aerosols in the troposphere and stratosphere. The cold atmospheric temperatures yield an extremely low-power thermal emission spectrum which requires high sensitivity to measure accurately, and AKARI results will be compared to previous derivations of Neptune’s thermal structure from the Voyager radio science investigation (RSS, Lindal et al 1990), infrared spectrometer (IRIS, Conrath et al 1998) and the Infrared Space Observatory (ISO, Feuchtgruber et al 1999; Fouchet et al 2003; Burgdorf et al 2003).
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