Disk-integrated spectra of Neptune were recorded on 2007-05-13, between 1.8 and 13 microns, using both the prism and the grism of the Infrared Camera on board ISAS/JAXA's AKARI satellite. The spectral resolving power was about 140 in the interval 2.5 - 5 microns and between 20 and 50 at other wavelengths. As Triton was included in the field of view at a distance of only 6 arcsec in the cross-dispersion direction, we had to subtract its contribution to the near-infrared spectra.From the mid-infrared spectra we determine the stratospheric temperature and the CH4 mixing ratio. We find good agreement with the observations obtained with the infrared spectrometer aboard Spitzer on 2005-11-20, with possible differences in the measured emission of ethane and methane between the two observatories being less than 10%. In contrast to this we find in the near-infrared flux densities up to a factor five weaker than previous measurements, confirming the presence of meteorological variations. At short wavelength, an emission peak at 4.7 microns can be interpreted as due to fluorescent emission of CO. A similar emission feature has been detected previously on Uranus (Encrenaz et al, A&A, 2004). As on Uranus, the fluorescent dominant emission comes from CO (2-1) band, and fits the observed peak. The contribution from the (1-0) band is strongly self-absorbed, and weaker. Synthetic spectra are calculated from a non-LTE radiative model, including solar radiation absorption, self-absorption in the resonant fluorescent (1-0) band, and frequency redistribution from vibrational CO bands. The spectra are compared to constrain the vertical variation of CO abundance as measured from submm range (Hesman et al, Icarus, 2007), the information from the fluorescence emission coming from as deep as the 1 bar level.
AAS/Division for Planetary Sciences Meeting Abstracts #40
- Pub Date:
- September 2008