Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dps....38.3906n&link_type=abstract
American Astronomical Society, DPS meeting #38, #39.06; Bulletin of the American Astronomical Society, Vol. 38, p.555
Astronomy and Astrophysics
Astronomy
3
Scientific paper
On March 20, 2006 Cassini's Visual and Infrared Mapping Spectrometer (VIMS) observed an occultation by Saturn of the star α CMi (spectral type F5IV), at 55 deg north latitude. Full resolution spectra spanning the range 0.85-5.1 microns were obtained every 80 ms, corresponding to a vertical sampling interval of 400 m in Saturn's atmosphere. At a range of 450,000 km, the projected stellar diameter was only 12 m, with a Fresnel zone diameter of 50 m at a wavelength of 3 microns.
Absorption by the strong methane fundamental at 3.3 microns is detected beginning at a pressure level of 0.02 mbar, with weaker CH4 bands appearing at progressively deeper levels. Below 0.3 mbar refractive defocussing becomes significant, and dominates the lightcurves at continuum wavelengths. Strong scintillation is observed between 0.7 and 5 mbar, indicative of refractive focussing by stratospheric turbulence and/or breaking gravity waves. A model including differential refraction by a mixture of 88% H2 and 12% He at a stratospheric temperature of 142 K plus absorption by CH4 at a mixing ratio of 0.0045 provides a good fit to the VIMS spectra below the 0.5 mbar level, but at higher altitudes the methane mixing ratio decreases. Below the 5 mbar level we see an abrupt and total loss of signal at all wavelengths, which we attribute to the onset of absorption by a relatively dense haze layer.
The major discrepancy between the observed and model spectra, which are based on the correlated-k coefficients of Irwin etal. (2005), is an additional absorption at 3.4 microns which may be due to an extended optically-thin layer of organic aerosol particles. A very similar absorption is seen in VIMS data from a Titan solar occultation (Bellucci, etal. this meeting).
This work was supported by NASA under a contract with the Cassini-Huygens Project.
Cassini Vims Team
Gierasch Peter J.
Hedman Matthew M.
Nicholson Philip D.
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