Saturn's atmosphere in the visible and near-infrared, 1986-1989

Abstract

This work describes observations of Saturn's atmosphere in the visible and near infrared (450-1000 nm) including four hydrogen quadrupole lines, 17 methane absorption bands ranging over three orders of magnitude in absorption strength, an ammonia absorption band, and the absolute calibrated continuum spectrum. All observations have complete coverage of Saturn's disk, in latitude as well as in center-to-limb position. The accuracy of the data is comparable or better than previous data. This data set gives a quite complete description of Saturn's atmosphere in the visible and near infrared at the spatial resolution of ground based observations. While the main data were acquired in 1988, small changes between 1986 and 1989 were determined also. An atmospheric model is given which fits all observations within estimated errors. It has clear gas at the top of the atmosphere, an extended haze layer and a reflective cloud at the bottom. Pressure levels and the haze optical depth were determined as a function of latitude. The single scattering albedo spectrum of the particles (most likely ammonia ice crystals) is also given for each latitude. The methane mixing ratio is (3.0 ± 0.6) x 10⁻³, the ammonia mixing ratio is (1.2 + 0.8/-0.6) x 10⁻³ below the ammonia condensation level. Room temperature methane absorption spectra do not fit the observed spectra for any cloud structure. A cold temperature methane absorption spectrum is determined under the assumption that methane band strengths are temperature invariant, but not necessarily the absorption coefficients at each location across the band. It indicates that the absorption coefficients are typically 20-30 per cent stronger in the center of a band and up to a factor of two weaker in the wings. This spectrum should be useful in the interpretation of methane observations of all the giant planets and Titan.