Longitudinal variations in the Saturnian atmosphere: I. Equatorial region

Abstract

Ground-based and Voyager observations in and out of methane and ammonia bands are analyzed to search for longitudinal variations in Saturn's equatorial region. A model with a reflecting layer at 2.1 bars, an extended haze to 170 mbar, and an overlying thin stratospheric haze is adopted. Two sets of data are analyzed, a set of ground-based observations covering the 6000-Å spectral region and a set of Voyager 1 images obtained with the orange and methane filters. The spectral variations are not consistent with a variation in the height of the reflecting layer. They are modeled by variations in the single scattering albedo of the haze and in the specific abundance of gas in the haze. The ground-based spectra, having a spatial resolution of 21,000 km, are consistent with a specific abundance of gas in the haze of 14 ± 1 km-am per mean free path and a haze single scattering albedo in the continuum at 6055 Å of 0.0090 (±0.006) with a longitudinal variation of ±0.003. The single scattering albedo derived from the ground-based observations at 6475 Å is 0.993 (±.006) with a longitudinal variation of ±.003. The Voyager data, having a spatial resolution of ∼500 km, are consistent with a specific abundance of gas in the haze between 10 and 24 km-am. The larger variation in the specific abundance derived from the Voyager data set is due to the larger relative uncertainty in the intensity (2%) for Voyager as compared to 1% for the ground-based spectra. We derive a methane mixing ratio of 2.2 (+0.8/−0.2) × 10 −3, representing a C/H ratio which is enhanced by a factor of 2.3 over the solar value. Our estimate of the ammonia mixing ratio, 4.5 × 10 −4, is a lower limit due to our assumption that ammonia exists at its saturation vapor pressure everywhere above the reflecting layer. There is no conclusive evidence that there are longitudinal variations in the structure of the Saturnian atmosphere in the Equatorial Region on a scale greater than 600 km.