Analysis of the near-IR spectrum of Saturn: A comprehensive radiative transfer model of its middle and upper troposphere

Spectra from 1.7 to 3.3 mu m acquired at the NASA Kuiper Airborne Observatory include two of Saturn's near-LR atmospheric transmission windows that are at least partially obscured by telluric H2O and CO2 absorptions at ground-based telescopes. This entire spectral region was fitted to a model that included gaseous absorption by H-2, CH4, NH3, and PH3 and the effects of multiple scattering by haze. The objectives were to determine accurate elemental abundance ratios (e.g., C/H, P/H, etc.) and to characterize the size, distribution, and composition of the haze particles in Saturn's atmosphere. The results for C/H and P/H are 8.5 x 10(-4) and 4.3 x 10(-7), respectiv- ely. No evidence of gaseous NH3 was found. The upper limit to the NH3 mixing ratio at Saturn's radiative-convective boundary is approximate to 10(-9). Ammonia is decidedly undersaturated at atmospheric pressures lower than approximate to 1 bar. The upper limit to gaseous NH3 at 3 mu m is extremely low compared to detected amounts derived from observations at visible, mid-IR, and microwave wavelengths. These differences can be reconciled on the basis of different mechanisms for spectral line formation in these disparate spectral regions. A search for solid phase NH3 was also negative. From thermochemical arguments it has been widely assumed that NH3 ice crystals comprise the upper clouds on Saturn, although no incontrovertible spectroscopic proof has ever been presented. Strong bands of solid NH3 at 3 mu m therefore offer an important test of this assumption. Saturn's observed spectrum was placed on an absolute reflectivity scale which then could be compared with synthesized spectra of candidate haze particles. The calculations demonstrated that the reflectances of pure, polydisperse NH3 ice crystals with effective radii ranging from 0.1 to 2.25 mu m are not compatible with Saturn's 3-mu m spectrum. A reasonable fit to Saturn's continuum spectrum can only be achieved by using bright, micron-sized scattering haze particles mixed in with H-2, CH4, and PH, in Saturn's middle and upper troposphere. (C) 1997 Academic Press.