Near-infrared spectroscopic studies of the troposphere of Saturn.

Abstract

Spectra from 1.7-3.3 μm acquired at the NASA Kuiper Airborne Observatory include 2 of Saturn's near-IR 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 H₂, 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, respectively. No evidence of gaseous NH₃ was found. The upper limit to the NH₃ mixing ratio at Saturn's radiative-convective boundary is ≈ 10⁻⁹. Ammonia is decidedly undersaturated at atmospheric pressures lower than ≈ 1 bar. The upper limit to gaseous NH₃ at 3 μm 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 μm 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 μm are not compatible with Saturn's 3 μm spectrum. A reasonable fit to Saturn's continuum spectrum can only be achieved by using bright, micron-sized scattering haze particles mixed-in with H₂, CH₄, and PH₃ in Saturn's middle and upper troposphere. This research was supported by NASA grant NAG2-206 and GSRP grant NGT-50782.