ICARUS 127, 190–212 (1997) ARTICLE NO. IS975688 Analysis of the Near-IR Spectrum of Saturn: A Comprehensive Radiative Transfer Model of Its Middle and Upper Troposphere DANA X. KEROLA,HAROLD P. LARSON, AND MARTIN G. TOMASKO Lunar and Planetary Laboratory, The University of Arizona, Tucson, Arizona 85721 E-mail: dkerola@lpl.arizona.edu Received April 5, 1996; revised October 11, 1996 H 2 and CH 4 (see Fig. 1). Within that broad bandwidth there are two transmission windows in the giant planet’s Spectra from 1.7 to 3.3 mm acquired at the NASA Kuiper Airborne Observatory include two of Saturn’s near-IR atmo- spectrum, centered at 2 and 3 em, each of which provides spheric transmission windows that are at least partially ob- a different diagnostic opportunity. Sunlight penetrates to scured by telluric H 2 O and CO 2 absorptions at ground-based different depths in the two windows. Furthermore, differ- telescopes. This entire spectral region was fitted to a model ent trace constituents have bands coincident with one or that included gaseous absorption by H 2 , CH 4 , NH 3 , and PH 3 another of these windows. This work represents the first and the effects of multiple scattering by haze. The objectives time a comprehensive, self-consistent model has been cre- were to determine accurate elemental abundance ratios (e.g., ated for Saturn across this entire spectral region. We em- C/H, P/H, etc.) and to characterize the size, distribution, and ployed the technique of spectrum synthesis in order to composition of the haze particles in Saturn’s atmosphere. The match as closely as possible a calculated spectrum with the results for C/H and P/H are 8.5 3 10 24 and 4.3 3 10 27 , respec- tively. No evidence of gaseous NH 3 was found. The upper observations. This approach is based on an atmospheric limit to the NH 3 mixing ratio at Saturn’s radiative-convective model which incorporates both absorption and scattering boundary is P10 29 . Ammonia is decidedly undersaturated at by gaseous and condensed constituents. Previous studies atmospheric pressures lower than P1 bar. The upper limit to often were conducted in narrower bandwidths, they lacked gaseous NH 3 at 3 mm is extremely low compared to detected essential laboratory measurements, or they did not include amounts derived from observations at visible, mid-IR, and mi- the effects of scattering by aerosols. We have improved crowave wavelengths. These differences can be reconciled on on past investigations by fully including the variations in the basis of different mechanisms for spectral line formation absorption by gases as a function of pressure and tempera- in these disparate spectral regions. A search for solid phase ture, by employing the most up-to-date spectral line param- NH 3 was also negative. From thermochemical arguments it has been widely assumed that NH 3 ice crystals comprise the upper eters wherever they are available, by placing the Saturn clouds on Saturn, although no incontrovertible spectroscopic observations on an absolute intensity scale, and by treating proof has ever been presented. Strong bands of solid NH 3 at as accurately as possible the effects of multiple scattering. 3 mm therefore offer an important test of this assumption. One major objective of this work was to determine the Saturn’s observed spectrum was placed on an absolute reflecti- distribution and chemical composition of Saturn’s tropo- vity scale which then could be compared with synthesized spec- spheric haze (see Section IV). The other main focus has tra of candidate haze particles. The calculations demonstrated been the determination of the gas composition in Saturn that the reflectances of pure, polydisperse NH 3 ice crystals with (see Section V). The spectral line optical depths for the effective radii ranging from 0.1 to 2.25 mm are not compatible main molecular species H 2 , CH 4 , PH 3 , and NH 3 were calcu- with Saturn’s 3-mm spectrum. A reasonable fit to Saturn’s continuum spectrum can only be achieved by using bright, lated. Retrievals of the abundances of the first three gases micron-sized scattering haze particles mixed in with H 2 , CH 4 , were possible through a simultaneous fitting of their near- and PH 3 in Saturn’s middle and upper troposphere.  1997 Aca- IR absorptions. For NH 3 , a stringent upper limit was de- demic Press duced. Historical Perspective I. INTRODUCTION Ground-based, airborne, and space-based attempts at remote sensing of molecular species have been fruitful, Objectives We modeled Saturn’s 1.7- to 3.3-em wavelength region, particularly for Jupiter and Saturn. For example, before 1970 only H 2 , CH 4 , and NH 3 had been positively identified which is defined primarily by gaseous absorptions due to 190 0019-1035/97 $25.00 Copyright  1997 by Academic Press All rights of reproduction in any form reserved.