Spatially resolved methane band photometry of Jupiter: II. Analysis of the South Equatorial Belt and South Tropical Zone reflectivity

Abstract

This work presents results and analysis of center-to-limb variations and absolute reflectivity measurements of Jupiter's South Equatorial Belt (SEBs) and South Tropical Zone (STrZ) in three narrowband methane filters and three nearby continuum filters. The observations and data reduction are reported in Paper I. The data were analyzed in terms of plane-parallel but vertically inhomogeneous atmospheric models. Diffuse reflecting-scattering models (RSM) and two-cloud models (TCM) with and without an additional high, thin haze layer (required from Pioneer observations) were computed. Computations of multiple scattering were performed with a doubling technique. Anisotropic phase functions derived from Pioneer 10 photometry were used. Observations in the strong 8900-Å band severely constrain the position of the upper cloud top. To fit both the center-to-limb variations and absolute reflectivity, the STrZ cloud top must lie between 0.55 bar total pressure, if the aerosols are concentrated (small scattering mean free path), and 0.43 bar for the RSM model with 8 to 10 m-am CH 4 per unit cloud optical depth. The 8900-Å data also constrain the cloud optical depth. If the cloud particles are concentrated, the top cloud must have optical depths between 1.5 and 2.5. The data at 7250 and 6190 Å are well suited to specify the level of the lower cloud. TCM models with concentrated aerosols have lower cloud-top pressure between 2.4 and 2.7 bars in the STrZ. To account for the small but significant differences between observations of the STrZ and SEBs, several configurations are allowed. An RSM model for the STrZ and a TCM model for the SEBs would constitute the greatest possible structural differences. RSM models were not satisfactory for the SEBs. If both the STrZ and SEBs are regions where the aerosols are concentrated, the upper cloud is slightly deeper (by 0.03 to 0.08 bar) in the SEBs; the cloud thickness is less (0 to 15%); and the lower cloud is deeper (by 0.4 to 0.8 bar). A forward scattering haze layer of the type derived from analysis of Pioneer 10 photometry is needed in the present STrZ and SEBs models at the 0.1-bar level to account for the limb darkening in the continuum. The haze could be concentrated in a thin layer or spread diffusely above the cloud top with little effect on the pressure level of the top cloud. A CH 4/H 2 mixing ratio of 1.2 to 1.5 × 10 −3 is estimated from computations by W. D. Cochran of the hydrogen quadrupole absorption strength for present models. The smaller value was used to assign pressure levels stated above.