Microwave imaging of Saturn's deep atmosphere and rings

Citation

Grossman, Arie William (1990) Microwave imaging of Saturn's deep atmosphere and rings. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/svtf-j306. https://resolver.caltech.edu/CaltechETD:etd-10302008-151649

Abstract

This work presents an analysis of microwave images of Saturn's atmosphere and rings. Interferometer observations at wavelengths of 0.27, 2.01, 3.53, 6.17, and 20.13 centimeters and precise application of synthesis imaging techniques yielded brightness and polarization maps of unsurpassed resolution and sensitivity. Linear polarization is detected from the ring ansea, and brightness variations in the deep atmosphere and the rings are revealed for the first time. The disk-integrated spectrum of Saturn is interpreted within the context of a radiative transfer model that requires the NH3 mixing ratio to take on a value of 0.9 to 1.1 x10[superscript -4] (0.5-0.6 times solar) directly below the ammonia ice cloud at a pressure of 1.4 bar. The NH[subscript 3] mixing ratio increases with depth to a value of 5.0 to 6.5 x10[superscript -4] (2.9-3.7 times solar) at a pressure of 6 bar. The variation of NH3 with depth can be entirely accounted for by the presence of 11-14 times solar abundance of H[subscript 2]S, which reacts with NH[subscript 3] to produce a substantial NH[subscript 4]SH cloud. Latitudinal variations in brightness temperature indicate that the saturated vapor abundance of ammonia decreases by 50% from equator to pole within the cloud deck. At greater depths the latitudinal variations of ammonia are consistent with alternating zones of concentration and depletion caused by vertical motions. An apparent depletion in northern mid-latitudes is well-correlated with a decrease in infrared opacity and depressed cloud top levels, indicating deep-seated downwelling. The size, composition, and shape of particles comprising the rings of Saturn are constrained by modeling the emission, scattering, and extinction of radiation by the rings. The observations can be fit by a incremental power-law particle size distribution with exponent in the range 2.6-3.0 for the combined A and B rings, assuming a classical many-particle-thick layer. The wavelength dependence of the optical depths places a strict lower limit of 1 cm on particle sizes in the classical rings. Observations of thermal emission from the rings further constrain the mass fraction of uniformly mixed silicate impurities to be less than 1%. Azimuthal variations in brightness and linear polarization rule out the possibility that the particles are smooth, convex objects, and favor a model in which the particles are irregularly shaped.

Thesis Files