Saturn's atmosphere: structure and composition from cassini/cirs

This thesis describes the analysis of thermal infrared spectra from Saturn measured by the Composite Infrared Spectrometer (CIRS) onboard the orbiting Cassini spacecraft (Flasar et al., 2004).  Many thousands of spectra in both the far (10 - 600 cm^-1) and mid (600 - 1400 cm^-1) infrared, acquired over the first three years of operations (2004-2007), have been analysed using a radiative transfer and optimal estimation retrieval algorithm (Irwin et al., 2004).  CIRS-spectra were used to deduce the vertical temperature structure and distribution of gaseous species with latitude throughout Saturn's upper troposphere and stratosphere and to infer the bulk composition of the gas giant. The temperature structure and para-hydrogen distribution over Saturn's northern (winter) and southern (summer) hemisphere were presented in Fletcher et al., (2007a). Hemispherically-symmetric small-scale temperature structures associated with zonal winds (Vasavada et al., 2006) are superimposed onto the seasonal temperature asymmetry for pressures greater than 100 mbar.  Seasonal asymmetries of stratospheric temperatures, tropopause height, radiative-convective boundary, para-H₂ distribution and the inferred aerosol properties are discussed.  The existence of Saturn's hot south polar vortex was confirmed and a similar north polar hotspot was observed for the first time.  Infrared images were also used to study the temperatures associated with the north polar hexagon for the first time.  The temperature field was used to study Saturn's zonal, meridional and vertical wind velocities in the upper troposphere. Absorption and emission features in high spectral resolution (0.5 cm^-1) data were used to derive vertical distributions of PH₃, NH₃, CH₄, HD, ¹³CH₄ and CH₃D.  Saturn's carbon and phosphorus were found to be equally enhanced over solar abundances; the degree of ammonia saturation was found to be uniform with latitude, and HD is apparently depleted in the upper troposphere.  This work significantly improves the accuracy of elemental abundances and key isotopic ratios over previous studies and investigates the sensitivity of results to the poorly constrained vertical structure and optical properties of Saturn's tropospheric haze.