Vertical structure modeling of Saturn with high spectral resolution imaging

A series of narrow-band images of Saturn was acquired on 7-11 February 2002 with an Acousto-optic Imaging Spectrometer (AImS) at 158 wavelengths between 500 and 950 nm. The 3.67-meter Advanced Electro-Optical System (AEOS) telescope at the Maui Space Surveillance Complex (MSSC) on the island of Maui was used for the observations.

With the unique data sets of high spectral agility and wide spectral coverage, the cloud structure and aerosol properties of Saturn's southern hemisphere are extensively studied at -10 °, -31°, and -42 ° planetographic latitudes.

To obtain physical parameters in Saturn's atmosphere, theoretical center-limb profiles are fitted to the observed data at a total of 23 wavelengths across the 619-, 727-, and 890-nm methane bands at each selected latitude. Twelve cloud models are used to examine the influences of modeling assumptions on the results. A simultaneous multi-wavelength multi-variable fitting algorithm is adopted in varying up to 9 free parameters to explore the vast multi-dimensional parameter space, testing a total of ~12,000 initial conditions in modeling the cloud structure at each latitude.

>From the results of the modeling simulations, the following major conclusions are derived. The upper tropospheric cloud is optically thickest and located highest near the equator. There is a cloud layer below the upper tropospheric cloud over the entire southern hemisphere of Saturn. If the upper cloud aerosols are assumed as spherical ammonia ice particles, the lower size limit of the aerosols is about 0.5 mm near the equator and 0.2-0.4 mm over the southern mid-latitudes of Saturn. The lower tropospheric cloud is responsible for Saturn's yellowish color. A significant brightening of Saturn's southern hemisphere occurred at a deep 890-nm methane band in 2002.

The ongoing observations by the Cassini spacecraft over a wide spectral range and from various phase angles will further constrain Saturn's cloud structure and aerosol properties.