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P. A. Yanamandra-Fisher, G. S. Orton, B. M. Fisher (JPL/CIT)
During the Voyager epoch (1980/81) and since, much of the southern hemisphere (0 -- 20\circ) was obscured by the rings. In 1995, the Earth and Sun crossed Saturn ring plane. Hence, until the next decade Saturn's southern hemisphere is observable from Earth, providing an opportunity to observe those latitudes previously obscured. High--resolution observations of Saturn were acquired at a wide range of wavelengths (0.4 -- 24 \mum) with various instruments (CCD, NSFCAM, QUIRC, MIRLIN, MIRAC2, Spectrocam 10) from several observatories (NASA/IRTF, UH88, Palomar, JPL/TMO) from 1995 to current apparition. The development of belt/zone structure was observed at all latitudes, particularly at southern latitudes not previously observed by Voyager. The 4.78 -- 5.30 \mum window on Saturn probes the troposphere in the 2 -- 5 bar pressure range in the absence of clouds, similar to that of Jupiter; and is a mixture of thermal radiation emanating from the deep atmosphere of Saturn overlaid by a reflected sunlight contribution. Due to the intrinsic low flux from Saturn and its distance from Earth, until recently it has been difficult to obtain high spatial/spectral images of Saturn in the 5-\mum window and lack of sensitive detectors. Radiometric calibration of our December 1997 data indicates that the thermal flux from most of the planet is characterized by brightness temperatures of 180 -- 185K, where as the colder regions, both in the bands and discrete regions exhibit brightness temperatures of 172 -- 175K. From observations of these 5-\mum cold spots (which could possibly represent a negative counterpart to the 5-\mum Jovian hot spots), we estimate a zonal wind speed of 173ms-1, slower than the known zonal wind speeds of ~ 300ms-1, interpolated from Voyager data (Ingersoll {\em et al}., 1984). Our observations can be interpreted as (1) the thermal radiation from the deep atmosphere is insensitive to the overlying haze composed of sub--micron sized particles or (2) the zonal winds possibly decrease with altitude.