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R. D. Lorenz (LPL, University of Arizona), E. F. Young (Southwest Research Institute, Boulder CO.), M. T. Lemmon (Dept. of Physics, Texas A & M University, TX)
We report observations of Titan by WFPC2 and STIS in December 2000 which show several dramatic aspects of seasonal change on Titan. First, Titan's appearance in 1994-1997 at 889nm (in a deep methane band, so probing only high altitudes) was dominated by a 'smile', with the southern limb brightest. By 2000 the smile had faded, with the brightness more uniform across the disk. At 953nm, the southern limb is still the brightest, suggesting enhanced tropospheric opacity, perhaps due to sedimentation of C4N2 ice during the polar night and acting as condensation nuclei for ethane and methane. A faint remnant of this polar hood is visible as a UV-dark ring around the south pole, much like the north polar collar seen by Voyager 2. Titan's north-south asymmetry has reversed, with the southern hemisphere brighter at green wavelengths (as during the Voyager era) - the asymmetry has reversed within 5 years of equinox, whereas a strictly sinusoidal cycle would not reverse until 7.5 years afterwards. It seems the asymmetry 'flips' rapidly after equinox, consistent with a model of the haze driven by thermally-direct Hadley winds from the summer hemisphere. Comparison of latitude-resolved STIS spectra from 1997 and 2000 show considerable variation of the asymmetry with wavelength and time. Both the wind-driven upper haze cycle, and the progressive sedimentation of condensate at lower altitudes in the southern hemisphere, should decrease the opacity there, improving Cassini's view of the sunlit south.
Proposals 7321 and 8580 were supported by NASA through STScI, operated by AURA under NASA contract NAS5-26555.
The author(s) of this abstract have provided an email address for comments about the abstract: rlorenz@lpl.arizona.edu