Seasonal Change on Titan Observed with the Hubble Space Telescope WFPC-2

Recent observations with the Wide-Field Planetary Camera (WFPC-2) on the Hubble Space Telescope (HST) show an unexpectedly rapid change in the atmospheric albedo contrast between the north and south hemispheres. In 1994 at blue wavelengths, the north was around 15% brighter than the south, and was expected to fall to about 12% in 1997, but has dropped to only 6% brighter. At some other wavelengths, the contrast has reversed, which was not expected until 2002. The interhemispheric contrast has a time dependence that varies with wavelength; contrast changes in blue lag behind changes in violet and yellow/red. The rapid change and the phase variation with wavelength are consistent with ground-based photometry. A physical model of the transport of high-altitude dark haze by meridional winds is a better description of Titan's behavior than the simple sinusoidal models used to date. Investigation with a radiative transfer model indicates that haze number density changes above 160-km altitude are compatible with the observed hemispheric albedo difference, and require particles >0.1 (mu)m in radius. Key Words: Titan; atmospheres, dynamics; image processing; photometry; radiative transfer.