Titan's smile and collar : HST observations of seasonal change 1994-2000

We observed Titan with the Hubble Space Telescope in November 2000 using the Wide-Field Planetary Camera (WFPC2) and the Space Telescope Imaging Spectrograph (STIS). Considerable change is apparent between these and observations in 1994-1997 : in particular the North-South asymmetry at visible wavelengths has reversed, with the southern hemisphere brighter, as during the Voyager epoch. The asymmetry shows considerable variation with wavelength in the near-infrared : in the 889nm methane band the asymmetry (which formerly resembled a,smile') has reversed in the last 3 years, while at 953nm the southern limb is still bright. A south polar collar, dark at ultraviolet wavelengths, is apparent some five years after equinox.