TITANS NORTH-SOUTH ASYMMETRY FROM HST AND VOYAGER IMAGING - COMPARISON WITH MODELS AND GROUND-BASED PHOTOMETRY

New measurements of Titan's hemispheric brightness asymmetry from I-IS T images from 260 to 1040 nm show that the contrast is strongest near 450 nm (blue) and, with the opposite sign, at 889 nn (methane band). C omparison with a full Titan year of disk-integrated albedo data indica tes that the seasonal cycle in asymmetry is smooth, but is insufficien t to explain the variation in albedo, and a twice-per-year global albe do enhancement 50% larger than the hemispheric asymmetry amplitude is required, as noted by other workers. We also report measurements of li mb-darkening (strongest at red wavelengths) and note that the transiti on zone between the ''hemispheres'' lies in the northern, brighter hem isphere at present. Comparison of models to the HST data set indicates that a change in aerosol number density above 70 km, and largely belo w 120 km, is the likely mechanism and is probably driven by aerosol tr ansport by meridonal and vertical winds. (C) 1997 Academic Press.