Titan's north-south asymmetry from HST and Voyager imaging: Comparison with models and ground-based photometry

New measurements of Titan's hemispheric brightness asymmetry from HST images from 260 to 1040 nm show that the contrast is strongest near 450 nm (blue) and, with the opposite sign, at 889 nm (methane band). Comparison with a full Titan year of disk-integrated albedo data indicates that the seasonal cycle in asymmetry is smooth, but is insufficient to explain the variation in albedo, and a twice-per-year global albedo enhancement 50% larger than the hemispheric asymmetry amplitude is required, as noted by other workers. We also report measurements of limb-darkening (strongest at red wavelengths) and note that the transition zone between the "hemispheres" lies in the northern, brighter hemisphere at present. Comparison of models to the HST data set indicates that a change in aerosol number density above 70 km, and largely below 120 km, is the likely mechanism and is probably driven by aerosol transport by meridonal and vertical winds.