CASSINI IMAGING OF TITAN

Cassini has completed five close Titan flybys as of July 2005, which provided imaging of the anti-Saturn and the sub-Saturn equatorial regions. Two distant flybys provided good views of the south-polar region, currently in late summer. The Imaging Science Subsystem (ISS) can best see down to the surface in a narrow bandpass filter at 938 nm, but the haze optical depth (~2) makes mapping at high resolution a challenge. We acquire multiple (3-5) images of each footprint with long exposure times to achieve a high signal-to-noise ratio, use techniques to enhance the surface contrast, and mosaic sets of images. Only albedo variations can be seen, not topographic shading, and the smallest features resolved have dimensions of ~ 1 km. The flyby geometries to date have enabled coverage of ~50% of Titan at 3 km/pixel or better. No obvious surface changes have been detected. We have mapped 6 circular patterns that could mark impact structures; two of these have been imaged by Cassini RADAR, confirming that they are craters. If these observations are representative, then Titan's uppermost crust must be geologically young, less than ~500 Myr. The south-polar region is of special interest due to relatively abundant convective clouds; we speculate that linear markings could be fluvial channels or valleys and that an especially dark region with smooth boundaries (~230 x 70 km) could be a hydrocarbon lake or lakebed. Cloud locations and motions hint at surface controls, such as methane sources. Albedo streaks in the equatorial regions suggest that eolian processes are important. Cassini plans include 39 additional close flybys of Titan in the nominal mission (through May 2008); we expect to acquire near-global coverage and to search for changes in repeat coverage.