EARLY CASSINI IMAGING OBSERVATIONS OF SATURNIAN MOONS

The Cassini ISS team presents preliminary results from new images of the Saturnian moons, especially Phoebe and Titan. Phoebe, with a mean radius 107.3 km and a retrograde orbit far from Saturn (mean distance 12.95 x 10⁶ km), is a primitive body perhaps like those in the Kuiper belt, but instead captured by Saturn. New measurements of the mass and volume acquired during the June 11 flyby give Phoebe a bulk density of ~1.6 g/cc, consistent with a mix of rock, ice, and organic material. Craters cover the moon and range from 100 km diameter down to resolution limits (~50 m). Landslides have exposed bright patches of nearly pure water ice, showing that pods of ice have segregated from the other materials. Near-global coverage of planet-sized Titan (2575 km radius) acquired during approach to Saturn revealed 100-km-scale dark linear markings in the equatorial region. Cassini passed within 339,000 km of the south polar region on July 2 and acquired images with scales down to almost 2 km/pixel, but, in spite of special filters, the photochemical haze limited resolution to no better than 10 km. The haze also washes out topographic shading, and the albedo markings have proven difficult to interpret. The south polar region appears different from equatorial regions, with brightness patterns roughly concentric or radial to the pole, perhaps due to atmosphere-surface interactions. A field of clouds about 400 km wide was observed near the south pole, with individual clouds growing and dissipating during just a few hours. The first close targeted flyby of Titan on October 26 will return much more detailed images of an equatorial region, including the Huygens landing site.