Saturn's Icy Satellites: Observations From Cassini/Huygens

The so-called icy satellites of Saturn include all the orbiting objects in the system larger than a ring particle ( ∼microns to meters) and smaller than Titan, which, with a density of 1880 kg m^-3, is an icy satellite also, but whose planet-scale size and massive atmosphere make it an object for special attention. Although the densities of the satellites are still subject to considerable uncertainty, most of the icy satellites are apparently more ice-rich than massive Titan, or the icy satellites of Jupiter. Some of the smallest, co-orbital and shepherd, satellites may be almost pure water ice in composition, in addition to having high porosities. Voyager images of the satellites revealed that most show evidence for internal activity and some degree of resurfacing. Included in this array of interesting worlds are two particularly intriguing moons - Enceladus and Iapetus. Enceladus appears heavily modified geologically in spite of its small size, and Iapetus has the largest hemispheric albedo dichotomy known. As Cassini/Huygens entered the Saturn system it performed its first close icy satellite encounter with Phoebe on June 11, 2004. Phoebe's measured mass and volume yield a density of 1600 kg m^-3 ± 100. The high rock fraction implied by this density, spectral evidence for water ice and carbon dioxide on its surface and images showing a surface of mixed bright and dark material all suggest that Phoebe originated not in the Saturn system but in the solar nebula beyond the asteroid belt, and may be related closely to Kuiper Belt Objects. The results of initial Cassini observations of the icy satellites and plans for upcoming encounters will be reviewed.