SATURN'S ICY MOONS: FROM CASSINI (GIOVANNI) TO CASSINI (ORBITER)

Saturn’s family of 8 mid-size icy satellites (those between 200 and 2000 km diameter) form with 5 similar Uranian satellites a distinct class of Solar System objects. Prior to 1980, these were little more than fuzzy discs with icy surfaces but Voyager 1 and 2 revealed diverse bodies with unique geologic histories. Mapping was incomplete with resolutions seldom better than 1 km (except on Rhea and Miranda), and resolvable coverage at ~25-60%. The Cassini orbiter imagery has allowed complete global mapping of Saturn’s icy moons down to 400 m or better and revealed a host of unsuspected features and processes. E-ring dust from Enceladus paints leading hemispheres, magnetospheric particles the trailing hemispheres, and high-energy electrons the equators of Mimas and Tethys. Extensive networks of graben cross Mimas, Dione, Rhea, and Tethys, likely due to global expansion. Smooth plains of Tethys and Dione appear to be volcanic in origin. Relaxed craters on any of these moons betray an earlier era of enhanced heat flow. A gigantic equatorial ridge spanning Iapetus and a narrow surficial equatorial deposit of bluish material on Rhea are likely due to an ancient ring system reaccreting on the surface. Extensive tectonism on Enceladus is merely part of a global history of active geology dating back several Gyr. Currently active resurfacing at the south pole is associated with several long narrow ridge structures; the sources for several dozen discrete vents jetting icy particles into Saturn orbit. High surface temperatures indicate the subsurface may be partly liquid. The heat source driving this activity and on the other satellites remains unresolved.