SATURN'S ICY SATELLITES: GEOLOGICAL VARIETY AND GEOLOGICAL PUZZLES

The Cassini spacecraft has conducted close flybys of many of Saturn's icy satellites in the first year of operation in orbit. Morphologic and compositional data show that these objects encompass a wide variety of internal and surface processes. Enceladus, ~ 500 km diameter, shows several styles and ages of tectonics, exhibits viscous relaxation of craters, as well as an overall gravitational equilibrium shape. This shape, however, suggests that it may not be strongly diffeentiated. Iapetus' global shape is not presently relaxed, but retains dimensions indicative of an earlier, much more rapid rotation rate. Internal models are highly constrained by the rotation history and low mean density. Small satellites, less than 150 km mean radius, lack possible internal heat sources, and are largely shaped by craters. Some of the small satellites, however, show evidence of complex cycling of regolith components. Possible causes of the unexpected features on Enceladus and Iapetus are discussed and placed in the context of the geology of other planetary bodies.