The six major icy satellites of Saturn have been known for many years through astronomical observations, although the Voyager encounters dramatically increased our knowledge of these bodies. All are composed of water ice with various amounts of silicate rock, and all have nearly circular orbits near the equatorial plane of Saturn.
The smallest of the icy moons, Mimas, is pockmarked with craters. A giant crater 130 kilometers (80 miles) across dominates this image returned from Voyager 1. A slightly larger impact could have shattered the moon into several fragments.
Unlike Mimas, the surface of Enceladus reveals a long and complex history. Highly cratered ancient areas exist, but are separated by curved grooves and younger, uncratered surfaces. Because of the moon's ice composition, it is doubtful that internal radioactive heating melted the crust. Instead, gravitational stresses from the moon Dione may heat the interior.
Internal cooling of Tethys and impact cratering have created an appearance similar to Mimas. However, the surface is crossed by a large trench system that extends across three quarters of the circumference of the moon.
The satellite Dione has the highest density of any of the Saturnian moons (1.4 grams per cubic centimeter), and perhaps the largest amount of rock material. Broad light streaks extend across the moon, possibly formed by upwelling of water along cracks in the crust.
This high resolution image of Rhea shows parts of its surface to be highly cratered, and hence very old. The bright areas on the inner walls of the craters may be fresh ice exposed by landslides.
The outermost of the major icy satellites, Iapetus, has dark and light sides that have long been known from telescopic observations. The Voyager encounters showed that the dark side has a jagged boundary with the bright, heavily cratered hemisphere. The origin of the dark coating remains a mystery, although it may be created by eruptions of methane.