The six major icy satellites of Saturn have been known for many years through astronomical observations, although the Voyager and Cassini 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. The giant Herschel 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.
False color images of Saturn's moon, Mimas, reveal variation in either the composition or texture across its surface.
In this Cassini image, Mimas appears flattened on the right as the crater Herschel is viewed from the side.
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.
NASA Press Release #P23955.
The plumes of Enceladus
Roughly parallel faults, informally called tiger stripes, are found near the moon's south pole. Geysers of water vapor spurt out from these fractures. Evidence suggests that Enceladus may host a global ocean under its icy crust that may feed the plumes, making this environment a candidate in the search for simple, microbial life beyond Earth.
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.
NASA Press Release #P24065.
Cassini imaged a large impact crater on Tethys named Odysseus. The crater is 280 miles (450 km) across.
Two moons, Tethys and Enceladus, line up in relation to Cassini's imager. Tethys is 660 miles (1062 km) in diameter and Enceladus is 313 miles (504 km).
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.
NASA Press Release #P23113.
With the higher resolution imaging of Cassini, the bright streaks could be seen in more detail than was possible from Voyager. Named chasmata, these features are linear fractures and icy cliffs.
Rhea is a highly cratered, and hence very old moon.
NASA Press Release #P23356.
Bright streaks on Rhea may be similar to the chasmata of Dione.
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.
NASA Press Release #P23961.
More detailed view of Iapetus from Cassini. The dark material is still puzzling. Theories for its origin include thermal cycles, sweeping up material from the moon Phoebe, and eruptions of hydrocarbons. The large impact crater at bottom left is 280 miles (450 km) in diameter. Several other large craters are present across the surface of Iapetus.
An enigmatic ridge spans the equator of Iapetus. It rises up to 8 miles (13 km) in places.
