LECTURE 5: THE SATURNIAN MOONS
Saturn has 60 named natural satellites, many of which were discovered only recently and may be as small as two-to-three kilometers across, plus hundreds of observed "moonlets" only a few tens or hundreds of meters across in the A Ring. Seven of the moons are large enough to have achieved hydrostatic equilibrium and so would be considered dwarf planets if they were in orbit about the Sun; indeed one of them, Titan, is massive enough to retain an atmosphere denser than our own. A precise number of moons can not be given, as there is no objective boundary between the countless small anonymous objects that form Saturn's ring system and the larger objects that have been named as moons.
- Mimas, the "Death Star" moon, with a deep crater one fifth its diameter;
- Enceladus, a bright, striped moon whose geysers are the source of the E Ring;
- Tethys, with a canyon running three-quarters its circumference that formed when an internal ocean froze;
- Dione, with wispy terrain on its trailing hemisphere.
Saturn's largest moons all orbit beyond its E Ring and can thus be considered a distinct group. They are:
- Rhea, which may have its own ring system;
- Titan, a huge moon, with methane lakes, sand dunes made of organic material, and an atmosphere thicker than Earth's;
- Hyperion, a smallish, tumbling moon that looks like a sponge;
- Iapetus, a two-toned, walnut-shaped moon with an equatorial ridge.
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