Overview

• Origin of name: The Greek god Cronos, son of Uranus and Gaia; A Titan, ruler of the world until he was dethroned by his own son Zeus/Jupiter.
• Orbit and appearance: Radius 9.45 AU [eccentricity is small], period = 29.5 years; Less bright than the inner planets and some stars; Along the orbit rings change in appearance because of their tilt.*
• Size: 95 Earth masses, 9.4 Earth radii; the most flattened planet, and the least dense [density 0.7 that of water!].
• Rotation: Rapid (about 10 h), differential (faster at the equator); causes an equatorial bulge; [the rotation axis and rings are inclined 27°, a long-standing mystery; could it be due to the influence of Neptune's orbit?].

  Exploration

• Early history: Galileo first saw the rings, but interpreted incorrectly what he saw; Christiaan Huygens in 1659 figured out their geometry, and Gian Domenico Cassini studied them more extensively in 1675.
• Early missions: 1970's, Pioneer 11; 1980-81, Voyager 1 and 2 (found most of the Moons).
• Currently: The Cassini spacecraft, launched 1997, arrived in 2004 and is orbiting Saturn and its moons, with ESA's Huygens probe that will descend on Titan.

Surface and Interior

• General appearance: Cloud belts, wind and turbulence; Thicker, but clouds form deeper, under methane haze in the top layer, so Saturn is less colorful than Jupiter.
• Composition: Mostly made of H (70%), He, methane, and ammonia; Colder than Jupiter, but hot inside.
• Core: It has a larger icy/rocky core than Jupiter.
• Energy: Produces more than it receives, from He precipitation toward the center (Saturn is cold enough for it to condense, contrary to Jupiter's case).
• Magnetic field: Strong, with a very extended magnetosphere.

  Rings

• Main structure: Starting from outside, A, Cassini division, B, C rings; Very thin (less than 100 m), tilted; Made of particles and water ice, ranging in size from small specks to several meters.
• Location: From 1 to 2.3 planet radii, inside the Roche limit.
• More detailed: There are ringlets, density waves, and moving spokes; Small gaps are cleared by moonlets, and some wider gaps caused by resonances (like the Cassini division and the moon Mimas); The F ring is controlled by the shepherd moons.
• Origin: Not original material; Catastrophic event? Reddish color indicates they may be remnants of a shattered body from the outer Solar System; Not stable, probably replenished by collisions.

Moons

• General appearance: They are not aligned and are easier to spot when they cross the thin rings; A complex set of satellites, with 50 known moons (the most recent ones announced in 2005; some don't have names yet), many of which have interesting motions; Most are covered by ice and rotate synchronously.
• Small moons: From Pan (20 km in diameter) to Phoebe (220 km), a dozen or so; Interesting ones are Hyperion's chaotic rotation, Janus and Epimetheus' orbit swapping, the orbit sharing moons, ...
• Medium-sized moons: Six of them have diameters between 380 and 1530 km [Rhea, two-colored Iapetus, Dione, Tethys, Enceladus, Mimas].
• Titan: Seen by Huygens in 1655; slightly bigger than Mercury, at 20 planet radii; We can't see its surface, because of its thick atmosphere (with N, Ar, methane, ammonia, and 470 mph winds!), Very cold (seems to have extended areas of icy bedrock), but may have lakes of liquid methane and pre-life conditions, and has been considered as a possible spaceship stopover (A C Clarke); The Huygens probe landed on it in Jan 05.