Saturn

Saturn is the Roman god of agriculture. The planet is second largest after Jupiter, most famous for its ring system. 

Christian Huygens was the first to realize the rings were not solid, but in fact orbiting debris in 1655. The widest and brightest ring is named Ring B, with Ring A fainter and further out. Incredibly, the rings are only 100m thick, compared to a radius of 135,000km. There is a narrow gap in Ring A called Encke's Division, through which the moon Pan orbits. Atlas orbits Ring A's outer edge. The Voyager probes show a new outer ring, Ring F. Ring F is patrolled by Prometheus and Pandora. These moons shepherd the rings. The moon Mimas helps to clear the Cassini Division by its gravitational pull.

Saturn is roughly 75% hydrogen and 25% helium, similar to Jupiter, and has a rocky core. It rotates rapidly and consequently bulges at the equator. The core temperature is 12000K due to gradual gravitational compression, known as the Kelvin-Helmholtz mechanism, which also happens in Jupiter and Neptune, but not Uranus. Cloud tracking shows wind speeds of up to 1800kmph.

Semimajor axis (km) 1433.53x106 Perihelion (km) 1352.55x106 Aphelion (km) 1514.50x106 Mean orbital speed (km s-1) 9.69 Sidereal orbit period (days) 10759.22 Tropical orbit period (days) 10746.94 Synodic period (days) 378.09 Length of day (hours) 10.656 Sidereal rotation period (hours) 10.656 Obliquity to orbit (deg) 26.73

Semimajor axis (AU) 9.53707032 Eccentricity 0.05415060 Inclination (deg) 2.48446 Longitude of ascending node (deg) 113.71504 Longitude of perihelion (deg) 92.43194 Mean longitude (deg) 49.94432 Discoverer Unknown Date of Discovery Classical J2000 Data from NSSDC

Saturn was first visited in 1979 by Pioneer 11 as a flyby. This was followed by Voyager 1 in late 1980, then its twin Voyager 2 also flew by, this time in 1981. Cassini/Huygens was launched in 1997 and Cassini continues to orbit Saturn while Huygens landed on Titan in January 2005.

There is orbit sharing among some of the moons, for example, Janus and Epimetheus share their track. Similarly, Tethys shares orbit with Telesto and Calypso, both much smaller moons. Saturn has 34 named moons, of which Titan is the largest.

Titan, alone of all the Solar System's moons, has an atmosphere composed of mostly nitrogen and some methane and argon. The atmosphere has 1.5 bar pressure and surface temperature is roughly 94K. The moon itself is thought to be 50% rock and 50% ice. There are traces of organic chemicals, including water.

Iapetus is the third largest moon and is probably mostly ice. It is curious because the leading face is much darker than the trailing one, probably coated in runaway material from Phoebe. Iapetus is also inclined at 15 degrees to Saturn's equator, the only moon apart from Phoebe not in the equatorial plane.

Mimas is also curious in that it has a huge crater, Herschel, with radius almost one third the radius of the moon itself. It is also pockmarked by other craters.