Saturn is the sixth planet from the Sun and the second largest, after Jupiter. Like Jupiter, Saturn is a "gas giant" planet, composed mainly of liquid and gaseous hydrogen, with no solid surface. But what makes Saturn so unique and impressive is its spectacular system of rings.
Observation and Exploration
Saturn presents one of the most stunning views that can be seen through a small telescope. Even experienced observers never tire of looking at this amazing object. Its rings are spectacular; even a small telescope will show them. Modest-sized telescopes (6 inches and larger aperture) can show much detail in the ring system. There are prominent bright rings, separated by gaps or dark areas called divisions. The most famous and easily-observable is the Cassini Division, which separates the A and B rings.
Saturn's average distance from the Sun is 9.5 times the Earth's, and it takes 29.5 years to complete one orbit. Saturn's equator and ring system are inclined by about 27 degrees to its orbital plane, so our perspective angle of the rings changes over Saturn's orbital period. When the Earth crosses the ring plane, the rings are seen edge-on and all but disappear; this occurs every 14.7 years.
With an equatorial diameter of 119,300 kilometers (74,130 miles), Saturn is almost nine times as large as the Earth. Saturn rotates very quickly; its day is only 10 hours and 39 minutes long. This rapid rotation makes Saturn appear visibly flattened at the poles.
Like Jupiter, Saturn also possesses a number of moons that can be seen with a small telescope. The largest and brightest of these is Titan, which appears like an 8th-magnitude star moving around Saturn over the course of its 16-day orbital period. Dione, Rhea, and Tethys are also visible, though fainter than Titan.
Saturn has been visited by a number of spacecraft: Pioneer 11 in 1979, Voyager 1 and 2 in 1980-81, and most recently the joint NASA-ESA Cassini mission. Cassini entered orbit around Saturn in 2004, and is currently engaged in an eight-year mission to study the planet, its rings, and its moons.
Atmosphere and Composition
Like Jupiter, Saturn is a "gas giant" planet. It is composed mainly of hydrogen and helium, in gaseous form at the cloud tops, and as a compressed liquid or in "liquid metallic" form further down. Like Jupiter, Saturn is also believed to have an Earth-sized rocky core. Interestingly, the bulk density of Saturn is less dense than water: if you could find a bathtub large enough to hold it, Saturn would float.
As with Jupiter, Saturn possesses both a powerful magnetic field and an internal heat source. The former emanates from the convective motion of electrical charges in the planet's liquid metallic hydrogen interior; and the latter powers its turbulent atmospheric weather.
Like the other gas giants, Saturn does not have a uniform rotation rate. It rotates faster at the equator than at the poles. Saturn's atmosphere has the same banded appearance as Jupiter's, but appears blander, with fewer of the complex ripples, spots, and storms that are present on the larger gas giant.
Ring System
The rings of Saturn were first seen by Galileo through his crude telescope, although he was unable to identify what they were. The first person to understand that Saturn was surrounded by a thin, flat ring was Christian Huygens, in 1665. As telescopes improved, more rings became visible; these were named the "A ring", "B ring", etc. The list today includes rings C, D, E, F, and G. Several dark divisions or "gaps" separate the bright rings; the largest of these are the Cassini and Encke divisions.
Although Saturn's rings extend out to 75,000 miles (120,000 km) from Saturn's surface, they are less than 100 meters thick. They are made up of countless billions of particles of almost pure water ice, ranging in size from grains of sand to house-sized boulders.
The Voyager spacecraft discovered that the rings are not uniform. Thousands of density variations - "ringlets" - exist within the lettered rings. Their intricate structure is the result of complex gravitational interactions and orbital resonances between the ring particles and a number of small moons orbiting within, and just outside, the ring system. Voyager discovered several small moons associated with the prominent gaps in the rings. Those moons act as "shepherds" whose gravity herds the ring particles back into the main body of the rings. The narrow outer F-ring, in particular, shows clumps and braids, generated by gravitational interactions with the nearby small moons Prometheus and Pandora.
Saturn's rings may be very old, dating back to the formation of Saturn itself. The most likely theory of their origin is that they are the debris of a moon, roughly the size of Mimas, which was disrupted by an enormous collision. The remaining ring particles orbit Saturn within its "Roche limit", where tidal forces prevent them from accreting back into a single, large, solid body.
Moons of Saturn
Saturn has an extensive system of moons. Nine were known before the Voyager spacecraft flybys in 1980 and 1981. Another nine were discovered by those spacecraft, and some of those small, inner moons are responsible for the divisions and structure in the rings.
Earth-based observations, and the Cassini orbiter, have revealed numerous asteroid-sized moons. As with Jupiter, many of the outer moons have retrograde and highly inclined orbits. As of mid-2008, Saturn has 48 named moons and another 12 objects awaiting "official status" as moons. There will undoubtedly be more discoveries of these smaller, asteroid-sized objects to come.
Saturn's large moons have proven to be as diverse and interesting as Jupiter's. Titan, in particular, is the second-largest moon in the solar system (after Jupiter's Ganymede), and is the only one known to possess a thick atmosphere. The Cassini spacecraft dropped an instrumented probe named Huygens into Titan's atmosphere in 2005.