Even when viewed through a small telescope, Saturn and its ring system is one of the most unique objects in the sky. With a large modern telescope in good observing conditions, the planet appears as a light yellow and gray banded oblate spheroid. Like the other giant planets--Jupiter, Uranus, and Neptune--the visible planet is the cloud top of an extensive gaseous atmosphere.

Saturn is one of the giant outer planets, which are characterized by their large size, low density, and corresponding extensive atmospheres. Current models of the interior indicate that below the relatively thin opaque cloud layer is an extensive, clear hydrogen-helium atmosphere. Data on the internal heat flux, the detailed gravity field, and the observed upper-atmosphere hydrogen-helium ratio satisfy a model of the interior where the ratio of hydrogen to helium decreases with depth. The gas density gradually increases downward, and the gas transforms into a liquid. Further down the pressures increase to a critical level, and there the hydrogen becomes metallic. A small core of silicate material probably exists at the center.

The Saturnian atmosphere is characterized by counter-flowing easterly and westerly jet streams that, at the equator, reach a speed of 480 m/sec (1,070 mph) relative to the clouds at 40 degrees latitude. The zonal jets do not change appreciably with time, but smaller-scale spots, waves, and eddies were seen on Voyager spacecraft images to change a time scale of hours. Such smaller features are usually hard to observe on Saturn because of an obscuring haze layer above the planet's cloud surface. One northern hemisphere feature, however, known as the Great White Spot, does become significantly noticeable for Earth- based viewers about every 29 years. The spot is apparently an upwelling of ammonia-rich materials; the ammonia then crystallizes at this greater height to produce the white color. The spot sometimes expands until it becomes a band of clouds girdling the planet.

Traces of methane, ethane, phosphine, and acetylene also exist in the hydrogen-helium atmosphere. Various colors that have been observed probably result from chromophores being produced by the interaction of such trace elements as sulfur or carbon compounds with ionospheric charged particles and lightning. This condition of chemical nonequilibrium is produced by vertical mixing, driven by heat from the gravitational energy released by the precipitation of liquid helium. Saturn radiates 2 to 3 times the heat absorbed from the Sun.

Saturn has a strong, dipolar magnetic field tilted only 0.7 degrees from the rotational axis. The subsolar magnetopause is 6.38 million km (3.96 million mi.) from Saturn on the average. A magnetic tail extends in the direction away from the Sun much like cometary plasma tails. Saturn's magnetic field traps charged particles coming from the solar wind. These particles move along magnetic-field lines but are absorbed by satellites and ring particles. The charged particles that impinge on the ionosphere create airglow emissions.

Saturn is far enough away from the Sun to retain the light elements (hydrogen and helium) and therefore has solarlike chemical abundance. Saturn's mass, unlike that of the Sun, was not large enough to initiate the fusion process, and Saturn, unlike Jupiter, did not give off enough excessive heat to drive out water from the inner satellites.

Saturn's white rings were first seen by Galileo Galilei in 1610; his small, imperfect telescope showed the planetary disk flanked by what he first interpreted as being two smaller bodies. Christiaan Huygens correctly theorized (late 1650s) the ring nature of these alleged "companions." James Clerk Maxwell mathematically demonstrated (1857) that the rings were composed of many small, unconnected particles, each orbiting near Saturn's equatorial plane.

The classical designations for the rings are based on the gross ring components identified from the ground, but the Voyager spacecraft have shown the ring system to be highly structured. The radial particle-density distribution changes over distances of hundreds of meters. Estimated sizes of individual particles range from tens to hundreds of centimeters. The ring plane has a maximum thickness of 1 to 2 km (0.6 to 1.2 mi.). Spectroscopy shows the presence of water ice, which probably covers rocky silicate cores.

The dynamics of the rings are not presently well under stood. The theory of satellite resonances predicts that particles whose orbital periods are integral fractions (such as 1/2 or 2/3) of the periods of the satellites become either locked into or perturbed out of a particular orbit, but only a few of the observed gaps can be explained in this manner. Voyager showed eccentric ringlets and asymmetrical kinks in some ringlets. The kinks in the F ring gave it a braided appearance, but despite the proximity of two small satellites, the kinks do not seem directly related to them. Voyager also showed irregular spoke-like features in the B ring that are composed of very small, strongly back-scattering particles temporarily out of the ring plane. Because of the detection of electrostatic discharges from the rings at radio wavelengths, these particles are thought to be electrically charged and thus forced out of the ring plane by Saturn's magnetic field.

The rings may be the debris from satellites or comets broken apart by tidal forces. Another hypothesis is that Saturn's tidal forces and the perturbations of various satellites prevented the material left over from the formation of Saturn to accrete into further satellites.

Saturn has the most extensive satellite system in the solar system. Not counting the myriad ring particles, more than 20 bodies orbiting around Saturn have so far been identified. Six can be easily seen through the telescope.

Titan is the largest Saturnian satellite and, among all solar system satellites, is second in size only to the Jovian satellite Ganymede. It is the only satellite with a substantial atmosphere, although Neptune's Triton has a much thinner one. The highest-resolution Voyager images show several haze layers that together obscure the surface. (For a discussion of this major satellite, see the article of that name.)

The other satellites of Saturn tend to have low densities (1 to 1.5 g/cu cm, or 62 to 94 lb/cu ft) and high albedos, or surface reflectivities (0.4), indicative of water-ice-dominated bodies. Water frost has been detected spectroscopically on the surface of most of these satellites. With the exception of Phoebe, Iapetus, and Hyperion, they are in nearly circular, direct, low -inclination orbits. All of these satellites, with the exception of Enceladus, have highly cratered, old surfaces. The larger satellites have two distinct crater populations, perhaps a result of different sources and types of impacting bodies or of changes in the impact behavior of the satellite surfaces as they evolved.

Mimas is dominated by a crater 130 km (81 mi.) in diameter, or one-third of its own diameter. The impact that produced such a large crater on Mimas, weakly bound by its own gravity, was near the limit of major disruption. The remaining sur face is heavily cratered, with grooves that may have been either formed when the large crater was formed or developed by tidal interactions following accretion. Using density measurements gathered by the Voyager 1 spacecraft, scientists have found that Mimas may have a small rocky core with a thick mantle of water ice.

Enceladus is unusually smooth and free of craters. Its albedo is possibly the highest in the solar system, the result of a substantially resurfaced ice crust. Because the orbits of Dione and Enceladus are in resonance, the latter has an eccentricity forced by Dione. This produces strong tides and tidal heating, which may have kept the ice in a more fluid state during the postaccretion impact phase. The smoother regions have groove and ridge terrain that is very similar to regions on the icy Jovian satellite Ganymede.

Tethys is heavily cratered and has an approximately 1,000-km- long (620-mi.) valley running roughly north-south. The terraced walls of the valley suggest crustal layering. With a bulk density of 1.0 g/cm3 (62 lb/ft3), Tethys is mostly water ice. Great expansion forces during freezing may have been responsible for the valley.

Dione is about the same size as Tethys but has a higher density. High-albedo streaks or wisps on the dark hemisphere may be frost deposits produced by water escaping from the interior. The crater density is generally lower than on Mimas. Dione shows leading- and trailing-hemisphere asymmetries in albedos, probably caused by "gardening" effects due to the sweeping up of postaccretion-phase debris.

Rhea also shows large albedo variations and has wispy markings. Differences in the size-frequency distribution of craters in bright and dark terrains indicate that the darker terrain is older. Hyperion is relatively small, irregular in shape, and heavily cratered. The long axis is not oriented toward the planet as would be expected in the dynamically stable case. Iapetus's albedo varies from 0.1 to 0.5 for the leading and trailing hemispheres respectively. The dark hemisphere is quite red, similar to the Jovian satellite Callisto. This hemisphere may be the result of selective impacts by carbonaceous chondritic material, but the albedo boundary is inexplicably sharp. Phoebe is in an eccentric, retrograde orbit expected of a body captured by the Saturn system rather than being formed there. It is a very-low-albedo, irregular-shaped body much like C-type asteroids.

The coorbital satellites Epimetheus and Janus occupy essentially the same orbit between Mimas and the F ring. Their orbital radii differ by less than their diameters, but collisions are prevented by mutual gravitational interaction. These satellites are both irregular in shape and cratered. They were perhaps once one body that was torn apart by a large impact. The satellite Atlas orbits just outside the A ring and acts as a gravitational barrier that defines the outer edge of that ring. The satellites Prometheus and Pandora orbit just inside and outside the F ring and probably confine the particles in that ring. These two "shepherding" satellites control the F ring. Telesto and Calypso are in orbits with periods nearly the same as that of Tethys; they are located near the stable Lagrangian points 60 degrees ahead (L4) and 60 degrees behind (L5) Tethys in its orbit. The satellite Electra librates (oscillates) about the Dione L4 point and has an irregular shape.

View a close-up image of Saturn.