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Saturn's ringsA system of coplanar rings in Saturn's equatorial plane, with an overall diameter of possibly 960 000 km, measured to the outer edge of the outermost (E) ring and including the space occupied by Saturn itself. The ring system is tilted at nearly 27° to Saturn's orbital plane. It was seen indistinctly by Galileo in 1610 and was recognized for what it was by Christiaan Huygens in 1656. Huygens published a description of the ring system three years later. But the true nature of the rings remained a matter of speculation until Pioneer 11 and Voyagers 1 and 2 visited Saturn in 1979–81.
The seven discernible main rings are now known to consist of thousands of narrow ringlets arranged concentrically and containing innumerable individual particles, chiefly of water ice. The particles are up to several centimeters or, in ring B, even a few meters in size, and each is a satellite of Saturn. The particulate nature of the rings is confirmed by the fact that the inner edge of each ring orbits Saturn faster than the outer edge. The rings are in general less than 1 km thick and almost disappear when edge-on to the Earth, as happens every 15 years or so, when the Earth crosses the rings' plane. Properties of the seven rings are given in the table.
The rings are designated by alphabetical letters reflecting the order of their discovery. The very faint and tenuous D ring is the closest to Saturn. There is no truly well-defined inner edge so that it would appear to extend right down to the cloud tops of Saturn. The C (or Crepe) ring, also known as the Dusky ring, fills the gap between the D ring and the inner edge of the brightest ring, ring B, at 25 000 km above the cloud tops. It is a complicated, ‘grooved’ region with a large number of discrete ringlets. There are at least two particularly noticeable gaps, the outer of which, the Maxwell Gap, is 270 km wide; these gaps contain narrow eccentric ringlets. Ring B is the brightest part of the entire ring system and is opaque when seen from the Earth. In the central portion of the ring spokes have been seen, measuring 10 000 km by 2000 km wide. The spokes move radially and they are thought to be created by the interaction of the local magnetic field with the particle distributions in the rings. There are also electrostatic (lightning) discharges in this region that may also be related to the spoke formation. The particles in ring B are reddish in color and may be as large as a few meters in diameter. These particles are quite different from those of the C and D regions.
The Cassini Division separates the B and A rings and is one of the most prominent features of the ring system. It is not empty of particles, but those to be found there are probably dust grains rather than specs of ice. The origin of the 4200-km gap may be related to the perturbation effect of the satellite Mimas, which is in a 2:1 orbital resonance with particles at the inner edge of the Cassini Division and the particles in the B ring area. A 285- to 400-km wide region near the inner edge of the Cassini Division, the Huygens gap, was discovered on Voyager photographs. The A ring is not as bright as its inner neighbor. It contains a large number of ringlets and minor gaps. These are quite separate from the principal division in the A ring, known as the Encke Division. This gap also contains several ringlets, and although only 325 km wide is visible from the Earth with moderate telescopes. The 35-km wide Keeler Gap lies in the outer A ring. The outer edge of ring A is a sharp boundary at a distance of 77 000 km above the cloud tops, which may be related to the presence of a tiny shepherd satellite, Atlas. The four rings have a thickness of 10–100 meters and are surrounded by a large rarefied cloud of neutral hydrogen extending to about 60 000 km above and below the ring plane.
Beyond the main rings are several diffuse ring systems. The braided F ring is not circular and its width varies over a range of 30–500 km. Two shepherd satellites, Prometheus and Pandora, are located on each side of the F ring, and in 2004 the Cassini/Huygens spacecraft discovered three other satellites in the F ring's vicinity, and two tenuous rings: R 2004/S2, close to Prometheus, and R 2004/S1, co-orbital with Atlas. Still farther out is the tenuous G ring , lying between the orbit of Mimas and that of the two coorbital satellites Janus and Epimetheus. The distance between the F and G rings is about 30 000 km. Finally there is the E ring , which is very wide, possibly 300 000 km, and like the G ring is much thicker than the other rings; the brightest part of the E ring lies just inside Enceladus' orbit.