Planetary Ring Systems
Structure Of The Rings
Prior to the Voyager mission, astronomers thought that Saturn had at most six different rings, labeled A through F. Voyager photographs show an amazing amount of unexpected detail in Saturn's rings. There are hundreds of individual ringlets in the 43,470 mi (70,000 km) wide main rings. The smallest may be as small as the 1.2 mi (2 km) width that the Voyager camera was able to resolve. (An even finer structure was discovered by another Voyager instrument which monitored brightness in a star that was occulted by the rings.) The very narrow F ring appeared braided to the Voyager 1, but the braids disappeared for the Voyager 2 nine months later.
Most of the complex structure appears to be the result of the combined gravitational forces of Saturn's many moons. Astronomers think that Saturn's moons cause resonance effects that perturb ring particles out of positions where the particles would have orbital periods exactly equal to a simple fraction (e.g., one-half, one-third, etc.) of the period of one of the moons, thus creating gaps. Two small moons may also act together as shepherding moons to confine ring particles to a narrow ring. Shepherding moons have also been observed in the rings of Uranus. Some of the ringlets of Saturn are spiral-shaped, rather than circular, and are thought to be created by spinal density waves, again triggered by gravitational forces due to the moons.
In addition to the many ringlets, Saturn's rings also showed unexpected spokes, pointing away from the planet, that do not travel around Saturn at the orbital speed as ring particles do. These dark spokes appear to be small particles that are swept along by Saturn's magnetic field as the planet rotates.
Saturn's rings are highly reflective, reflecting roughly 60% of the incident light. Therefore, the individual ring particles are probably ice or ice coated. These chunks of ice average about 3.3 ft (1 m) in diameter, with a likely range of sizes from dust grains to about 33 ft (10 m). The total mass of the rings is about 1016 kg, roughly equivalent to an icy moon 6.2 mi (10 km) in diameter.
The ring systems of Uranus and Neptune are much less extensive. One of Uranus' 11 rings is 1,553 mi (2,500 km) wide, the rest are only several kilometers wide. The widest of Neptune's five rings is 3,726 mi (6,000 km). These rings are narrower and more widely separated than those of Saturn. The individual particles are much darker, reflecting only 5% of the incident light, so they are more likely dark rock than ice. Jupiter's ring is composed of tiny dark dust grains produced by erosion from the inner moons.
There is still much we don't know about planetary rings. What is their origin? Are they short lived or have they lasted the five billion year history of the solar system? What causes the structure in the ring systems? The Voyager mission represented a beginning to our study of planetary rings. Future space missions will help us better understand ring systems.
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Paul A. Heckert