Planetary Ring Systems
Planetary Ring Systems
Planetary ring systems are large numbers of particles orbiting about a planet in a flat disc-shaped region. A peek at Saturn through a small telescope reveals the solar system’s jewelry, a breathtaking system of rings. These rings consist of a large number of individual particles orbiting Saturn. The diameter of Saturn’s ring system is about 167,670 mi (270,000 km), a little less than the distance between Earth and the moon. Yet the rings are only a few hundred meters thick. Saturn has the only ring system that can see directly from Earth. Jupiter, Uranus, and Neptune, do however all have ring systems. So rings do seem to be a common feature of giant gas planets.
Italian astronomer and physicist Galileo Galilei (1564–1642) almost discovered Saturn’s rings in 1610. His new telescope revealed something on either side of the planet. Galileo’s drawings almost look as if Saturn had grown a pair of giant ears. Galileo was seeing but not quite resolving the rings of Saturn. In 1655, Dutch mathematician Christiaan Huygens (1629–1695) correctly described Galileo’s appendages as a flat system of coplanar rings that were not attached to Saturn. In 1675, Italian astronomer and engineer Giovanni Domenico Cassini (1625–1712) first noticed structure in the ring system, a gap now called Cassini’s division. He also first suggested that the rings are composed not of a solid body but of individual particles orbiting Saturn. In the nineteenth century, Scottish physicist James Clerk Maxwell (1831–1879) proved mathematically that Cassini’s suggestion must be correct. In 1895, American astronomer James Edward Keeler (1857–1900) observed the orbital speed of different parts of Saturn’s rings, finally proving they are a large number of individual particles. In 1980, the Voyager spacecraft sent back amazing pictures of the rings, showing a wealth of detailed structure.
The rings around Uranus were discovered next. On March 10, 1977, four groups of astronomers observed Uranus pass in front of a star and occult it, in hopes of learning something about Uranus as the starlight dimmed. To their surprise, the star winked, several times, both before and after the occultation. Winking once would suggest a moon, but several symmetric winks before and after suggested rings. The group of astronomers from Cornell University, led by James Elliot, obtained the most complete data and discovered five rings. In 1978, four additional rings were found during another occultation. The Voyager flyby in 1986 confirmed the previously discovered rings and found two more for a total of 11. The rings of Uranus were later observed from the Earth, with infrared telescopes, which reveal the long wavelength emission from the icy ring particles. On August 14, 1994, the repaired Hubble Space Telescope photographed Uranus showing, but not fully resolving, the rings.
In 1979, the Voyager 1 and 2 flybys discovered a very thin ring around the planet Jupiter that is not observable from Earth. By 1979, Saturn, Uranus, and Jupiter were known to have rings. What about Neptune? Voyager 2 did not fly past Neptune until 1989. To avoid waiting ten years to see if Neptune had rings, astronomers observed occultations of Neptune. Perhaps rings could be discovered indirectly from Earth as for Uranus. Some observations seemed to show rings; others did not. The mixed results suggested partial rings. In 1989, the Voyager photographs finally revealed that Neptune does indeed have a ring system. However, the rings vary in width. Narrower parts of the rings would be harder to detect from Earth, so the occultations gave mixed results. It is not know why these rings vary in width.
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 that 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.
Occultation— When a moon or a planet passes in front of a star.
Rings— Systems of particles orbiting a planet.
Shepherding moons— Small moons thought to confine ring particles to a particular ring by their gravitational forces.
Voyager— A pair of unmanned robot spacecraft that left the Earth in 1977 to fly by all the gas giant planets (Jupiter, Saturn, Uranus, and Neptune).
Saturn’s rings are highly reflective, reflecting roughly 60%percnt; 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 astronomers do not 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 the scientific study of planetary rings. Future space missions will help scientists better understand ring systems.
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Paul A. Heckert