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Saturn and Its Rings by Andrew Fraknoi

"But, of course, Saturn does have rings—a spectacular golden array of rings that stretch as far from edge to edge as the distance from the Earth to the Moon, yet may be no thicker than a coconut palm is tall." — Timothy Ferris in Seeing in the Dark (book version, Simon & Schuster 2002)
Saturn
Voyager 2 true-color picture of Saturn, taken on July 21, 1981, 21 million miles away. (JPL/NASA)

Perhaps the most imposing sight you can see through a telescope is the planet Saturn, with its magnificent system of rings. Many dedicated amateur astronomers say they were turned on to stargazing by seeing Saturn through a telescope. And Saturn is impressive in more ways than just its looks.

One of the giant planets in the outer solar system, Saturn contains enough material to build 95 Earths. Its diameter is 75,000 miles—large enough to fit more than nine Earths across it. Yet for all its bulk, Saturn is a lightweight planet. Made mostly of the simplest gases in the universe, its average density is less than that of water—so if you had a bathtub big enough, Saturn would float in it.

The giant planet takes only about 10 hours to spin once—a day less than half of our much smaller Earth's. This rapid spin plus heat rising from its interior create powerful and complex weather systems in Saturn's atmosphere. The storms get particularly intense every 30 years—one Saturnian year—when summer comes to the ringed planet's northern hemisphere. In 1990, the largest of the season's storms could be seen even with modest telescopes. It was amateur astronomers who first alerted professional scientists that a huge storm was brewing. It eventually spread to encircle the Saturn globe.

These days, a sophisticated spacecraft called Cassini is conducting close-up observations of the Saturn system, orbiting the planet and recording images and data about the atmosphere, the rings, and Saturn's moons. You can see some of the pictures it is sending back at: saturn.jpl.nasa.gov/multimedia/images/index.cfm

Saturn has a swarm of moons that interact with the rings. As of mid-2007, 59 moons had been identified, most of them relatively small. One moon, however, called Titan, is the second largest in the solar system, and bigger than Mercury, Eris, and Pluto. It has a smoggy atmosphere and rivers of liquid swamp gas (methane) appear to flow on its cold surface. Bright and rather ruddy looking, Titan can be readily discerned out beyond the rings; it looks like a star in smaller telescopes, while larger ones show it is a disc.

Saturn's rings are made up of billions of icy pieces, organized by their mutual gravity and the interfering gravity of nearby moons into thousands of strands and ringlets. The ring particles range in size from smoke particles to the bulk of a small truck. Occasionally, an especially strong interaction between a moon and the ring particles will produce a noticeable gap in the rings; some of the gaps appear in the pictures on this page. When the edge of a ring or gap is especially sharp, there is a good chance that a shepherd moon is responsible. Just as shepherds and shepherd dogs keep flocks of sheep from straying, shepherd moons keep ring particles from moving away from the orbit they mark out, leaving a clear ring edge in their wake.

Spokes in Saturn's rings
Stephen O'Meara's Drawing of the Spokes in Saturn's Rings

As described on Seeing in the Dark, in the mid-1970's, Stephen O'Meara, a young amateur astronomer with phenomenal observing skills, noticed dark radial features on the rings of Saturn that reminded him of spokes in a bicycle wheel. Although they had been spotted by a few earlier observers, the consensus was that they had to be optical illusions. Scientists reasoned as follows: Every chunk of ice in the rings orbits Saturn at its own rate, with inner bodies—which, being closer to Saturn, experience a stronger force of gravity—moving faster than the outer ones. If you flew a spaceship over the rings and painted a radial line across them, the inner part of the line would move ahead while the outer part lagged behind, and the line would quickly disappear. So O'Meara was unable to get his drawings of the spokes published. Nobody took them seriously.

Then, in 1979, the Voyager 1 spacecraft flew by Saturn and took close-up pictures of its rings with unprecedented detail. Clearly visible on some of the pictures were long straight spokes across the rings, much like the ones O'Meara had sketched.

Saturn's rings
Voyager 2 image of the spokes in Saturn's B Ring (JPL/NASA)

No one fully understands the reason these spokes exist, but the fact that they do NOT drift apart means they must be connected with the spin of Saturn and not with the motion of the many ring chunks. One theory, mentioned in the film, is that Saturn's magnetic field captures any small pieces of icy dust that have an electric charge and levitates them above the rings while stringing them out into straight marks that trace lines in the magnetic field. Magnetically lifted above the main rings, such particles respond to the rotation rate of Saturn's magnetic field rather than of the rings themselves.

The evidence suggests that spokes appear mainly during certain seasons of the long Saturn year, perhaps in response to the changing angle at which sunlight hits the rings. This may explain why the Cassini space probe, unlike Voyager, found only a few, dim spokes. The facts are hard to pin down, but what is clear is that the patient observations of a young amateur astronomer helped generate an intriguing new branch of Saturn ring science, and that the Lord of the Rings is very likely nowhere near finished spinning puzzles for Earth's observers.

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