Know Your ABCs
Even the most modest telescope reveals the beauty of Saturn's rings. A 60-millimeter (3-inch) refractor at 50-power will clearly show them. [For details on observing the rings, see "Running Rings Around a Planet," Mercury, September/October 1996, p. 9.]
Eerie rings. This is, in effect, a blow-up of the photo above. The black stripe in the outer part of the 'A' ring is the Encke division. The white stripe just outside the 'A' ring is the infamous 'F' ring, discovered by the Pioneer 11 space probe in 1979. 'F' might as well stand for "flummox", because unlike other rings, which are generally smooth and curved, the 'F' ring has lumps, knots, and kinks -- perhaps due to electromagnetic forces and the influence of little moonlets. Photo courtesy of NASA JPL.
From Earth, we can see the three main rings. Ring 'A', the outer band, and 'B', the widest and brightest section, are both visible in any telescope (see photo below). They are separated by the Cassini division, a broad gap that may seem narrow, but is about as wide as North America. The division looks as black as the sky, but does contain some particles. The gap is caused by the gravity of Saturn's moon Mimas. Ring particles orbiting in the gap have what is called a "resonance" with Mimas -- they complete exactly two orbits for every one Mimas makes. On each orbit, the gravity of Mimas pulls the particles a slight amount, and over time these nudges accumulate, largely clearing out the area.
With this ring I thee amaze. From the ground, a telescope can make out the basic structure of Saturn and its rings. The dim 'C' ring barely appears on this image, but is easier to spot on other ground-based images. From above, the rings would look circular, but from our viewing angle they are tilted. Photo courtesy of Lick Observatory.
Ring 'C', the innermost of the main rings, is so dim that it takes a large telescope to reveal it. Also known as the "crepe ring," ring 'C' is a phantomlike structure extending about halfway toward the planet from the inner edge of ring 'B'.
Conveniently for observers on Earth, the rings are tipped by Saturn's 27-degree tilt. Because the rings ride exactly above Saturn's equator, we would only ever see their edge if Saturn had a tilt of, say, 3 degrees, as Jupiter does. As it is, we are treated to a majestic cycle of ring visibility during Saturn's 29-year orbit of the Sun. The current cycle began with the rings seen edge-on last year. They will be fully open with the south side presented to our view in 2002, then edge-on again in 2009. The north side of the rings is at maximum tilt toward us in 2016, then edge-on in 2025.
Saturn itself is not wobbling around to create these changes. Like Earth, Saturn's rotation axis is essentially fixed in space over the centuries. What we are seeing is Saturn's seasons -- first spring and summer in the southern hemisphere, then spring and summer in the northern hemisphere [see "To Every Season There is a Reason," The Universe in the Classroom, winter/spring 1995].
As smooth and simple as the rings seem from Earth, they look much more complicated close-up. When the twin Voyager spacecraft flew by Saturn in the early 1980s, they discovered that the main rings consist of hundreds of ringlets (see photo below). Many of gaps between these ringlets are generated in much the same way as the Cassini division [see "Rally Around the Ring," Mercury, March/April 1995].
Looking groovy. If you still remember what the grooves on a phonograph record look like, then Saturn's rings will remind you of them. The rings consist of hundreds of narrow ringlets and gaps, as shown in this Voyager 2 image, which looks down on the rings at an angle. Most of the ringlets are several hundred kilometers wide. They, in turn, consist of trains of rocks and icy chunks in orbit about Saturn. Photo courtesy of NASA JPL.
A few years before the Voyager flybys, astronomers discovered that the other giant planets -- Jupiter, Uranus, and Neptune -- also have rings, though they are pale facades compared to the sparkling beauty of Saturn's adornment. If any one of these other ring systems surrounded Saturn, it would be almost invisible to us on Earth. The rings of Jupiter are dull and diffuse, those of Uranus and Neptune downright dark -- among the least reflective objects in the solar system. On the other hand, those other ring systems have their own subtle beauty. Uranus's rings, for example, are slender and oval in shape, which indicates that they are engaged in a complex gravitational dance with nearby moonlets.
Once upon a time, Earth might have had a ring, too. But without nearby moonlets to resupply the ring with new material, the ring would have disappeared slowly but steadily as material fell into our atmosphere or was blown out into space.
There is much yet to be learned about Saturn, which is why the Cassini spacecraft is now on its way there. Built by an American and European partnership and launched in October, Cassini is due to reach Saturn in 2004 and swing into orbit for a four-year tour [see "Saturn and Titan on the Eve of Cassini-Huygens," Mercury, September/October 1997]. Soon after it arrives, it will drop a set of scientific instruments into the atmosphere of Titan, Saturn's largest moon.
Titan is the only satellite in the solar system with a substantial atmosphere. In many ways, it is a deep-freeze version of Earth. Its air is much like ours: primarily nitrogen, with almost the same surface pressure, but with the rather brisk temperature of -180 degrees Celsius (-290 degrees Fahrenheit). High up in the atmosphere are thick, orangish clouds -- a smog produced by natural chemical reactions of methane and other compounds. These clouds cloak Titan's surface, so no one knows for sure what is down there. It is probably a mix of methane and water-ice glaciers, with lakes of liquid ethane, a form of liquid natural gas. Given the Earth-like atmosphere and carbon-based compounds, some scientists think living organisms, or lifelike chemical reactions, might lurk on the surface.
Cassini will take tens of thousands of close-up images of Saturn, its rings, and moons. It's safe to say that the exploration of Saturn has just begun.
TERENCE DICKINSON is the editor of SkyNews, the astronomy magazine of Canada's National Museum of Science and Technology. Since he got interested in astronomy -- he says a meteor did it, at age 5 -- he has written a dozen books, produced a weekly astronomy column for the Toronto Star, worked at planetariums in Toronto and Rochester, appeared on numerous radio and TV shows, and served as editor of Astronomy magazine. Last year Dickinson received the ASP's Klumpke-Roberts Award for his efforts to bring astronomy to the public. His email address is email@example.com.
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