Fabulous Saturn, showpiece of the solar system is the sixth planet from the sun. It is also the second largest planet exceeded only by Jupiter in that regard. Named after the Roman god of agriculture, Saturn is roughly 10 times the diameter of Earth and almost 10 times as far removed from the sun. Extremely massive at 95 times Earth's mass, it is still the least dense of all the planets, for that mass is achieved with a volume that is 763 times that of Earths.
Mankind has been aware of Saturn from ancient times; it is not clear which culture make the discovery. It is generally accepted that Galileo was the first to view it through a telescope, realizing that the planet had a distinctly peculiar appearance without actually discerning the rings. That honor went to Christiaan Huygens in 1659. Saturn’s beautiful and complex ring system make it the most easily recognized of all the components of the solar system; for well over 300 years it was thought that Saturn alone possessed this phenomenon.
In 1977, the faint rings of Uranus were discovered, those of Jupiter and Neptune shortly thereafter. Still, none of the other planetary ring systems approaches the splendor and complexity of that belonging to the sixth planet.
The ring system is indeed complex, because it involves not only Saturn and the rings but also a growing flock of moons and moonlets ranging in size from mighty Titan to tiny bodies not yet named. There are more than 60 recognized at present and with the Cassini space vehicle still on the job around Saturn and more missions on the way the list is expected to grow.
Some of the moons have profound influences on ring structure and these moons as a group are known as “guardian” or “shepherd” moons. Both descriptions are apt because these moons help the rings maintain their form and structure, as well as that of the gaps between like the Cassini division or the Encke and Huygens gaps.
Some of the guardian moons also cause unusual ring structures apparently out of all proportion to their size. Daphnis for example has barely a five mile diameter and in located in the Keeler gap on the outer edge of Saturn’s A ring. Its miniscule gravitational field is yet sufficient to perturb the ring to the point of generating “waves’ one mile tall on a thin ring edge barely 30 feet in thickness. Of course, the rings are composed primarily of dust and ice crystals and are sensitive to mild gravitational fields as a result.
The most dramatic interaction between the guardian moons and the rings is the function of keeping the rings on track, by forcing the particles to stay in their own grooves, if you will. The method by which this is thought to occur is quite interesting and is tied to the physics of energy and motion.
At first glance, it might seem that a ring with an orbit which is enclosed by two guardian moons, one inside its orbit, closer to Saturn, would be drawn inward towards the planet by the gravitation of the inner moon. Similarly, one might expect the outer moon to draw the ring outward. In reality, just the opposite is what takes place. The ring is pressed outward by the inner moon and inward by the outer one.
Objects, whether they are moons or ring components closer to the planetary body will orbit at faster speeds than will objects farther out and will therefore possess a higher kinetic energy, or energy of motion. The outer object by virtue of being farther from the planet; will have a greater potential energy. When one adds potential energy and kinetic energy together, it turns out that potential energy is the larger component, and that the object farther away has the greater total energy.
Changing the total energy of an orbiting body changes the distance of the object’s orbit from the planet; increasing the energy moves it out, decreasing it draws it in. And this is just what the guardian moons do.
The inner moon is moving faster than the particles of the ring closest to it, and drags them ahead with its gravitational field, increasing each particles total energy and speed and forcing them farther from the planet.
The outer moon’s gravitational field however, drags the ring particles backwards as it is moving more slowly then they are, decreasing the total energy and dropping the particle closer to the planet. The net result is a well groomed and shepherded ring, courtesy of the guardian moons!
There are many, many rings within the total Saturnian ring system, with more being discovered all the time. More and more moons and moonlets are being discovered as well as more space mission vehicles like Cassini and the Voyagers stop by and take their measurements and pictures. All of this information leads to greater understanding of the fascinating mechanics of Saturn, its rings, and its guardian moons.