Ring Structure of Saturn
We have a lot to learn before we fully understand planetary rings. Saturn's rings are the brightest and therefore more famous than Jupiter's and Uranus' rings. The total number of rings is seven, and each one was given a letter between A and G for its name. Three of the rings, A, B and C, are visible from Earth with a telescope.
Saturn's rings were first discovered by Galileo in the 1600's, although at the time he didn't know what they were. In 1655, the astronomer Christian Huygens predicted that Galileo had seen rings. Later on, more powerful telescopes proved Huygens right.
In 1675, a scientist named Cassini found what appeared to be a gap between the A and B rings. This gap was later called the Cassini division. In the 1800's a third, faint ring was found and named C. It wasn't until 1979 that we found the E, F, and G rings, when the Pioneer 11 and Voyager spacecrafts flew by Saturn. They also found a smaller gap between the A and F rings, called the Encke division.
These rings still remain somewhat of a mystery to scientists. We now know that Saturn's gravity keeps the small particles that make up the rings in place. It also prevents the chunks of ice and rock from combining to form moons. Every planet has something called a Roche limit, which is a specific distance away from the planet. Depending on the planet's gravitational force, anything inside the limit can't combine to create larger objects. That is why the rock particles are mostly a few centimeters in size. However, most of Saturn's moons are outside of the Roche limit, which is why they stay together.
Two of Saturn's moons, Prometheus and Pandora, are called shepherd satellites. They are both tiny moons located on either side of the F ring. They work against each other, pushing the particles inside the F ring towards the middle of the ring. The result is a small, compact F ring.
An enormous new "ring" was discovered in 2009. The tenuous Phoebe Ring is about 100 times larger than the main ring system. The ice and dust in the ring apparently comes from the odd moon Phoebe, and may cause the strange coloration of the surface of Iapetus.