The Rings of Saturn

PIA17172 Saturn And Its Rings with Earth, Mars, and Venus

Image Credit:

NASA / JPL-Caltech / Space Science Institute

 

The first image that pops in people's heads when they think of Saturn is probably a planet with rings. And they wouldn't be wrong. Saturn's most famous feature are its impressive rings and in my opinion, the most striking feature of any planet in our Solar System. So what exactly are the rings and where did they come from?

The rings were first discovered by Galileo in 1609 when he saw these objects on the sides of Saturn that he called "ears". His telescope was not good enough to resolve the ears into a disk and to see the rings clearly. It wasn't until 1659 when Christian Huygens was able to resolve the rings into a disk and see that the rings were not attached to Saturn physically.

 For the next few centuries, it was believed that the rings were a solid torus around Saturn. No evidence was observed to make anyone think differently. In 1859, James Clerk Maxwell, famous for his four equations of electromagnetism (which you can learn about here), proved mathematically that a solid ring would be unstable and not be able to orbit around Saturn. It wasn't until 1895 that two astronomers, James Keeler at Allegheny Observatory outside of Pittsburgh, PA (where I used to work) and Aristarkh Belopolsky of Pulkovo Observatory near Saint Petersburg, Russia, independently spectroscopically determined that the rings were not solid, but made up of many particles. Using the Doppler effect, which is that effect that causes waves to change wavelengths based on the speed of the observer, the source of the wave, or both, they both were able to show that the outer rings travel slower than the inner rings. This would not be possible if the rings were solid. If the rings were solid, the angular velocity of the inner parts of the rings and the outer parts of the ring would have to be the same, and they showed that this was not true. Maxwell's mathematical prediction was true.

The rings themselves are made up of both rocky dust and ice particles, depending on where in the ring structure the particles are located. The rings orbit (in general) above Saturn's equator. Much like the rings of Jupiter, the particles must be continually replenished by micrometeorite collisions with the moons of Saturn, adding particles to the rings while parts of the rings are dissipated by Saturn's gravity or the gravity of the nearby moons. The rings are believed to have been first formed when Saturn was formed when small planetessimals were within the Roche limit of Saturn and could not consolidate into moons.

The rings were originally named in the order in which they were discovered, starting with A. But as more were found, the newer ones were given proper names. Starting with the innermost ring, the ring system is broken down in this manner:

• The D Ring: the fourth ring discovered in 1980 by Voyager 1 is a very faint ring system. Its distance from Saturn ranges from 66,900 km to 74,510 km
• The C Ring: the third ring discovered in 1850 by George and William Bond. Its distance ranges from 74,658 km to 92,000 km and will be discussed in more detail in its own post.
• The B Ring: the second ring discovered and the most massive of the rings. Its distance ranges from 92,000 km to 117,580 km and will be discussed in more detail in its own post
• The Cassini Division: a space between the B ring and the A ring discovered by Giovanni Cassini in 1675. Its range is from 117,580 km to 122,170 km and again, will be discussed further in its own post.
• The A Ring: the first ring to be discovered when Huygens first detected the rings as rings. It ranges from 122,170 km to 136,775 km and will be its own post
• The Roche Division: the gap between the A Ring and the fainter F Ring. There is material in this division, but is so thinly populated that we do not see it very well. The moon Atlas orbits in this division. Its range is 136,775 km to 139,380 km.
• The F Ring: thin ring orbiting outside the Roche Division. It has a small range or 30 to 500 km but orbits around 140,180 km from Saturn. It is kept in place by two small moons, Pandora and Prometheus and will be discussed in detail when talking about those two moons.
• The Janus/Ephimetheus Ring: a ring that is maintained by the moons Janus and Ephimetheus. It was discovered by the Cassini spacecraft in 2006. It ranges from 149,000 km to 154,000 km.
• The G Ring: faint ring with a bright inner edge. Halfway between the F Ring and the E Ring, it has the moonlet Aegaeon orbiting nearby. It ranges from 166,000 km to 175,000 km.
• The Methone Ring Arc: not a full ring, but a 10° arc orbiting around Saturn. It shares an orbit with Methone and was detected for the first time in September of 2006. It orbits abour 194,230 km from Saturn
• The Anthe Ring Arc: not a full ring, but a 20° arc orbiting around Saturn, much like the Methone Ring Arc. It shares an orbit with Anthe and was detected for the first time in June of 2007. It orbits abour 197,665 km from Saturn
• The Pallene Ring: shares an orbit with the moon Pallene at around 211,000 km to 213,500 km. It was discovered by Cassini in 2006.
• The E Ring: the last of the lettered rings, though the fifth discovered. The second outermost ring, but the outermost orbiting equatorially with Saturn. It is very wide and is between the orbits of Mimas and Titan. There are moons that orbit within the ring and they are tinted by particles from the ring. It orbits between 180,000 km to 480,000 km, by far the widest of the rings.
• The Phoebe Ring: the outermost ring orbiting just to the interior of the moon Phoebe. It was discovered in October of 2009 by NASA's infra-red Spitzer Space Telescope and orbits at an angle of 175° to the equator of Saturn, so it also orbits retrograde. It orbits between 4 million and 13 million km from Saturn and will be discussed more in its own post.