Saturn has the most extensive and beautiful ring system of any planet in the solar system.

Discovering Saturn's Rings

When Galileo observed Saturn with his first primitive telescope, he saw bumps on the side of the planet. He was not however able to resolve these bumps into rings. Christiaan Huygens observed Saturn in 1655 with an improved telescope. For the first time he was able to resolve the image well enough to identify the rings.

What Are Saturn's Rings Made of?

Saturn's rings are composed of small chunks of ice individually orbiting Saturn. We know that orbiting chunks form the rings because their Doppler shift gives us their speed. The speed is correct for individual particles orbiting Saturn.

Reflecting about 80% of the light that strikes them, the brightness of Saturn's rings tells us that they are chunks of ice rather than rock. The chunks of ice are as large as a few dozen meters down to less than a millimeter.

How Big is Saturn's Ring System?

At about 275,000 kilometers, the diameter of the easily visible portions of Saturn's ring system is a little less than the distance between the Earth and Moon. The outermost E ring with a diameter of 960,000 kilometers is larger but not easily visible from Earth.

Despite this large size, the rings are very thin. The rings are at most a few hundred meters thick and possibly as thin as tens of meters. When Saturn passes in front of a star, it is possible to see the star's light shining through the rings.

How Many Rings Does Saturn Have?

Prior to the space program, astronomers observed relatively little structure in the rings. The individual ring structures were named using the first few letters of the alphabet. The largest ring structures, the A and B, rings were separated by a dark band named the Cassini division, after it's discoverer.

When NASA's Voyager probes flew by Saturn, photos of the rings revealed detail beyond the wildest expectations of astronomers. The larger A and B rings had structure consisting of tens of thousands of small individual rings. The smaller F ring had poorly understood kinks and braids.

Two of the most likely causes of Saturn's ring structure are resonances with Saturn's larger moons and small shepherd moons.

Dark spokes in the rings move at the rate Saturn rotates on its axis rather than at the rate particles orbit Saturn. These dark spokes are probably metallic and swept along by Saturn's magnetic field as the planet rotates.

Origin and Age of the Rings

The Roche limit, about 2.4 times Saturn's radius, is the minimum distance a stable moon can exist. Inside this limit tidal forces (the difference in gravitational forces between the two sides of the moon) from Saturn will pull the moon apart. Saturn's rings fall inside the Roche limit.

The individual particles comprising Saturn's rings cannot be a stable moon. Either tidal forces ripped apart a moon, or prevented one from forming.

Voyager and Hubble observations of Saturn's rings led astronomers to think that the rings were relatively young and were perhaps the result of a moon ripped apart by tidal forces. The most recent observations from the Cassini mission however suggest that the rings are composed of very old material that is continually recycled. If this idea proves correct, the rings may be debris that was unable to collect into a moon.

Whatever their origin, Saturn's rings are truly the crown jewels of the solar system.

Further Reading

Chaisson, E, and McMillan, S. Astronomy Today 5th ed., Pearson, 2005.