VI.

The Ring System

When the Italian scientist Galileo saw Saturn’s ring system through a small telescope in 1610, he did not understand that the rings were separate from the body of the planet. He described the rings as handles (ansae). The Dutch astronomer Christiaan Huygens was the first person to describe the rings correctly. In 1655, desiring further time to verify his explanation without losing his claim to priority, Huygens wrote a series of letters in code, which when properly arranged formed a Latin sentence that read in translation, “It is girdled by a thin flat ring, nowhere touching, inclined to the ecliptic.”

The rings are named in order of their discovery, and from the planet outward they are known as the D, C, B, A, F, G, and E rings. The Cassini probe discovered an additional faint ring between the G ring and the F ring in 2006. The main rings are now known to comprise more than 100,000 individual ringlets, each of which circles the planet. The visible rings stretch out to a distance of 136,200 km (84,650 mi) from Saturn's center, but in many regions they may be only 5 m (16.4 ft) thick. They are thought to consist of aggregates of rock, frozen gases, and water ice ranging in size from less than 0.0005 cm (0.0002 in) in diameter to about 10 m (33 ft) in diameter—from dust to boulder size.

The apparent separation between the A and B rings is called Cassini's division, after its discoverer, the French astronomer Giovanni Cassini. Voyager's television showed five new faint rings within Cassini's division. The wide B and C rings appear to consist of hundreds of ringlets, some slightly elliptical, that have ripples of varying density. The gravitational interaction between rings and satellites, which causes these density waves, is still not completely understood. The B ring appears bright when viewed from the side illuminated by the Sun, but dark on the other side because it is dense enough to block most of the sunlight. Voyager images have also revealed radial, rotating spokelike patterns in the B ring. These spokelike patterns appear to be seasonal and were not visible when Cassini began orbiting Saturn in 2004. The patterns may be caused by electrostatic effects that elevate tiny particles above the ring plane. The spokes may reappear when the angle of the rings to the Sun changes.

Scientists continue to debate the age of the rings. The planet Saturn itself formed about 4.5 billion years ago. The relatively fresh appearance of the rings as seen by Voyager seemed to indicate that the rings were created as recently as 100 million years ago, a time when dinosaurs roamed the Earth. Results from the Cassini probe, however, suggest to some researchers that parts of the ring system may date back billions of years. The material in the ring is constantly being recycled, forming small moonlets that later break up, accounting for the fresh and bright appearance of many of the particles.

One theory about the origin of the rings is that a comet or an asteroid smashed a small moon that orbited the planet. The debris from the collision spread out to form the main body of the rings (rings D through A). Images from Cassini confirm that the rings contain chunks and particles of rock and ice in a full range of sizes as expected from a collision. Similar collisions with small moons may have happened at different times, supplying material to the rings. The tenuous G ring is associated with an arc of icy material that may supply its particles. The faint E ring that stretches from the orbit of the moon Mimas past the orbit of the moon Rhea is mainly made of tiny particles released by active geysers on the moon Enceladus and is constantly being renewed.

The gravitational pull from some of Saturn’s moons helps shape parts of the rings. The Cassini division is thought to be caused by the moon Mimas, which orbits Saturn once for every two orbits made by ring particles near the gap. Some rings are shaped by small so-called shepherd moons that orbit inside gaps in the rings or along the edges of rings. The gravitational pull of some of these small moons tends to keep the ring debris in place. In other cases, small moons keep some rings narrow or cause the rings to have braided or scalloped shapes.