First spied in 1619 by Galileo, who described them as "handles," Saturn's rings remained a mystery until 1655, when the Dutch scientist Christian Huygens realized they were a system of rings around the planet. More than 300 years later, they were unveiled in all their individuality by the Voyager space probes in the 1980s. In the photo at left, the shadows of the A, B, and C rings mark the almost featureless surface of Saturn, shown with the tiny moons Tethys and Dione. The false-color image at right reveals more than 60 bright and dark ringlets in the C ring and suggests the differing surface composition of the particles making them up. Voyager's data also gave scientists clues to the gravitational forces (below) that maintain the narrow bands, spiral patterns, and twisted braids that characterize the rings.

Shepherd Moons

In this greatly exaggerated view, looking directly down on the ring plane, we see the actions of two moonlets on ring particles. A small outer moon drags on particles drifting outward, robbing them of orbital energy so that they drop to lower orbits. The inner moon, traveling faster than particles farther out, tugs the particles in their direction of motion. This transfers angular momentum and boosts the slower-moving particles into higher orbits. The one-two action of the shepherds keeps the particles confined to a narrow band.

Gravitational Waves

In this simplified cross section of a ring, spiral density waves alternately bunch particles and disperse them (near right, center, and far right). Also shown is a bending wave, a phenomenon triggered by interaction with an orbiting moon that pushes the ring sheet into crests and troughs.