Saturn’s rings have posed a big challenge ever since Galileo first laid eyes on them in 1610 through his 20-power telescope. He interpreted his first observation as a spherical planet with two smaller spheres beside it, two years later he no longer saw the smaller spheres, and still later he reported seeing parts of several ellipses (see drawings). He had no way of knowing that he was seeing a thin tilted ring system that can appear quite different from one year to the next as the relative position of Earth and Saturn changes.

Sketches of Galileo’s early observations of Saturn’s rings; first he concluded there was a moon on each side of the planet later he saw half-ellipses.

Huygens in 1625 proposed a thin, flat, solid ring—a rigid disk—which did explain the periodic disappearance of the ring, when Earth lies in its plane and the ring is seen edge-on. In 1675, Cassini observed a gap in the ring system, which is now called the Cassini Division, and he correctly hypothesized that the rings are composed of small particles. This particle model gained support in 1849, when Roche determined the distance, called the Roche limit, at which a satellite held together by gravity would be broken apart by tidal forces.

This Voyager image shows the tiny moon Pan, only 20 km in diameter, barely visible after considerable image enhancement (Mark Showalter and the PDS-Rings Node)

Gravitational forces on the near and far side of a moon, as shown by the two red arrows. The difference in these two forces can rip a moon apart if it is too close to a planet.

Tidal forces originate in the inverse square dependence of the gravitational force on distance. As shown in the diagram, Saturn pulls more on the nearer part of a satellite and less on the more distant part, resulting in a force that tends to rip the moon apart if it is close enough to the planet. For satellites of Saturn, the Roche limit is about 2.4 times the planet’s radius, as measured from the center of the planet, which places nearly all of the ring system within this limit. Solid objects can exist within the Roche limit, since they are held together by more than gravity—by atomic and molecular forces—but moons inside the limit must be small, and indeed the four Saturnian moons within the ring system have radii ranging from 10 km to 50 km, with the smaller of these closer to the planet’s surface.

Also, from Kepler’s Laws, a satellite’s period squared goes as the radius cubed, so the ring particles closer to the planet have shorter periods than those further out, ruling out a solid ring. This differential rotation was first observed in 1895.

Observation of Saturn made a great leap in 1980-1981, when the Voyager 1 and 2 flybys provided a wealth of new data, including incredibly detailed images that delighted both scientists and the public alike. All this new information led to a much better understanding of the details of the ring system, including the explanation of how small “shepherd” moons form some of the narrow gaps. The image, from Voyager, shows the tiny moon Pan, just 20 km across, which produced the gap where it is found.