Saturn's magnificent rings are one of the greatest mysteries of the solar system. Other planets—Uranus, Neptune, and Jupiter—have rings. But none are as massive, complex, or diverse as Saturn's rings, which make them the solar system's best laboratory for studying planetary rings in action. Cassini's mission includes orbits ideally suited to studying the rings, especially during the last year of its four-year tour. At that time, the spacecraft will orbit almost directly over Saturn's poles, providing scientists with excellent views of the rings.
Scientists using Earth-based telescopes discovered five of Saturn's seven rings, including the four main rings (A, B, C, and D), which extend from Saturn's upper atmosphere outward about 64,000 kilometers (40,000 miles). The fifth ring is the scattered E ring, which extends from just inside the orbit of Enceladus to as far away as Titan's orbit. NASA's Pioneer-Saturn spacecraft, which flew by Saturn in 1979, found the narrow F and G rings.
The Voyager probes discovered that the rings look like thousands of narrow ringlets. However, the main rings (except for a few gaps) are actually continuous rings that change in optical depth (the number of ring particles per unit area), giving the appearance of individual ringlets. In other words, the main rings' particles orbit closer together in some places and farther apart in other places, creating areas that look like alternating dense and less dense ringlets. The apparent ringlets range in size from about 2 to 100 kilometers (1 to 60 miles) in diameter. Cameras aboard the Voyager spacecraft also revealed that Saturn's rings consist of icy particles that may be as tiny as dust grains or as large as chunks the size of large houses or even mountains. The rings themselves, however, are paper-thin, no more than 90 meters (300 feet) thick in most places.
Cassini's cameras have photographed Saturn's rings in greater detail than ever seen before. At their best, the cameras can obtain a resolution of about 90 meters (300 feet), about the length of a football field. (Resolution is the ability of a lens to produce separate images of objects that are close together.) The images have revealed, for example, the existence of seven new ringlets, five orbiting within gaps between main rings.
Cassini scientists have marveled at the images of such unexpected and intriguing ring features as straws and ropy areas. These structures appear to be long clumps of particles that, for some unknown reason, have become jammed together. The straws extend from 1 to 2 kilometers (0.62 to 1.2 miles) in length, while the ropy areas may be 9.6 to 19 kilometers (6 to 12 miles) long.
Cassini's cameras also revealed the “wake” created by the tiny moonlet Pan, Saturn's innermost moon, as it speeds like a motorboat in water through a passage in the A ring known as the Encke Gap. In the images, the wake appears as scalloping along the inner edge of the gap. Scientists believe that Pan maintains the Encke Gap by sweeping away any particles that fall into it. After Cassini entered Saturn's orbit, the cameras discovered three faint new ringlets in the Encke Gap. These ringlets suggest that Pan is not the only moonlet maintaining openings through the Encke Gap. Cassini's cameras also discovered a new moonlet in the Keeler Gap, a gap near the outer edge of the A ring.
Cassini has shed some light on the development of the rings. The rings may have formed from fragments of one of Saturn's moons after a collision with another moon or with a comet, meteor, or other object traveling through space. They also may be the remains of comets or meteors that ventured too close to Saturn and were captured and then torn apart by Saturn's gravity.
Before Cassini, scientists knew that the rings are only a few hundred million years old, much younger than Saturn and the rest of the solar system, which formed about 4.6 billion years ago. However, Cassini found that some regions of the rings contain fresh material that was released in the last 10 million to 100 million years. Where did these additions come from? The fresh material might have been created when a larger ring particle was broken apart, releasing cleaner, purer material.
Cassini also discovered that the rings might lose material to Saturn's moons. On Oct. 29, 2004, Cassini cameras caught the moon Prometheus stealing particles from the slender F ring. Prometheus—and its partner, Pandora—are among Saturn's shepherd moons, pairs of moons that orbit a short distance apart, keeping ring material confined between them. As Prometheus reached its closest approach to the F ring, a thin streamer of ring particles linked the two. These images showed, for the first time, the complex interaction between the shepherding moons and the F ring.