Image Credit: NASA/JPL/Space Science Institute
Saturn’s ring system is divided up into 7 major divisions with alphabetic designators in the order of discovery. From the innermost ring to the outermost ring the designators are D, C, B, A, F, G and E. With a thickness of about one kilometer (3,200 feet) or less, they span up to 282,000 km (175,000 miles). Each major division is further subdivided into thousands of individual ringlets.
The F and G rings are very thin and difficult to see while the A, B, and C rings are broad and quite visible. Between the A and B rings is the largest gap called the Cassini Division, a gap measuring 4,700 km (2,920 miles), which actually contains fainter rings.
Each ring orbits at a different speed around the planet.
The system is composed of 93% water ice with traces of simple organic compounds as methane or ethane and 7% amorphous carbon.
There are billions of ring particles in the entire ring system. The total mass in the rings is about the size of a medium mass moon. The sizes of the particles in the rings vary, from microscopic particles in the E and D rings, to sand- and pebble-sized particles in the C and F rings, to cobble- and boulder-sized particles in the A and B rings. Some particles in the B ring could be much larger, up to a few tens of meters across.
Frequent collisions between ring particles would tend to break big chunks into smaller ones, but big chunks may gather smaller chunks around them under the force of gravity. Little moonlets (particularly small natural satellites within the rings) may constantly form and be broken apart. Cassini’s images of the F ring may have shown this process in action.
The rings show a tremendous amount of structure on all scales; some of this structure is related to gravitational perturbations by Saturn’s many moons (62 known moons orbit the planet; 53 are officially named, not including the hundreds of, but much of it remains unexplained.
The Voyagers found that the rings were not necessarily circular, and even found rings that appeared to be braided. They found further that the outer ring was kept in place by the gravitational interaction of two small shepherd moons lying just inside and outside it, and that at least some of the other rings are kept narrow by similar small shepherding satellites.
Voyager 1 and Hubble saw “spoke” features running across the B ring, but they had disappeared by 1998. Cassini first sighted the spokes in the summer of 2005. That timing coincides with the changing of Saturn’s seasons.
There are two main theories regarding the origin of the rings. One theory is that the rings are remnants of a destroyed moon of Saturn, pieces of comets or asteroids that broke up before they reached the planet.The second theory is that the rings are left over from the original nebular material from which Saturn formed. Some ice in the central rings comes from the moon Enceladus’ ice volcanoes.