$$\require{cancel}$$

# 7.9 Ring Systems of the Giant Planets

People have been familiar with the rings around Saturn since the invention of the telescope. Today we know that not just Saturn, but all four giant planets have ring systems. There are many questions about the rings, and we understand only some of the answers. What are they made of? How did they form? Why is Saturn's ring system so much more prominent than the others? What explains differences in ring systems from one planet to another? Rings are complex dynamical systems that we don’t yet understand completely. Someday, the answers to these questions may also lead to a better understanding of the dynamics of other disks of particles. For example, it may inform us about the solar nebula, and the process of planet formation within it.

Saturn's faint ring structures come out during an eclipse. Click here for original source URL

Christian Huygens sketches of Saturn, noting changes in the orientation of the rings as the planet circles the Sun. Click here for original source URL.

Galileo's sketch of Saturn and it's rings. He did not know at the time that these were rings, and he assumed that Saturn was a three planet system. Click here for original source URL.

The beautiful ring system of Saturn has intrigued scientists since 1610, when Galileo first turned his telescope on Saturn. With his primitive optics, he could make out only a fuzzy blob on either side of Saturn's disk, and he drew Saturn as a triple planet. Not until 1655 did Christian Huygens realize that the "blobs" were actually a ring system encircling the planet along its equator. Seen with a modern backyard telescope, Saturn's rings present a changing appearance from year to year in a 29-year cycle. Because the plane of the rings is tipped with respect to the plane of the solar system, we see the rings from different directions as Saturn orbits around the Sun every 29 years.

Saturn's rings are not solid flat sheets, but are composed of billions of separate pieces of ice and rock. We can see stars and the planets themselves through the rings, which illustrates their diaphanous nature. The ring particles are much too small to be seen individually from the Earth. Each particle is a tiny moonlet on its own elliptical orbit around the planet, following Kepler's laws of orbital motion. Kepler’s laws dictate that each particle orbits at a speed that depends on its distance from the planet. So the rings do not orbit as a whole, like a record on a record player. Instead, the inner parts of a ring system orbit the planet faster than the outer parts.

Natural-color mosaic of? Cassini?narrow-angle camera images of the unilluminated side of Saturn's D, C, B, A and F rings (left to right) taken on May 9, 2007. Click here for original source URL.

Saturn's ring structure is very complex. Scientists had expected that the rings would be relatively smooth and uniform, but they were surprised when Voyager's close-up photos showed thousands of individual thin ringlets, often separated by gaps of different widths. The widest gap, the Cassini Division, is easy to see with an amateur telescope from Earth. It’s named after its discoverer, Italian astronomer Giovanni Domenico Cassini, who also discovered four of Saturn’s moons in the 17th century.

Saturn’s ring particles are bright reflective pieces of ice, with some dust and dirt mixed in. They range from dust to golf ball-sized to house-sized. The larger particles are probably rare. A visitor to Saturn's ring system would be surrounded by an amazing swarm of icy bodies orbiting at high speeds around the planet. No probe has yet traveled into such a dangerous environment, although Cassini took a calculated risk in 2004 when it shot through a gap in the rings at 50,000 mph! The spacecraft sustained 100,000 particle hits in just five minutes, but the particles were like cigarette smoke so no damage was done. Although this ring system is 274,000 kilometers (171,000 miles) from tip to tip, dynamical forces keep the rings less than about 100 meters thick. This is incredibly thin — imagine something the size of a pizza that’s a thousand times thinner than a human hair!

Each giant planet has its own distinctive set of rings, although only Saturn's are bright enough to be easily detected from the Earth. The other three ring systems are so faint that they were not discovered until the 1970s and 1980s, primarily by observations from the two Voyager space probes. These rings are so thin they appear almost transparent. This is because the ring particles are spaced so widely apart that very little light is reflected.

Images of the jovian main ring obtained by the New Horizons spacecraft in backscattered (top) and forward-scattered (bottom) light. A faint outer ringlet is just outside the orbit of Adrastea. A gap situated between the orbits of Metis and Adrastea is clearly visible. Metis is just inside the bright outer (~1000 km) part of the ring. Click here for original source URL.

Jupiter's main ring has a sharp outer edge and a diffuse inner edge. It’s much fainter and narrower than Saturn's rings. Inside the main ring is another component of the ring system, the “halo.” As the name suggest, the halo is a broad, diffuse, donut-shaped ring of particles. Outside the main ring, there are two additional “gossamer rings,” even fainter than the main ring. These are bounded by the orbits of two of Jupiter’s small satellites, Amalthea and Thebe. Jupiter’s rings are composed of dark, microscopic dust particles. They are probably composed of rocky, carbonaceous material, as opposed to Saturn’s highly reflective, icy ring particles. The sparseness of the rings and the low albedo of the particles explain why we can’t see them from Earth.

A pair of?Voyager 2?images of Neptune's ring system. Click here for original source URL.

The image is a merge of two images of Uranian rings obtained by Voyager 2 spacecraft in 1986. The forward-scattering image (left) shows dust in the ring system; while back-scattering image (right) shows distribution of larger bodies. ? and ? rings are matched. The difference in the position of the ? ring is caused by its eccentricity. Click here for original source URL.

The rings of Uranus as seen by the Hubble Space Telescope. Click here for original source URL.

Uranus's rings present a different model from Saturn or Jupiter. Here, instead of a broad sheet, the ring particles are concentrated into nine or more separate string-like rings, separated by wide empty spaces. Neptune's rings are similar to Uranus's rings. However, the rings of Neptune have clumps of particles that form distinct arcs. Voyager 2, the only spacecraft to visit the outermost planets, returned images of several new rings around these planets, along with satellites “shepherding” them. Occultations are another way to study these distant rings. When a star passes behind the rings, its light dims momentarily, giving information