Ring Systems Of Planets

Type of physical science: Astronomy; Astrophysics

Field of study: Planetary systems

A planetary ring system consists of relatively small particles that fan out from the planet in the form of a disk, orbiting as a unit around the planet. Planetary rings are common in the outer solar system; each of the four gas giants has a ring system.

Overview

The rings of Saturn are stunning and are perhaps the most beautiful images in the night sky. They were first observed in 1610 by Galileo in Padua, Italy. Yet, Galileo's telescope was not the best of astronomical instruments, and what he sketched were two spheres on either side of the planet. Galileo thought he had discovered that Saturn was a triple planet. The matter was clarified in 1655, when Dutch observer and telescope maker Christiaan Huygens clearly saw Saturn's rings through his improved instrument. Later, in 1675, Jean-Dominique Cassini, supervisor of the Paris Observatory, discovered that there was a structure to the rings--an opening in the ring about two-thirds of the way out from the planet. The gap became known as the Cassini division. The outer ring was called the A ring and the inner ring, the B ring.

The rings of Saturn were of intense interest among scientists through the eighteenth and nineteenth centuries. New gaps and subrings were subsequently discovered and named. In 1789, English scientist Sir William Herschel estimated the rings to be no more than 500 kilometers thick. By 1850, astronomers' telescopes could resolve that the rings were largely transparent and that the edge of the planet could be seen through the formation. In 1848, French mathematician Edouard Roche demonstrated mathematically that if a satellite (moon) orbited too close to a planet, tidal forces from the planet would tear it apart into small pieces (or would not allow a moon to form from small pieces). This limit would become known as the "Roche limit." The rings of saturn fell inside the Roche limit. Roche boldly suggested that Saturn had captured a small satellite that had been broken into billions of tiny pieces.

In 1857, the University of Cambridge offered a prize to settle the question of whether the rings were rigid, fluid, or made up of small pieces of matter "not mutually coherent." Scottish physicist James Clerk Maxwell presented a mathematical argument that won the prize. In his proof, Maxwell demonstrated that any solid ring would be torn apart by gravitational forces. He demonstrated that it could not be a liquid; thus, the rings of Saturn were composed of countless individual particles, each in its own independent orbit around the planet.

From the mid-nineteenth century until 1979, discoveries about Saturn's ring system were largely limited to discovery of new divisions, and it was postulated that the rings were no more than 15 kilometers thick. In 1977, rings were discovered by a stellar occultation method around the planet Uranus. In 1979, the U.S. Voyager spacecraft discovered rings around Jupiter.

Voyager later confirmed Uranus' rings and in 1989, discovered diminutive rings around Neptune.

A comprehensive understanding of all planetary ring systems can be made from the study of Saturn's rings, although the composition of each of the giant planets' rings has not been made as completely as Saturn's.

The Saturnian ring composition is nearly completely water ice fragments from microscopic in size up to the size of large boulders. The composition of the rings of Jupiter, Uranus, and Neptune, however, is not known. Their materials appear much lower in reflectivity.

Because the rings of these planets were so small and diffuse, a good definition could not be made and will have to await subsequent visits.

Saturn's rings begin only 32,000 kilometers above the planet's clouds and extend outward to 230,000 kilometers. The rings are no more than a few hundred meters in thickness.

There are seven major rings separated by what are still known as "gaps" (including Cassini's gap between the A and B rings). A closer inspection by Voyager revealed that the major rings consist of tens of thousands of ringlets that resemble grooves on a vinyl phonograph record. The ringlets are not cleanly separated from one another but appear to exhibit the property of wave propagation through the structure. Each ringlet differs in width from 100 kilometers to less than 1 kilometer.

The rings are composed almost exclusively of ice fragments whose diameters range from submicrometers to 10 meters or more. The denser regions of the rings appear to be composed largely of smaller particles, while in the gaps, the larger, meter-sized particles dominate. There may be many ring particles as large as 50 kilometers across within the ring system, although the extent of these particles is not known.

One of the most astonishing discoveries of Voyager was the tremendous dynamic complexity of the ring system. One of the dynamic effects discovered by Voyager was the spokelike effect seen in the motion pictures made by Voyager. The radial spokes extend outward like spokes on a bicycle wheel and rotate with the rings. The spokes tended to form, widen, then disappear after about an hour. The most widely accepted theory is that the spokes are formed by electrostatic forces between the submicrometer particles and the planet. They are short-lived because the orbital period of the inner particles is faster than that of the outer particles of the spoke.

Another of the discoveries of Voyager was the existence of what have become known as "shepherd satellites." These are tiny moons (not visible from Earth-based telescopes) that orbit on the outside fringes of the rings within some of the gaps. The minuscule gravitational field of these tiny moons is enough to push the ring particles back into the rings and thus define the rings' outer edges, hence, the name "shepherd." Shepherd satellites also exhibit some rather interesting effects on the rings. In the case of the F ring, two shepherd moons (1980S26 and 1980S27) confine the ring. The F ring appears to be discontinuous in places, is intertwined in others, and knotted and lumpy in others. The current theory is that the F ring's complexity is caused by the slight eccentricity in the orbits of the two shepherding moons. It is speculated that over time, variant gravitational interactions cause the structural convolutions. The trivial rings of Neptune exhibited similar discontinuities and irregularities, but there was not enough resolution or coverage to discern shepherding satellites.

Even before the Voyager encounter and the discovery of the superior structural details, a concept known as satellite "ring resonance" was advanced to explain what could be observed from Earth-based telescopes. This concept describes the shape of the rings and gaps to be determined not only by the orbits of individual ring particles but also by the gravitational influence of Saturn and her major moons. This theory reasons that the moons Mimas and Enceladus have a particular influence on particles of the rings so that the Cassini division is created by the gravitational interaction of the moons on the particles in the ring. The discoveries of Voyager supported part of the resonance theory, but the shepherding satellites and other gaps that had no resonance explanation in Saturn's system and that of the other giant planets left some question about its ultimate effect.

The rings of Saturn are flattened along the equatorial belt because of countless energy-dissipating collisions between particles that have over the millennia all but eliminated up-and-down motions. Such collisions do not affect the circular orbital motion of the particles, hence, the net effect is a disklike flattening. Astronomers had not expected Jupiter to have rings.

Before the Voyager flight, rings had never been observed from Earth. As Voyager passed between Jupiter and the outbound, it looked back on the giant planet and photographed the ring particles in reflected sunlight. It found that the Jovian rings were, for all practical purposes, invisible. They blocked only one ten-thousandth of the sunlight passing through them. There are two main rings around Jupiter, at 47,000 and 53,000 kilometers above the cloud tops. One ring is 5,000 kilometers wide, while the outer ring is only 800 kilometers in width. The spacecraft was unable to detect thicknesses, but they are estimated to be from 1 to 30 kilometers. The Voyager scientists estimated that most of the Jovian ring particles are probably made up of dust-size pieces, each in an individual orbit around Jupiter with an orbital period from five to seven hours.

Because the tiny, dark particles are in unstable orbits and are constantly falling in toward Jupiter, the rings are probably constantly renewed by the tiny moon Adrastea, which was also discovered by Voyager.

The rings of Uranus were actually detected before the Voyager spacecraft arrived.

Voyager discovered five well-defined rings around Uranus and four other less well-defined rings.

The innermost Uranian ring appeared quite compact and dense, while the outermost ring was quite diffuse. Aside from the nine distinct rings around Uranus, from fifty to one hundred nebulous dust bands were discovered in outer orbits containing very small particles. It is speculated that the Uranian ring system began when ten to twelve small moons less than 200 kilometers in diameter began to break up. As they broke up in low orbit, the fragments began to collide with one another, forming the dust bands and main ring system. Thus, the Uranian ring system may be constantly replenished so that it may have a lifetime of millions of years.

The Voyager spacecraft flew past Neptune in the summer of 1989 and discovered that this gas giant planet also has a distinct ring system. Voyager found five complete rings around Neptune. The exceptionally thin, bright, innermost ring of Neptune was found to be more dense than the others, some 17 kilometers wide with a distinct, more diffuse component extending 50 kilometers in toward Neptune. In the outermost rings, distinct points of light were observed, suggesting they are embedded with tiny, icy moonlets, which may act as shepherding agents, as in the Saturnian system. These tiny moonlets are estimated to be about 10 or 15 kilometers in diameter. The outer rings of Neptune are much wider than the inner rings; the third ring is quite dispersed and tenuous but is some 2,500 kilometers across.

The rings of all the giant planets may have been created by one of several mechanisms.

They may have been formed from the breakup of a satellite destroyed by tidal forces. The rings may have been created when a satellite and asteroid or comet collided in orbit. Or, the rings may be merely particles left over from the formation of the planet inside the Roche limit that could not accrete into a satellite because of the tidal forces.

Why the rings of Saturn are so different from those of the other giant planets may be explained by different material origins. It is possible that the rings around Saturn were created when a satellite of icy origin was broken up, while the dark rings of Jupiter represent the remains of a body made of much darker material. None of these conjectures will be proved until pieces of the rings can be directly analyzed.

Applications

The rings around Saturn are the best understood of the planetary ring systems because their existence was well known long before the Voyager probes were designed. Many of the instruments on Voyager were designed and emplaced to study only the Saturnian rings.

Fortunately, much of the information gained at Saturn can be applied to the other giant planet ring systems, even though the other planets' rings appear much different.

The evolution of planetary ring systems may all be the same. A careful analysis of ring system material should eventually demonstrate this fact. The dynamics of ring systems seem to be similar. Braided and discontinuous rings were seen in all ring systems. Shepherd satellites were discovered in at least two of the systems. The spokes seemed to be confined to Saturn, which may have something to do with the fact that the mass of material in the Saturnian system was significantly higher than in any of the others.

The question that applies to the ring system's total mass is an important one and awaits resolution. It may be that ring systems have definite lives. If there is no shepherding action, many of the ring particles may be spun out of the ring system by multiple piece encounters, and random gravitational and tidal effects from the planet and its larger moons. Hence, it may be that the Saturnian ring is relatively young when compared with the other planets' rings and hence appears to be better developed. A robust, more massive ring system, thus, may be a sign only of its youth, while a devitalized ring system such as seen on the other planets may be evidence of their advanced age. This hypothesis is speculative, at best.

A study of ring systems has direct applications to the study of forming planetary systems. In this comparison, Saturn can be visualized as a forming sun, while the ring particles can be seen as the developing solar nebula of dust and particles. Such a system has been theorized for the solar system's development some 5 billion years ago. In this comparison, the behavior and dynamics of the particles in planetary ring systems can be compared with the early solar system. Even though the Saturnian ring particles are prevented from accretion by confinement within the Roche limit, some comparisons may be made with other dynamic system components.

Such a comparison is not only valuable for direct association to the solar system but also valuable elsewhere in the galaxy. A careful study of the dynamics of planetary ring systems may ultimately be used to calculate the probability of other nebular systems developing around other solar systems. An extension of such estimates will enable more accurate calculation of the number of planetary systems, where the planets develop with respect to the star, and perhaps even how many planets could be expected to develop. This leads to other estimates, such as the number of Earthlike planets that may develop out of such processes.

Context

The Saturnian rings have long held a special place in the history of science. They have always been the crown jewel of the solar system and are unparalleled for their spectacular beauty.

The aspect and application of science to the rings began with sketches by the first scientist to view them, the inventor of the telescope, Galileo.

The rings became the focal point for one of the first evaluations of the nature of the solar system from Earth. The mathematical evaluation of the rings accomplished by Maxwell foretold of an era of scientific estimate using available data without direct visitation. The robot probes Voyager and Pioneer extended the reach to the ringed planets and greatly enhanced the methods of remote study. Again, the rings played a central part in these missions and the assessment of the nature of the solar system. The rings continue to hold an important part in science. Using the rings for modeling the early solar system and other solar systems in the galaxy is a further advantage.

Evaluating the mass of data returned by the robot probes will continue to keep theorists busy for years to come. The enigmatic spokes, braided rings, and discontinuities each represent problems that are still far from being solved. The aspect of ring resonance and the concept of ringlets as manifestations of a kind of particulate periodicity also have not been totally modeled.

These uncertainties will require further investigation and mathematical analysis.

Some of the questions will almost certainly require direct return of samples and other visits by robot and manned probes. These explorations to the rings will undoubtedly continue and will provide astronomers and planetary scientists with exciting scientific opportunities for decades.

Principal terms

CASSINI DIVISION: the largest gap in the Saturnian rings directly viewable from Earth-based telescopes

GAPS: spaces in the rings caused by tidal effects between the moons, the planet, and shepherd moons

RING RESONANCE: the effect of the gravity of the planet, its moons, and the particles on the rings; the principal effect of the resonance is formation of discrete rings and gaps

ROCHE LIMIT: the closest orbital distance a satellite can approach a planet or other body before the tidal forces of the larger body break it apart

SHEPHERD SATELLITES: (also shepherd moons): the tiny satellites responsible for gravitationally restraining ring particles in their defined rings

Bibliography

Cornell, James, and Alan P. Lightman. REVEALING THE UNIVERSE. Cambridge, Mass.: MIT Press, 1982. This book discusses in some detail specific areas of observational and theoretical astronomy. A discussion of Saturn and its ring system is included, specifically with respect to structure and composition. Somewhat technical, and is best approached by those with a scientific background. Illustrated.

Hartman, William K. ASTRONOMY: THE COSMIC JOURNEY. Belmont, Calif.: Wadsworth, 1978. This book has the feel of a textbook but is much less rigid than most texts. Heavily illustrated, it approaches astronomy in all its basic features. Offers a detailed discussion of ring systems of Saturn and the other giant planets. Very readable by anyone, even those with a minimal grounding in science.

Hunt, Garry, and Patrick Moore. SATURN. New York: Rand McNally, 1982. This book by map maker Rand McNally is designed to be a picture book, but has a wealth of descriptive analysis, including a three-part analysis of the ring system. It has a complete set of color and black-and-white photographs, as well as detailed charts.

Morrison David. VOYAGES TO SATURN. Washington, D.C.: Government Printing Office, 1982. This NASA publication examines the Voyager encounters with Saturn. Includes details of Voyager's Jupiter encounter and the Jupiter rings. Outlines the history of rings and ring theory. Contains photographs and is indexed and easy to read.

Zeilik, Michael. ASTRONOMY: THE EVOLVING UNIVERSE. New York: Harper & Row, 1982. This book is a fine review of astronomy in general, but has detailed sections on the planets. Included is an analysis of ring systems. Very readable, complete with photographs, illustrations, and index.

Couplings and Resonances in Planetary Orbits

Satellites of Planets