First Ring Around Jupiter Is Discovered

Date March 4-7, 1979

Jupiter became the third planet known to possess a ring; it was photographed by the Voyager 1 probe and raised new questions about the physical properties of the Jovian system.

Locale Pasadena, California

Key Figures

• Raymond L. Heacock (b. 1928), project manager on the Voyager Imaging Team
• Edward C. Stone (b. 1936), project scientist on the Voyager Imaging Team
• Bradford A. Smith (b. 1931), leader of the Voyager Imaging Team leader

Summary of Event

The exciting discovery in 1977 of a system of rings around the planet Uranus raised the possibility that other large planets in the outer solar system might possess such features. Proposals to search for rings around Jupiter, however, initially did not attract much enthusiasm. Prior to the Uranian discovery, several prominent astrophysicists had developed elaborate models to explain why Saturn, apparently alone among the outer planets, has a ring system. Because conditions for distant Uranus and Neptune were not well known—the Uranian ring discovery came inadvertently during sensitive measurements of the planet’s occultation (blocking) of starlight from the Kuiper Airborne Observatory at an altitude of more than 12,000 meters—many of these models depended on contrasting circumstances at Saturn with those in Jupiter’s system (known as the Jovian system): atmospheric characteristics, positions of satellites, radiation emissions, and the like. Before 1977, there was a fairly broad consensus in the scientific community that Jovian conditions would not allow for a ring system. In any case, such a system, if significant, surely would have been observed long ago around Jupiter, a planet substantially larger than Saturn and only about half as far from Earth.

There remained, however, the possibility of a tenuous ring system (possibly one with unusual properties that would pose further challenges to standard models) that may have escaped telescopic detection or gone unnoticed by observers not expecting to find it. Since the 1950’s, there had been occasional reports of what looked to some observers like an unexplained shadow on the surface of Jupiter in certain photographs. Moreover, in December, 1974, Pioneer 11 recorded anomalous radiation levels near the Jovian equator. Although not equipped with photographic equipment, Pioneer 11 carried instruments to locate and measure radiation drops in the Jovian satellites’ “shadows”—the result of charged particles being absorbed by the satellites and thus reducing background radiation. Pioneer 10, which passed Jupiter in December, 1973, identified these areas, thus providing parameters for general radiation levels near Jupiter (which turned out to be exceedingly high).

Pioneer 11 replicated these experiments, but in the process also recorded an unexpected drop in radiation levels on both the inbound and outbound legs of the flyby, at about 1.8-1.9 Jovian radii, suggesting absorption of charged particles by solid matter. Pioneer 11 data also showed a drop in micrometeoroid impacts with the spacecraft as it approached the Jovian equator. One explanation would be that some mechanism reduced the general levels of dust in low Jovian latitudes and concentrated this material near the equator.

On the basis of these rather enigmatic readings, members of the Voyager Imaging Team (Raymond L. Heacock, Edward C. Stone, and Bradford A. Smith), preparing flyby routines for two spacecraft in 1979, proposed a search for a ring system. Team managers, although dubious, finally consented to a one-exposure photographic search to be programmed for Voyager 1. On March 4, 1979, Voyager 1 found the ring in the predicted location. After careful analysis, the team announced the discovery three days later. Within days, the University of Hawaii observatory on Mauna Kea confirmed the ring by ground sighting.

The Jupiter ring system is some 9,000 kilometers in breadth and only 30-35 kilometers thick. Its outer edge, which Voyager showed to be sharp and well defined, was 58,000 kilometers from the planet. Its close proximity to Jupiter (less than two planetary radii) suggested it must be composed of relatively large chunks of material. At this distance, forces of radiation and Jupiter’s particle ionization effects would reduce quickly the angular momentum of dust and small particles and send them into the atmosphere. The sharp outer edge of the ring, the team speculated, may be caused by the gravitational resonance with the small Jovian satellite Amalthea. (Such resonances disturb stability of small objects and tend to limit them to certain orbital paths.)

Voyager 2, now hastily reprogrammed to study the ring, presented new problems with its photographs taken July 9-11, 1979. These showed a complex ring system, much brighter and dustier than had been expected. Clearly, much of the material in the rings consisted of dust and small particles only a few microns in diameter. Particles of this size could survive only a few centuries, possibly less, amid the maelstrom of radiation and ionization in the vicinity of Jupiter. Unless the ring system was a very recent phenomenon—a proposition counter to the thinking of most planetary scientists—it had to be receiving new material continuously from somewhere else in the Jovian system.

An early candidate as a material source for the rings was astonishing volcanic activity on Io, one of the major Jovian satellites. Photographs of Io showed huge volcanic eruptions in progress, even as the Voyager spacecraft passed through the system. Some scientists proposed that material entered the rings not only from Io but also from a great many sources, including satellites and micrometeoroid bodies. In the complexity of resonances and forces emanating from the large number of Jovian satellites, the ring system was a kind of broom sweeping up particles from throughout the system.

Further observation established that, although some material might be entering the rings from a variety of exotic sources, replenishment, for the most part, came from larger bodies inside the ring system. Examination of Voyager data revealed the presence of a small satellite, christened Adrastea, less than 50 kilometers in diameter and orbiting very near the outer edge of the ring system, and still another, smaller satellite (Metis) in a very similar orbit. Scientists generally have accepted the theory that the rings contain many objects perhaps hundreds of meters in diameter—Voyager’s cameras could resolve individual bodies only down to about 1 kilometer—slowly being pulverized by radiation and micrometeoroid bombardment.

Adrastea and Metis were important discoveries for another reason: They were the first to be observed of a classification of moons or satellites generally called “shepherds.” Their presence prevents the spread of ring particles and confers on many ring systems remarkably clean and sharp boundary zones. Some researchers already had theorized the presence (eventually confirmed) of shepherd moons to explain characteristics of the ring system around Uranus, and similar bodies are now known to exist in the Saturn system. Discovery of the Jovian shepherds was particularly fortunate at the time, since location of the more plentiful shepherds of rings around Saturn proved a more difficult task than expected.

Significance

Detection of the Jovian rings provides both an object lesson in the nature of thought processes at work in scientific communities and important new information about the character of the outer solar system. The enormous quantities of data returned by the Voyagers also raised entirely new questions concerning planetary and solar system origins.

Clearly, some scientists became so entrapped in the theoretical models required to explain the long-known rings of Saturn that they failed to interpret correctly several kinds of pre-Voyager data suggesting the presence of Jovian rings. One wrote off the drop in radiation levels near the equator of Jupiter (first recorded by Pioneer 10 six years before Voyager 1 photographed the rings) as a false reading, possibly caused by equipment malfunction. The Hawaii team that obtained ground confirmation of the rings—using only a moderately large reflector with a mirror diameter of 2.2 meters—once alerted to look for the rings, accepted photographic evidence that most likely would not have led to the same conclusion before the Voyager 1 mission. The ring discovery showed that theories and models, valuable as they are for maintaining the elements of rigor and verification in research, occasionally lead scientists to look only for what they are predisposed to find and to overlook that which they previously have ruled out.

The Jupiter ring system, in itself, turned out to be a relatively minor chapter in a burst of discovery about the outer planets. The Uranus observations in 1977 and Voyager’s flyby of Jupiter in 1979 shattered ring theories based on Saturn models and collectively implied that ring systems are typical of the giant planets beyond Mars. The subsequent arrival of Voyager 2 at Saturn released a torrent of information about ring systems and underscored the relative simplicity of the Jovian ring. Voyager went on to explore the Uranus system in 1984, as astronomical evidence of a ring around Neptune appeared, and the Neptune ring was confirmed by direct observation in 1989.

The ring system around Jupiter has been incorporated into a far more complex set of models than most scientists could have perceived prior to the Voyager missions. Ring systems are not the smooth and uniform objects they were once thought to be. Instead, they contain myriad structures, many of which still elude explanation. The rings of Jupiter, although hardly as visually spectacular as the Saturn system, nevertheless appear to be more typical of ring formation and history than the major rings of Saturn. The Jupiter ring system, those of Uranus and Neptune, as well as some of the minor rings of Saturn, are all made up of extremely small particles and some, astonishingly close to their primaries, raise a host of unsolved questions. By far the most unsettling aspect of these rings is that many of them appear to be much younger than the estimated 4.5-billion-year age of the solar system. So, too, do most of the tiny shepherd moons, particularly those well inside the Roche limit, wherein solid bodies should not be able to survive the gravitational stresses caused by their primaries.

Computer models suggest that satellites with diameters of about 20-50 kilometers, lodged amid the debris of the rings and subject to fierce radiation, could not survive much longer than one billion years. Smaller bodies might be even younger, perhaps only a few thousand years old.

Jupiter’s ring system, like most, is ever changing. Material attaches to the shepherd moons, while other material is stripped away by radiation and other effects. Shepherd moons may sweep clean for a time wide swaths in the particle belts. Most of the rings appear doomed to destruction by the forces around them, but others will replace them. The rings of Jupiter belong to a planetary system almost as complex as the solar system itself, a dynamic and incredibly varied system the study of which has breathed new life into planetary sciences.

Bibliography

Burns, Joseph A., Mark R. Showalter, Jeffrey N. Cuzzi, and James B. Pollack. “Physical Processes in Jupiter’s Ring: Clues to Its Origin by Jove!” Icarus 44 (1980): 339-360. Discussion of the physical forces involved in creating and maintaining Jupiter’s tenuous ring system. Recounts early theories proposed before discovery of the shepherd moons and the mechanics of the shepherd system.

Cuzzi, Jeffrey N., and Larry W. Esposito. “The Rings of Uranus.” Scientific American 257 (July, 1987): 52-66. The best discussion in popular scientific literature of the dynamics of narrow, dusty rings such as those around Uranus and Jupiter. Particularly useful on the nature of shepherd moons.

Elliot, James, and Richard Kerr. Rings: Discoveries from Galileo to Voyager. 2d ed. Cambridge, Mass.: MIT Press, 1987. Highly readable and nontechnical account of the series of ring discoveries around Jupiter, Saturn, and Uranus from 1977 to 1986. Authors describe these as one of the major scientific events of the late twentieth century and discuss problems and theories derived from them. Useful account of technology involved in the discoveries and of the human element of excitement in the research teams.

Esposito, Larry W. “The Changing Shape of Planetary Rings.” Astronomy 15 (September, 1987): 6-15. Excellent summary, by the chair of the Voyager Rings Science Working Group, of a decade of discovery. Shows that the trend of research supports theories that the Jovian rings—and probably those of Uranus as well as some Saturnian rings—may be much younger than their primary planets.

Harland, David M. Jupiter Odyssey: The Story of NASA’s Galileo Mission. London: Springer-Praxis, 2000. A detailed scientific and engineering history of the Galileo program, but also includes extensive discussion of Voyager program events and results.

Irwin, Patrick G. J. Giant Planets of Our Solar System: Atmospheres, Composition, and Structure. London: Springer-Praxis, 2003. Provides an in-depth comparison of Jupiter, Saturn, Uranus, and Neptune, incorporating data obtained from astronomical observations and planetary spacecraft encounters.

Jewitt, D. C., and G. E. Danielson. “The Jovian Ring.” Journal of Geophysical Research 86 (1981): 8691-8697. Early summary of apparent physical properties of the rings and questions raised of origin and maintenance under measured conditions.

Owen, Tobias, et al. “Jupiter’s Rings.” Nature 281 (1979): 442-446. The initial report from the research team on the existence of the rings. Summarizes properties and problems presented by the discovery.

Smith, Bradford A., et al. “The Jupiter System Through the Eyes of Voyager.” Science 204 (1979): 927-950. Summary of observations of the Voyager spacecraft, which places the rings in context with other knowledge about the Jovian system.