Planetary ring systems

A planetary ring system consists of enormous numbers of relatively small particles that fan out from a planet in the form of a disk, orbiting as a complex unit around that planet. Planetary rings are common in the outer solar system; each of the four “gas giant” planets has a ring system of different ages and degrees of complexity.

Overview

The rings of Saturn are stunning, perhaps among the most beautiful features to observe in the night sky. They were first discovered in 1610 by Galileo Galilei in Padua, Italy. Galileo’s telescope was not the best of astronomical instruments. What he sketched were two spheres, one on each 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, through his improved instrument, Saturn's rings. Later, in 1675, Gian Domenico Cassini, supervisor of the Paris Observatory, discovered that there is structure to the rings. He also discovered that Saturn has multiple rings, with gaps between them. The largest gap exists about two-thirds of the way out from the planet; this gap later became known as the Cassini division. The outer ring from the Cassini division is called the A ring, and the ring on the inner side was termed the B ring.

The rings of Saturn were of intense interest to scientists throughout 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 five hundred 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 Édouard Roche rigorously proved that if a satellite orbits too close to a planet, tidal forces from that planet would tear it apart into small pieces. The planet also would not allow a satellite to form from small pieces inside this distance. This limit is known as the “Roche limit.” The rings of Saturn fall inside the Roche limit. Roche boldly suggested that Saturn had captured a small satellite that had then been broken into billions of tiny pieces by the gas giant’s gravitational forces.

In 1857, the University of Cambridge offered a prize to settle the question of whether the rings are 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 also demonstrated that the rings could not be a liquid. Thus, the rings of Saturn are 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 finding new divisions, and it was postulated that the rings are no more than fifteen kilometers thick. In 1977, rings around the planet Uranus were observed using a stellar occultation method. In 1979, the National Aeronautics and Space Administration’s (NASA’s) twin Voyagerspacecraft discovered rings around Jupiter. Voyager 2 confirmed Uranus’s rings and, in 1989, discovered faint rings and ring arcs around Neptune.

Much about all planetary ring systems can be learned from the study of Saturn’s rings, although the composition of each of the giant planets’ rings has not been determined as completely as has Saturn’s. The Saturnian ring system is composed almost completely of water ice fragments, ranging from microscopic particles to boulder-sized bodies, whereas the composition of the rings of Jupiter, Uranus, and Neptune, while believed to be dust particles, is not fully known. Saturn’s rings begin only 66,970 kilometers from the planet's center and extend outward to 480,000 kilometers. The rings are thin, ranging from less than ten meters to about one kilometer thick—with the exception of the E ring at more than two thousand kilometers. There are seven major rings separated by what are known as “gaps” (including the Cassini division 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. There are spiral density waves as well as bending waves.

Saturn's rings are composed almost exclusively of ice fragments whose diameters range from submicron-size to ten meters or more. 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 fifty kilometers across within the ring system, although the extent of these particles is not known. The Cassini spacecraft, launched in 1997, continues to provide images from its orbit of Saturn that reveal more structure to the planet’s complex ring system.

One of the most astonishing discoveries made by the Voyager missions is the tremendous dynamic complexity of Saturn's ring system. An unexpected dynamic effect found by Voyager is the spoke-like effect seen in movies assembled out of individual sequences of images taken by the spacecraft. Radial spokes extend outward like spokes on a bicycle wheel. They rotate with the rings. Spokes tend to form, widen, and then disappear after about an hour. The most widely accepted theory is that spokes are formed by electrostatic forces between the submicron-size 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 Voyager discoveries about Saturn is the existence of “shepherd satellites.” These are tiny satellites (not readily 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 satellites is enough to push ring particles back into the rings. They help define the rings’ outer edges (hence the name “shepherd”). Shepherding satellites also exhibit some rather interesting effects on the rings. In the case of the F ring, two shepherding satellites (1980S26 and 1980S27, later named Pandora and Prometheus, respectively) confine the ring. The F ring appears to be discontinuous in places, is intertwined in others, and is knotted and lumpy in others. One theory is that the F ring’s complexity is caused by the slight eccentricity in the orbits of the two shepherding satellites. It is speculated that, over time, variant gravitational interactions cause the structural convolutions. The other Jovian planets have shepherd satellites as well.

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 indicates that the shape of the rings and the location of the gaps are determined not only by the orbits of individual ring particles but also by the gravitational influences of Saturn and its major satellites. This theory postulates that Mimas and Enceladus have a particular influence on the particles of the rings so the Cassini division is created by the gravitational interaction of the satellites on the particles in the ring. The discoveries of Voyager support part of the resonance theory, but shepherding satellites and other gaps that have no resonance explanation in Saturn’s system and that of the other giant planets left some questions about its ultimate effect. Those questions were left for the Cassini orbiter to investigate.

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 vertical displacements. Such collisions do not affect the circular orbital motion of the particles; hence, the net effect is a disk-like flattening.

The ring system of Jupiter was found to be faint and largely made of dust. Between Voyager and Galileo spacecraft observations, it has been determined that Jupiter’s ring system has four components, each of which is shepherded by a moon. There is an inner halo ring, a relatively thick torus-shaped collection of particles. The main ring is very thin and relatively bright. It consists of three ringlets, one just outside the orbit of Adrastea, one just inside the orbit of Adrastea, and one just outside the orbit of Metis. Then there are two gossamer rings that are rather wide, thick, and very faint. One is associated with the satellite Amalthea, and the other with the satellite Thebe; these gossamer rings are believed to be composed of material coming from their associated satellites.

Astronomers had not expected Jupiter to have rings. Before Voyager, rings surrounding Jupiter had never been observed from Earth. As Voyager passed behind Jupiter on its outbound journey, it looked back on the giant planet and photographed the ring particles in reflected sunlight. It found that the Jovian rings absorb all but one ten-thousandth of the sunlight incident upon them. There is a torus of thick particles that is not particularly bright, and the small satellites Amalthea and Thebe produce the particles that compose the two gossamer rings. Voyager was unable to detect ring thicknesses precisely, but they were estimated to be between one and thirty kilometers wide. Later measurements have shown that the width of the main ring is approximately 6,440 kilometers, with a thickness that varies between thirty and three hundred kilometers. Jupiter's rings begin at about 100,000 kilometers from the center of the planet and extend outward to about 214,200 kilometers. Voyager scientists estimated that most of the Jovian ring particles are probably made up of dust-sized pieces, each in an individual orbit around Jupiter and 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 satellite Adrastea, which was also discovered by Voyager.

The rings of Uranus were actually detected before the Voyager spacecraft arrived at that planet. Voyager discovered five well-defined rings around Uranus and four other, less defined rings. From Voyager images, the innermost Uranian ring appears quite compact and dense, while the outermost ring is quite diffuse. Later discoveries determined that Uranus has thirteen rings. Aside from the distinct rings around Uranus, from fifty to one hundred nebulous dust bands have been discovered in outer orbits containing very small particles. It has been speculated that the Uranian ring system began when small satellites less than two hundred 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. Given that the planet's ring system appears to be young, it must be renewed continuously by small collisions. Thus, with this constant replenishment, the Uranian ring system may have a lifetime of millions of years.

Voyager2 flew past Neptune in August 1989 and discovered that this gas giant planet also has a distinct ring system. Voyager found what was believed to be incomplete ring arcs around Neptune. It was later determined that Neptune has five complete rings as well as five partial ring arcs. The bright ring arcs, which extend from the Adams ring, were the first part of Neptune's ring system to be discovered. The rings and ring arcs range from 41,900 kilometers from Neptune's center to 62,900 kilometers. Three of the rings—Le Verrier, Arago, and Adams—are less than one hundred kilometers wide; the other two rings, Galle and Lasell, range in width from two thousand to five thousand kilometers. The rings are believed to be composed of dark material similar to what's been observed in Uranus's ring system. In the outermost rings, distinct points of light were observed by Voyager, suggesting that the rings are embedded with tiny, icy moonlets, which may act as shepherding agents, as in the Saturnian system. The stability of the ring arcs is likely related to the satellite Galatea, which orbits inside the Adams ring. The satellite Despina, located inside the Le Verrier ring, may act as a shepherding moon for this ring.

The rings of all the giant planets may have been formed by one of several mechanisms. Ring particles may have been accumulated from the breakup of a satellite destroyed by tidal forces; they 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 broke 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.

Methods of Study

The rings around Saturn are the best understood of the planetary ring systems because their existence was well known long before the Voyagers were launched. Much of the information gained at Saturn by the Voyagers and later the Cassini orbiter can be applied to the other giant planet ring systems, even though the other planets’ rings appear much different.

As an orbital platform capable of examining the complex ring system around Saturn with sensors viewing in several portions of the electromagnetic spectrum in addition to the visible, the Cassini spacecraft has verified a number of the Voyager findings and has provided even newer insights into ring dynamics, the spokes phenomenon, and gravitational interactions between ring particles and shepherding satellites. Cassini arrived in Saturn orbit on July 1, 2004. It was initially supposed to have only a four-year mission, but, in light of the spacecraft's good health, its mission was extended until September 2017. NASA disposed of the spacecraft by deliberately flying it into Saturn's atmosphere. In addition to all the refinements of Saturn’s ring structure, Cassini produced hints of a tenuous ring system around Saturn’s second-largest satellite, Rhea. Cassini used its Magnetospheric Imaging Instrument (MIMI) to infer these thin rings based on a depletion of energetic electrons near that satellite. MIMI data indicated three drops in electron concentration symmetrically located about Rhea, suggesting that particles from decimeters to meters in size are absorbing the electrons. The rings are too tenuous and dark to image directly.

The evolution of planetary ring systems may all be the same, which could eventually be demonstrated through careful analyses of ring system material. The dynamics of ring systems seem to be relatively similar. Braided and discontinuous rings have been seen in all ring systems. Shepherding satellites likely influence all known ring systems (Jupiter, Saturn, Uranus, and Neptune) as well. Spokes seem to be confined to Saturn, which may have something to do with the fact that the mass of material in the Saturnian system is significantly higher than in any of the others.

The question of a ring system’s total mass is an important one. It may be that ring systems have definite lives. If there is no shepherding action and no collision replenishment, many of the ring particles may be spun out of the ring system by multiple piece encounters, along with random gravitational and tidal effects from the planet and its larger satellites. It may be that the complex Saturnian ring system is relatively young when compared with the other planets’ rings, as it appears to be better developed and its youngest particles may only be one hundred million years old. This hypothesis, however, remains speculative at best—scientists also believe that Saturn's rings could have at first formed during the formation of Saturn itself. Additionally, it is believed that the rings of Uranus and Neptune are young, Uranus's dating back to no more than six hundred million years.

A study of ring systems has direct applications to the study of newly forming planetary systems. In this comparison, Saturn can be visualized as a forming star, while the ring particles can be seen as the developing stellar nebula of dust and particles. Such a system has been theorized for our solar system’s development some 4.6 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 with the solar system, but is 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 make possible more accurate calculation of the number of planetary systems, where extrasolar planets develop with respect to their star, and perhaps even how many planets could be expected to develop.

Context

The Saturnian rings have long held a special place in the history of science. Because of their spectacular beauty, they have always been considered to be the crown jewel of the solar system. The application of science to the study of those rings began with sketches by the first scientist to view them, Galileo. The rings became the focal point for one of the first evaluations of the nature of the solar system from Earth. Mathematical evaluation of the rings accomplished by Maxwell foretold of an era of scientific investigation without direct visitation.

In February 2023 astronomers observed a previously undiscovered system of rings that lay at the edge of the Earth's solar system beyond Neptune. The discovery revealed the oddity of a ring system surrounding not a gas giant, but a minuscule dwarf planet named Quaoar approximately half the size of Pluto. Furthermore, the ring system is not in relatively close proximity to the host planet but is removed a distance equal to seven times Quaoar's radius. This is twice the Roche limit. This discovery has now thrown previously accepted theories concerning ring theories into flux.

The Pioneer, Voyager, Galileo, and Cassini spacecraft have extended humanity’s reach to the ringed planets and have greatly enhanced our methods of remote study. Evaluating the mass of data returned by robotic probes will continue to keep theorists busy for years to come. Enigmatic spokes, braided rings, and discontinuities each present problems that are far from being solved. Ring resonance and the concept of ringlets as manifestations of a kind of particulate periodicity also have been modeled. Some of the remaining dynamic and compositional questions will almost certainly require the direct return of samples and other visits by robotic probes. Explorations of the rings will undoubtedly continue, and they will provide astronomers and planetary scientists with exciting scientific opportunities for decades.

Bibliography

Beatty, J. Kelly, Carolyn Collins Petersen, and Andrew Chaikin, eds. The New Solar System. 4th ed. Cambridge: Cambridge UP, 1999. Print.

Bortolotti, Dan. Exploring Saturn. New York: Firefly, 2003. Print.

Bredeson, Carmen. NASA Planetary Spacecraft: Galileo, Magellan, Pathfinder, and Voyager. New York: Enslow, 2000. Print.

"Cassini." NASA Solar System Exploration, 25 Sept. 2023, solarsystem.nasa.gov/missions/cassini/science/rings. Accessed 26 Sept. 2023.

Davis, Jason. "Your Guide to the Rings of the Solar System." The Planetary Society, 8 Dec. 2022, www.planetary.org/articles/rings-of-the-solar-system. Accessed 25 Sept. 2023.

De Pater, Imke, and Jack Jonathan Lissauer. Planetary Sciences. 2nd ed. New York: Cambridge UP, 2010. Print.

Elkins-Tanton, Linda T. Uranus, Neptune, Pluto, and the Outer Solar System. Rev. ed. New York: Facts on File, 2011. Print.

Encrenaz, Thérèse, et al. The Solar System. 3rd ed. New York: Springer, 2004. Print.

Harland, David M. Mission to Saturn: Cassini and the Huygens Probe. New York: Springer Praxis, 2002. Print.

Hartmann, William K. Moons and Planets. 5th ed. Belmont: Thomson Brooks/Cole, 2005. Print.

Irwin, Patrick G. J. Giant Planets of Our Solar System: An Introduction. 2nd ed. New York: Springer, 2009. Print.

"Jupiter: Rings." Solar System Exploration. NASA, 23 Apr. 2013. Web. 19 July 2013.

Karttunen, H. P., et al., eds. Fundamental Astronomy. 5th ed. New York: Springer, 2007. Print.

Lea, Robert. "‘Impossible’ New Ring System Discovered at the Edge of the Solar System.'" Scientific American, 11 Feb. 2023, www.scientificamerican.com/article/impossible-new-ring-system-discovered-at-the-edge-of-the-solar-system/#. Accessed 26 Sept. 2023.

Miner, Ellis D., Randii R. Wessen, and Jeffrey N. Cuzzi. Planetary Ring Systems. New York: Springer Praxis, 2006. Print.

Morrison, David. Voyages to Saturn. NASA SP-451. Washington: NASA Scientific and Technical, 1982. Print.

"Neptune: Rings." Solar System Exploration. NASA, 20 Aug. 2012. Web. 19 July 2013.

"Saturn." NASA Solar System Exploration, 25 Sept. 2023, solarsystem.nasa.gov/planets/saturn/in-depth/#." Accessed 26 Sept. 2023.

"Saturn: Rings." Solar System Exploration. NASA, 17 July 2013. Web. 19 July 2013.

"Uranus: Rings." Solar System Exploration. NASA, 20 Aug. 2012. Web. 19 July 2013.