By use of ground-based telescopes and the Voyager 2 spacecraft, astronomers learned that Neptune has rings made of dustlike particles as well as a unique incomplete ring.
In 1979, the Voyager 1 space probe discovered a ring of material around the planet Jupiter. This meant that of the solar system’s four gas giant planets Jupiter, Saturn, Uranus, and Neptune all but Neptune were known to have rings. Most astronomers, thus, expected to find rings at Neptune also.
The search for Neptunian rings was done by viewing stellar occultations.
In 1968, E. F. Guinan of Villanova University had observed an occultation of Neptune from New Zealand. The primary record of the event was accidentally lost, but a secondary record had been made on punched cards. These cards were not as precise as the primary record, which had been made on a paper strip chart, so they were set aside and ignored for more than ten years. When the cards were finally examined, they were discovered to be surprisingly informative; they showed that the star had flickered distinctly before disappearing behind Neptune. The fact that the starlight fluctuated rather than went out completely suggested one of two things: that a moon had partly blocked the star an extremely unlikely occurrence, considering the tiny disk of such a moon as seen from Earth or that the star had passed behind a diffuse ring. Guinan announced the discovery of rings at Neptune in 1982.
In 1981 and 1983, more Neptunian occultations were observed, but neither provided conclusive evidence of rings. Another occultation took place on July 22, 1984. This one was viewed by two astronomical teams in Chile about 100 kilometers apart. André Brahic of the University of Paris led the group at the European Southern Observatory in Cerro La Silla, while William B. Hubbard of the University of Arizona headed the group at the Cerro Tololo Inter-American Observatory. Both astronomers reported the same results: Before passing behind Neptune, the star flickered for about one second. This could not have been caused by a satellite; the observatories had seen the occultation at the same time from slightly different angles. This required either the absurd coincidence of two moons simultaneously passing the star in the views of the respective telescopes or the existence of a stream of debris orbiting Neptune a ring. Once again, data suggested that Neptune had a ring system, but this one had a surprising twist. After the star passed behind the ring and then Neptune, it should have flickered again as it passed the ring a second time; however, there was no flicker. Somehow, the ring did not totally encircle the planet; the stream of particles formed only a portion, or arc, of a ring.
Neptune’s rings, as captured by Voyager 2 on August 25, 1989.
Such a finding should have been impossible. Ring particles closest to a planet orbit much faster than those farther away, because gravity is stronger closer to the planet. As a result, a ring arc would be unstable: It would spread itself eventually into a full ring. Even if the arc were of recent origin such as a moon coming too close to Neptune and being torn apart by the planet’s gravity theoreticians calculated that it would become a ring in only three to ten years. However, when Hubbard, Brahic, and their colleagues compared previous occultations that had provided no evidence of a ring, they concluded that their ring arc “encircled” only about 10 percent of Neptune. To astronomers, it seemed more likely that the ring arcs were millions of years old perhaps as old as the solar system itself than the possibility that the fragments had been discovered during the few years in which they would exist.
Several scientists searched for mechanisms that could maintain the arcs. Most of these theories relied on the gravitational effects of small moons called “shepherds,” like those known to orbit close to the rings of the other gas giants.
As Voyager 2 approached Neptune for its 1989 flyby, the mission astronomers viewed its progress from the Jet Propulsion Laboratory
There are four Neptunian rings, made primarily of tiny particles best described as dust or smoke. The three ring arcs, made of rocky particles perhaps as large as a fist, are embedded in the outermost ring, strung together in one 35-degree sector. Two rings are a few kilometers thick and are 62,900 and 53,200 kilometers in radius, respectively. Two more are broader; one has a radius of 41,900 kilometers, and the other extends from the inner thin ring out to nearly 59,000 kilometers. Also circling Neptune is a broad sheet of fine material called the “plateau,” which may overlap all the rings and is so diffuse that it is barely visible in enhanced Voyager photographs that show clearly the other very faint rings.
Each time the Voyager probes examined a planet, the mission astronomers saw a ring system unlike any of the others. Jupiter has a single thin ring, while Saturn has thousands forming a band more than 66,000 kilometers wide. Saturn’s rings are bright enough to be seen in archaic telescopes, while the meter-sized boulders that make up Uranus’s eleven thin rings are blacker than coal dust. Neptune’s rings are made of invisible smoke, dust, and streams of debris that cannot be explained fully yet. As Carolyn Porco
Voyager images of the cratered and fractured surfaces of the moons of the outer planets have led scientists to believe that these moons are frequently bombarded, sideswiped, and pushed into new orbits by passing comets from beyond Pluto’s orbit. Such violence can easily turn a satellite into a ring of debris.
This, however, does not solve the mystery of the ring arcs: that is, if they are newly created (astronomically speaking) or if they have circled Neptune for billions of years. If the ring arcs are a long-lasting feature, astronomers need to determine what holds them together. Also, if the culprit is a shepherd moon, it is puzzling why Voyager 2 did not reveal it. The Voyager scientists believe that Saturn’s broken ringlets could be influenced by moons too small for Voyager to see. It may be that the same holds true for Neptune. If the ring arcs were formed recently perhaps since 1979 and are transitory, then the timing of the arcs’ creation was extremely fortunate for Voyager, because experts believe that the arcs would persist for less than a decade. If this was the case, however, astronomers are wondering what Guinan recorded in 1968.
One theory, advanced by Larry Esposito
Another of Esposito’s theories is that rings, although stable, are fluid. The debris may shift continually and reshape itself as a result of random collisions and side-swipes between ring particles and shepherd moons, so that the ring does not maintain a uniform thickness and density indefinitely. Ring arcs and incomplete ringlets, then, are a transitory stage in the life of a planet’s rings.
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Kerr, Richard A. “Voyager Finds Rings in Need of Rejuvenation.” Science 245 (September 29, 1989): 1451. Part of a special section on Neptune, this article discusses the theory that a planet’s rings are born, decay, die, and are replaced by new rings. Informative piece for all readers. -
_______. “Why Neptunian Ring Sausages?” Science 245 (September 1, 1989): 930. Larry Esposito is a proponent of the theory that planetary rings are continually being shaped and reshaped by gravity and impacts with the bodies. This short article about Esposito is a sidebar to a piece about the Voyager encounter with Neptune and its moon Triton. For a wide audience. -
Littmann, Mark. Planets Beyond: Discovering the Outer Solar System. 2d ed. Mineola, N.Y.: Dover, 2004. Examines the discovery and exploration of Uranus, Neptune, and Pluto. Numerous sidebars explain important concepts without interrupting the narrative; some of the sidebars are first-person accounts by the researchers involved. The book describes the early evidence of Neptunian rings. A new chapter covers Voyager 2’s flyby. For a general audience. -
Miller, Ron, and William K. Hartmann. The Grand Tour: A Traveler’s Guide to the Solar System. 3d ed. New York: Workman, 2005. This general-readership book contains fascinating paintings and an unusual format. It examines briefly all bodies in the solar system at least 1,600 kilometers in diameter, in size order from Jupiter down to the largest asteroids. -
Smith, B. A., et al. “The Neptune Ring System.” Science 246 (December 15, 1989): 1431-1437. One of a group of articles by the Voyager 2 science team about the findings from Voyager’s encounter with Neptune. Unfortunately, the article does not address the question of the origin of Neptune’s rings and arcs. Other articles in this group examine Neptune’s chemistry, weather, moons, and the like. Highly technical; written for scientists, but suitable for college students.
Astronomers Discover the Rings of Uranus
Voyagers 1 and 2 Explore the Outer Planets
First Ring Around Jupiter Is Discovered
Pluto’s Atmosphere Is Discovered
Cassini-Huygens Probe Is Launched