Saturn's ring system

Data transmitted by Pioneer 11, and in greater detail by Voyagers 1 and 2, revolutionized the understanding of Saturn’s complex ring system previously obtained by observations from Earth-based telescopes. This information revised models based on those earthbound investigations dating back to the discovery of the rings more than three centuries ago. Hubble Space Telescope and Cassini orbiter studies then built upon data returned by earlier spacecraft and revealed an even more complex ring system at Saturn.

Overview

Pioneer 11, Voyager 1, and Voyager 2—three probes launched between 1973 and 1977—all encountered Jupiter before their trajectories were directed toward Saturn. Pioneer 11 was also known as Pioneer Saturn during its Saturn flyby, as it was a pathfinder for the more sophisticated two Voyager spacecraft. Voyager 2, launched after Pioneer, would have to pass through Saturn’s ring plane at a distance of 2.86 Saturn radii (about 112,000 kilometers above the surface) in order to be put on a trajectory for a potential Uranus flyby. Although beyond the main rings, there was the possibility that a tenuous ring existed in this region. Such a ring would pose a threat to any spacecraft passing through at high speed. The decision was made to have Pioneer 11 cross the ring plane at this distance to determine whether it was safe for the more valuable Voyager spacecraft to come.

The Cassini spacecraft was launched on October 15, 1997. Cassiniarrived in Saturn orbit on July 1, 2004 and immediately began to add to the Voyager findings. Cassini provided deeper insights into ring dynamics, the spokes phenomenon, and gravitational interactions between ring particles and shepherding satellites. Cassini was initially planned as a four-year mission, but, as the spacecraft continued to operate and provide scientifically valuable data, its mission was extended until September 2017. NASA disposed of the spacecraft by deliberately flying it into Saturn's atmosphere.

Pioneer 11 survived the crossing of the ring plane on September 1, 1979, at a distance of 2.82 Saturn radii. The success of this maneuver was evident from the continued reception of the spacecraft’s radio transmissions before, during, and after the ring crossing. Pioneer 11 then swung around the planet, crossing the ring plane a second time (about two-and-a-half hours after the first crossing) at a distance of 2.78 Saturn radii. Again, although it did encounter debris, there was no detectable damage during this crossing.

During its Saturn flyby, Pioneer 11 transmitted images of the rings made by an imaging photopolarimeter, an instrument that produces images by means of polarized light. In this case, it made images at two visible wavelengths, one in the red region of the and another in the blue. These images could be processed at the receiving station to simulate color pictures. As the spacecraft approached Saturn, the exceeded that of earthbound observations. At a distance of about one million kilometers, a new ring was detected in one of the images. It was named the F ring. It was too narrow and too close to the outer edge of the A ring to have been seen from Earth. Pioneer 11 also collected data on Saturn’s rings by using transmitted sunlight, a method not possible for observations made from Earth. With this method, images are analogous to a photographic negative; gaps in the rings appear bright instead of dark, while the dense, bright parts of the rings appear dark. A week after the first encounter, the Saturn flyby was essentially complete, and Pioneer 11 was on its way on an escape trajectory that would take it out of the solar system.

As the planet moves about the sun, the orientation of the ring plane relative to Earth varies. In early 1980, prior to the Voyager 1 flyby, Saturn’s rings appeared sideways in Earth-based observations. The main rings were mostly invisible, because they appeared so thin from that perspective. Conditions were favorable, however, for detecting faint satellites and diffuse rings that would otherwise be lost in the glare of the main rings. A faint ring was detected beyond the F ring by astronomers during this period. This ring, now called the E ring, had first been discovered in 1966, the last time that the rings were edgewise, by a then-unknown astronomer, Walter A. Feibelman. At that time, its existence was called into question by other observers. Pioneer 11 vindicated Feibelman.

The Voyager imaging system represented a tremendous improvement over that of Pioneer 11. It consisted of two television cameras. One was outfitted with a wide-angle lens, while the other had a narrow-angle lens. These cameras were mounted on a scan platform that could be aimed continuously at the target, a process referred to as "slewing" or "motion compensation." This prevented the smearing of images due to the rapid motion of the Voyager spacecraft and was accomplished without using propellant to reorient thrusters to move the spacecraft. Pioneer 11’s imaging had to with the spin-stabilized spacecraft and could only record the subject once during each revolution. Voyager images could be transmitted more rapidly in black and white or in color. They also had higher resolution.

During October 1980, Voyager 1 was traveling toward Saturn at an average rate of 1.3 million kilometers per day. At the end of the month, it was 17 million kilometers from the planet, and the improved resolution brought to light new details within the rings. The principal rings, particularly the B ring and C ring, were found to consist of narrow concentric rings. The "ringlets," as they were called, proved to be so numerous that they suggested the analogy of grooves on a phonograph record. The main gap in the ring system, the Cassini division between the A ring and B ring, was not a total void. It contained some ringlets of its own.

The B ring exhibited some curious radial streaks referred to as "spokes," which rotated with the ring while changing their shapes. This was a completely baffling phenomenon. To study them more carefully, Voyager 1 was programmed to make images every five minutes over a period of ten hours. In the process, two small satellites were discovered. One orbited along the outer edge of the F ring, the ring that had been discovered by Pioneer 11, while the other orbited along the ring’s inner edge. The specific location of these satellites is considered essential to the permanence of the F ring. The F ring itself also has a curious property: it is braided.

During its encounter with Saturn’s rings on November 12, 1980, Voyager 1 swung around Saturn and crossed the ring plane twice without damage. Before being directed to escape the solar system, it transmitted spectacular images of Saturn and its rings. The number of ringlets that could be detected with the Voyager instrumentation was estimated to have an upper limit of one thousand. Voyager 1 discovered two more rings, the D ring, which replaced the C ring as the innermost ring, and the G ring, beyond the F ring discovered by Pioneer 11. Evidence for the G ring had been obtained by Pioneer 11. Its charged particle detector registered a decrease in intensity that could have been caused by absorption by the ring particles. Voyager 1 confirmed the existence of the E ring by photographing it directly.

As a result of discoveries made by Voyager 1, the program of Voyager 2 was revised for focused studies of the rings. A decision was made to program the second Voyager to collect data during a stellar of Saturn’s rings. During the occultation, Voyager 2 would be positioned above the ring plane, focusing on the selected star through Saturn’s rings. Delta Scorpii, the star, was bright enough to be seen through the partially transparent rings. One of Voyager’s instruments, the photopolarimeter subsystem (PPS), would be focused on the star for about two hours. The PPS would not form images. It would measure the rapid fluctuations in the brightness of the star. These fluctuations would be caused by the ring particles in the line of sight momentarily blocking the light from the star. Data could be analyzed to map variations in ring structure. Voyager 2 also scanned the area for small satellites that might be embedded within the rings.

As Voyager 2 approached Saturn, images of the rings achieved an extraordinary resolution of ten kilometers. Still, there was no evidence of embedded satellites. The number of observed ringlets increased with the improved resolution. On August 25, 1981, the occultation of Delta Scorpii was successfully observed. As seen from Earth, Voyager then proceeded to cross the ring plane behind Saturn. The successful crossing could not be confirmed until radio communications were resumed about one hour later. Voyager 2 was then directed toward Uranus. Shortly afterward, ground controllers discovered that the scan platform had lost its motion and had been shut down by the onboard computer. Many of the programmed images had not been acquired, although some good images of the F ring were obtained before the scan platform failed. Quick actions by the Voyager team salvaged much of the anticipated scientific investigations as Voyager 2 left Saturn.

As Cassini approached orbit about Saturn, it made its closest planned encounter with the planet’s rings. Looking at the B ring, Cassini noted a puzzling lack of spokes. This strongly suggested that the spoke phenomenon might be a seasonal effect. Later in the mission, Cassini captured additional images of the spokes.

Cassini also examined gaps in the rings and noted far more structure in them than had the Voyagers and found additional shepherding satellites. For example, inside the forty-two-kilometer-wide Keeler gap within the A ring, in May 2005, Cassini discovered a small satellite. This body clears out material within this gap. In 2006, Cassini imaged a very faint dust ring located near the satellites Janus and Epimetheus. It was believed that this five-thousand-kilometer-wide ring is composed of particles liberated by meteoroids impacting these two satellites. That same year, another faint dust ring was found existing close to the Pallene. This ring was only twenty-five hundred kilometers in radial extent. It too was suspected to be composed of particles generated by collisions of objects, specifically with Pallene.

Knowledge Gained

Three of Saturn’s major rings (D ring, F ring, and G ring) were discovered with the help of deep space probes. The existence of the faint E ring was confirmed. The rings, in order of increasing distance from the planet, are D, C, B, A, F, G, and E. Saturn’s rings were first named in alphabetical order in 1850: A, B, and C for the three rings then known. The convention now is to name the rings in the order of their discovery.

Three narrow gaps in the rings were also found: one in the C ring; one at the inner edge of the Cassini division, which separates the A ring and B ring; and a third one in the A ring, close to its outer edge.

While individual ring particles have not been observed directly, the distribution of particle sizes can be estimated from measurements of the transmission of light and radio waves through the rings and scattering, or reflection, from the rings. Being an orbital platform, Cassini was able to conduct multiple radio occultation measurements. The sizes range from microns to about ten meters. Spectroscopic measurements confirm that these particles are primarily composed of ice, with some impurities. Scientists estimate the thickness of the rings to be about ten meters on average. This is relatively thin since the ring system is over two hundred thousand kilometers wide.

Rings discovered by deep space probes differ from the ones first identified by ground-based observation. The outer two rings, G and E, are composed of particles in the micron range and do not have the ringlet structure. They are diffuse and are probably around one hundred meters in thickness. It is believed that Voyager 2 passed through the G ring when it was leaving Saturn. Its plasma wave detector recorded a large increase in intensity in the vicinity of the ring. The increase was apparently caused by the spacecraft colliding with ring particles as it traveled through the ring at ten kilometers per second. Impacts would have vaporized the particles, producing puffs of ionized gas (plasma) that would have been recorded by the detector.

Braided strands of the F ring observed by Voyager 1 were found by Voyager 2 to have changed to parallel strands. They changed back to braided strands by the end of the encounter, one example of the dynamic nature of the rings. Two so-called serve to confine the ring. They are elongated rather than spherical. The long axis of the larger one is about 140 kilometers. Cassini was able to observe the dynamic behavior of the F ring over the course of its many orbits about Saturn.

Of all the ring components seen in the outer solar system, Saturn’s F ring displays the most unusual and dynamic activity. Features in it can be seen to change on timescales ranging from just hours to several years. Studies using Cassini’s ultraviolet imaging during stellar occultations, found at least a dozen objects within the F ring ranging from twenty-seven to ten thousand meters in size. Data suggested that these features were aggregates that had temporarily clumped together, the supposition being that within the rings material coalesces and breaks apart due to gravitational and collision processes, respectively. In mid-2008, researchers using Cassini observations, published a paper in the journal Nature that attributed the unusual characteristics of F ring dynamics to perturbations created by small moonlets, making the F ring the ring location in the found thus far where a serious number of collisions happen regularly. In some ways, F ring dynamics provide a window into the early solar system’s protoplanetary disk at a time when collisions of small particles were needed to drive planetary formation.

A Cassini image taken in 2004 appeared to have identified a five-kilometer-wide moonlet that may have produced some of the largest of the observed jets in the F ring. The extended Cassini mission also revealed hundreds of "mini-jets," low-speed collisions of smaller objects ranging in size from 0.8 kilometers to 160.9 kilometers.

The D ring appeared to be a collection of ringlets too faint to be seen from Earth. It has a relatively small percentage of micron-sized particles. Analysis of Voyager data revealed three ringlets. These were designated D68, D72, and D73 in increasing distance radially from the planet toward the C ring. Cassini again revealed dynamic behavior and a more complex structure. Twenty-five years after its discovery, D72 was observed to be fainter and at a location two hundred kilometers closer to Saturn. The gap between D73 and the C ring was not empty. Cassini recorded fine structure with ripples of material thirty apart. Vertical ripples in the C and D rings are likely the residual effects of cometary impacts in 1983.

Apparently unoccupied gaps around the C ring had long puzzled scientists until 2010, when Saturn's equinox enabled researchers to obtain unprecedented images of the "tsunami" effect exerted on the C ring by Titan's gravitational field during its sixteen-day orbit.

The Cassini division, when viewed in transmitted light, appeared to have five rings in its central region and a gap on each side separating it from the A ring and B ring. The gap at the inner edge has an eccentric ringlet, which was scanned for small satellites that might confine it. None were found, but Cassini uncovered even more structure in the gap than revealed by the Voyagers.

The B ring, the largest and brightest of the “classical” rings, has the most elaborate structure, literally thousands of ringlets. Spokes are transient features that appear bright in transmitted light but dark when viewed from the sunlit side. They consist of micron-sized particles. When electrically charged, they interact with Saturn’s magnetic field, a process that explains some of their properties. Evidence was collected that suggests the ringlets themselves are manifestations of a wave, propagating through the ring in the form of a spiral. This spiral might, in turn, produce the observed density variations. Cassini provided a great deal of evidence for not only density waves in the rings but also torsion waves.

Context

History records that in 1610, Galileo was the first person to observe Saturn through a telescope. He described it as having a close satellite on either side. Later observers used the Latin word ansae ("handles," in the sense of cup handles) to describe what they saw. This term is still used to refer to the parts of the rings that are visible on either side of the planet. Christiaan Huygens, based on observations in 1655, concluded that Saturn was surrounded by a thin ring not attached to the planet. Gian Domenico Cassini, discovered the main gap in the ring system in 1675, showing that there were two rings, now called A and B. The C ring was discovered in 1850.

In 1867, Daniel Kirkwood, an American astronomer, applied a resonance theory he had developed to explain the existence of the Cassini division. A ring particle in the division orbits Saturn with a period one-half that of Saturn’s satellite Mimas, which is farther from the planet. Every other period, the particle passes Mimas in the same part of its orbit and is affected by a pulling it outward. This periodic or resonant force would clear the ring of particles. The process is actually more complicated. Additional satellites have to be considered, and the gravitational interaction is not as simple as described. In some instances, the force might produce a ringlet by causing a spiral density wave to form, as mentioned with respect to the B ring. Originally, this type of wave was introduced to explain the structure of spiral galaxies, such as the Milky Way. Just as the complexity of the ring system was completely unexpected, the concept that Saturn’s rings may have some similarities to spiral galaxies, despite their enormous difference in size, could not have been anticipated before the Saturn encounters.

Prior to Pioneer 11, Voyager 1 and 2, and Cassini, Saturn’s rings could be described only in relatively simple terms. It had been established that the rings consist of discrete particles in orbit around the planet. Spectroscopic measurements showed that the inner parts of the rings rotate faster than the outer parts. Thus, it was clear that the rings were not solid disks.

In addition, it had been proved theoretically that any natural satellite large enough to be held together simply by the force of its own gravity would be fragmented by tidal forces exerted on it by the planet if it was closer than about 2.4 times the radius of the planet. This inner limit is known as the Roche limit, named for the nineteenth-century French mathematician Édouard Roche. All rings, except the F ring and E ring, are at a distance greater than 2.4 Saturn radii away from the planet. Roche suggested that the rings were formed by fragmentation of a satellite that came too close to the planet. Spacecraft data support another possibility. A number of Saturn’s icy satellites were found to be pockmarked by impact craters. Ring particles could be remnants of the debris resulting from the collisions that produced such craters.

Voyager 2 successfully completed its Uranus encounter on January 24, 1986, and continued on its route to Neptune, passing through that system in August 1989. This spacecraft is unique in having made observations at close range of the four known planetary ring systems: those of Jupiter, Saturn, Uranus, and Neptune. The extensive data set from the Saturn encounters made by four spacecraft form the basis for a unified model of planetary rings in general.

Since the extension of the Cassini mission in 2008, there have been several follow-up studies of the new and exciting discoveries it initially uncovered about the complex ring system of Saturn. For instance, Cassini, in November 2005, was directed to use its magnetospheric imaging instrument (MIMI) to observe the planet’s in the vicinity of its second-largest satellite, Rhea; although MIMI found three specific diminishments of energetic particles located symmetrically about Rhea that led to the supposition that Rhea might be the only satellite to have its own ring system, analysis of Cassini images from 2008 and 2009 later disproved this hypothesis.

In July 2023 NASA directed its most powerful space-based instrument, the James Webb Space Telescope, toward Saturn. The images that resulted were both stunning and immediately added to Saturn’s body of data. The Webb telescope incorporated new technologies, such as the Near-Infrared Camera (NIIRCam) which showed new perspectives of the planet. The NIIRCam was particularly effective in showing the distinction among Saturn’s seven rings. Also in 2023, an additional 62 moons were discovered orbiting the planet. By that point in time, Saturn was observed to have a total of 146 moons. Surpassing the 95 moons orbiting Jupiter, Saturn was acknowledged as having more moons than the rest of the planets in the solar system combined.

After collecting evidence through a series of complex simulations, researchers published a study in the Astrophysical Journal in 2023 that furthered the support of a hypothesis that suggested Saturn’s rings were formed by the collision of two planetary bodies millions of years ago. This, in addition to other scientific findings in the early twenty-first century, led some scientists to believe that Saturn’s rings were formed much more recently than the existing estimates that placed their age at billions of years old.

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