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Space Topics: SaturnThe Alphabet Soup of Saturn's RingsAlthough Saturn's rings were first observed by Galileo in 1610, his telescope was not powerful enough to unambiguously resolve the ring system. It was Huygens who, around 1655, recognized that Saturn was "girdled by a thin, flat ring, nowhere touching it." Cassini observed the most prominent of the known divisions among the rings in 1675. Now, thanks to centuries of telescopic observation and especially to the photographs captured by the two Voyager spacecraft, we recognize seven distinctly different rings orbiting Saturn, each of them composed of innumerable ringlets. Here's a closer look at the "Lord of the Rings."
The Main (Inner) RingsThe Hubble Space Telescope image below shows all of the amazing detail in the rings that can be seen from Earth. The rings are lettered more or less in the order that they were discovered, not in their order from Saturn. (Their actual order from Saturn is D, C, B, A, F, G, E.)
When we think of Saturn's rings, we are usually imagining the globe of Saturn surrounded by its brightly reflective disk. This disk is composed of two rings, the outer A ring and inner B ring, separated by the Cassini division. These main rings are astonishingly thin. Though they spread hundreds of thousands of kilometers across, they are less than 100 meters thick. That’s thinner than a piece of tissue paper spread across a football field.
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Saturn's A ring
This Voyager 2 image of Saturn's A ring shows it to be fairly smooth. The widest black gap in the orange ring is the Encke gap. You can see another division very close to the outer edge, the Keeler gap, and above the outer edge, the moon Prometheus. The Cassini Division lies at the extreme lower right of this image, which was made by compositing green, violet, and ultraviolet images captured on August 23, 1981. Credit: NASA / JPL |
The Cassini Division
In between the A and B rings is a black gap called the Cassini division. This gap is kept mostly free of large particles because of gravitational interactions with the moon Mimas. Objects at this distance from Saturn orbit twice for every one time Mimas orbits; the gravitational shove that Mimas gives the particles every time they meet pushes them into different orbits, opening up a gap in the rings. There are diaphanous ringlets of fine dust in the Cassini division, however.
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Through the dark side of the rings
In this unusual view, Cassini peers through the ring system from its unlit side to a crescent view of the globe of Saturn on August 2, 2005. The view makes clear the differences in density between the main rings. At the top is the thready C ring. In the center, the darkest band is the B ring, which is completely opaque in places. Next is the Cassini division and its thin lanes of dust. The A ring is semi-transparent, and includes a prominent narrow gap, the Encke gap. Finally, at the outer edge, is the F ring. In this viewing geometry the F ring is actually bright as seen from the unlit side, because it is extremely tenuous (so it allows nearly all light through), and its fine dust particles scatter light forward to the observing spacecraft. Credit: NASA / JPL / Space Science Institute |
The B Ring
The B Ring is the inner of Saturn's two brightest rings. Unlike the A ring, which permits some light to pass through it, the B ring is dense enough to be opaque to light. It is so dense that its particles cannot avoid touching each other, and behave more like a fluid than they do like individual particles. The particles appear to be organized into uncountable ringlets, many of which are eccentric (not circular). Many of these ringlets are actually all part of one spiral structure wrapping tightly around and around Saturn. No matter how closely Cassini looks at the B ring, it sees more waves, ringlets, and structure. The B ring must be a dynamic and changing place.
Panorama of Saturn's rings
A spectacular panorama across Saturn's ring system was captured by Cassini on December 12, 2004, at a distance of approximately 1.8 million kilometers (1.1 million miles). The scale in the image is 10.5 kilometers (6.5 miles) per pixel. The view reveals myriad ringlets, gaps, wave patterns, and subtle color variations across the rings, particularly the B ring, which lies at the center of the view. Credit: NASA / JPL / Space Science Institute |
Ring Spokes
Voyager 1 and Hubble saw "spoke" features running across the B ring, but they had disappeared by 1998. Cassini first sighted the spokes in the summer of 2005. That timing coincides with the changing of Saturn’s seasons.
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Spokes in the B ring movie
This movie is made from many separate Voyager 1 images of the B ring. Spokes are apparently a seasonal phenomenon; they were readily visible to the Voyagers, when Saturn was near its equinox, but were not visible to Cassini when it arrived during Saturn's southern summer. Credit: NASA / JPL |
The C Ring
Next inside from the B ring is the C ring. It is much fainter than the A and B rings and appears to be made of lots of separated, dark, dusty ringlets. The C ring is also known as the "Crepe Ring" because of its relative transparency. In Cassini images of Saturn, the C ring can be seen to cast fine, thready shadows across Saturn’s globe. The best views of the C ring itself are obtained when the Sun illuminates the side of the rings opposite to the viewer, because the fine dust particles that make up the C ring scatter light forward to the viewer, much like dust motes in a sunbeam.
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Close view of Saturn's C ring
On October 29, 2004 Cassini gazed down upon the C ring from the unlit side of Saturn's rings. In this geometry, fine particles in the C ring scatter light forward, making the otherwise faint C ringlets appear bright. The noticeably dark Maxwell gap is near the upper right. It contains a bright, narrow ringlet called the Maxwell ringlet. Credit: NASA / JPL / Space Science Institute |
The D Ring
Interior to the C ring is a very dim fuzz of material called the D ring. It's not visible in most Cassini or Voyager photographs; long exposures must be taken at favorable geometries to bring out detail. Cassini has discovered that although some ringlets within the D ring have stayed in the same place since Voyager saw them, one of the ringlets has moved 200 kilometers (120 miles) closer to Saturn in the 25 years that separate the two missions at Saturn.
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Changes in the D ring from Voyager to Cassini
A view of the D ring from Cassini is compared to a view from Voyager at similar viewing geometry. The boundary between the C ring and D ring is marked on the Cassini image by a green line. The C ring is overexposed in the lower left corner of the Voyager image. The outer D ring feature called D73 lines up perfectly between the two images, separated by 25 years. So does an inner D ring feature called D68, near the right sides of the two images. But in the middle, the brightest feature in the Voyager image, called D72, does not line up with any feature in the Cassini image. The inset view shows a region outside D73 with its closely spaced wavelike features, so regular that they could be the grooves in a vinyl record. Credit: NASA / JPL / Space Science Institute |
The F Ring
Outside rings A, B, C, and D, but lying actually quite close to the A ring, is the narrow F ring. It was discovered by Pioneer 11 in 1979. It was theorized that a thin, narrow ring could be held together by so-called "shepherd moons" -- one orbiting inside the ring and one orbiting outside. Voyager 1 discovered the F ring's shepherds, Prometheus and Pandora, in 1980. Although it's narrow and dim, the F ring displays a lot of interesting structure, including clumps and moving ringlets. One of the F ring clumps may be a tiny moon that actually passes back and forth through the F ring on each eccentric orbit.
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Prometheus and the F ring The tiny moon Prometheus excites complex structures in the F ring. In this view the F ring separates into five distinct strands. The inner strands are disrupted by Prometheus' occasional forays into them as it travels on its elliptical orbit. Credit: NASA / JPL / Space Science Institute |
Pandora and the F ring Pandora and the F ring pose in a Cassini view captured on May 4, 2005. The night side of Pandora is illuminated by Saturnshine. The F ring is separated into five or six distinct strands, and there are prominent kinks in its central strand. Credit: NASA / JPL / Space Science Institute |
The Outer Rings
Outside the D, C, B, A, and F rings are two more rings of a very different character. The G ring (closer to Saturn) and the E ring (farther away) spread out over a much larger vertical distance than the super-flat inner rings. They are also many times more tenuous and transparent than the inner rings.
Credit: NASA / JPL |
The E Ring
The E ring was first discovered telescopically in 1967, and its presence was confirmed by the Pioneer 11 flyby in 1979. It is a thick disk of very fine icy or dusty material, with the individual particles only one micron (a millionth of a meter) across. Five of Saturn's seven largest moons are embedded within it: Mimas, Enceladus, Tethys, Dione, and Rhea. Cassini has discovered that active vents on Enceladus’ south pole are the source of the material in the E ring. The vents produce tiny particles of water ice, and the motions of the moons and magnetic field of Saturn spread the material out from Enceladus into a broad doughnut around Saturn.
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Saturn's E ring
This photo of Saturn was captured from a University of Hawaii telescope during the ring plane crossing on August 10, 1995. Saturn and its interior rings have been blocked from view so that their brightness won't wash out the image. The E ring is a very faint diagonal line extending to the left and right. On the E ring you can see four spots where icy moons reflected so much light that they saturated the camera's detector: from left to right, they are Rhea, Dione, Mimas, and Tethys. Source Credit: Institute for Astronomy, University of Hawaii |
The G Ring
The G ring was discovered by the Voyager 1 spacecraft in 1980. It is extremely thin, but unlike the E ring it is probably made of "macroscopic" particles (that is, particles that you can see without a microscope). Consequently it would be much more dangerous for a spacecraft to fly through the G ring than through the E ring. Cassini has discovered that the G ring contains “arcs,” regions of denser ring material.
An arc in Saturn's G ring
These three images of the tenuous G ring were taken by Cassini about 45 minutes apart on May 24, 2005. In the first image, a bright arc is visible at the bottom edge of the ring. In the middle image, the bright arc has rotated around to the ansa (left side) of the ring. In the right image, the arc has moved up and to the right. The origin of this arc is unknown. Credit: NASA / JPL / Space Science Institute |
What are the rings made of?
The main rings -- D, C, B, A, and F -- are quite bright and incredibly thin, no more than 10-30 meters thick compared to their expanse of 275,000 kilometers (165,000 miles) across. (If the A ring was as wide as a football field, it would be as thick as a piece of tissue paper.)
From the colors of the rings, scientists know that they are made of chunks of icy material, contaminated with a small amount of dust. Observations of the rings with infrared and radio waves have shown that the sizes of the particles in the rings vary, from microscopic particles in the E and D rings, to sand- and pebble-sized particles in the C and F rings, to cobble- and boulder-sized particles in the A and B rings. Some particles in the B ring could be much larger, up to a few tens of meters across.
Frequent collisions between ring particles would tend to break big chunks into smaller ones, but big chunks may gather smaller chunks around them under the force of gravity. Little rubbly moonlets may constantly form and be broken apart. Cassini’s images of the F ring may have shown this process in action.
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"Dynamical Ephemeral Bodies" in the rings of Saturn
An artist's conception of the ephemeral chunks and agglomerations of icy material making up Saturn's A ring. Credit: © William K. Hartmann |
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