Online Astronomy eText: The Planets
The Rings of Saturn
(also see Pictures of Saturn, The Structure of Saturn, The Satellites (Moons) of Saturn)

Artist's rendering of the rings of Saturn (NASA, JPL, Planetary Photojournal)


A more detailed view of the inner, brighter rings, comparing visual and radio observations
Click here for a much more detailed view NASA, JPL, Planetary Photojournal)

A view of Saturn and its rings from above, showing the shadow of the planet on the rings. The sunward (daylit) part of the planet is overexposed, so that the partial lighting of the night side caused by reflection from the rings is visible. (This image is a composite of several Cassini spacecraft images, so artifacts caused by combining the images are also visible, particularly on the night side.) (Cassini Imaging Team, SSI, JPL, ESA, NASA, apod070306) An additional ring was recently discovered, in the orbit of Phoebe, which see for a discussion of the new ring.

The Appearance of the Rings
      The rings of Saturn consist of a sheet-like distribution of icy particles, most about the size and composition of snowballs, orbiting Saturn in individual, but very similar, and almost exactly circular orbits, at various distances from the planet. All of the particles are well inside a region defined by the Roche Limit, within which the gravity of Saturn tends to tear apart large objects. They are continually colliding with each other, and building up to larger sizes, and at the same time, continually breaking down, through collisions and their gravitational interaction with Saturn, so that the distribution of particle sizes is roughly constant, over time.
      The rings are over 120,000 miles in diameter, but are very thin, being no more than a few hundred yards thick at any given place, although bending waves, caused by the interaction of the ring particles with each other, and various moons, cause the "surface" of the rings to fluctuate up and down by several times that distance. As a result, when the rings are viewed "edge-on", they can essentially disappear from view. How this works is shown in the diagram, below:


How Saturn's rings are oriented, at various points in its orbit.
(Modified from Chaisson, "Astronomy Today")

     Saturn has an axial tilt (the inclination of its axis of rotation, to its orbital plane) of 27 degrees. When it is tilted "toward" the Sun (as in 1987), or "away" from it (as in 2003), we see the planet, and its rings, from above, or below, and the rings look like a broad, elliptical band (actually, the Sun sees things in that way, but since we are so much closer to the Sun than Saturn is, our view is almost the same). But when it is "sideways" relative to the Sun (and us), as it was in 1980 and 1995, we see the rings "edge-on", and they may almost disappear. In the photograph below, which is a composite of several images taken by the Hubble Space Telescope between 1996 and 2000, we see Saturn from below. In the earlier images, at the bottom left, when Saturn was more sideways to the Sun, the rings are a very skinny ellipse. In the later images, at the top right, when Saturn was tilted away from the Sun, the rings are a fatter ellipse. In the photograph below the composite, Saturn is viewed at the time of the Earth's ring-plane crossing (that is, the time when the rings are exactly edge-on, as seen from the Earth), and the rings virtually disappear from view. In fact, they would completely disappear, if it were not for the fact that the images are considerably enhanced.


The changing appearance of Saturn, as it moves around the Sun.

     This sequence of pictures shows (from lower left to upper right), the appearance of Saturn from 1996 to 2000. When more edgewise (1996, lower left), the rings look like a skinny ellipse. When seen more from below (2000, upper right), the rings look like a fatter ellipse. (R. G. French (Wellesley College) et al., Hubble Heritage Team (AURA / STScI / NASA), apod030405)



The ring-plane crossing of May 22, 1995

     As the Earth moved through the ring-plane, the rings almost completely disappeared. However, the shadow of the rings is visible on the planet, because the Sun was nearly 3 degrees above the plane of the rings.
      In these images, the portion of the rings to the west (right) of Saturn is shown with enhanced brightness. The top image was taken a little while before the Earth crossed the ring plane. Tethys and Dione are visible on the left, and Janus on the right. The middle image was taken very close to the ring-plane crossing, and the rings are only 25% as bright. Rhea is visible to the left, and Enceladus in the enhanced-brightness box on the right. The bottom image, taken 96 minutes later, after the HST had gone around the Earth again, is after the ring-plane crossing. Rhea and, since it had moved in front of the planet as seen from the Sun, its shadow, are visible on the left.
     (Note: These images were taken using light at the 8922 Angstrom (infrared) methane absorption band.) (A.S.Bosh, A.S.Rivkin, HST, NASA, hubblesite.org)



Two more views of the ring-plane crossing of 1995

     In the top picture, taken by the Hubble Space Telescope on August 6, 1995, the rings are barely visible. Titan and its shadow are visible on the left, while Mimas, Tethys, Janus and Enceladus are on the right.
     In the bottom photo, taken on November 17, 1995, the rings are slightly tilted again. Dione is on the lower right, and Tethys is on the upper left.
     A careful examination of the right side of the rings, in the bottom photo, shows a thin dark streak pointing down to the right; this is the shadow of Dione, and shows that the Sun is 'down to the right', relative to our line of sight. In contrast, in the top image, the relative position of Titan and its shadow shows that the Sun is 'up to the left', relative to our line of sight. This change in the position of the Sun relative to our line of sight is not due to Saturn's motion, which is very slow, but to our own movement, around the Sun during the three and a half months between the two dates. (E. Karkoschka (University of Arizona), HST, NASA, apod981018)


Another view of Saturn, with its rings seen exactly "sideways" by the Cassini spacecraft, so that the rings are edge-on, completely disappear "above" the planet, and are visible only as a thin line across its center, and as broad shadows on the left. The small dot above center, near the rings, is 300-mile wide Enceladus. (Cassini Imaging Team, SSI, JPL, ESA, NASA, apod060503)


     Saturn as viewed by the Cassini spacecraft, a day after its August 11, 2009 equinox. At the equinox, the plane of the rings, which is the same as the equatorial plane of the planet, is exactly in line with the Sun, so the rings cast no significant shadow on the planet, and are barely lit by the Sun, making them much darker than usual. (The extremely thin line of shadow running along the equator is due in part to the slight tilt acquired during the day after the equinox, and in part to wavelike structures in the rings caused by gravitational interactions with Saturn's inner moons.) Two views of the planet are shown: above, the entire planet in a composite of 75 images, and below, a closer view of the sunward half of the upper image. (Cassini Imaging Team, ISS, JPL, ESA, NASA, apod090930)




Saturn's rings near its equinox, nearly edge-on to the Sun, and therefore unlit
(Cassini Imaging Team, ISS, JPL, ESA, NASA, apod090825)


Saturn's shadow and rings, close to its equinox
(Cassini Imaging Team, ISS, JPL, ESA, NASA, apod090901)

     Saturn and its rings, seen from its shadow, by the Cassini spacecraft. In this exaggerated-color image, the night side of the planet is lit by light reflected from its rings, while that portion of the rings in the shadow of the planet blocks portions of that reflected light, appearing as darker bands. Meanwhile, the rings still in sunlight (on the right and left), glow brightly (especially in this high-contrast image) as sunlight glints off them, and lights them almost as noticeably as seen from the Earth (which happens to be the small dot just above the brightest portion of the rings, on the left side of the image below). (Cassini Imaging Team, SSI, JPL, ESA, NASA, apod061016)


     The left side of the previous image, magnified to show greater detail. Aside from the bright inner rings, broad diffuse rings surround the planet at greater distances. The outermost, broadest band shown is centered on the orbit of Enceladus, and is caused by ice crystals emitted from that satellite by water-ice geysers, near its southern pole. (credits as for the image above)


     Dark spokes in Saturn's B ring, taken by the NASA Voyager 2 spacecraft on August 22, 1981, when the spacecraft was 2 1/2 million miles from the planet. Narrow spokes, which seem to have been recently formed, rotate with the planet (counterclockwise in this view), regardless of their position in the rings, whereas larger, diffuse spokes, which were presumably formed earlier, rotate with the rings -- faster closer to the planet, and slower further away. This suggests that the spokes consist of charged particles lifted above the ring plane by the planet's magnetic field (which rotates with the planet). Some mechanism -- perhaps lightning strikes, perhaps collisions between larger bodies -- produces a large amount of charged material, and scatters it across a wide area. For a while, its particles rotate with the magnetic field, but as they lose their charge, the spokes grow more diffuse, their motion becomes more like the rings' motion, and they fade away (usually, within a few days of their origin). The appearance of the spokes depends upon the viewing angle. If viewed from the sunward direction, so that light is reflected back toward the observer, the spokes appear dark against the lighter background of the rings, as in this Voyager image. If viewed from the opposite direction, as in the Cassini image below, so that light is scattered toward the observer, but away from the Sun, the spokes appear lighter against the darker background of the rings. (Voyager, NASA)


     When the Cassini spacecraft arrived at Saturn, no spokes were visible in its rings, leaving various controversies about the spokes' nature and origins unsettled. Recently, however, spokes have been seen, as in the above image, taken from the shadow side of the rings, so that the spokes are viewed as light, against the darker background of the rings. It is hoped that further study of the spokes may finally reveal their origin, and more certainly establish their physical nature, and the forces controlling their behavior. (CICLOPS, JPL, ESA, NASA apod061127)

Why Are The Rings So Flat?
     As noted above, the rings of Saturn are incredibly flat -- in most places, only a few hundred yards thick, compared to more than a hundred thousand miles breadth. For this to occur, the particles that make up the rings -- countless numbers of icy bodies of various sizes -- must all have very nearly the same velocity as they orbit the planet. Any difference in their motion, whether radial (in or out, relative to a perfectly circular orbit), tangential (faster or slower, relative to the circular orbital velocity in their vicinity), or vertical (up or down, relative to the plane of the rings), must be less than a hundred thousandth of a percent of their average velocity. In other words, although orbiting Saturn at a hundred thousand miles an hour, their relative velocities, in a given region, must be only a hundredth of a mile per hour.
     The reason for this can be seen by imagining what would happen if there were particles which had a substantial vertical motion, relative to the rest of the ring particles. Such objects would have orbits which move upwards on one side of the orbit, and downwards on the other side, and on opposite sides of the planet, pass through the ring plane. If there were very little material in the way, they might be able to do this indefinitely, and if this were possible, undoubtedly many objects would do so, if only as a result of collisions with other objects, and the rings would be much thicker. But instead, since there is so much material in the rings that in many places, they can block the light of distant stars, the odds are that any particles with significant vertical motions would collide with particles in the ring plane almost every time they had to pass it. And in those collisions, their vertical motion would be "damped out" by being shared with the objects they collided with, and the objects those objects collided with, and so on.
     A similar thing would happen if there were significant radial or tangential variations in velocity, and so in any given region, all differential motions must be as small as the thickness of the rings is, compared to their breadth.

Saturn's Influence Within the Rings
     Although the differential motions inside the rings must be small, there must be occasional collisions between ring particles. Given the slow speeds with which they strike each other, it is more likely that such collisions will build larger and larger objects (this growth through collision is referred to as accretion), even if the objects are made of relatively loose, fragile structures, as many of the icy bodies in the outer Solar System appear to be. In fact, it has been estimated that most of the material within a given part of the rings might well combine to form small moonlets, a few miles in diameter, within periods of a few weeks or months. There is, however, a problem with this -- namely, the immense mass of Saturn, and its correspondingly immense gravity.
     The main effect of Saturn's gravity is, of course, to keep the ring particles in orbit around it; but just as the Moon raises tides on the Earth (and the Earth on the Moon, albeit "frozen" tides), Saturn can raise tides within the larger bodies surrounding it. For very small objects, the difference in gravity due to Saturn on the side of the object closer to Saturn, and on the side of the object further from Saturn, is insignificant, even in comparison with the weak forces which hold the icy bodies together, simply because of their "stickiness". But if they were to grow to several miles diameter, the tidal forces would become much larger; and in fact, any object within a certain distance of Saturn -- referred to as the Roche Limit after its discoverer -- would be stretched by tidal forces from the planet greater than any gravitational attraction it exerts on itself, and as soon as it was large enough, further collisions would tend to shatter it into smaller pieces, rather than build it up to larger size.
     As it happens, virtually all of the ring material lies well within the Roche Limit for Saturn, so although collisions tend to build things up on short time scales, eventually the larger objects tend to break down into smaller pieces, and on the average, the ring particles remain as they are. There is, however, another factor, which promises to end the existence of the rings as we know them, within a few tens or hundreds of millions of years. That factor is the outer atmosphere of Saturn, which although incredibly rarefied at the distance from Saturn of its rings, can exert very small frictional forces on the ring particles, particularly in the inner parts of the rings. As a result, there is a slow infall of material toward the planet, which over very long times will remove most of the material from the rings, and erase their spectacular appearance. This implies that the rings must not be a permanent feature, but were probably formed by the passage of a comet or some other icy body too close to Saturn to remain in one piece, in the not too distant past; "not too distant" in this case, meaning sometime within the last few hundreds of millions of years.

The Influence of Saturn's Moons Within the Rings
     Some of the moons of Saturn can also exert gravitational forces on the rings. The most significant of these is Mimas, which is directly responsible for the Cassini Division -- the large, dark, nearly empty area which divides the outer rings from the inner rings. The Division is caused by a resonance between the orbital period of ring particles that are in that region, with the orbital period of Mimas. (For now, see Mimas for a discussion of this interaction.)

More to Follow, Later
     (1) a discussion of the density waves within the rings
     (2) a discussion of the "spokes" of electrostatic particles sometimes seen there
     (3) a comparison of the short-term accretion of the ring material to the buildup of the planets, in the early Solar System, similar to the following paragraph, but with more explanation (mostly placed, however, with the material on the origin of the Solar System):
A Comparison of the Rings of Saturn to the Solar Nebula
     A similar thing would have happened in the early Solar System, as we will see when discussing the origin of the Solar System. At that time, the now solid portions of the planets would have existed as gases, or as innumerable bits of microscopic dust -- ices, soots and metal oxides -- orbiting the Sun, in the Solar Nebula. With such huge amounts of gas and dust, interactions between the materials in a given region would guarantee that the motions were nearly uniform, and collisional velocities would be very small, allowing things to build up very rapidly. A similar thing occurs in the rings of Saturn, as we will see in class, but because of the close proximity of Saturn (all of the rings are within a region defined by the Roche Limit, where Saturn's gravity is stronger than the gravity of the ring particles), objects that build up to large sizes are more likely to be torn apart, than to grow still further. In the Solar Nebula, the Roche Limit for the Sun was in a region where no solid materials could exist, because of the Sun's high temperature, so it was not a factor; and as objects rapidly built up, through collisions, there was no limit on how large they could grow, save for the available materials.

(older) Detailed images of the rings, showing density and radial waves

Above, an overall image (the visual image near top of this page)
Below, larger views of portions of the rings