Main Ring Structure

Radial structure As mentioned above, one of the major uncertainties about Saturn's main rings has been: Just how much material do the thickest parts contain? For example, during Voyager, the tilt angle of the rings was only a few degrees as seen from Earth and the Sun, so neither sunlight nor Voyager's radio transmission could penetrate the densest regions, and the single observation of a star shining through the rings was limited by noise [15], so we have remained largely ignorant of its properties in many locations. Cassini was planned to arrive when the rings were tilted nearly wide open to the Sun and Earth for two reasons. First, sunlight is more easily able to penetrate the dense B ring, so images and spectra can be obtained from its unlit side. Second, the large opening angle relative to the Earth means that Cassini's radio occultations—passing its radio transmissions at three wavelengths through the rings—also can penetrate the thickest portions of the B ring. Figure 10.4 gives a sense of the very dense central core of the B ring. This same structure is revealed in

Figure 10.4(a). Image of Saturn's northern hemisphere, covered by the shadows of the main rings. (See next page for a different enhancement of this image.)

much greater detail by still-unpublished radio occultation observations. That is, the central region of the B ring is just as opaque to the probing radio waves, and the same slightly more transparent channels are seen within and around it. The meaning of this structure will need to await further analysis.

We can also look directly at the unlit face of the rings as revealed in Cassini images, such as shown in Figure 10.5. This image was taken of the north (unlit) face of the rings, and shows how the dense B ring blocks most of the sunlight, which at this time was falling on its southern face, from getting through. The moderate optical depth A ring is of intermediate brightness, and the least optically thick rings—the C ring and Cassini Division—appear the brightest because they are optically thin enough for us to see through them to the lit particles on the southern face.

Spiral density and bending waves One of Voyager's most important discoveries about the rings (see Chapter 9) was how moons create spiral waves which propagate slowly

Figure 10.4(b). A brightness-enhanced and stretched view of the same image, showing a very dense central core of material in the B ring, which seems to be cut by a few lower density channels.

through the rings. Each moon has a well-defined orbit period—the time it takes to orbit Saturn. In the rings, the orbit period changes continuously with distance from the planet; material closer to the planet orbits in a shorter time. Thus, there is a large, but countable, number of places where the ring-particle orbit periods are a simple integer fraction of some moon's period; these locations are called orbit resonances [16]. At resonances, the small gravitational forces from a moon act repeatedly on the same ring material, and after some number of orbits cause noticeable disturbances in its behavior. These disturbances create condensations of ring material, and the tiny gravitational forces from these condensations generate further disturbances in particle motions which propagate away from the resonance in a tightly coiled spiral that looks like a watchspring (Figure 10.6). Spiral density waves, where the wavelength decreases outwards, are common; moreover, for moons which are also inclined relative to the rings, the resulting up-and-down forcing creates spiral waves of vertical motion— corrugated or flapping structures called spiral bending waves, where the wavelength decreases inwards (Figure 10.6 shows examples of both). The wave patterns are fixed

Figure 10.5. Saturn's main rings from the northern (unlit) face. The dense B ring appears the darkest. The A ring, at left, contains two empty gaps; the most noticeable is the Encke gap. The Cassini Division lies between the A ring and the very dark, and dense, B ring. Its outer portion is brighter in this view, partly because the particles there are more reflective than those in the inner Cassini Division. The outermost part of the C ring is visible to the right, echoing the appearance of the Cassini division.

Figure 10.5. Saturn's main rings from the northern (unlit) face. The dense B ring appears the darkest. The A ring, at left, contains two empty gaps; the most noticeable is the Encke gap. The Cassini Division lies between the A ring and the very dark, and dense, B ring. Its outer portion is brighter in this view, partly because the particles there are more reflective than those in the inner Cassini Division. The outermost part of the C ring is visible to the right, echoing the appearance of the Cassini division.

in the frame of reference rotating with the moon which causes them, so ring material, which moves more quickly than the moons, actually moves through these wave crests and troughs. The spirals can have multiple arms, depending on the order of the resonance (the number of orbits of ring and moon needed for them to become aligned again).

Much of the theory of spiral waves is in fairly good shape (Section 9.5), so their observed properties can be used as powerful tools to measure the properties of both the moon that causes them and the local ring material through which they move. For instance, the local wavelength, or distance between crests, is a measure of the local surface mass density of the rings (grams per square centimeter), which tells us the total mass of the rings. Knowing both this property, and the optical depth of the rings, tells us the typical particle size (because larger particles have more mass per unit area). Or, if we have independent information on the particle size—for instance, from a radio occultation—we can infer the density of the particles and thus something about their composition. Moreover, the amplitude of the waves (their contrast or strength, from

Figure 10.6. Spiral density and bending waves in Saturn's rings. Each wave is caused by a single orbital resonance with a different moon. Here, the bright, broad wavetrains are caused by Mimas—a density wave at right center and a bending wave at left. The next smaller wavetrains are caused by Pandora and Prometheus. The finely spaced lines throughout are each individual wavetrains—here not resolved—caused by Pan in the nearby Encke gap, which is much closer and much smaller. In addition to the images, dozens of stellar and radio occultations profile the structure of these waves at different longitudes.

Figure 10.6. Spiral density and bending waves in Saturn's rings. Each wave is caused by a single orbital resonance with a different moon. Here, the bright, broad wavetrains are caused by Mimas—a density wave at right center and a bending wave at left. The next smaller wavetrains are caused by Pandora and Prometheus. The finely spaced lines throughout are each individual wavetrains—here not resolved—caused by Pan in the nearby Encke gap, which is much closer and much smaller. In addition to the images, dozens of stellar and radio occultations profile the structure of these waves at different longitudes.

crest to trough) tells us the mass of the moon that causes them. From this and images of the moon which give us its volume, we can determine the density of the moon (Section 10.5).

Density waves damp out as they propagate, because their energy of motion is dissipated in collisions between particles at the small random velocities discussed later in this section. The damping length of a wavetrain can thus tell us something about the

Spiral Density Waves Ring

Figure 10.7(a). Closeup images of spiral density waves (above and left part of 10.7(b)) and spiral bending waves (right half of 10.7(b)) in Saturn's A ring, taken by the Cassini ISS instrument during the short SOI time period when the spacecraft was flying immediately above the rings. The images are small sections of spirals that are as tightly wrapped as watchsprings. Because these are taken from the unlit face of the rings, opaque regions appear darker. The image on the opposite page—Figure 10.7(b)—is a higher resolution piece of the terrain covered by Figure 10.6; the image above shows a grainy structure dubbed "straw".

Figure 10.7(a). Closeup images of spiral density waves (above and left part of 10.7(b)) and spiral bending waves (right half of 10.7(b)) in Saturn's A ring, taken by the Cassini ISS instrument during the short SOI time period when the spacecraft was flying immediately above the rings. The images are small sections of spirals that are as tightly wrapped as watchsprings. Because these are taken from the unlit face of the rings, opaque regions appear darker. The image on the opposite page—Figure 10.7(b)—is a higher resolution piece of the terrain covered by Figure 10.6; the image above shows a grainy structure dubbed "straw".

local random velocities, or collisional velocities, between ring particles. Both stellar occultations by VIMS and UVIS, and radio occultations by RSS—all still under analysis—have revealed perhaps hundreds of new spiral waves and have traced the damping of many of them much more carefully than possible with Voyager observa-

Figure 10.7(b)

tions [17]. Some waves are now seen to propagate more than 600 km—much farther than expected and consistent with predictions of theories that many scientists had previously treated with skepticism. Two extreme closeups of spiral waves are shown in Figure 10.7(b), taken by the Cassini ISS team just after the SOI burn, having a spatial resolution of only 200m/pixel!

One interesting new observation of interest revealed in Figure 10.7(a) is the speckled dark blotches seen at the lower left, in the troughs (bright, low-density valleys) between the opaque, dense crests of density maxima. These appear to be densely packed clumps of particles, jammed and stuck together as particle trajectories converge in the crests of density waves. Emerging from the dense crests, these plugs of ring material—as long as a freight train and thicker in diameter—eventually scatter apart as they collide with other particles. Understanding this effect will help us understand the physics that transpires in the dense crests of density waves.

Main ring irregular structure Given the complexity of the irregular structure [18] and the variety of data required to unravel it, much of which is still under analysis, it's a little early for specific advances to be evident. Theoretical advances in this general area have been substantial since Voyager, however. A number of studies have converged on a process called the "pulsational instability" or "overstability" as an explanation for irregularly spaced, fine-scale structure in the rings [19]. This process relies on a combination of viscosity and self-gravity of the ring material, acting in opposition. Self-gravity tends to make dense structures become denser, depleting surrounding regions, but if viscosity increases with increasing density faster than gravity does, viscous diffusion spreads the dense material out again, causing the adjacent regions to become denser in their turn. A number of theoretical models show that very fine scale structure can indeed develop if the optical depth is sufficiently high. The length-scale of the structure is very fine—some tens of ring thicknesses in size, or perhaps a few hundred meters. Also, the models show a transient structure, sloshing back and forth without ever forming a permanent, fixed pattern—but always present. In fact, Voyager observations (Chapter 9) had already shown that the finest scale structure in the B ring—that seen at its outer edge—was not azimuthally symmetric but varied with longitude. Whether this longitudinal variation also implies temporal variation is still not known, and more observations and analysis are needed. It has been suggested that, over time, larger-scale structures might grow or "anneal" from the fine-scale structure, but models show that the fine-scale structure always remains underneath.

Cassini observations don't confirm all these predictions, but there are some interesting correspondences. Figure 10.8 compares two moderately high-resolution images of the lit face of the B ring, showing the irregular structure in much better detail and higher sensitivity than seen by Voyager. The images have the same resolution, and it is readily apparent that there is abundantly more very fine scale structure near the outer edge of the B ring than in its center.

Moreover, Cassini ISS images taken at SOI, having a resolution of 200m/pixel, do show fine-scale structure in abundance, in several places—and preferentially where the ring optical depth is the largest (Figure 10.9, two right panels). At locations only a few hundred km away (Figure 10.9, two left panels), the familiar irregular structure is seen with a scale of 100 km or more, which entirely lacks this ultrafine structure. So, in some ways the data are in accord with theoretical predictions that viscous overstability (pulsational instability) will occur in only the most optically thick regions; however, we still seem to need a different mechanism to generate the longer (few hundred km) scale irregular structure that permeates the B and inner A rings where the optical depth is only a little bit lower. Furthermore, we need to understand why the optically thickest regions of all—in the middle of the B ring—don't seem to show as much of this fine-scale structure as regions in the outer B ring and inner A ring, where the optical depth is actually smaller. Cassini radio occultation data and stellar occultation data will ultimately contribute heavily to unraveling this story, providing higher spatial resolution and multiple cuts across the rings at different longitudes and times. However, these data remain in the early stage of analysis.

Figure 10.8. Two images (this and next page) of the lit face of the B ring, taken at nearly the same time and with the same spatial resolution, showing its irregular structure. In the middle B ring (this page) we see abundant structure with scale of a few hundred km, but little if any very-fine-scale structure. In the outermost B ring (next page), terminating with the empty Huygens gap, its ringlet, and the bands of the Cassini Division, abundant ultrafine scale structure is seen. The lower images on both pages are enlargements of about a quarter-scale central part of the upper ones, again both at the same scale.

Figure 10.8. Two images (this and next page) of the lit face of the B ring, taken at nearly the same time and with the same spatial resolution, showing its irregular structure. In the middle B ring (this page) we see abundant structure with scale of a few hundred km, but little if any very-fine-scale structure. In the outermost B ring (next page), terminating with the empty Huygens gap, its ringlet, and the bands of the Cassini Division, abundant ultrafine scale structure is seen. The lower images on both pages are enlargements of about a quarter-scale central part of the upper ones, again both at the same scale.

Figure 10.8 (cont.). Outer B ring. Note the abundant fine-scale structure compared with the middle B ring shown on the previous page.

Cassini Soi Ring

Figure 10.9. Cassini images of irregular structure in Saturn's B and A rings, taken during the SOI period. These images have a resolution of 200 m/pixel and show the rings from their unlit face, so darker regions tend to be more opaque. In the upper two panels, we see two regions in the inner B ring—one where the structure has a smooth, 80-km scale, with no trace of fine-scale structure, and the other where km-scale structure is everywhere. Similar fine structure with km scale is seen in the inner A ring, which is also optically thick, but not in nearby regions.

Figure 10.9. Cassini images of irregular structure in Saturn's B and A rings, taken during the SOI period. These images have a resolution of 200 m/pixel and show the rings from their unlit face, so darker regions tend to be more opaque. In the upper two panels, we see two regions in the inner B ring—one where the structure has a smooth, 80-km scale, with no trace of fine-scale structure, and the other where km-scale structure is everywhere. Similar fine structure with km scale is seen in the inner A ring, which is also optically thick, but not in nearby regions.

C ring and Cassini Division The C ring, Saturn's innermost ring, is an enigma of another sort. In many—if not most—ways, the C ring most closely resembles the Cassini Division—which is not at all empty but a ring, of sorts, in its own right. Their similarities are evident in plots of their optical depth as a function of radius (Chapter 9). Both regions contain several empty gaps, and otherwise empty gaps containing eccentric, sharp-edged ringlets. A few of the empty gaps in the C ring are identified with known satellite resonances, but just as many are not. The C ring also contains several fine examples of spiral density waves which have no assigned cause [20]. Moreover, the C ring presents us with a quite beautiful and simple, but completely unexplained, series of moderate optical depth "plateau" features that are nearly regularly spaced and nearly symmetrically placed about its outer (unexplained) Maxwell gap (Figure 10.10). This structure is also evident in Figure 10.19 [21]. Cassini has obtained a number of images of these regions, has taken several surveys for small embedded moonlets (none have as yet been found), and is starting to do full azimuthal studies to search for telltale structure that might provide evidence for moonlets too small to see directly. Radio and stellar occultations have also begun to sample the region, again with no specific conclusions as yet released regarding the cause for this intriguing structure.

Figure 10.10. The outer C ring, surrounding the Maxwell gap with its embedded, eccentric ringlet (right center). The bright plateau features are not surrounded by gaps or set off in any way, yet maintain their moderately sharp edges in spite of the expected smoothing by diffusion and spreading. They are about 300 km wide. There is no known resonant structure to explain their observed symmetry about the Maxwell gap.

Figure 10.10. The outer C ring, surrounding the Maxwell gap with its embedded, eccentric ringlet (right center). The bright plateau features are not surrounded by gaps or set off in any way, yet maintain their moderately sharp edges in spite of the expected smoothing by diffusion and spreading. They are about 300 km wide. There is no known resonant structure to explain their observed symmetry about the Maxwell gap.

Main ring azimuthal structure As described in Chapter 9, the A ring has long been known to display an azimuthal asymmetry which is thought to be caused by the varying cross-sections of elongated structures, caused by ongoing gravitational instabilities, which are angled to the line of sight. Cassini's many stellar and radio occultations, and even the structure in the ring temperature variations, are showing these differences in wonderful new detail, although no results have yet been published. Figure 10.11 illustrates how this works. Occultation traces passing through the rings at different longitudes result in different apparent optical depths, because the ring structure is not uniform. As more occultation traces are made at different longitudes and elevation angles, it is becoming possible to model the horizontal and even the vertical extent of these structures, and ultimately more will be learned about the particle size distributions which make them up as well [22].

Figure 10.11. Schematic of a stellar (or radio) occultation trace through a patch of rings in which stranded gravitational instabilities, or gravity wakes, have formed. The wakes have a constant typical pitch angle relative to the direction to Saturn, and thus their angle relative to the observer depends on their longitude. Occultation traces passing through longitudes as seen at left let more light pass through, and appear to have lower optical depth, than occultation traces through regions appearing as at right, where more of the starlight is blocked.

Figure 10.11. Schematic of a stellar (or radio) occultation trace through a patch of rings in which stranded gravitational instabilities, or gravity wakes, have formed. The wakes have a constant typical pitch angle relative to the direction to Saturn, and thus their angle relative to the observer depends on their longitude. Occultation traces passing through longitudes as seen at left let more light pass through, and appear to have lower optical depth, than occultation traces through regions appearing as at right, where more of the starlight is blocked.

Main ring vertical structure; ring volume density One of the remarkable things about Saturn's rings is how thin they are vertically for their enormous radial extent (Chapter 9, Figure 10.12). Ever since their discovery, the fact that they essentially vanish when seen edge-on from Earth has fascinated observers. For the last few decades it has been

Figure 10.12. Where'd they go? Although nearly as far from one side to the other as from the Earth to the Moon, Saturn's rings are locally only a few tens of meters thick. Even what we see here, imaged from Cassini in an equatorial orbit, is light scattered from rippling vertical structure in the rings—spiral bending waves a few km thick—and vertically inclined rings such as the F ring. At the top are the shadows of the main rings on Saturn's northern (winter) hemisphere. The blackest shadow is from the densest (B) ring. The upper shadow is from the A ring, showing light coming through the Encke gap.

Figure 10.12. Where'd they go? Although nearly as far from one side to the other as from the Earth to the Moon, Saturn's rings are locally only a few tens of meters thick. Even what we see here, imaged from Cassini in an equatorial orbit, is light scattered from rippling vertical structure in the rings—spiral bending waves a few km thick—and vertically inclined rings such as the F ring. At the top are the shadows of the main rings on Saturn's northern (winter) hemisphere. The blackest shadow is from the densest (B) ring. The upper shadow is from the A ring, showing light coming through the Encke gap.

realized that even other forms of vertical structure in the rings—spiral bending waves, inclined ringlets, and so on—mask the local ring thickness in edge-on observations.

The true local thickness of the rings is determined by the small vertical velocities of particles—the larger these are, the higher particles wander in their orbits, the thicker the rings are, and the lower is the volume or packing density of the particles. Theoretical models which include inelastic collisions between particles imply that the rings—especially the opaque rings where collisions are common—can't be more than a few times the size of the largest particles in vertical thickness, and thus that the packing density of particles can't be extremely low.

For decades, these dynamical expectations have been hard to reconcile with the opposition effect of the rings (Chapter 9) in which the ring brightness increases sharply when the Sun and Earth line up and sunlight is scattered directly backwards to the Earth (also known as the zero-phase effect). This was originally attributed to a shadow-hiding process, and implied that the particles were widely separated in a low-volume density ring. Another theory emerged, based on laboratory observations, in which the opposition effect is caused by interference of photons which travel along opposite but identical trajectories, which implies that shadow hiding plays no role and thus that the ring volume density does not need to be extremely low [23]. The predictions of these two theories were quite different regarding how the effect should vary with the reflectivity of the ring particles. Cassini observations of this effect were planned in order to resolve this difficulty. Figure 10.13 illustrates these observations. In Figure 10.13, we see a Cassini ISS image of the rings taken by the spacecraft with the Sun immediately behind it, so the light is directly scattered backwards. This geometry allows photons from the Sun to traverse slightly different paths having the same total distance between the rings and the camera, allowing them to add coherently together when the phase angle (the angle between the Sun and the camera as seen from the rings) becomes extremely small (i.e., at the bright spot). The test of this effect was whether it would behave the same for particles of high and low albedo, and for rings of high and low optical depth.

The VIMS team observed the same geometry simultaneously, and while their spatial resolution is not as good as that of Cassini's cameras, they have a broader coverage in wavelength and particle reflectivity which has allowed them to conclude that hiding of shadows is not, after all, responsible for this effect and, instead, coherent backscattering apparently is ([24]). This reconciles the observations with dynamical expectations, but leaves us with one less way to measure the actual volume density of the rings.

Diffuse rings Saturn's diffuse, outlying rings have never been in the spotlight, lacking the exotic structure of the main rings. Yet, they are of profound importance because Cassini spends a considerable time simply crossing through them, and we need to understand the nature and abundance of their particles. Furthermore, these modest structures have become ever more interesting because of their time variations.

The E ring: This, for example, is dominated by tiny and short-lived grains; this aspect, plus its association with the unusual moon Enceladus, which Voyager showed to have flow-like structures on its surface, were clues that something perhaps unusual

Figure 10.13. The "opposition effect'' is apparent in this Cassini ISS image of the main rings, with the C ring at lower left and the Cassini Division and A ring at the top right. The Sun is directly behind the spacecraft, and the bright spot in the B ring is the directly backscattered light from the Sun. Its small angular size (less than a degree) is an indication of "coherent back-scattering'' in the grains of the ring particles rather than a measure of their spacing. Images like this were taken to track this bright spot as it moves across the face of the entire ring system. When their analysis is complete we will have a better understanding of exactly how the opposition effect varies from one location to another.

Figure 10.13. The "opposition effect'' is apparent in this Cassini ISS image of the main rings, with the C ring at lower left and the Cassini Division and A ring at the top right. The Sun is directly behind the spacecraft, and the bright spot in the B ring is the directly backscattered light from the Sun. Its small angular size (less than a degree) is an indication of "coherent back-scattering'' in the grains of the ring particles rather than a measure of their spacing. Images like this were taken to track this bright spot as it moves across the face of the entire ring system. When their analysis is complete we will have a better understanding of exactly how the opposition effect varies from one location to another.

was occurring in and around this ring (Chapter 9). Cassini found, months before even getting to Saturn, that the E ring had new surprises in store. The UVIS instrument routinely maps the entire Saturn system for fluorescence from atoms which might be degassed from various rings or moons. Between the end of 2003 and March 2004, the UVIS instrument saw a huge increase in the abundance of atomic oxygen in and around the E ring ([25], Figure 10.14). Now that Cassini has discovered active jets and plumes of dust and ice being degassed from the near-surface of Enceladus [26], it becomes easier to understand such time variation as merely a consequence of sporadic eruptions from Enceladus. This is an idea that actually goes back several decades, but—in the absence of any hard observations of Enceladus—was regarded as somewhat speculative at the time [27]. Other odd and unconfirmed features have been seen

Figure 10.14. Abundance of atomic oxygen in the Saturn system by UVIS in early January 2004 (top) and in February 2004 (bottom). The band outlined in white is the E-ring core. The amount of oxygen that appeared in this one month (and then disappeared again in the next month) is comparable with the entire mass of all the known E-ring micron-sized grains. This puzzle has apparently been resolved by Cassini's discovery of active degassing of water from the moon Enceladus, which orbits in the core of the E ring and presumably supplies its micron-sized ice grains.

W)

Figure 10.14. Abundance of atomic oxygen in the Saturn system by UVIS in early January 2004 (top) and in February 2004 (bottom). The band outlined in white is the E-ring core. The amount of oxygen that appeared in this one month (and then disappeared again in the next month) is comparable with the entire mass of all the known E-ring micron-sized grains. This puzzle has apparently been resolved by Cassini's discovery of active degassing of water from the moon Enceladus, which orbits in the core of the E ring and presumably supplies its micron-sized ice grains.

in or near the orbit of the E ring [28]; perhaps Cassini will be able to confirm such features during its 4-year tour.

G ring arc: Orbiting between the huge, broad E ring and the main rings is the G ring—thought to be rubble left over from disruption of a former moonlet [29]. Actually discovered by Pioneer 11 from its influence on Saturn's Van Allen belts, it was first imaged by Voyager (Chapters 2 and 9). Cassini has been watching the G ring carefully prior to encounter, as scientists and mission planners carefully chose and monitored a trajectory for Cassini to avoid being sandblasted by ring particles on our first approach to Saturn. Since that time, the G ring has been observed in a number of geometries. It remains a diffuse band of material, in about the same place as seen by Voyager, but seems to have given birth to a new feature—an azimuthally incomplete arc of debris, concentrated at certain longitudes (Figure 10.15). Probably, whatever still-unseen moonlets remaining within the G ring's diffuse core—providing

Figure 10.15. Saturn's G ring is several thousand km wide, but its core is primarily visible here. Note in this time sequence an extended clump or arc, starting at bottom center in the left image, moving to the ring ansa or tip in the middle image, and continuing to the far arm in the right image. This feature has been tracked in a number of other images.

Figure 10.15. Saturn's G ring is several thousand km wide, but its core is primarily visible here. Note in this time sequence an extended clump or arc, starting at bottom center in the left image, moving to the ring ansa or tip in the middle image, and continuing to the far arm in the right image. This feature has been tracked in a number of other images.

material to replenish the ring by constant micrometeoroid erosion—can also collide or be hit with a big enough meteoroid to throw off large amounts of debris. The orbital rate of the arc is not yet well enough known to say much about its dynamics. It will be of interest to see how long it lives and how it spreads.

The D ring: Lying even closer to Saturn than the C ring, the D ring was glimpsed only once during each Voyager flyby. It is composed of a number of faint ringlets, each somewhat reminiscent of the G ring (Figure 10.16). However, Cassini observations

Figure 10.16. Comparison of structure in the D ring as seen by Voyager and Cassini. The bright material in the lower image is the inner edge of the C ring. Two of the D ringlets align, but its formerly brightest one has either disappeared or moved and been replaced by a relatively low brightness feature. The insert shows a new kind of very regular pattern seen at the very outermost edge of the D ring.

Figure 10.16. Comparison of structure in the D ring as seen by Voyager and Cassini. The bright material in the lower image is the inner edge of the C ring. Two of the D ringlets align, but its formerly brightest one has either disappeared or moved and been replaced by a relatively low brightness feature. The insert shows a new kind of very regular pattern seen at the very outermost edge of the D ring.

showed that one of these ringlets has either changed location or vanished entirely to be replaced by another nearby fainter ringlet, or perhaps by a gap, even while others seem to be found in the same place. Moreover, a new kind of regular pattern, which has no explanation, has been seen at the very outside edge of the D ring, just inside the C ring.

The particles making up the D ringlets don't contain much mass, so it's not implausible for some impact to have simply blown apart a big boulder into rubble that has spread and become eroded to produce dust; however, understanding how the previously dominant ringlet could just vanish so quickly is more difficult. Modeling of absorption by local boulders is needed to see if it can remove dust in the two decades since Voyager, while remaining invisible today.

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