Acknowledgement. We thank Science for their permission to use an excerpt from:

Smith, B. A., et al. 1986. Voyager 2 in the Uranian System: Imaging Science results Science 233 (4759), 43-64. (Excerpt from pp. 48-55.)

Copyright AAAS, July 4, 1986.

Voyager 2 in the Uranian System: Imaging Science Results

The Uranian Rings

Earth-based stellar-occultation observations of the Uranian rings have been until now the dominant source of our knowledge of this distant ring system (17). Analysis of these near infrared observations indicated the existence of nine narrow, sharp-edged rings of relatively high optical depth (tau >= 0.3). In order of increasing orbital radius from the center of Uranus, they have been unofficially referred to as the 6, 5, 4, alpha, beta, eta, gamma, delta, and epsilon rings. All but the eta ring are eccentric; all but the eta, gamma, and epsilon rings are inclined. The most inclined rings, 6, 5, and 4, achieve maximum heights above Uranus' equatorial plane of 24 to 46 km. These three rings are also among the narrowest; their modeled mean widths, corrected for the effects of diffraction and finite star diameter, are all 3.5 km or less. The eta ring, equally narrow, is accompanied by a shoulder 55 to 60 km wide and of variable optical depth (tau<= 0.1). The Earth-based data also showed the epsilon ring to be by far the widest and most eccentric of the rings. Its eccentric radial amplitude is 404 km, five times larger than that of the next most eccentric ring (ring 5).

All nine rings vary in width and optical depth with position around the ring; in most cases these variations are consistent with a radially integrated mass density independent of azimuth. However, only the widths of the epsilon, alpha, and beta rings vary systematically. The epsilon ring width ranges from 20 km at periapse to 96 km at apoapse. The alpha and beta ring widths range from approximately 5 to 12 km; these extremes occur about 30 degrees from the periapse and apoapse respectively (18, 19).

Earth-based imaging observations of the Uranian rings at 2.2 micrometers (20) and at visual wavelengths (21) yield, for both spectral regions, a reflectivity integrated over the entire known ring system of only 1.6 to 2.0%. In actuality, these observations are mostly of the epsilon ring, because it alone contains 70% of the surface area of the rings. Such imaging observations place an upper limit on the optical depth of 0.0015 at 2.2 micrometer for any diffuse material that is uniformly distributed over the 9300-km extent of the ring system; the upper limit on the product of width and optical depth for narrow rings, observed from Earth-based stellar occultations, is approximately 0.2 km (18, 19).

The primary objectives for imaging of the Uranian rings were (i) to observe ring phenomena on azimuthal scales smaller than what could be observed from Earth; (ii) to search for additional rings, both diffuse and narrow, within and surrounding the known rings; (iii) to obtain information about the particle size distribution by observing over a range of phase angles (approximately 12 degrees to 172 degrees) much wider than that observable from Earth (about 0 degrees to 3 degrees); (iv) to determine the reflectivity and color of each ring; and (v) to search for new satellites---those within the rings that might be responsible for ring confinement and those between the rings and Miranda that might play a part in affecting the rings' kinematic behavior.

Inbound, low phase angle observations. Voyager observations during the few days before encounter were made at phase angles ranging from 14 degrees to 25 degrees and at image resolutions of 20 km per line pair or more. Substantial improvements in the stability of the spacecraft since the Saturn encounter made it possible to obtain several unsmeared images of the rings with exposures as long as 15.4 seconds. Two such low phase angle images of the ring system are combined as a mosaic in Fig. 9A. Targeted to capture the epsilon ring at its widest point, this image resolves the ring into two bright inner and outer regions separated by an area of intermediate surface brightness, features observed previously in Earth-based stellar occultations. Except for the epsilon ring (Fig. 9B) and the broad outer shoulder of the eta ring (Fig. 9C), all rings were unresolved in even the best unsmeared low phase angle frames. From occultation modeling of the ring shapes, the alpha and beta rings are predicted to be 10 and 12 km wide, respectively. Although Voyager images show systematic azimuthal variations in total integrated brightness around the alpha and beta rings, and although frames such as those in Fig. 9A contain evidence for small-scale azimuthal variations in the brightnesses of the remaining rings (including the broad component of eta) more detailed analyses and modeling are required to obtain information about longitudinal variations in the surface brightness of the rings.

For the resolved rings, the ring particle albedo may be determined once the optical depth, or fractional cross-sectional area of the particles, is known. It has been pointed out recently that these fractional cross sections are about one-half the generally accepted values derived from Earth-based stellar occultations, indicating a particle surface albedo of 0.04 to 0.05 at visual wavelengths (22). Initial analysis of Voyager images of the epsilon periapse and apoapse points has given comparable low albedo values, assuming periapse to be opaque and angular variation of particle albedo to be zero. Similarly, low particle albedos are inferred for the resolved, low optical depth component of the eta ring. These estimates of a few percent for the geometric albedo of the particles should be improved as more accurate calibrations of the imaging data and direct measurements of the ring optical depths from the Voyager stellar and radio occultation experiments become available.

To gain further information about the optical properties of the rings, several color sequences were acquired. Because of the very low ring surface brightness, the smear characteristics, and the low throughput of the color filters, the average signal level in these color images was less than 1 DN for all but the epsilon ring. (A DN, or data number, is the smallest unit of digitized image brightness.) Brightness values were averaged along each ring over as large an azimuthal extent as possible to increase the signal-to-noise ratio. We averaged the results for two sets each of green, violet, and clear images. The reflectivity at the apoapse of the epsilon ring, which is the only resolved ring, is identical in all filters to within the measurement uncertainty (~10%). We conclude that the epsilon ring is gray. Absolute reflectivities of the remaining rings could not be measured reliably in these frames. When scaled relative to the epsilon ring, apparent differences color were initially seen; however, the total integrated light from each ring differed between filters by less than twice the estimated uncertainties. Thus, a more careful analysis will be needed to determine whether any color variations exist.

Figure 9A also shows a narrow-angle image acquired from 1.12e6 km (resolution ~20 km per line pair) that contains the first Uranian ring discovered by Voyager. The ring, designated 1986U1R, has a typical reflectivity (23) of only about 1e-4. At the level of the noise, hints of radial structure can be seen. This and five subsequent views of the ring (including those in Figs. 10 and 13) were acquired at different azimuths and are consistent with a circular ring of orbital radius 50,040 +/- 15 km [in a system defined by the absolute radii assigned to the known rings (19)] Another previously unknown ring was detected in backscatter outside the beta ring. This feature, whose brightness is roughly one-tenth that of the beta ring, is faint in the three images in which it is seen and is situated at an orbital radius of about 45,736 km. No evidence of eccentricity has been found. Searches for other features in inbound images have not yielded credible positive results, even though extremely faint features at or below the noise level are suggested in some images.

Ring plane crossing: Intermediate phase angle observations. Figure 10 is a mosaic of three in a series of four images taken from a distance of 1.18e5 km and a subspacecraft latitude of approximately -5 degrees about 11 minutes before Voyager crossed the ring plane. All nine previously known rings are visible, as is 1986U1R. The innermost image of this series (Fig. 11A) reveals a broad band of material inside ring 6 never reported in Earth-based observations. The significant differential smear in this frame is due to the relative motion of the spacecraft and the rings. The exposure time of this image was 7.68 seconds, resulting in about 15 pixels of smear on the far arm of the rings and significantly more on the near arm. Nonetheless, because the smear is dominantly in the direction perpendicular to the ring plane, the natural radial width of any feature in this geometry is preserved at the ring ansa. Measured at this location in the image, the broad sheet of material covers a radial range of 37,000 to at least 39,500 km; the uncertainty in these numbers is approximately 200 km. Radial structure may be present in this feature, but further inspection is necessary to confirm it. This diffuse inner ring has an I/F value (2) in the range of 1e-4 in this image, which was taken at an inclination angle of less than 5 degrees (Fig. 11B). For phase angles of about 90 degrees, the reflectivity of a particle (23) is probably on the order of 1e-2 whether the particle is microscopic or macroscopic; therefore, the optical depth of the diffuse sheet is probably in the vicinity of 1e-3. However, without further knowledge of the particle size, we cannot even begin to estimate the mass of this feature.

A view of the rings silhouetted against the planet, taken with the wide-angle camera from a ring-plane elevation angle of 9.8 degrees about 30 minutes before closest approach, is shown in Fig. 12A. The spacings of the 6, 5, and 4 rings are markedly different in this image than in the basically pole-on view of the inbound images (Fig. 9). This is a direct consequence of the inclinations of the rings and the position of the spacecraft with respect to the nodal lines of rings 5 and 6, which differed at the time of encounter by 160 degrees. The configuration of the rings in this geometry (Fig. 12B) matches predictions for ring positions from Earth-based models (19). Any discrepancies in the match are comparable to the uncertainties in the inclinations assigned in those models. Consequently, although a view from low spacecraft latitude provides a far better geometry from which to measure and refine inclination than does the pole-on geometry of Earth-based observations made during the past 10 years, we do not expect that Earth-based values of inclination can be improved by Voyager imaging results alone.

The gamma and 5 rings in Fig. 12A are significantly less conspicuous relative to other rings than in, for example, Fig. 9. This difference may be due merely to different combinations of width and optical depth between the rings at the longitude of the image (Fig. 12B). Specifically, the PPS observations (24) show that the gamma ring is far narrower and more opaque than its neighbors; that it is completely unresolved readily explains the appearance of Fig. 12A.

High phase angle observations. After Voyager emerged on the antisunward side of the rings, a sequence of images was acquired to take advantage of the forward-scattering brightness-enhancement of the micrometer-size particles that have been seen in the Jovian and Saturnian ring systems. The image in Fig. 13 was taken through the clear filter of the wide-angle camera at a phase angle of 172.5 degrees and a range of 2.25e5 km from the region of the rings shown. The planet was then occulting both Earth and the sun as seen from Voyager; the lack of scattered sunlight provided the opportunity to obtain this long-exposure (96-second) image to search for extremely low optical depth material. The image was targeted at the location on the rings where the component of the spacecraft smear in the radial direction vanished. The combination of orbital and spacecraft motions in the azimuthal direction amounted to 1400 km in 96 seconds, smearing out azimuthal variations below this scale. The differential radial smear in the rings from left to right across the frame results from the curvature of the rings. In spite of the large azimuthal smear, a suggestion of azimuthal structure can be seen in the frame.

Using the best current model (19) for the nine known rings, the spacecraft trajectory data, and the predicted camera direction, we computed the predicted locations of the rings in the frame. Once the locations for the alpha and beta rings are aligned with the bright features in Fig. 13, the remaining rings fall at the indicated positions. All nine main rings are identifiable in the image; although the narrow component of the eta ring is not seen as a bright feature, its broad component is easily visible. The closest point to Uranus on the ring plane in this image is about 39,800 km, roughly the same as the outer radius of the broad sheet of material detected at ring plane crossing (Fig. 10). The brightest feature in Fig. 13 falls at the position of 1986U1R, the newly discovered tenth ring. The feature is 38 km wide and is bounded on either side by sharp edges. Structure in the vicinity of 1986U1R may be associated with structure seen faintly in backscatter. Just inward of 1986U1R is a dark but not completely empty lane coinciding approximately with the orbit of 1986U7. The structure of this lane has important implications for the behavior of small particles under the simultaneous disturbing influences of a shepherding satellite, gas drag, and plasma drag; all such forces may affect small particles in orbit around a planet that has a displaced and highly inclined magnetic field and extended exosphere (25- 27).

The amount of detail in this frame and its spatial scale is perhaps the most unusual result of Voyager's ring observations at Uranus. Features as narrow as the resolution limit (12 km per pixel) are visible. However, with the exception of the nine previously known rings, the two new ones discovered by Voyager in backscattered light, and the features outside the beta ring, none of the remaining bright narrow features in this frame can be seen in backscattered light. Moreover, the brightness of this region is low; the apparent reflectivity, I/F (2), is typically 1e-4, with the brighter regions typically 1e-3. These brightnesses are far too high to be produced by rings of macroscopic particles that would not have been detected either by Earth-based stellar occultation or direct reflectivity observations or by inbound Voyager imaging observations. Rather, the brightness requires the strong forward-scattering characteristic of wavelength-sized "dust" particles such as that observed in the Jovian ring and in various regions of Saturn's rings. This obvious lack of correlation between regions of high dust density and of large particles is discussed below.

Search for small satellites. The Voyager 2 cameras were used to search for satellites not observable from Earth that might be associated with the ring system. It had been suggested that the nine extremely narrow rings might be confined by pairs of small satellites (28) that apply torques to the ring particles to counteract their natural spreading tendency. Imaging sequences were planned to search the areas between Miranda and the epsilon ring as well as within the ring system.

The images revealed ten new satellites, one orbiting just inside the epsilon ring and the remaining nine orbiting outside the rings and inside the orbit of Miranda (Table 2). The largest, outermost, and first discovered, 1985U1, orbits about halfway between the epsilon ring and Miranda. Except for 1985U1, most of these objects are only a few pixels wide in the images. Estimating albedos and diameters is difficult at such low resolutions; the values given Table 2 represent minima for albedo and maxima for diameters.

The ten small satellites lie in a series of concentric, nearly circular orbits, roughly coplanar with Uranus' equatorial plane. Figure 14 shows two of the new satellites, designated 1986U7 and 1986U8, straddling the epsilon ring in an image taken from a range of 4.1e6 km. The nine smallest satellites all appear to have reflectivities of a few percent, comparable to the reflectivities of the ring particles and 1985U1.

Although there appears to be a weak clustering of semimajor axes between 58,000 and 70,000 km, the small satellites (excluding the two ring shepherds) are distributed roughly uniformly between the rings and 1985U1. This distribution is different from that of Saturn's small satellites, which include three Lagrangian satellites (Tethys and Dione), two coorbitals, and three ring shepherds. Another major difference between the small satellites of Uranus and of Saturn is that the Uranian bodies are all dark, whereas the Saturnian bodies are all bright. This may suggest significant differences in formation conditions, in subsequent evolution, or in processes operating on the small Uranian satellites compared to their Saturnian counterparts.

Bounds on undetected satellites in the rings. Several imaging sequences were scheduled in the last 3 days of Voyager's approach to search for satellites embedded in the ring system. The most useful sequence consists of 150 narrow-angle frames shuttered virtually continuously over a 13.5-hour period beginning about 3 days 5.5 hours before closest approach; the range to the planet during the sequence was 4.2e6 to 3.5e6 km. These images were targeted at a fixed nonrotating position on the rings so that all ring material and embedded satellites would be covered in the sequence. The 13.5-hour period of observation spanned about 2.2 orbital periods of material at ring 6 (radius ~42,000 km) and about 1.6 orbital periods of the epsilon ring. In addition, because the orbital velocity of material in the 6 ring is about 12 km sec^(-1), even the fastest ring object would take 30 to 35 minutes to cross the narrow-angle field of view. Because the frames were shuttered approximately every 6 minutes, an object should be observable in at least four or, more likely, five or six frames, making possible the confirmation or rejection of the identification of new satellites.

Subsequent spot coverage with the narrow-angle camera imaged approximately 80 percent of the orbital longitudes of the rings. Although shuttered to within 1 day and 1e6 km of closest approach, this sequence has insufficient redundant narrow-angle coverage; thus we could not confirm or reject suspect identifications. Predictions of the location of candidates in earlier narrow-angle frames will eventually provide a means of their confirmation or rejection. For this initial report, we therefore establish a bound on the size of undetected satellites on the basis of the earlier 150-frame sequence. By comparison with the resolved image of the inner epsilon shepherd 1986U7, the radius of the outer shepherd 1986U8 is estimated to be 25 km (Table 2). The outer shepherd was observed to be about 8 DN above background in a 15.36-second, narrow-angle, clear-filter frame taken at about 3.6e6 km during the 150-frame sequence. The shepherd's image was actually a streak 8 pixels long, and therefore the effective exposure was only 2 seconds. Orbital motion alone accounted for about 5 pixels of the streak's length. Therefore, an integration time of only 2 or 3 seconds will occur in most cases (except for those in which other random motions significantly add to or cancel the smear). A 2-DN, 8-pixel streak in such a frame would also be easily visible; therefore, an object about 15 km in radius would be detectable unless its apparent reflectivity was considerably lower than those of the known shepherds (about 2% at these phase angles). Because a 1-DN, 8-pixel streak would also be visible, redundant frames may allow the detection limit to be lowered to a radius of about 10 km (reduced by square root of 2). We therefore adopt a current detection limit of a 10-km radius for the entire region of the main rings. We expect that careful analysis of the nonredundant spot coverage will eventually allow reduction of this diameter limit by about a factor of 2.

Discussion of the ring observations. The ring systems of the outer planets differ considerably in particle properties, overall optical depth, and radial structure. The Voyager data have shown that the particle albedos in at least the epsilon and eta rings are low (about 5%), comparable to the low albedos of the newly discovered small satellites associated with the rings. These direct results are in good agreement with estimates made of ring material albedo using Earth-based observations before Voyager data were obtained (21, 22). Initial analyses of the relative brightnesses of the other rings suggest similarly low particle albedos. The dark, colorless surfaces of the Uranian ring particles are important in the study of the origin and evolution of the ring-satellite system; they contrast sharply with the brighter and definitely reddish surfaces of the ring particles and satellites of Jupiter and Saturn. The nine main Uranian rings, although similar in kinematics and structure to the narrow ringlets observed in Saturn's rings (29), are also unusual in that their fractional abundance of dust is low. From high phase angle Voyager images we estimate the areal fraction of micrometer-sized particles in the main rings to be of order 1e-3 to 1e-4. For comparison, optically thick parts of Saturn's rings contain several percent dust (30), and Jupiter's ring is thought to have roughly comparable optical depths due to dust and macroscopic particles (31).

The presence of micrometer-sized dust particles is of interest primarily because such small particles are short lived in any ring environment. Thus their presence indicates local sources for example, larger particles that can survive the processes of drag, sweepup, and erosion that can rapidly remove micrometer-sized particles. Contrary to our suspicions at the time of encounter, closer investigation of the magnetic field perturbations shows them to be of questionable importance (32). However, the results of the Voyager UVS investigation at Uranus (6) indicate that, at the eta ring for example, the extended Uranian exosphere and (extrapolated) neutral atom densities are high enough to remove a 1 micrometer particle in about 2000 years by gas drag alone (26). This rate is equivalent to the removal of several 10-m radius objects per year or of one 25-km radius object in the age of the solar system. Thus the dust may well be in steady state, being regenerated by some process at the same rate at which it is lost from the system by gas drag. Because of the rapid orbital decay of micrometer-sized particles, no shepherding process would be effective on the inferred dust shown in Fig. 13.

Possible sources of dust in moderately opaque rings include locally active, internal processes such as collisions caused by spiral-wave damping or shepherding by nearby small satellites (30). Another such process may be collisions among parent bodies that are more energetic in the optically thin inter-ring regions and that thereby liberate more dust upon collision.

Alternatively, the creation of dust may result from the direct impact of meteoroids onto as yet unseen parent bodies only a few kilometers across [as is likely for the Jovian ring (31)] or onto debris belts maintained by such parent bodies in "horseshoe orbits" (33), thereby preventing rapid orbital decay caused by gas drag. The low optical depth of such belts of parent macroscopic particles would be more easily maintained in horseshoe orbits than by gravitational torques, whose operation requires collisional damping. Even such bands of macroscopic debris as may currently exist may themselves be only transient products of recent large meteoroid impacts. In this "external production" scenario, the lack of correlation of the brightest dusty features with the main rings of high optical depth (Fig. 13) would require the mechanism that removes the dust to be more efficient in the main rings than elsewhere.

The dust in the rings (Fig. 13) exhibits considerably more fine structure and an optical depth that is orders of magnitude higher than the dust in the Jovian ring and in Saturn's G and E rings. The Uranian dust rings are similar in structure to the D ring of Saturn. Both Saturn's D and the Uranian rings contain multiple, narrow, well-defined bands of material of low optical depth with no obvious dynamical means of maintenance. Like Saturn's D ring (34), the brightness of the dusty features in the Uranian rings, as seen in forward scattering, correlates poorly with the optical depth of the underlying larger particle material (34).

Regardless of the process responsible for the optically thin dusty bands, the nine main rings require some active shepherding process that will function in the presence of the inevitable energy loss that accompanies their frequent interparticle collisions; evidently they require some form of gravitational torque. A major contender for the role of maintaining narrow, high optical depth rings has been the shepherding mechanism proposed by Goldreich and Tremaine (28) after the discovery in 1977 of the narrow, opaque Uranian rings. The process involves excitation and damping of orbital eccentricities in ring material by nearby satellites; the net result is a transfer of angular momentum that keeps the ring material away from the satellites.

There are several manifestations of the shepherding process that involve similar physics but produce quite different results. One of the most important factors is whether the perturbing satellite is close enough to its "flock" for overlap of multiple Lindblad resonances (35). This is the case for Saturn's Encke gap, which is cleared by an embedded shepherd (36). However, when the satellites become so far from the ring that the distance between successive Lindblad resonances becomes larger than the interaction width of the resonances, the ring edges may be maintained by discrete inner and outer Lindblad resonances with the shepherding satellites (37). This process is much more likely to take place in the Uranian ring environment than at Saturn because the Uranian rings are several times closer to their parent planet; the correspondingly larger gradient of angular velocity leads to lower wavenumber (more widely spaced) resonances for a given ring-satellite separation. In this case, a satellite could only control the ring's edge in the same manner that Mimas controls the outer edge of Saturn's B ring (38).

We searched for Lindblad resonances of the outer and inner epsilon shepherds that fall near the epsilon ring. These locations were computed from the mean motions determined from multiple imaging observations of each satellite. The spacing of the inner Lindblad: resonances of 1986U8 is approximately 200 km at the epsilon ring; the spacing of the outer Lindblad resonances of 1986U7 are about; 60 km at the ring-much greater than either the resonance width [(M/M_U)^0.5]R (where M is the ring mass, M_U is the mass of Uranus, and R is the distance between Uranus center and the ring) (35) or the ring's width. Thus, the resonances seem to be nonoverlapping. A more complete understanding of the relation of such resonances to ring edges must await the refinement of the ring radii from Voyager tracking data and of satellite orbital elements. However, as judged from current values, it does appear that high wavenumber inner and outer Lindblad resonances lie at the outer and inner edges of the epsilon ring, respectively, to within the uncertainties; they may therefore be responsible for this ring's confinement.

We are still left with the problem of the confinement mechanism of the eight inner main rings. Analyses of the irregularities in their radii, widths, and precession rates, a determined from Earth-based stellar occultations, have established limits on shepherding satellite radii of about 10 km (19); this limit is comparable with results presented here based on our best fully redundant coverage.

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