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

Lane, A. L., et al. 1986. Photometry from Voyager 2: Initial results from the Uranian atmosphere, satellites, and rings. Science 233 (4759), 65-70. (Excerpt from pp. 66-69.)

Copyright AAAS, July 4, 1986.

Photometry from Voyager 2: Initial Results from the Uranian Atmosphere, Satellites, and Rings

Stellar Occultations of the Uranian ring system. On 24 January 1986 universal time (U.T.) the Voyager 2 PPS performed two Uranian ring occultation experiments. For each occultation, 0.27-micrometer UV-filtered light from the selected star was sampled 100 times per second (1, 7) while the star was in the vicinity of the previously known nine rings (8). Each occultation geometry was unique (Fig. 4). Because on both ingress and egress each star was blocked by the rings, we obtained four profiles of the epsilon and delta rings and two profiles of each of the other rings. These occultation paths provided data for questions concerning the orientation, eccentricities, and inclinations of the rings. It also raised the question of the azimuthal continuity of the individual rings.

Table 1 lists characteristics of the nine previously known Uranian rings, the newly discovered ring 1986U1R (9), and several new features in the PPS data. For ease of presentation, we have converted subsets of our data into two-dimensional images on the assumption of azimuthal symmetry in the rings (Fig. 5). These images represent false-color pictures of what an observer or camera might see from a point where the resolution is the same as that provided by the PPS stellar occultation.

Our profiles confirm the broad structure seen from Earth (8) at low resolution. At higher resolution, we see a multitude of new structure. Comparison of ingress and egress data for the epsilon ring, for example (Fig. 6), shows that the new features are real, although the exact shape and relative opacity varies with longitude. By comparing the epsilon ring's mean optical depth observed by the PPS at the four longitudes sampled by the stellar occultations, we determine that the total amount of material is constant to approximately 10%, even though the measured width varies from 20 to 90 km. For several of the other rings, the variability of material with longitude is much greater. Our results also agree with ground-based observations (8) if their inferred optical depths are divided by 2 to allow for diffraction (10).

The edges of the epsilon and gamma rings are sharp. At the outer edge of the epsilon ring, the counting rate makes the transition from its free-space value to nearly 0 in a radial distance of less than 40 m. The inner edge shows this same transition in about 500 m. Because the line of sight from the PPS to these stars is not perpendicular to the ring plane, the ray from the star at any instant samples material at different radii from Uranus. The sharpness of the transition from opaque to transparent thus provides an upper limit to the vertical extent of the ring at the edge (1). We find an upper limit on the vertical extent of about 150 m or less at the epsilon ring's outer edge. Because our measured optical depth in the epsilon ring exceeds the value possible for a single layer of particles, the particle centers must be spread in altitude to provide the observed opacity. If we only assume that the particles are spherical and do not assume that there is any particular particle size distribution, then the largest abundant particles must be three to four times smaller in radius than the vertical extent of the rings; that is, they must have radii of 30 m or less. Additional assumptions can add constraints that lower the limit even further, but these are model dependent.

Much of the new structure seen in the epsilon ring is probably caused by satellite perturbations. The resemblance of the features to density-wave peaks is striking (Fig. 7): they are sharp, narrow, extend to an opacity of 2.5 or more, and are separated by broad regions where the opacity is lower. The quasi-periodic structure is also reminiscent of waves. The fit of wave crest position to the density-wave model, however, is not satisfactory for our data and requires more study.

In addition to the information extracted from the epsilon ring data, we can state several other conclusions from the data in Table 1 and Figs. 8 and 9.

1) The rings are not azimuthally homogeneous. Even the delta and gamma rings, which are nearly circular (11), exhibit extremely different spatial distributions of material at the different occultation longitudes measured by the PPS.

2) The narrow component of the eta ring, detected and measured at several longitudes by ground-based occultations, is virtually undetectable in the PPS ingress cut for the beta Persei occultation at an 80% confidence level on the nondetection. No environmental or instrumental changes occurred during the time period corresponding to the region where the eta ring should have been. This narrow component of the eta ring may be discontinuous.

3) A large number of narrow, low optical depth, incomplete rings or arcs exist throughout the spatial region sampled by the PPS stellar occultations, including a 3000-km region outside the epsilon ring. The examples of this class of features (Fig. 9) are each more than 5 sigma detections, meaning a better than 96% probability that each is real. These features may be similar to the proposed arc-like nature of the Neptunian rings (12).

4) The 6 ring, the innermost of the rings detected by ground-based observation, appears to have at least one companion ring arc of equivalent optical depth and width.

Unlike the situation for the Saturn stellar occultation (1), the Uranian rings are fully illuminated by sunlight as they occult the star, and some sunlight is scattered by the ring particles into the PPS. The beta Persei occultation geometry provides an upper limit on the amount of scattered light observed and hence a limit on the abundance of small particles. Our results show that less than 0.1% of the epsilon ring's opacity comes from particles smaller than 1 micrometer (dust). Likewise, if a uniform sheet of dust-like material exists between all pairs of rings, its mean opacity must be 1e-4 or less.

Our data on the epsilon ring and data from other Voyager experiments (6, 9) suggest the following picture of the Uranian rings. The ring particles are dark and large, and lack dust. Small particles created by collisions are rapidly swept out of the ring system and fall into the planet. Comparing our measured opacity to dynamical estimates for the ring mass (13), we find a mean particle size of 20 cm. Physical constraints (14) establish the number density of ring particles as 1e-4 to 1e-1 m^-2; the average separation of particles is 2.5 to 10 times their radius. The variability of ring opacity with longitude and the discovery of partial ring arcs suggest that the Uranian rings are dynamic and perhaps quite young.

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