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

Lane, A. L., et al. 1989. Photometry from Voyager 2: Initial results from the Neptunian atmosphere, satellites, and rings. Science 246 (4936), 1450-1454. (Excerpt from pp. 1452-1453.)

Copyright AAAS, December 15, 1989.

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

Sigma Sagittarii stellar occultation of Neptune's ring system. On 24 August 1989, 5 hours before the closest flyby of Neptune, the PPS observed a stellar occultation of the Neptune ring system by the star sigma Sgr. The occultation measurement was performed with the 0.26 micrometer UV filter and linear polarizing filter. Light from the star was sampled every 0.010 s in a manner similar to that used at Saturn and Uranus (2, 3, 26). The Neptune ring system was probed over a radial range from approximately 75,000 to 42,000 km, which includes the main region of ring arc detections from ground-based measurements (27-29). The occultation path provided information for only a single ingress cut of a portion of the Neptune ring system. The occultation geometry is shown in Fig. 5. The radial resolution during the occultation ranged from about 80 m at the beginning of the occultation to about 10 m at the end of the occultation measurement. Using a similar methodology at Uranus, with the same star, we were able to unambiguously detect rings whose equivalent depths (integrated product of normal optical depth and ring width) were >=0.1 km (3, 26). At Neptune the slant optical depth measured by the PPS is 2.7 times higher than for an identical ring at Uranus because of the oblique viewing geometry, making relatively narrow lower optical depth rings easier to detect. However, the lower radial resolution of the Neptune occultation makes detection of narrow rings more difficult.

The sigma Sgr occultation grazed the leading edge of one of the three imaging-detected bright regions located within ring 1989N1R (8) at a radial distance of about 63,000 km from the center of Neptune (Fig. 5). The PPS optical-depth profile of 1989N1R (Fig. 6) is characterized by a condensed core near the inner edge of the ring. A similar structure was observed in the 20 August 1985 occultation observed in Hawaii from the IRTF (Infrared Telescope Facility) and CFHT (Canadian, French, Hawaiian Telescope) observatories, but at this time the core appeared near the outer edge of the ring (27, 29). From the PPS data, the ring is about 50 kilometers in radial extent. At 1.5-km resolution, the normal optical depth of the core is about 0.05 with a width of 9 to 11 km. Outside the core the optical depth ranges from about 0.01 to 0.02. The entire ring as measured by the PPS has an equivilent depth of 0.78. The equivalent depths of the ground-based occultations are 1.30 to 2.15 km (27), which are approximately twice the value measured by the PPS. This is explained by the effects of diffraction (30) if the size of the particles is larger than a few centimeters.

A number of other features were observed at a lower level of significance, with the most interesting one near 53,000 km, close to the reported location of 1989N2R (8). This feature has an average normal optical depth of ~0.006, a width of ~110 km, and an equivalent depth of about 0.7. Although such a feature could occur statistically four times in the total occultation data set, it is quite unlikely that it should appear by chance within 200 km of the reported location of 1989N2R. We are currently applying a variety of statistical techniques to the entire data set to elucidate other possible regions of ring material.

The PPS profile of 1989N1R has strong similarities to our profile of the Saturnian F ring (2) and to the Uranian delta and eta rings, with the radial profiles displaying a condensed core and adjacent wave-like oscillations (3, 26, 31). All of these rings exhibit a narrow core and have a broad, low-density companion. Azimuthally, both the F ring and the eta ring are clumpy (32,33), and the eta ring varies by a factor of two in its equivalent width (26, 34). These variations have been attributed to nearby satellites. In the case of 1989NlR, a single satellite, 1989N4, was discovered by Voyager 2 (8) just interior to the orbit of 1989N1R.

Numerical simulations of the ring response to an isolated resonance from a nearby satellite show that the collisional response forms a sharp density enhancement at the resonance (35). The width of this sharp feature is approximately the width of the resonance that is given by the region in which the perturbed particles overlap: w_0 ~= 1.8a[M_s/M_p)]^(-1/2), where M_s and M_p are the satellite and planet masses, and a is the semi-major axis of the ring (36). For an isolated resonance from 1989N4, w_0 ~= 20 km, which is in reasonable agreement with the measured 9 to 11 km (above). A smaller, closer satellite than 1989N4 could also create this structure as long as its resonances are isolated. For example, an unseen moon with radius 10 km must be more than 200 km from the ring. Brophy, Esposito, and Stewart (35) propose this mechanism to create the morphology of the Uranian delta ring, so the similarity of the delta ring core to the 1989N1R PPS profile is significant. The width and exact radial position should vary with azimuth, so comparisons with other data would be useful in testing this hypothesis. It is also possible that the satellites creating these resonances are also important in explaining the clumps seen in 1989N1R, similar to the phenomena simulated for the F ring by the work of Showalter and Burns (37) and Kolvoord and Burns (38).

If the Neptunian rings represent one part of a collisionally evolving system, as proposed by Esposito and Colwell (39) for the Uranian rings, we can compare the size distribution of material near Neptune to that near Uranus [fig. 1 in (39)]. Using the PPS measurements of equivalent depths, the measured sizes of the newly discovered satellites, and the reported optical depth and width of dusty rings (8), we find that the surface area in each of these components (dust, ring particles, 1989N2 through 1989N6, and 1989N1) is ~1e15 cm^2. This is consistent with these components being the observable part of a power-law size distribution with q ~= 3, where n(a)da = Ca^(-q), where da is the number of particles in the size interval [a, a+da]. However, since the size distribution of the ring particles is still unknown, this must be treated as a preliminary result (40). Further, without a measured size distribution, we cannot estimate the mass of the Neptunian rings. However, the total area of the Neptunian rings is about 1% of that of the Uranian rings, despite the fact that the amount of material in small moons near the ring is very similar in both systems. Thus we see that the Neptune ring system is markedly deficient in ring material by several orders of magnitude when compared to the Uranian ring system. One source of such material could be a moon-shattering event large enough to provide the amount of material such as that seen in the Uranian rings [2e-17 cm^2, figure 1 in (39)], which might imply that this kind of event has not happened as recently in the Neptunian system.

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