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

Tyler, G. L., et al. 1986. Voyager 2 Radio Science observations of the Uranian system: atmosphere, rings, and satellites. Science 233 (4759), 79-84. (Excerpt from pp. 81-83.)

Copyright AAAS, July 4, 1986.

Voyager 2 Radio Science Observations of the Uranian System: Atmosphere, Rings, and Satellites

Rings of Uranus. Ring occultation data were obtained on both sides of the planet (Fig. 1). Our initial results are based on reduction of dual-frequency data from the 64-m antenna at Canberra, which covered the region of the previously known rings. These data have been corrected for diffraction and examined at an elective radial resolution of 200 m (21).

The nine previously known rings were readily detected and studied at both 3.6 cm and 13 cm in the radio data the first time that they have been observed at centimeter wavelengths. The characteristic maximum normal radio depths (tau) observed for each ring vary from approximately 0.8 to 8 (Table 1). The upper range of these values is surprisingly large (22), indicating that the Uranian ring system contains substantial amounts of material in the form of particles having sizes comparable to or larger than the radio wavelengths. In addition, there are a number of other, weak observations of possible rings.

We focus further discussion on the epsilon ring because it displays a number of properties found in the others as well as a newly discovered scattering phenomenon. Figure 3 displays results for points near periapsis and quadrature in the orbit of the ring particles, corrected for diffraction at a resolution of 1.0 km. The change in width of the ring with location is readily apparent. Although the large-scale morphology of the ring at the two observation points is similar, there are significant changes in the internal small-scale structure. Near periapsis (Fig. 3A), tau is typically 2.5 to 3, with a maximum of about 8 at 3.6 cm. Signals at the two wavelengths are differentially attenuated at the point of maximum tau. Near quadrature (Fig. 3B), tau is approximately 1 and increases to about 2.5 near both edges. Integrated values of extinction across the epsilon ring (Table 1) differ by about 1% at 3.6 cm and by about 23% at 13 cm (23).

The almost complete independence of the integrated extinction over a 3.3:1 variation in width requires that the extinction efficiency of the individual particles be conserved around the ring. We know of no way in which this can be achieved in regions of tau significantly greater than 1 by other than a collection of well-separated particles (24). It follows that the epsilon ring must have a considerable thickness normal to the ring orbital plane (25). Furthermore, the differences in detailed radial structure between immersion and emersion indicate structure that varies markedly with the true anomalies of particles in their orbits.

Considerable small-scale structure in the epsilon ring is revealed at 200-m resolution (Fig. 3B), inset. A wavelike feature can be identified over 6 to 9 km from the left edge of the inset. A similar structure has been found at about the same relative position in the immersion data.

An unexpected aspect of the epsilon ring observations has been detection of strong near-forward scattering at 3.6 cm as Voyager passed behind the ring near quadrature on occultation emersion. Such scattering has not been detected from the epsilon ring near periapsis, nor has it been observed during either occultation at 13 cm. It has not been seen in connection with any other ring at either wavelength at Uranus or Saturn.

The epsilon ring data have been plotted as a time sequence of power spectra. The scattered signal appears in the spectra as a single localized feature of bandwidth approximately 100 Hz drifting at about -5 Hz sec^(-1) with respect to the directly propagating signal. The total received scattered power is about 0.5% that of the direct signal. It persists for a time that coincides with passage of the spacecraft antenna beam over the epsilon ring. Although the scattering is clearly associated with the epsilon ring, we can rule out scattering by independent discrete objects and three-dimensional collections of objects. Scattered signals from objects in Keplerian orbits within or near the epsilon ring would drift in frequency at three times the observed rate; the actual Doppler signature requires that the scattering center move radially outward or in a retrograde orbit (or both). Further, the large power received requires that the radar cross section of the scattering canter be approximately 6e15 m^2---an impossibly large value for a localized scattering center (26).

We may be able to explain this anomalous signal in terms of organized structure within epsilon ring rather than by scattering from a discrete object. By presuming that the scatter originates from elongated structure, possibly canted to the azimuthal coordinate, we obtain a match to the frequency drift and a correspondence with the observed bandwidth. The signal strength and duration suggest that the effective scale along the ring of this structure be comparable to or larger than the Fresnel scale (~2 km) and that the radial scale be on the order of meters; the scattering source is thus essentially two-dimensional. We find the indications for such ordered structure within epsilon ring to be extremely intriguing as well as somewhat paradoxical in view of the apparent lack of order in the larger scale extinction profiles shown in Fig. 3. Physical mechanisms invoked to explain the azimuthal asymmetry observed in Saturn's A ring may be relevant (27).

Table 1 also gives values for the other previously known rings. Most of the observations show no clear differences in integrated depth at the two wavelengths; further comparison awaits more complete analysis of the relatively noisy 13-cm profiles. Two measurements show larger integrated tau at 3.6 cm; two others, however, show higher integrated tau at 13 cm, which is puzzling. No ring shows strong wavelength variation at both radio measurement locations.

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