STRUCTURE AND PARTICLE PROPERTIES OF SATURNS E-RING

We have systematically reanalyzed a large subset of the available photometric data on Saturn's E Ring, including Voyager images and star tracker data, Earth-based CCD images, photometry, and spectrophotometry. In order to compare data sets with vastly different vi viewing geometries on this broad, vertically extended ring, we have calibrated every observation based on a common three-dimensional model. The ring shows a density peak at a location indistinguishable from the Enceladus orbit, with a radial offset of <3000 km. We have found that a very simple power-law model describes the ring's normal optical depth profile with orbital radius. The ring shows a general increase in vertical thickness with distance from the planet, ranging from 6000 km FWHM at its inner bound to nearly 40, 000 km at its outer. However, this trend is violated near the density peak, where a localized decrease in thickness by ∼30% is observed. The ring is also displaced northward from the planet's equatorial plane by 310 ± 25 km in one set of Voyager images, although the overall nature of the ring's vertical asymmetry is unclear. No power-law size distribution is compatible with the available photometry. A narrow distribution of slightly nonspherical particles of radius 1.0 ± 0.3 μm provides the best fit to the data. This highly peculiar size distribution clearly indicates that the ring does not originate from collisional or disruptive processes, and is therefore unlike any other known ring. Hence, we can give some credence to the possibility that the E Ring originates in "geyser-like" eruptions from the surface of Enceladus. Depending on the precise particle size chosen, the ring's peak normal optical depth is found to be 1.5(±0.4) × 10-5, corresponding to a geometric cross-section per unit area of 5.3(±1.3) × 10-5. Macroscopic bodies comprise less than 1% of the ring's optical depth. © 1991.