Acknowledgement. We thank Science for their permission to use an excerpt from:
Stone, E. C., et al. 1982. Voyager 2 Encounter with the Saturnian System. Science 215 (4532), 499-504. (Excerpt from pp. 501-502.)
Rings. Voyager 2 returned significant information on the rings, both because it was possible to measure the amount of light from the star Delta Scorpii transmitted through the rings (the stellar occultation) and because it obtained much higher resolution images of the illuminated face of the rings. The stellar occultation measurements by the photopolarimeter provided a typical radial resolution of ~< 300 m. It was found that even at this scale very few gaps exist in the rings. The edges of the rings at the gaps that do exist are so sharp, however, that the rings must be ~< 200 m thick at those locations.
Since there are so few gaps, most of the radial structure in the B ring must be due to variations in the optical thickness of the rings resulting from density waves, gravitational instabilities, or dynamical instabilities. Density waves are excited by the gravitational effects of the Saturnian satellites and propagate outward from the resonant orbits where the ring particles orbit Saturn in synchronism with the satellite. For example, at the 2:1 resonant point with satellite 1980S1, there is a series of outward propagating density waves with characteristics which indicate that in that region of the B ring there is ~ 60 g of material per square centimeter of ring area and that the relative velocities of the ring particles are ~< 1 mm sec^-l.
The outer edge of the B ring is elliptical with a difference of ~> 140 km in the semimajor and semiminor axes, a somewhat larger difference than predicted by the 2:1 resonant interaction with Mimas. However, the axes of the ellipse rotate with the orbital motion of Mimas, as predicted. There are numerous radial variations with scales of 20 km in the outer B ring which are also noncircular, presumably the result of waves or instabilities. Radial features with scales of 500 km, however, appear to be circular.
In almost every case where clear gaps appear in the rings, eccentric ringlets are found. All seem to exhibit azimuthal brightness variations, due at times to variable radial extent of the rings and at other times to "kinks" or concentrations of ring particles or to nearly complete absence of ring material at various azimuthal positions. Two separate discontinuous ringlets were seen in the A ring gap near 2.21 Saturn radii (R_S) from the center of Saturn (1 R_S = 60,330 km = 37,490 miles). At high resolution, at least one of these ringlets is multiply stranded.
Although the presence of gaps and eccentric ringlets suggests that there are imbedded moonlets that are responsible, a systematic search for small satellites in the ring gap at the inner edge of the Cassini division yielded negative results. The nondetection sets an upper limit to the diameter of any unseen moonlets of 5 to 9 km (3 to 6 miles), depending on the assumed surface reflectivity. Although no systematic searches were conducted in the other ring gaps, the limited regions that were imaged revealed no moonlets.
The multistranded narrow F ring received considerable attention in replanning the Voyager 2 observations. At 15 km (9-mile) resolution, the F ring was seen to consist of one bright strand and four fainter ones, each ~ 70 to 100 km across, which, at least in that segment of the ring, did not appear to be braided or intertwined. The photopolarimeter stellar occultation showed that even the brightest strand was subdivided into numerous narrower strands about 3 km wide, possibly as a result of larger particles in the ring. Clumps of material in the F ring were observed over 15 orbits. The clumps are quasi-uniformly distributed about the ring every ~ 9000 km, a spacing that coincides with the relative motion of F ring particles and the shepherding satellites in one orbital period.
The "spokes" or clouds of micrometer-sized particles observed between 1.72 R_S and the outer edge of the B ring were also studied. Time-lapse sequences by Voyager 2 revealed that the spokes that are narrow and radial in alignment (hence presumably more recently formed) very nearly corotate with Saturn's magnetic field. Broader, less radial spokes appear to be remnants from earlier epochs (perhaps as much as several orbits earlier) and follow Keplerian orbits. In some cases, "new" spokes may be reprinted over preexisting ones. Spoke formation is not limited to regions near the planetary shadow, although spokes are more readily seen at the ring ansa near which ring particles have most recently exited the shadow. Spokes are also seen at high phase angles in reflected Saturn light on the unilluminated face of the rings, suggesting that their formation may not be dependent on photoelectrically induced charging effects.
Voyager 2 provided higher resolution color images of the entire ring system than were previously available. These reveal color differences within each of the major rings in addition to the differences between rings noted in Voyager 1 images. The color differences may be due in part to differences in optical depth between various parts of the rings, but may also indicate small compositional differences that have been preserved over geologic time. v
The general dimensions of the main rings are given in Table 1, based on Voyager 1 and Voyager 2 imaging, radio science, and ultraviolet observations. More precise numbers will eventually be available from Voyager 1 radio science and Voyager 2 photopolarimeter measurements. However, some of these features may not have constant radial distances from Saturn.
Improved images of the D, G, and E rings were also obtained by Voyager 2, providing better measurements of their positions, optical depths, and radial extents.
Last updated Feb-27-1997