Acknowledgement. We thank Science for their permission to use an excerpt from:
Sandel, B. R., et al. 1982. Extreme Ultraviolet Observations from the Voyager 2 Encounter with Saturn. Science 215 (4532), 548-553. (Excerpt from pp. 550-552.)
Ring occultation. A double occultation of the star delta Sco (HD143275, B0.5 IV, V = 2.32) by the rings of Saturn was observed by the UVS. The first occultation preceded the entrance of delta Sco into Saturn's dayside atmosphere and covered the portion of the C ring between 1.43 and 1.29 R_S. The second covered the exit of delta Sco from the top of the atmosphere through the F ring, all within the shadow of the planet. The apparent radial velocity of the star through the rings was ~ 10 km sec^-1, giving a resolution of about 3 km. The line of sight to delta Sco intercepted the ring plane at an angle of 28.71 degrees, so that observed optical depths are multiplied by the sine of this angle to obtain normal optical depths (tau_n). The fact that there is no stellar signal shortward of 912 Angstroms allows us to accurately estimate our dark count level. This together with a strong signal (900 counts per 0.32-second spectrum) makes possible a wide dynamic range in the determination of absolute optical depths.
Spectra averaged over portions of the A, B, and C rings as well as the Cassini division exhibited only neutral (spectrally flat) absorption between 912 and 1700 Angstroms. Because of the short absorption path length, this result imposes little significant constraint on the density of molecular species. However, it does indicate the probable lack of a significant population of wavelength (0.1 micrometer) sized particles.
The C ring consists of a large number of easily recognizable ringlets of varying width and optical depth embedded in a generally unbroken ring of lower optical depth. The underlying optical depth of the C ring ranges from very low (tau_n < 0.05) for its inner portions to tau_n ~ 0.15 in its outer portions. The optical depths we have measured between ringlets are comparable to those found by Esposito et al. (14) from the Pioneer 11 results. In Fig. 4 we show preliminary normal optical depths plotted against planetocentric ring radius for the C ring. These results, which have been averaged over 50 km, show a great deal of structure because of the embedded ringlets of higher optical depth.
A comparison of the two occultations over the same portion of the C ring shows that the C ring is remarkably symmetrical with respect to the center of Saturn. This portion of the C ring (1.43 to 1.29 R_S) contains six prominent ringlets. Five of these features show an almost identical appearance on both sides of the planet. Radii (15) of these five features agree to within 7 km as determined from the preliminary spacecraft trajectory. In Fig. 5 we show the normal optical depths observed for three of these features on both the occultations. The innermost of these ringlets, located at 77,865 km, consists of a pair of very narrow ringlets of large optical depth (tau_n > 1) separated by 3 to 6 km. On the dayside these ringlets have a total width of ~ 30 km, whereas on the nightside they are 19 km in total width and displaced 32 km closer to the planet. Figure 5 shows the eccentric ringlet as it appears in both the ingress and egress occultations. The C ring terminates in a 50-km-wide ringlet (tau_n ~ 0.4) followed by a sharp transition to large optical depth (tau_n ~ 1) within 6 km. This transition to the B ring is located at a radius of 92,066 km.
The detailed analysis of the UVS exit occultation data beyond the inner part of the B ring is not yet complete; however, several facts are apparent. In contrast to the C ring, the B ring can be characterized as having extensive regions of large optical depth interspersed with narrow regions of low optical depth or gaps. In about 40 percent of the B ring, between 1.72 and 1.89 R_S, no stellar signal was observed, indicating an average normal optical depth greater than 3.5. A preliminary look at the data in this region reveals no evidence of holes or gaps in the 1- to 3-km range having significant transmittance. The outermost portion of the A ring has a normal optical depth of tau_n ~ 0.6. This region abruptly terminates in a few highly absorbed spectra indicating a ringlet with tau_n > 1 and less than 3 to 6 km in width. This ringlet is located at a radius of 136,786 km.
Ring albedo. The UVS observed an extremely low level (<<1 R per angstrom) of reflected sunlight from the ring particles. This spectrum of the rings is nearly identical with a direct UVS spectrum of the sun as viewed through the occultation port. We have used the absolute solar spectrum of Donnelly and Pope (16) to estimate the absolute albedo of the rings at several wavelengths. In doing this we have assumed that the portion of the rings which fills our field (principally the B ring) is optically thick and that the rings act as a Lambert surface in reflecting far-ultraviolet photons. These assumptions yield absolute albedos of 3.5 percent at 900 Angstroms and 5 percent at 1304 and 1410 Angstroms. These represent lower limits because of the assumption of optically thick rings. Laboratory measurements between 1200 and 1375 Angstroms show that the reflectance spectrum of water ice is relatively flat (17) with an absolute albedo of ~ 5 percent to within a factor of 2. We are aware of no published measurements of the reflectance of water ice below 1200 Angstroms. However, our comparison with the direct Voyager solar spectrum indicates a relatively flat reflectance spectrum between 1100 and 600 Angstroms. One puzzling aspect of this observation was our failure to observe a similar albedo spectrum during the Voyager 1 encounter. Our viewing geometries were similar; however, the elevation of the sun with respect to the ring plane was 3.6 degrees at the time of Voyager 1 whereas it was 8 degrees for Voyager 2. This may indicate a strong dependence on solar elevation angle.
The presence of solar albedo from the sunlit side of the rings effectively limits the detectability of any emission from the vicinity of the rings which might be associated with the electrical discharges noted by the planetary radio astronomy experiment (18). We have used several hours of postencounter integration time obtained on the underside of the B ring, in the shadow of the planet, to search for such emission. Except for the case of the O I 1304 Angstroms line our upper limits for neutral and ionized O are now a factor of 2 lower than those reported from Voyager 1 (1). For H I Lyman beta our upper limit is 0.7 R.
Last updated Feb-27-1997
