Observations of Mimas and Enceladus during the Saturn Ring Plane Crossing of August 1995

B.J. Buratti, J.A. Mosher (JPL/Caltech), P.D. Nicholson (Cornell University), R.G. French (Wellesley College), C. McGhee (Cornell University)

The satellites of Saturn were observed with the 60-inch telescope at Palomar Mountain during the Saturn Ring Plane Crossing (RPX) on August 9-12, 1995. Observations were obtained with a 1024 square CCD and a methane filter to reduce light from Saturn. During the four nights more than 250 images were obtained. These images enable a rare opportunity to obtain photometric measurements of Saturn's two inner medium-sized satellites, Mimas and Enceladus. Telescopic observations of albedo variegations on Mimas have never been obtained, while Enceladus was previously observed by Franz and Millis (Icarus vol. 24, p. 433, 1975). Voyager lightcurve observations of both satellites were obtained by Buratti and Veverka (Icarus vol. 58, p. 254, 1984); good orbital coverage was attained for Enceladus, but Mimas's coverage was limited to only one hemisphere (90-270 degrees). Our telescopic observations of Mimas are also somewhat limited (due to the similarity of Mimas's orbital period to a terrestrial day), but we did obtain measurements at longitudes not imaged by Voyager.

Voyager observations of Mimas (Buratti and Veverka, op. cit.) show no significant albedo variegations on the satellite. Our preliminary reductions, obtained by producing ratios of the brightness of Mimas with Titan (which has a well-established lightcurve), confirm this view and extend it to areas unseen by the spacecraft.

Our observations of Enceladus show that this satellite has albedo variegations of at least 10% on its surface. Unlike Tethys, Dione, and Rhea, Enceladus is brighter on the trailing side rather than the leading side. This result is in agreement with both the Voyager observations and the groundbased results of Franz and Millis. However, the amplitude of our derived lightcurve is less than that obtained by Franz and Millis.

Work funded by NASA.