17.01

Saturn Ring Profiles from the August 1995 HST Data

P. D. Nicholson (Cornell)

During the second crossing of Saturn's ring plane by the Earth in August 1995, HST observations were obtained with WFPC2 from August 10, 13:15 UT through August 11, 1:30 UT (Nicholson et al., [1996] Science 272, 509). On each HST orbit, a pair of 300 sec exposures was taken with the Wide Field camera which cover the entire ring system, plus a series of up to five 100 sec exposures with the Planetary Camera targeted to one ansa. All exposures were made with the FQCH4N 890 nm methane filter to suppress scattered light from Saturn. Prior to the Earth's crossing the plane at  hr UT, the radial brightness profiles of both ring ansae are relatively flat and smooth, increasing slowly outwards until they terminate abruptly at the radius of the F Ring (r=140,200 km). 30-50% increases in brightness at radii corresponding to the C Ring and the Cassini Division 7 hr before the crossing indicate a modest contribution from light diffusely transmitted through the main rings, but closer to the crossing time the profiles are essentially featureless and most of the edge-on ring brightness seems to be attributable to the F Ring. The average vertically-integrated reflectivity (I/F) of the rings at this time was found to be  km, with the the west ansa being brighter than the east. Within 45 min after the Earth had crossed the plane the central portion of the west ansa had brightened by a factor of two, while the east ansa had increased by less than 50%. By 23:50 UT, however, this asymmetry had largely disappeared, and in our final set of images the west ansa is fainter than the east ansa, an observation which it would be very interesting to confirm with ground-based observations. Linear fits to the pre- and post-crossing brightness over different radial regions yield crossing times which are significantly earlier on the west ansa, with the largest asymmetry occuring in the B Ring region, where the difference between east and west amounts to  min. An effect of this amplitude is much too large to be accounted for by the rings' Laplace plane warp of 140 m, and is most likely due to the F Ring's small inclination (Olkin & Bosh [1996] BAAS, 28, 1125).