A Photometric Model of the Inclined F Ring of Saturn

Abstract

Observations by Hubble Space Telescope (HST) of the rings of Saturn during the 1995 ring-plane crossing revealed a surprising asymmetry in the brightness of the east and west ansae. As in historical observations, the ring brightness was nonzero at the time that the rings are observed edge-on. We create a photometric model of the ring system with the F ring inclined to the main ring plane which re- produces the observed brightness of the rings and the asymmetry in ring brightness after the ring-plane crossing.

The F ring is modeled as "ribbon" 60 km tall, of indeterminate radial width, with a gaussian profile of optical depth as a function of height with an equivalent depth of D=10+/-4 km and a full width at half maximum of 13+/-7 km. This is the first estimate of the physical vertical thickness of the F ring, which we find is approximately 1000 times greater than the main ring thickness.

The model shows that as the Earth crosses the main ring plane, the F ring dominates the brightness of the system, and that the asymmetry in ring brightness after the ring-plane crossing is caused by asymmetric absorption of light from the main rings by the front of the F ring.

The model gives post-crossing asymmetries in good agreement with the HST data, but fails to reproduce the small asymmetries in ring brightness observed before the ring-plane crossing. The model profiles of ring brightness plotted vs. horizontal distance from the center of Saturn show many of the features observed in the HST profiles. However, the F ring in this model is longitudinally symmetric, and does not include the many clumps or arcs that have been observed in the real F ring. It therefore cannot reproduce many of the small-scale variations seen in the HST profiles. It may be that these small features are responsible for the asymmetries observed before the ring-plane crossing.