Azimuthal distribution of arcs and clumps in the F ring of Saturn during August 1995 ring plane crossing.

C. Ferrari, A. Brahic, S. Charnoz, P. Thebault (Europa, University Paris 7), F. Roddier (IFA, University of Hawaii)

Ground-based observations of the August 1995 Saturn's ring plane crossing with the University of Hawaii Adaptive Optics System have led to the discovery of several new objects orbiting around the planet. Their orbits are known with variable accuracy but most of them are compatible with the F ring. Some of these objects are clearly azimuthally elongated structures, possibly arcs, the others are not resolved. Planetary arcs are swarms of clumps, which largest particles are most probably unresolved kilometer-sized objects (Ferrari and Brahic, 1992, 1994, 1997). Are these objects evolving with time ? On which timescale ? The F ring arcs have been first observed during the Voyager encounters. New constraints on the evolution of the brightests of them, on a two-weeks timescale, at the epoch of the Voyager 2 flyby, are presented. A revised orbit of the F ring is used to derive an azimuthal distribution of the newly discovered objects at August 1995 epoch. This is compared with the azimuthal profile of the F ring observed three months later, during the Sun ring plane crossing (Nicholson et al., 1996). The possible nature and lifetime of these new objects is discussed.