The crossings of Saturn ring plane by the Earth in 1995: Ring thickness

The crossings of Saturn's ring plane by Earth were observed in the near inf rared on May 22 and August 10, 1995, from the 2.2-m telescope of the Univer sity of Hawaii, the 2-m telescope at Pic du Midi, France, and with the Adon is adaptive optics camera at the 3.6-m telescope of the European Southern O bservatory in Chile. Images from the Hubble Space Telescope, obtained in Au gust 1995, are also reanalyzed. The radial brightness profiles of the rings indicate that the outer and usually faint F ring dominates the edge-on bri ghtness of the system, thus hiding the vertical structure of the main rings within a few hours around the ring plane crossing. The photometric behavio rs of the A, B, and C rings and of the Cassini Division are analyzed, using a radiative transfer code which includes the illuminations by the Sun and by the planet. The F ring is modeled as a physically thick ribbon of height H, composed of large particles embedded in dust of fractional optical dept h f. The observed profiles, combined with previous results, can be explaine d if the F ring is both optically thick (radial optical depth similar to 0. 20) and physically thick (H = 21 +/- 4 km). We suggest that this vertical d istribution results from the interactions between ring particles and shephe rding satellites and/or from gravitational stirring by large bodies. The du st particles dominate the F ring's photometric behavior even in backscatter ed light (f > 0.80). Constraints on the particle properties of the other ri ngs are also derived. (C) 2000 Academic Press.