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S.M. Brooks, L.W. Esposito (LASP, University of Colorado), M.R. Showalter (Stanford University), H.B. Throop (Southwest Research Institute)
High phase angle, clear filter Voyager images of Jupiter's main ring clearly revealed an asymmetry in the brightness between the arms of the ring closest to and furthest from the spacecraft (Jewitt and Danielson, 1981, \emph{JGR}, \textbf{86}, 8691-8697). Specifically, the arm furthest from the spacecraft appears brighter than the near arm. Careful analysis of these images by Showalter \emph{et al}.\ (1987, \emph{Icarus}, \textbf{69}, 458-498) revealed this asymmetry to increase toward the ring ansa and to reach a maximum of 10%. No satisfactory explanation for this was found.
Ockert-Bell \emph{et al}.\ (1999, \emph{Icarus}, \textbf{138}, 188-213) reported a similar asymmetry in high phase angle Galileo images. However, the asymmetry in the Galileo images increases away from the ring ansa. The sense of the asymmetry is reversed from what was seen by Voyager; the near arm is brighter than the far arm. After modeling and subtracting contributions from the background and ring halo, we determined the maximum amplitude of this asymmetry to be 60%~ ±~15%.
The particle clumping and inter-particle shadowing thought to be responsible for the azimuthal, quadrupole asymmetry seen in Saturn's A Ring (Dones \emph{et al}., 1993, \emph{Icarus}, \textbf{105}, 184-215) is not a significant factor for Jupiter's optically thin rings. We were unable to identify any pattern in ring brightness as a function of longitude. Other mechanisms involving the light scattering behavior of individual particles are problematic. Therefore, we interpret the asymmetry as a localized enhancement in the number of ring particles. An increase of ~~10% in the ring's optical depth would be required to produce the observed asymmetry. We investigate the possibility that these enhancements are the debris produced by impacts onto main ring parent bodies by both other ring parent bodies and external impactors.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.