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J. A. Burns, M. Ockert-Bell (Cornell U), D. P. Hamilton (U.Maryland), M. R. Showalter (Stanford U), P. C. Thomas, J. Veverka (Cornell U), M. J. S. Belton (NOAO), T. V. Johnson (JPL), P. D. Nicholson (Cornell Univ), I. de Pater (UC-Berkeley), Galileo Imaging Team
Galileo images reveal that Jupiter's gossamer ring -- previously seen only in its Voyager discovery image -- is in fact two faint rings that are bounded by the orbits of the small ring-moons, Amalthea and Thebe. Very faint material continues past Thebe's orbit at 222,000 km, to ~250,000 km. When viewed nearly edge-on, the rings have a rectangular shape with enhanced intensities along the top and bottom edges, much like the IRAS dust bands. The gossamer rings have thicknesses that are comparable to the maximum elevations of their bounding satellites off Jupiter's equatorial plane. The unique morphology of the gossamer rings can be generated if the ring particles are collisional ejecta evolving radially inward (apparently due to Poynting-Robertson drag) from the satellites (orbital semi-major axis a, inclination i). Typical escape speeds off Jupiter's small satellites are many tens of m/s for Amalthea and Thebe, and between 0-10 m/s for Adrastea and Metis. Since escape speeds are << orbital speeds, ejecta following meteoroid impacts into a Jovian ring-moon initially resides in a slender tube surrounding the moon's orbit . Due to Jupiter's oblateness, this tube precesses in a few months; after a few years, the tube has smeared into a cylindrical band. When viewed from nearly in the equatorial plane, the band appears as a thin rectangle of length 2a and height 2ai, having its top/bottom edges enhanced by the particles' vertical epicyclic motions. As the grains drift inward, the rectangle will brighten toward the planet, and will contain an embedded bright "bow-tie" shape since the drifting particles retain their starting orbital inclinations. Since ring-moons of ~10 km radius are the most efficient source of collisional ejecta, Jupiter's main ring seems to be produced similarly by the equatorial ring-moons Adrastea and Metis.