The jovian rings: new results derived from Cassini, Galileo, Voyager, and Earth-based observations

Cassiniʹs Imaging Science Subsystem (ISS) instrument took nearly 1200 images of the Jupiter ring system during the spacecraftʹs 6-month encounter with Jupiter (Porco et al., 2003, Science 299, 1541–1547). These observations constitute the most complete data set of the ring taken by a single instrument, both in phase angle (0.5°–120° at seven angles) and wavelength (0.45–0.93 μm through eight filters). The main ring was detected in all targeted exposures; the halo and gossamer rings were too faint to be detected above the planetʹs stray light. The optical depth and radial profile of the main ring are consistent with previous observations. No broad asymmetries within the ring were seen; we did identify possible hints of 1000 km-scale azimuthal clumps within the ring. Cassini observations taken within 0.02° of the ring plane place an upper limit on the ringʹs full thickness of 80 km at a phase angle of 64°. We have combined the Cassini ISS and VIMS (Visible and Infrared Mapping Spectrometer) observations with those from Voyager, HST (Hubble Space Telescope), Keck, Galileo, Palomar, and IRTF (Infrared Telescope Facility). We have fit the entire suite of data using a photometric model that includes microscopic silicate dust grains as well as larger, long-lived ‘parent bodies’ that engender this dust. Our best-fit model to all the data indicates an optical depth of small particles of τs=4.7×10−6 and large bodies τl=1.3×10−6. The dustʹs cross-sectional area peaks near 15 μm. The data are fit significantly better using non-spherical rather than spherical dust grains. The parent bodies themselves must be very red from 0.4–2.5 μm, and may have absorption features near 0.8 and 2.2 μm.