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M. R. Showalter (SETI Institute), J. J. Lissauer (NASA Ames), I. de Pater (U. C. Berkeley)
We present recent results from HST on of the ring systems of Uranus and Neptune. Using the High Resolution Channel of the Advanced Camera for Surveys, we have detected the Adams and Leverrier rings of Neptune at visual wavelengths for the first time since the 1989 Voyager encounter. The leading arcs, Courage and Liberte, have almost completely vanished, indicating the continuation of a trend noted in recent Earth-based infrared detections. The trailing arcs Egalite and Fraternite remain, and photometry reveals that they are essentially equal in brightness to their appearance in the Voyager data. However, initial photometry of the Leverrier ring suggests that it is several times brighter than in 1989; this large change requires further investigation. We have detected every inner moon of Neptune except Naiad. All inner moons show significant ephemeris offsets (0.1 arcsec) from JPL's most recent ephemeris, NEP022; Nereid also deviates significantly from predictions. Naiad should be detectable in our data; we suspect that a much larger ephemeris error explains its current non-detection.
We continue to track the newly-discovered moons of Uranus, S/2003 U 1 and S/2003 U 2. Precise orbital solutions are in progress. The close orbital proximity of S/2003 U 2 to Belinda suggests its orbit may be unstable. Clumps and arcs within the ring system appear to be primarily periodic variations in the widths of known rings, related to modes of the delta and gamma rings, plus resonant perturbations by Cordelia and Ophelia. All the individual rings except 6, 5 and 4 can be resolved in our data; initial photometric results will be presented.
Support was provided by NASA from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, under NASA contract NAS5-26555
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.