RINGS OF URANUS 

M. R. Showalter, SETI Institute; and J. J. Lissauer, Ames Research Center,
NASA, report the discovery of two rings of Uranus, R/2003 U 1 and R/2003 U
2. Both are faint, dusty rings orbiting well beyond Uranus' previously
known ring system. R/2003 U 1 peaks in brightness at 97 700 km from the
center of Uranus, coinciding with the orbit of Uranus XXVI (Mab), which is
almost certainly the ring's primary source body. The ring has a broad
triangular profile, with an inner limit at 86 000 km--near the orbit of
Uranus XV (Puck)--and an outer limit at 103 000 km. R/2003 U 2 peaks in
brightness at 67 300 km, where no known moons are present. This ring also
has a triangular profile, terminating near the orbits of Uranus XII
(Portia) on the inside and Uranus XIII (Rosalind) on the outside. The rings
are visible in data from the High Resolution Channel of the Hubble Space
Telescope (HST) Advanced Camera for Surveys. They appear in 250-second
clear-filter (300-700 nm) images obtained on 2003 Aug. 25 (24 images), 2004
Aug. 20 (40 images), 2004 Aug. 26 (40 images) and 2005 Aug. 20 (18
images). When images from a single date are co-added, S/N > 10. Both rings
vary in brightness with the opening angle, as is expected for optically
thin rings. R/2003 U 1 has a peak reflectivity at opposition (times the
sine of the opening angle) of 9 x 10^(-7) and an estimated normal optical
depth of 9 x 10^(-6). R/2003 U 2 has a peak of 4 x 10^(-7) and an estimated
normal optical depth of 6 x 10^(-6). 

I. de Pater, University of California
at Berkeley; H. B. Hammel, Space Science Institute; and S. Gibbard,
Lawrence Livermore National Laboratory, write: "We report the detection of
a faint uranian ring at a distance of about 67 700 km from Uranus (outside
the epsilon ring). We identify this ring as the first ground-based
detection of R/2003 U 2, reported by Showalter and Lissauer above. We
obtained the images at K' band (2.2 microns) on 2005 Aug. 23 UT with the
10-m Keck II Telescope (+ NIRC2/AO) on Mauna Kea. The small ring opening
angle (about 8.4 deg) and small phase angle (0.4 deg, close to opposition),
along with a full hour of integration time, combined to make this faint
ring detectable. We also made a probable detection of this same ring in
Oct. 2005, but the ring opening and phase angles were less favorable. We
searched out to 115 000 km from Uranus in Oct. 2005 and saw no additional
rings. This will establish upper limits on the 2-micron brightness of any
material out to that distance, including R/2003 U 1 reported by Showalter
and Lissauer from HST data."

