doi:10.1016/j.icarus.2004.10.020 
Copyright © 2004 Elsevier Inc. All rights reserved.
The dynamic neptunian ring arcs: evidence for a gradual disappearance of Liberté and resonant jump of courage
Imke de Patera, 
, 
, Seran G. Gibbardb, Eugene Chianga, Heidi B. Hammelc, Bruce Macintoshb, Franck Marchisa, Shuleen C. Martina, Henry G. Roed and Mark Showaltere
aAstronomy Department, 601 Campbell Hall, University of California, Berkeley, CA 94720, USA
bLawrence Livermore National Laboratory, Livermore, CA 94550, USA
cSpace Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA
dO.K. Earl Postdoctoral Scholar in Planetary Science, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
eSTAR Laboratory, Stanford, CA 94305-9515, USA
Received 13 May 2004;
revised 1 October 2004.
Available online 22 January 2005.
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Abstract
We present Adaptive Optics observations of Neptune's ring system at 1.6 and 2.2 μm, taken with the 10-m W.M. Keck II telescope in July 2002 and October 2003. We recovered the full Adams and Le Verrier rings for the first time since the Voyager era (1989), and show that the overall appearance of these rings did not change much, except for the ring arcs. Both the location and intensity of all arcs changed drastically relative to trailing arc Fraternité, which has a mean orbital motion of 820.1118 ± 0.0001 deg/day, equal to that of Nicholson et al.'s (1995, Icarus 113, 295–330) solution 2. Our data suggest that all arcs may have decayed over the last decade, while Liberté, in 2003, may be on the verge of disappearing completely. The observed changes in the relative intensities and locations of all arcs further indicate that material is migrating between resonance sites; leading arc Courage, for example, has jumped 
8°, or, when adopting Namouni and Porco's (2002, Nature 417, 45–47) CER (corotation eccentricity resonance) theory, it advanced by one full corotation potential maximum. Overall, our observations reveal a system that is surprisingly dynamic, and no comprehensive theory exists as of yet that can explain all the observed intricacies.
Keywords: Planetary rings; Neptune; Infrared observations
Fig. 1. (a) Average image of the 28 July 2002 data (2.12 μm, 33 min total integration time), revealing satellites, ring arcs and the complete Adams and Le Verrier rings. The images have been high-pass filtered by subtracting the same image median-smoothed with a width of 50 pixels. This procedure removes diffuse scattered light, and brings out small-scale features (Neptune is highly saturated in this presentation). A 1-min exposure of Neptune itself at K′ is shown in the insert. (b) Average high-pass filtered image of the 03 October 2003 data (2.12 μm, 30 min total integration time). The insert shows a 1-min exposure of Neptune itself at K′-band. (c) Average high-pass filtered image of the 05 October 2003 data (1.63 μm, 23 min total integration time). The insert shows a 1-min exposure of Neptune itself at H-band.
Fig. 2. Longitudinal scan along the Adams ring, obtained from the deprojected image of 03 October 2003. The scan has been integrated in the radial direction over the full extent of the rings, as discussed in the text. Zero longitude was chosen to coincide with the center of arc Fraternité. Equivalent width (in m) is shown on the right.
Fig. 3. Radial profiles through the rings. These profiles were obtained by averaging over azimuth on each day, away from satellites and ring arcs. The radial profiles for the 2002 and 2003 epochs are shown in the upper panels, and the combined scan is shown in the lower panel.
Table 1.
Neptune ring data
Δ = geocentric distance; ro = heliocentric distance; all images are 1-min integrations in H (1.48–1.78 μm) or K′ (1.95–2.30 μm); the ring opening angle was −27.7°.
Table 2.
Flux densities and reflectivities for Galatea and the ring arcs
a Data from
Dumas et al. (2002).
b Data from
Porco et al. (1995) and
Karkoschka (2003).
c All uncertainties include a 10% photometric calibration error.
d I/F is calculated assuming a 79 km radius for Galatea.
e Combined ring arcs Fraternité and Egalité. The 2003 Keck data include a
0.4 μJy flux density from the (background) Adams ring itself.
f Assuming a neutral color for the rings, we scaled the 1.63 and 1.87 μm data according to the Sun's reflectivity (divided the observed flux densities by factors of 1.50 and 1.28, respectively), and Neptune's geocentric and heliocentric distance compared to those on Oct. 3, 2003.
g Assumed: 15 km radial extent; 25° in azimuth; normal optical depth
τ=0.1 Porco et al., 1995. The NICMOS reflectivity is for the entire ring arc system.
h Flux density of all arcs combined. The 2003 Keck data include a
0.5 μJy flux density from the (background) Adams ring itself.
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). This perturbation changes the effective mean motion (i.e., the mean motion averaged over a synodic period or longer). The correction is 0.0041°/day, which displaces the arcs 0.2 km farther away from Galatea than was previously calculated by C. C. Porco (1991, 
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