The gamma ring of Uranus
Abstract
The gamma ring has long been an outlier among the ten classical Uranian rings. Shortly after the discovery of the rings in 1977 (Elliot et al. 1977, Nature 267,328; Millis et al. 1977, Nature 267,330; Hubbard et al. 1980, Astron J. 845,1663), the gamma ring was identified as having a unique orbital characteristic (French et al. 1986, Science 231, 480). In addition to an eccentricity with amplitude ae=5.2 km, the ring possesses a pronounced m=0 "breathing" mode, wherein all ring particles reach periapse at the same time, producing the appearance of a ring that expands and contracts as a whole by +/- 5 km over a period of 7.54 hours. Even with the inclusion of these contributions to the time-dependent shape of the gamma ring's orbit, however, there remained unexplained residuals in the orbit fit (> 1 km) that were significantly larger those of the other Uranian rings. The gamma ring is among the narrowest, most opaque, and sharpest-edged of the rings, and thus the measurement of its orbital radius in occultation data is the most secure of all. Consequently, these large deviations could not be attributed to measurement error, and instead pointed to unmodeled dynamics at work. As part of developing a comprehensive orbit model for all of the ten classical Uranian rings from occultation observations obtained between 1977-2002 (see French et al. this conference), we securely identified three additional normal modes for the gamma ring: m=6 (associated with the 6:5 inner Lindblad resonance with Ophelia - see Chancia et al. 2019 AAS/DDA meeting 51/P9), m=1, and m=-2. Inclusion of these components in the orbit model for the gamma ring reduced the RMS post-fit residuals to a level comparable to that of the other rings. The gamma ring joins the select company of a very few of Saturn's narrow and optically thick ringlets in possessing both positive and negative wavenumber normal modes (French et al. 2016, Icarus 274, 131). The origin and maintenance of these composite normal modes are promising topics for future dynamical investigation.
- Publication:
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43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E.478F