Reassessing Narrow Rings at Uranus and Neptune

Workshop on the Study of the Ice Giant Planets (2014) 2033.pdf REASSESSING NARROW RINGS AT URANUS AND NEPTUNE. Matthew S. Tiscareno, Center for Radio- physics and Space Research, Cornell University, Ithaca NY 14853 ([email protected]). Introduction: Narrow rings are a unique assem- Arcs: The arcs of Neptune’s Adams ring remain an blage of matter [1], ephemerally narrow (as little as 1 enigma. While azimuthal confinement due to a reso- km in radial width, compared to ~100,000 km in diam- nance with Galatea remains plausible, post-Voyager eter) and not in a gravitational ground state (unlike a observations have not fit the simple model’s predic- planet, one would collapse if it stopped moving), yet tions, and the observed spatial frequencies of the arcs behaving as a coherent self-contained object on plane- were never an easy fit to the simple resonant model. tary lengthscales. Cassini observations at Saturn now provide a counter- Due to their close proximity, narrow rings provide example, as the G ring and several others do behave the best constraints on the interior structure of Uranus, exactly as predicted by the simple resonant model. The via their precession rates [2]. The main rings of Nep- Adams ring may turn out to be more like Saturn’s F tune, while also narrow, tend to be less sharp-edged ring and the ringlets in the Encke Gap [6], which have and richer in dust than those of Uranus. rich azimuthal structure that is clearly not associated Several aspects of narrow rings are surprising, in- with a resonant spatial frequency and is most likely cluding their radial confinement despite viscous due to embedded source moons [1]. spreading. Many narrow rings additionally exhibit Chariklo, a new analogue: The rings newly dis- sharp edges and globally coherent modes despite the covered around the minor planet 10199 Chariklo [7] expectation of differential precession across a ring’s are apparently narrow and close to each other. In this radial extent, and the Adams ring of Neptune exhibits they are like the rings of Uranus and Neptune, though arcs despite the expectation of differential orbit periods they are 100x smaller. Current data are of insufficient across its radial extent [1]. quality to distinguish whether Chariklo’s rings are ec- Radial Confinement: The classic mechanism for centric or circular, nor is it known whether they are radial confinement is the action of “shepherd moons” sharp-edged (like Uranus’) or dusty (like Neptune’s). on either side of a narrow ring. Indeed, the F ring of While the non-detection of shepherd moons is also Saturn and the ε ring of Uranus both have adjacent insignificant at this time, the examples of Uranus and moons, though in both cases it has become unclear Saturn tempt one to speculate that Chariklo’s rings are whether those moons really exert a stabilizing effect similarly self-sustaining. more than a perturbing one. All other narrow rings, Questions: Does Uranus’ system of narrow rings including the Maxwell and Huygens rings at Saturn depend only on itself for radial confinement and the and the 9 other main rings of Uranus, lack known generation of global modes? Or are yet-undiscovered shepherd moons, and that absence is becoming more shepherd moons required? Are the arcs of the Adams and more conspicuous as observational data accumu- ring confined by a nearby moon, or are they continu- late, so the nature of radial confinement may yet re- ously generated by embedded sources? Additional ob- main an open question. servations by telescopes and spacecraft, along with Global Modes: Several narrow rings of Uranus continued modeling, may lead to new answers for the- and Saturn have overall eccentric shapes (an m=1 se questions. And it remains to be seen whether the mode), and other rings exhibit modes ranging from rings of Chariklo will shed light on the rings of the ice m=0 to m=6 [2,3]. A few of these are explained by giants, or vice versa. resonant effects of distant moons, but most remain References: [1] Tiscareno MS (2013). Planetary mysterious. Analytical theory can explain the m=1 Rings. In Oswalt TD, French L, and Kalas P, eds., mode as due to the rings own self-gravity and mutual Planets, Stars, and Stellar Systems (Dordrecht: Spring- particle collisions in the presence of a non-zero eccen- er), 309–376 (arXiv:1112.3305). [2] French RG, Ni- tricity gradient de/da. Validating such theories with cholson PD, Porco CC, and Marouf EA (1991). In numerical simulations is difficult; global N-body simu- Bergstralh JT, Miner ED, and Matthews MS, eds., lations require a prohibitive number of particles be- Uranus (Tucson, Univ. Arizona Press), 327–409. [3] cause of the rings’ large radius but narrow width, while Spitale J, and Porco CC (2006). AAS/DDA Abstracts, traditional “patch” simulations are incapable of track- 37, 7.02. [4] Mosqueira I (1996). Icarus 122, 128–152. ing the global nature of the modes. Some investigators [5] Mosqueira I and Estrada PR (2002). Icarus 158, have used novel methods to simulate basic properties 545–556. [6] Hedman MM, Burns JA, Hamilton DP, of narrow rings [4,5]. and Showalter MR (2013). Icarus 223, 252–276 [7] Braga-Ribas F et al. (2014). Nature 508, 72–75. .

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