Fingerprints in Saturn's F Ring

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ASTROPHYSICS Fingerprints in Saturn’s F ring

A planet’s rings can be distorted by the gravitational pull of its satellites, and these complex interactions have been difﬁ cult to disentangle. Saturn’s moon Prometheus, however, has now been caught returning to the scene of the crime.

DOUGLAS P. HAMILTON Figure 1 Saturn’s F ring. This wide-angle camera view of the is in the Department of Astronomy, University of Maryland, F ring was captured by Cassini College Park, Maryland 20742, USA. shortly after the spacecraft was e-mail: [email protected] put into orbit around Saturn in July 2004. The outer edge aturn’s narrow, tortured F-ring confounded of the main rings are visible scientists when the fi rst images of it were on the left. Several strands of beamed back from the Voyager spaceprobes S F-ring material are visible, as more than 25 years ago. Th e ring — outside the main well as a broadly extended and band of Saturn’s rings — seemed to be composed more tenuous sheet of orbiting of three to four separate but possibly intertwined debris. The nearby satellite strands of material, confi ned radially by two nearby Prometheus is just interior to small satellites, Prometheus and Pandora. It has the ring at the upper left of the long been suspected that the gravitational tugs from image. Simulations of the ring’s these moonlets perturb the ring, contributing to its structure (such as the example bizarre appearance (Fig. 1). Th e details of this process, inset), showing a stream of however, remained murky for a quarter of a century particles pulled periodically until the Cassini orbiter arrived at Saturn last year. NASA/JPL/SSI towards Prometheus, match Now Carl Murray and others on the Cassini Imaging the exquisite detail of the Team present 1, in Nature, incredible images of regular new images1. structure in the F ring that, coupled with numerical simulations, reveal exciting new details of the of sharply defi ned narrow rings found primarily at gravitational interactions between ring and moons. Uranus and Neptune. Th irty years ago, Saturn was thought to be the When they were fi rst discovered, narrow rings only ringed planet, surrounded by three broad and came as a complete surprise because mutual collisions presumed featureless swaths of unused debris left among orbiting debris, which dissipate orbital energy over from the age of satellite formation. Within a while preserving angular momentum, lead inexorably single decade, however, this established order was to radial spreading. Th e presence of shepherding overturned by a rash of ground- and space-based satellites resolves this paradox, as these exchange discoveries. Th e three other giant planets were all energy and angular momentum with ring particles, found to have ring systems and, on closer inspection, slowly separating from each other while keeping ring the ‘featureless’ rings of Saturn broke up into material radially confi ned between them. Shepherding thousands of closely spaced ringlets with structure satellites were eagerly sought in Voyager images and as small as camera resolutions could reveal. Perhaps several examples were found: Cordelia and Ophelia most surprisingly, the rings of Jupiter were found to confi ne Uranus’s Epsilon ring, and Neptune’s Galatea be totally unlike those of Saturn, and the uranian and keeps the Adams’s ring confi ned while simultaneously neptunian ring systems were diff erent still. forcing its unusual azimuthal asymmetry. We now recognize three main types of planetary Nearly all isolated narrow rings are found rings in the Solar System. Th e fi rst are broad massive around the ice giants, but an exception is Saturn’s rings, replete with fi ne-scale structure, some of F ring — the closest and, accordingly, best-studied which is produced by embedded moonlets and some archetype. Located just 3,500 km exterior to the outer by gravitational resonances with both nearby and edge of the Saturn’s main rings, the F ring seems to be distant satellites. Saturn provides the only example confi ned by two shepherding satellites, Prometheus of this type. Second are broad sheets of dusty debris and Pandora. Th is system has been studied by a that are found in close association with small source number of scientists2–4, who found that a nearby satellites. All of Jupiter’s rings appear to be of this type, satellite imposes periodic structure on the ring with and additional examples are found around each of a characteristic wavelength, λ = 3πδa, where δa is the the other giant planets. Th e third ring type consists diff erence between the ring and moon orbital radii.

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Th us the perturbations of Prometheus, and perhaps of Prometheus’ orbit around Saturn, shear out with also those of the smaller and more distant Pandora, time, and preserve a record of the last few dozen are imprinted on the F ring. Similarly, small satellites encounters. And the periodic spacing of these features embedded in Saturn’s main ring clear out gaps and fi ts the wavelength prediction of λ = 3πδa. induce wavy features along the gap edges. In the current epoch, Prometheus aff ects only Prometheus makes regular forays into the inner the relatively broad sheet of background material and dusty edge of the F ring with each half-day orbit the innermost of the F ring’s denser strands. Over the around Saturn. But what happens when a satellite next four years, however, the most distant point of approaches its neighbouring ring even more closely? Prometheus’s orbit will march slowly outwards into Over 20 years ago, Nicole Borderies and colleagues5, the denser portions of the F ring. Th e gravitational followed later by Murray and Giuliatti Winter6, interactions will become more intense and should showed that long-term oscillations in the slightly result in more pronounced streamers and channels. elliptical orbits of both the shepherding satellites and When the dense strands themselves are signifi cantly the F ring itself would, over a decade-long timescale, perturbed, they might clump and display unusual bring Prometheus into the very core of the ring. Th e time-variable behaviour7. And what will happen when new images1 recorded by the Cassini Imaging System the shepherd begins to feed upon its fl ock? Time, and show the results of these strong interactions, and an on-call orbiting spacecraft , will tell. fresh numerical simulations1 using these data paint an REFERENCES elaborate picture. Both images and simulations show 1. Murray, C. D. et al. Nature 437, 1326–1329 (2005). that, as Prometheus moves close to the ring, a stream 2. Dermott, S. F. Nature 290, 454–457 (1981). of ring particles is pulled gravitationally towards the 3. Showalter, M. R. & Burns, J. A. Icarus 52, 526–544 (1982). moon, and long-lasting channels are opened in the 4. Kolvoord, R. A., Burns, J. A. & Showalter, M. R. Nature 345, 695–697 (1990). 5. Borderies, N., Goldreich, P. & Tremaine, S. Icarus 53, 84–89 (1983). ring, due to systematic kicks to ring-particle orbital 6. Murray, C. D. & Giuliatti Winter, S. M. Nature 380, 139–141 (1996). energies (Fig. 1). Th e channels evolve over the period 7. Showalter, M. R. Icarus 171, 356–371 (2004).

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