ScienceWeek

ASTRONOMY: ON SATURN'S RINGS AND PROTOPLANETARY DISKS

The following points are made by J.A. Burns and J.N. Cuzzi (Science 2006 312:1753):

1) Saturn's elegant rings embody all that is wondrous and exotic about the universe (1). Planetary rings are surely beautiful, but they also provide astronomers with nearby -- albeit imperfect --analogs (2) for the very remote, massive protoplanetary disks that surround many young stars and form the nurseries of extrasolar planets (3). Although the events deep within protoplanetary disks are obscured by dust and can only be inferred, ongoing observations of the Saturn system by the capable Cassini spacecraft (1) let us view -- in real time -- the complex dynamics that operate in our neighborhood's astrophysical disk. Some of the processes that Cassini has observed address questions relevant to the solar system's origin: how hordes of orbiting bodies crowd together, how material accumulates in such systems to form moons or planets, and how nearby masses disturb --and are perturbed by -- adjacent disks.

2) Saturn's rings and protoplanetary disks are both composed of innumerable small objects orbiting a dominant central mass. In the former case, centimeter-to meter-sized chunks of water ice orbit within a few planetary radii; in the latter, planetesimals (kilometer-to Mars-sized) circle at tens to hundreds of stellar radii. When such swarms of orbiting bodies collide with one another, they lose energy while conserving angular momentum. As a result, the systems flatten rapidly to thin disks that then inexorably spread radially unless some process halts their drift. In Saturn's rings, this evolution is interrupted at resonances, those specific orbital radii where the local orbital period is a simple ratio of a nearby perturbing satellite's orbital period. Resonances presumably play a comparable role in protoplanetary disks; indeed, the truncation of our asteroid belt's outer edge at a resonance with Jupiter gives evidence of a similar process having occurred in the early solar system. Adjacent to such resonant locations, a moon's gravity systematically perturbs a ring's mass distribution, thereby initiating spiral density or bending waves, which transfer angular momentum between the moons and the rings and ultimately drive them apart. An analogous process supposedly pushed many extrasolar planets unexpectedly close to their stars. This paradigm can be tested if, by mission's end, Cassini's precise observations identify changes in any moon's motion.

3) By causing satellites and rings to repel one another, this process can truncate rings and pry open gaps. Two of the most prominent clearings in Saturn's rings contain embedded moons: Pan amidst the broad Encke Gap and the less massive Daphnis lurking in the narrower Keeler Gap. The comparison between Saturn's two examples of moon/gap configurations indicates that our angular momentum scaling law is basically correct. By surveying the distorted edges of the gaps where these embedded moons reside, one can watch the shepherding mechanism at work. A similar mechanism is likely responsible for the inferred openings in protoplanetary disks. Observations of the interactions between the dominant moonlets and other debris in these gaps may elucidate how planets continued to grow even after they cleared openings in protoplanetary disks.

4) The elementary model in which ring edges are maintained by satellite resonances predicts that a ring's periphery should contain an integer number of graceful sinusoidal scallops. For example, Saturn's A ring, which halts at Janus' 7:6 resonance, should exhibit seven oscillations. To examine this, Cassini is scanning the various ring edges around the ring's full circumference with unprecedented radial and longitudinal resolution (4). The shapes of the main rings' perimeters grossly fit our picture, but they are not nearly as simple as had been forecast. Elsewhere too, such as along the rims of the Encke and Keeler gaps, wavy edges change amplitude and form, bobbing along in unanticipated ways, only to later regain their previous shapes. Near most crisp ring boundaries and in other strongly perturbed regions, where angular momentum is most effectively exchanged, images show that the ring's texture becomes ropy in places and strawlike elsewhere (1,5.

References (abridged):

1. Special Issue on Cassini at Saturn, Science 307 (25 February 2005)

2. J. J. Lissauer, J. N. Cuzzi, in Protostars and Planets II, D. C. Black, M. S. Matthews, Eds. (Univ. of Arizona Press, Tucson, AZ, 1985), pp. 920-958

3. Special Issue on Disks in Space, Science 307 (7 January 2005)

4. M. S. Tiscareno et al., paper presented at the American Geophysical Union fall meeting, 5 to 9 December 2005, San Francisco (abstract FM-P33B0245T2005).

5. J. E. Colwell, L. W. Esposito, M. Sremcevic, Geophys. Res. Lett. 33, L07201 (2006)

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