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Authors' Summary:
Saturn’s Small Inner Satellites: Clues to Their Origins

C. C. Porco et al.

Before Voyager 1 arrived at Saturn in 1980, it was thought that the icy particles composing Saturn’s rings were the detritus left over from the planet’s formation and that strong tidal forces close to the planet prevented them from aggregating to solids much bigger than a few meters across. The Voyager spacecraft discovered small moons orbiting within and just outside the main rings, and made observations that suggested a ring mass equivalent to that of Saturn’s 400-km-wide icy moon Mimas. These and other Voyager findings led many scientists to conclude instead that Saturn’s rings formed by the catastrophic collisional disintegration of one or several large icy bodies, perhaps preexisting moons. The resulting debris would have quickly settled into the equatorial plane to form a thin disk. It was possible that the different major rings—A, B, and C—formed from different progenitor bodies and therefore were of different ages.

However, Voyager results also implied that a Mimas-sized moon would survive without collisional disruption for so long that any ring-forming event producing that much debris would have had to happen in the early days of the solar system, ~4.5 billion years ago. Hence, ring B, by far the most massive, might be that old. Ring A, with less mass requiring a smaller, more easily disrupted progenitor body, might have formed more recently (1). The presence of moons 30 km in diameter near the outer part of the A ring, such as Pan and Atlas, remained a puzzle: The lifetimes of such small bodies against collision were expected to be shorter still. If disrupted since the A ring’s formation, they must have reformed. But how could they do so in the presence of strong planetary tides? It was therefore deemed likely that the small, irregularly shaped moons within and around the rings were shards left over from the original breakup of the rings’ progenitor body (or bodies) (2). Even the more distant, small moons of Saturn’s major satellites Tethys and Dione were expected to be irregular chips off their larger co-orbital companions.

Divining the origin and evolution of Saturn’s entire ring-satellite system was one of the principal goals of the Cassini mission. Cassini has surveyed the rings (within the orbit of Titan) for small bodies missed by Voyager and gathered information on their physical characteristics and orbits. Images have yielded sizes and shapes; masses have been determined through observed gravitational effects on other moons or on the rings.

Resize Image

View from the Cassini spacecraft of Saturn’s moon Janus (about 190 km across) on the near side of the rings and Prometheus (about 120 km across) on the far side. Both moons have a low density. Analytical results and computer, and simulations imply that Prometheus, and perhaps Janus, have grown by accumulating porous ring debris around a relic core.

Here, we show that the ring-region moons have high porosities and low densities, about half the density of water ice. Other Cassini results (3) imply that the ring particles themselves must have a similar low density. Beyond about the orbit of the F-ring shepherd Pandora, where tidal effects weaken, accretion of such porous material can proceed easily. So whether Janus, Epimetheus, or any of the other, much smaller, more distant moons were formed entirely from aggregation of smaller fragments or are monolithic collisional shards, or somewhere in between is unknown. However, closer to the planet, we find that spontaneous, unseeded aggregation of porous particles, even if broadly distributed in size, to form large, homogeneous kilometer-sized bodies is not feasible.

Instead we suggest, and computer simulations support, that the moons Pan and the newly discovered Daphnis (both within the outer A ring), Atlas (just beyond the A ring), and even Prometheus and Pandora (a few thousand kilometers farther out) all likely grew to their present sizes by the accumulation of porous ring material onto massive, denser cores that were one-third to one-half the present-day sizes of the moons. Although the cores themselves may be collisional shards, the present moons in their entirety cannot be. The cores of Pan and Daphnis were likely large enough to open their respective gaps in the rings, and their growth was largely finished before the gaps were cleared. A secondary stage of accretion apparently formed the equatorial ridges of Pan and Atlas (4), and presumably Daphnis as well, after the disk had thinned to its present ~20-m thickness.

How far in the past all this took place is hard to say, and depends on when and whether Pan and Daphnis were disrupted after their initial formation. However, if they suffered any disruptions at all, their cores must have remained more or less intact, or reformation would have been impossible. Perhaps the thick porous blankets of material surrounding these cores today has protected them from being blown to bits, because porous material can absorb impacts with little damage to the underlying structure (5). It remains to be tested whether the surfaces of Pan and Daphnis and the other moons in the ring region have the same composition as the rings, which would support this story. Also, crater size-frequency distributions on the larger saturnian satellites, especially Iapetus, will clarify how frequently collisions have occurred within the saturnian system, telling us whether the A ring is likely to be young and its small moons have been blasted apart and reaggregated since their original formation, or whether the whole system has been around since the dawn of the solar system.

Summary References

1. L. Esposito, Icarus 67, 345 (1986).
2. B. A. Smith et al., Science 215, 505 (1982).
3. M. Tiscareno et al., Astrophys. J. 651, L65 (2006).
4. S. Charnoz, A. Brahic, P. C. Thomas, C. C. Porco, Science 318, 1622 (2007).
5. E. Asphaug et al., Nature 393, 437 (1998).

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