Dec. 7, 2007 -- Small moons embedded in Saturn's ring system likely settled into orbit and then bulked up by absorbing bits of neighboring debris, an analysis of high-resolution imagery taken by the Cassini spacecraft shows.
The finding refines the long-held theory that the small members of Saturn's eclectic clan of satellites are shards of cosmic collisions pinned in place by the planet's gravity.
Studies of Pan, Saturn's innermost moon, and newly discovered Daphnis, a 4-mile-diameter world orbiting within Saturn's A-ring, show that they "almost certainly opened their respective gaps (in the rings) and then grew to their present size early on when the local ring environment was thicker than it is today," Carolyn Porco, head of Cassini's imaging team, wrote in today's issue of Science.
Proco, a researcher with the Space Science Institute in Boulder, Colo., and colleagues mapped the shapes and densities of more than a dozen small Saturn satellites, showing that the worlds grew as large as they could balancing the various gravitational effects. The researchers also took into consideration the diminished supply of building materials as the rings thinned out.
They found that computer models of that growth process matched the satellites' shapes better than models simulating collision forces.
"We think the only way these moons could have reached the sizes they are now was to start off with a massive core to which the smaller, more porous ring particles could easily become bound," Porco said.
The theory also offers an alternative explanation for how more distant moons, such as Janus and Epimetheus, formed. Scientists posit these moons were once closer to their parent planet and were sprung out to their present locations due to their gravitational effects on the rings.
The models now suggest Janus, Epimetheus and a clutch of other sibling moons could have formed entirely from ring debris even if they lacked a dense core, such as Pan's, to start off with. The outer moons also could be shards from a collision which, over time, bonded with additional matter.
A related study of unusual ridges in the equatorial regions of Pan and Atlas, another Saturn moon, may help scientists understand the forces at work in the formation of planets.
Cassini has been studying Saturn and its moons since 2004.
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