A MODEL EXPLANATION: In an artist's depiction of a new computer simulation, particles from Saturn's main ring accrete into moonlets just outside the gravitational stability limit [left]. Image: www.animea.com
Saturn is perhaps best known for its intricate ring system, but the giant planet also boasts a collection of moons, numbering in the dozens, that is nothing to sniff at. The largest, Titan, has helped draw a bit more attention to the Saturnian satellites in recent days, following an announcement that various chemical abundances on Titan were consistent with but not necessarily indicative of the presence of methane-dwelling, hydrogen-breathing life.
Now, research in the June 10 issue of Nature deals with a population of smaller Saturnian satellites—Atlas, Prometheus, Pandora, Janus and Epimetheus—linking the origin of those so-called moonlets to the celebrated rings themselves. The study presents the result of a computer simulation of Saturn's dynamic environment, demonstrating how the moonlets, which dwell just beyond the planet's famed main rings, could have formed from material oozing out of the rings and accreting into clumps. (Scientific American is part of Nature Publishing Group.) What is more, the moonlets appear to have coagulated in recent astronomical time, implying that more moonlets may be forthcoming.
"We explored the idea that accretion is possible and ongoing at the rings' outer edge," says lead study author Sébastien Charnoz, an astrophysicist at the University of Paris Diderot. "It is a common idea to say that the solar system had finished its formation by 4.4 billion or 4.5 billion years ago," he adds. "With this process we see that there is still accretion in the solar system."
The model tests a mechanism that had been proposed before but had been difficult to computationally confirm, Charnoz says—that the moonlets arise from icy debris spreading outward from the rings, past the point of gravitational stability. "Disks in astrophysics are like pancakes—they spread," he says, adding that collisions within the disk or ring drive the spreading detritus outward. Once the icy ring particles venture beyond about 140,000 kilometers from Saturn's center, they become unstable, clumping into tiny protomoons and then moonlets.
At present, the orbit of Janus, the largest of the inner moonlets at 180 kilometers across, keeps the rings within the gravitational stability limit. But as Janus continues to migrate outward the rings will expand as well, perhaps turning loose more icy material for new satellites.
The model reproduces a number of observed features of the moonlets—their apparent compositional similarity to the ring material, their positioning just outside the main rings' edge, and the seeming contradiction between their orbital motion and the position of the rings. "Everybody was saying there was a problem there," Charnoz says. "We know that the moons go away from Saturn and from the rings because of tidal interactions." But when astronomers ran orbital simulations back in time, it appeared that the moonlets would have recently plowed through the rings themselves. What the new model shows is that the moonlets formed at the edge of the rings from loose material there and then migrated outward to occupy their present orbits.
Derek Richardson, a computational astrophysicist at the University of Maryland, College Park, who did not contribute to the new research, says he finds the explanation fairly convincing. "The thing that's compelling is it simultaneously explains the small moons and the outer configuration of the main rings of Saturn," Richardson says.
Richardson notes that the new paper returns to a familiar theme in astronomical research of late—that the solar system is an ever-shifting place that has not yet finished maturing. "The rings are a dynamic place, and we are sort of seeing a snapshot in their continuing evolution," he says. "There's this constant interaction or dance between the outermost edge of these rather dense rings and these small moons."
At the same time, he says, the authors' model of Saturn's dynamics cannot perfectly capture the intricacies of the system. "It's based on simplified models of the dynamics," Richardson adds, because the processes at play are so complex and act over millions or billions of years. "They'll need to do follow-up simulations to really follow the details of what they are finding here," he says.