June 10, 2010

Saturn’s Rings Were Birthplace Of Tiny Moons

For decades, researchers have been puzzled over the origins of Saturn's baby moons, which, according to many models, are so small that they should have been blown to pieces long ago by impacts with asteroids and comets.

But a group of researchers from France and Britain now think they have the answer.

Saturn's icy rings might have given birth to the planet's odd-shaped, small moons, scientists now believe.

Scientists theorize that these unusual moons, some of which resemble flying saucers, may have clumped together from bits of ice and dust that make up Saturn's unique rings.

Many of the moons, which measure less than 35 miles across, should have been destroyed long ago by comets. And over time, moons tend to recede from planetary orbits, as indeed our own Moon is receding from Earth. But Saturn's moons are in such close orbit that they would have had to have formed practically inside the planet itself.

The small moons were initially believed to have been formed around the same time as the larger moons of the planet, such as Titan, some 4.5 billion years ago.

However, calculations of the orbits of five of the smallest moons of Saturn gathered just within and beyond the edge of the planet's main rings reveal that they are far too young to have been formed that long ago. These moons must be less than 10 million years old -- given that they are also largely unmarred by the impacts expected from meteoroids.

"There are still new objects forming in the solar system today," planetary scientist Sebastien Charnoz at the Paris Diderot University of France told Space.com. "We used to think everything was formed four, five billion years ago, but no! New objects are still forming today."

To prove the younger existence of Saturn's moons would normally require a numerical calculation that begins at the formation of the Solar System and tracks the satellite's every orbit since then -- all one trillion of them.

With no computer able to perform such a large-scale calculation, Charnoz and his colleagues created a simplified model of moon dynamics and reduced the ring to one dimension. Once they tested it by reproducing our own Moon's formation, they applied it to Saturn.

The model shows that Saturn's main A ring, which is about 75,000 miles from the planet's center and about 9,300 miles wide, feeds material to the empty region just outside. Here the material can clump together into baby moons. The bigger the moons get, the more Saturn's gravitational forces push them outwards.

The model not only shows why the biggest moons are farthest out, but also the possible origin of the mysterious F ring, a dusty region outside the A ring in which the baby moons currently orbit. According to Charnoz's group, the moons would have generated the F ring's dust through collisions with each other.

Heikki Salo, a planetary scientist at the University of Oulu in Finland, said the model's explanation of moon formation is "very beautiful". But he is not sure how much it helps to explain the wider, long-standing question of the age of Saturn's main A and B rings.

Salo believes it is possible that they formed within the past 100 million years or so, perhaps from a large comet hitting the planet, but it seems unlikely that such a body could have ventured close enough in the recent past. A more likely explanation is that they formed billions of years ago in the protoplanetary disk -- but in that case they would have become dirtied by numerous meteoroid impacts.

Salo also notes that the Charnoz model does not rule out the possibility that the rings are older, and that "recycling" of the ring material could explain their pristine condition. "The origin of the rings could therefore be pushed back to a more probable time frame, such as the era of late heavy bombardment 4 billion years ago," Salo says.

Derek Richardson, a computational astrophysicist at the University of Maryland, who was not involved in the Charnoz research, told the Sicentific Aermican that 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," he 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."

Charnoz and his colleagues published the study in the June 10 issue of the British journal Nature.


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