Published online 9 June 2010 | Nature | doi:10.1038/news.2010.286

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Mystery of Saturn's midget moons cracked

Their recent birth in rings may explain why moons were not pulverized by comets.

This montage of images of the Saturnian system was prepared from an assemblage of images taken by the Voyager 1 spacecraft during its Saturn encounter in November 1980. This artist's view shows Dione in the forefront, Saturn rising behind, Tethys and MimaSaturn's moons may have formed from icy lumps in the planet's rings.NASA / JPL

For decades, researchers have puzzled over the origin of Saturn's baby moons. According to conventional models, these moons are so small that collisions with comets should have blown them to pieces long ago. Now a group of researchers in France and Britain think they have the answer — and it lies in the planet's icy rings.

Accepted theory says that the giant planets, and their moons, slowly accreted out of a gaseous 'protoplanetary disk' around the Sun some 4.5 billion years ago. Yet Saturn's baby moons never quite fitted this picture. At less than 50 kilometres across, they ought to have been destroyed by comets over that period. And over time, moons tend to recede from the planets they orbit, as indeed our Moon is receding from Earth. But Saturn's moons are in such a close orbit that they would have had to have formed virtually inside the giant planet.

About six years ago, the Cassini spacecraft relayed images that hinted at an alternative origin. Sailing past Saturn's outer rings, it found lumps of ice up to 100 metres across, ten times bigger than the rings' other icy particles. For some researchers, the discovery called to mind another intriguing fact: that the moons and the rings share a composition of the purest ice in the Solar System. "When you put all this together, you had the strange feeling that something is going on in the rings' outer edge," says Sébastien Charnoz at Paris Diderot University, who was involved in the latest research.

Charnoz and his colleagues, who are based in Paris and at the University of Cambridge, UK, believed the baby moons formed from smaller lumps of ice sticking together on the outskirts of the main rings. That process, they thought, might have occurred more recently than had been thought — perhaps just 10 million years ago — after which the moons migrated outwards. To prove it, however, would normally require a numerical model that begins at the Solar System's formation and tracks the moons' every orbit since then — all one trillion of them.

Age concern

With no computer able to perform such a huge calculation, Charnoz's group created a simplified model of moon dynamics and reduced the ring to one dimension. Once they had tested it by reproducing our own Moon's formation, they applied it to Saturn. Their model shows that the main A ring, which is about 120,000 kilometres from Saturn's centre and about 15,000 kilometres 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 'tidal' forces push them outwards (see video).

The model reveals not only 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 reside. According to Charnoz's group, the moons would have generated the F ring's dust through collisions with each other. The group's study is published in Nature today1.

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Heikki Salo, a planetary scientist at the University of Oulu in Finland, calls the model's explanation of moon formation "very beautiful". But he is unsure how much it helps to address the wider, long-standing question of the age of Saturn's main A and B rings. It is possible that they formed in the past 100 million years or so, perhaps from a large comet hitting Saturn, 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 notes that the Charnoz group's model does not rule out the possibility that the rings are older, and that a "recycling" of 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," he says. 

  • References

    1. Charnoz, S., Salmon, J. & Crida, A. Nature 465, 752-754 (2010). | Article
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