How Saturn got its rings has long been a mystery, but a paper published in Nature this week put forward a new idea about where all that watery material originated. Dr. Robin M. Canup suggests that Saturn's rings could have originally been the icy outer layers of large satellite moons that were stripped away by Saturn's tidal forces as the moon spiraled into the planet itself.

There have been many theories about where Saturn's rings came from. One involves the disruption of a small moon, though that doesn't account for the fact that the rings are made almost entirely of ice. Another popular theory is that many comets, which are made of ice, got too close to Saturn. In the new paper, Canup points out that having enough comets run into Saturn to deliver the entire volume of the rings is unlikely.

Instead, Canup proposes that the ice originated from much larger bodies. She suggests that ice-encased satellites about the size of Saturn's largest moon Titan drifted too close to the planet, allowing the ice to be torn away by the forces at the Roche limit.

The Roche limit is a feature of every celestial body. It's a kind of destruction boundary that lies at a distance from the body's surface that's set by its gravitational pull. When a smaller object crosses the boundary, the tidal forces of the large body overcome the gravity holding it together, and the small object gets torn apart.

Using smooth particle dynamics, Canup modeled a large moon with an outer icy layer reaching Saturn's Roche limit. She found that, for a particular ratio of external ice to internal metal or rock, the icy layers of the moon could by stripped away once it's inside the Roche limit. Before the tidal effects could disrupt the rocky insides of the moon, though, it would crash into Saturn, leaving its ice behind in orbit.

The model would explain a number of phenomena other than the rings' origin. Canup's analysis showed that, as time passed, interparticle collisions within the rings would allow them to spread and decrease in mass while maintaining their ring shape. Contrary to other popular theories, this would indicate that the rings started out larger and denser than they currently are, and have lost volume over time.

As more time elapsed in the simulation, Canup found that the outer ring particles could conceivably accrete into whole icy moons. This would explain the origin of many of Saturn's interior moons, including Tethys, which, like the rings, are around 90 percent ice. (This isn't the first model to suggest this.)

Of course, if Canup is going to claim that icy comets flying in to populate Saturn's rings is unlikely, large moons that have just the right ice-to-rock ratio don't sound much more probable. But the ratio may not have had to be just so—the rings do currently have some rocky debris in them, which could be remnants of any interior moons that went in for a crash landing.

In fact, the origin of the non-ice material in the rings could turn out to be a better key to understanding their origin than the ice itself. The debris is often said to be meteoroid pollution, but Canup notes that, if a ring that started out several orders of magnitude more massive, it would have had enough mass and angular momentum that it shouldn't have picked up much pollution. The icy interior satellite moons should have prevented this as well.

NASA's Cassini spacecraft will help settle this matter by taking data on Saturn's rings, including the impact rate of their particles, and a direct measurement of the rings' total mass. Canup notes that the data will help determine how susceptible the rings are to pollution, and may indicate the history of their size.

Nature, 2010. DOI: 10.1038/nature09661 (About DOIs).

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