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Saturn stripped icy moons
to create rings

Posted: 13 December 2010

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A new study finds that the icy component of a Titan-sized moon disrupted by Saturn's gravitational pull likely formed the giant planet's rings.

The total mass of Saturn's rings is equivalent to the mass of a satellite some 500 kilometres across, but the rings are 90-95 percent water ice, unusual since objects in the outer Solar System are roughly half ice and half rock. Moreover, when they formed they were likely 100 percent pure ice, but have since been polluted by rocky meteoroid impacts.

Saturn's rings were likely caused by the giant planet stripping ice from a Titan-sized moon. Image: NASA/JPL/Space Science Institute.

The previous leading ring origin theory suggests the rings formed when a small satellite was disrupted by an impacting comet. “This scenario would have likely resulted in rings that were a mixture of rock and ice, rather than the ice-rich rings we see today,” says the paper’s author, Robin Canup of the Southwest Research Institute.

Canup's new computer simulations might have finally gotten to the bottom of how the rings formed. The models consider a large, differentiated satellite, that is, one with a distinct core of silicates and iron, surrounded by a mantle of water ice, spiralling in towards Saturn. As Saturn's gravitational pull takes hold, the icy mantle of the moon is stripped away into chunks a few kilometres to a few tens of kilometres wide, and subsequent collisions between the fragments shatter them into smaller pieces.

At the end of the formation of Saturn’s satellites, a Titan-sized satellite spirals inward toward the planet due to interaction with the primordial circumplanetary gas disk. As the satellite approaches the planet’s surface, planetary tides strip material from its outer icy layers, producing material that eventually forms a massive ice ring. The frames show material stripped from the satellite after [left] 8 simulated hours and [right] 25 simulated hours.The inner dashed circle indicates Saturn’s current mean radius, RS. Image: Image courtesy of Southwest Research Institute.

The silicate core of the moon would have been consumed by Saturn, leaving the icy boulders to disperse into a ring around the planet. But this process would have formed icy rings one thousand times more massive than the rings are today. A solution to this problem is that over time, the rings would have spread out, with some of the boulders joining together to spawn the planet's inner moons, up to and including Tethys, suggests Canup.

“The new model proposes that the rings are primordial, formed from the same events that left Titan as Saturn’s sole large satellite,” says Canup. “The implication is that the rings and the Saturnian moons interior to and including Tethys share a coupled origin, and are the last remnants of a lost companion satellite to Titan.”

The brightness of the rings today can also be explained by Canup's model – a ring with a greater mass would be less sensitive to the darkening effects of meteoroid bombardment. The Cassini extended mission will measure the impact rate within the rings to learn how quickly they become polluted, and will also measure the rings' current mass, the results of that survey should help consolidate Canup's models.