One of Saturn's oddest moons, Iapetus, acquired an equatorial ridge, planetary scientists suggest, from a ring of debris that once circled the frozen mini-world, the detritus of a destroyed satellite. A similar story may explain Saturn's rings, suggests another study.

At the American Geophysical Union meeting in San Francisco, Andrew Dombard of the University of Illinois Chicago, will today discuss the collapsed ring theory for Iapetus. Iapetus has puzzled astronomers since flybys by the international Cassini spacecraft in 2005 revealed its appearance. The 913-mile-wide moon is bifurcated by a ridge 12 miles high and 62 miles wide stretching across at least three-quarters of its equator.

"Some people have proposed that the ridge might have been caused by a string of volcanic eruptions, or maybe it's a set of faults," says theory co-author William McKinnon of Washington University in St. Louis, in a statement. "But to align it all perfectly like that — there is just no similar example in the solar system to point to such a thing."

Instead, Dombard and McKinnon, "propose that at one time Iapetus itself had a satellite, or moon, created by a giant impact with another big body," says the statement. Over time, gravitational forces would have ripped the moon's satellite apart, "forming a ring of debris around Iapetus that would eventually slam into the moon near its equator." Once, therefore, one of Saturn's moons may have possessed its own ring.

"Imagine all of these particles coming down horizontally across the equatorial surface at about 400 meters per second, the speed of a rifle bullet, one after the other, like frozen baseballs," says McKinnon. "Particles would impact one by one, over and over again on the equatorial line."

Depending on the size and distance of the moon's one-time satellite, the break-up of the ring and its deposition onto the surface of Iapetus could have take a billion years to play out, the planetary scientists suggest. Earth's moon, they note, orbits too far from our planet to ever endure such a wrenching finish.

A study released by the journal Nature on Sunday (and first reported in October), however, suggests that Saturn's rings likely sprang from tidal forces similarly shredding a large moon of the ringed planet to flinders over a period of about 3 million years

"Saturn has only one large satellite, Titan, whereas Jupiter has four large satellites; additional large satellites probably existed originally but were lost as they spiralled into Saturn," writes study author Robin Canup of the Southwest Research Institute in Boulder, Colo. "Planetary tidal forces preferentially strip material from the satellite's outer icy layers, while its rocky core remains intact and is lost to collision with the planet. The result is a pure ice ring much more massive than Saturn's current rings. As the ring evolves, its mass decreases and icy moons are spawned from its outer edge with estimated masses consistent with Saturn's ice-rich moons."

Most likely, Canup concludes, Saturn ripped apart a moon the size of its largest current one, Titan, to form its rings.

"This elegant model could provide the missing links between a suite of observational and theoretical results that have changed our understanding of Saturn's rings," write France's Aurélien Crida of the Université de Nice and Sébastien Charnoz of the Université Paris Diderot, in a commentary accompanying the report.

By Dan Vergano