Saturn's majestic rings and icy inner moons formed following the planet's collision with another moon the size of Titan, a new study suggests.
Titan is the largest remaining moon orbiting Saturn, and is about one-and-a-half times the size of Earth's moon.
The study, published today in the journal Nature, uses computer simulations to explain both the composition of Saturn's rings and its icy inner moons.
The lead author, Dr Robin Canup from the Southwest Research Institute in Boulder, Colorado, claims Saturn's gravitational forces stripped material from the outer icy layers of a Titan sized moon.
These eventually formed the icy inner rings and moons, while the rocky core of the moon would have fallen into the giant gas planet.
The low densities of Saturn's inner moons indicate they are unusually rich in ice, and her rings are between 90% and 95% water ice. This composition is unusual compared to the regular half-ice half-rock mixture normally found in this part of the Solar System.
Comet collision theory wrong
The leading ring-origin hypothesis suggests they formed when a small satellite collided with a comet.
But Canup says, "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".
Her idea links the formation of the rings to the formation of Saturn's moons. While Jupiter has four large satellites, Saturn has only one, the huge planet-sized moon Titan.
Canup says previous work suggests multiple Titan-sized moons formed around Saturn, but those orbiting closer in than Titan were lost as their orbits spiralled into the planet.
As the last of the large and now lost satellites neared Saturn, tidal flexing of the moon due to Saturn's gravity created heat, melting the ice. The rocky core was then pulled into Saturn's gaseous body.
To test the hypothesis, Canup used smoothed-particle hydrodynamics to simulate the tidal stripping of a thick icy shell from a Titan-sized moon.
Mutual collisions among the icy fragments would have driven the particles into a pure-ice ring, which over time became the Saturnian rings we see today.
In the simulations, inwardly spreading ring material is lost, while material spreading past the ring's outer edge accumulates into icy moons with estimated masses consistent with the inner moons seen today.
"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 common origin, and are the last remnants of a lost companion satellite to Titan", she says.
The Roche Limit
Macquarie University planetary scientist Dr Craig O'Neill says the paper shows how little we really know about the outer solar system.
"It highlights how much there still is to learn about something as distinctive and commonly viewed as Saturn's rings."
O'Neill says Canup's theory revisits a 160 year old idea by a French mathematician. The so-called Roche limit determines how close to a planet a body can get before it's pulled apart by that planet's gravitational tidal forces.
"Edouard Roche invented this to explain Saturn's rings, but it largely fell by the wayside over the years. Now Robin's paper has revisited this idea and it looks to be the real reason why the rings formed."