Two professors say that the unusual moons of Saturn could be explained by several large impacts in which several major satellites merged to form Titan.
Erik Asphaug of the University of California, Santa Cruz and Andreas Reufer of the University of Bern suggest that the Saturn system started with a family of major satellites comparable to the four large moons of Jupiter.
"We think that the giant planets got their satellites kind of like the Sun got its planets, growing like miniature solar systems and ending with a stage of final collisions," says Asphaug.
"In our model for the Saturn system, we propose that Titan grew in a couple of giant impacts, each one combining the masses of the colliding bodies, while shedding a small family of middle-sized moons."
Earth is thought to have undergone a similar kind of giant impact - indeed, two new theories relating to this have been published today. And, just as our moon consists of material similar to Earth's rocky mantle, the middle-sized moons of Saturn are made of material similar to Titan's icy mantle.
"Our model explains the diversity of these ice-rich moons and the evidence for their very active geology and dynamics," says Asphaug. "It also explains a puzzling fact about Titan, in that a giant impact would give it a high orbital eccentricity."
Through computer simulations, Asphaug and Reufer found that a merger of major satellites could liberate ice-rich spiral arms, mostly from the outer layers of the smaller of the colliding moons. Gravitational clumping would then lead to something like Saturn's current middle-sized moons.
"While we don't have a preferred timeframe for this origin scenario to play out, it could have happened recently if something came along to destabilize the Saturn system, triggering the collisional mergers that formed Titan," says Asphaug. "This 'something' could have been the close passage of a marauding Uranus and Neptune."