Making Saturn's Moons with a Bang

Impacts are a favorite tool for planetary scientists to create beautiful things.

The press had just gone wild with visions of a colossal impact creating the Earth-Moon system (10/18/2012), when Saturn cried, “Me, too!”  A spate of reports claims the improbable assortment of moons at the ringed planet are the result of another dramatic impact (at least in simulations).  The story was carried by Space.com, Live Science, Science Daily and Nature News (i.e., the usual suspects).

It’s not that the Saturn system “did” form this way; it’s that it “could” have, as Nature News‘ headline states: “Moon-merge model could explain Saturnian system;  Simulation suggests that the moons of Saturn were once more like Jupiter’s.”  The Jupiter system is hard enough to explain, but planetologists feel it is a little more orderly than Saturn’s odd collection of small icy bodies (with active Enceladus and inactive Mimas), big icy bodies, and Titan, the oddest of all.

With widely ranging densities and locations, Saturn’s six mid-size satellites are among the strangest in the outer Solar System. Measuring between 300 and 1,500 kilometres in diameter, the moons have varying characteristics: several are made almost entirely of frozen water; one, Enceladus, is rockier and geologically active; and some show evidence of submoons and rings. How did they get this way?

With their charming impact video, the Saturnian modelers are apparently trying to share the limelight with the Galvanized Lunacy club: “Earth’s moon is thought to have formed from a giant impact about 4.5 billion years ago. And just as the moon and Earth are geochemical twins, the half-dozen or so medium-size moons of Saturn are similar in composition to Titan’s icy mantle, researchers said.”

Science Daily included this gaffe by the modeler:

“What makes the Saturn system so beautiful and unique could be its youth,” Asphaug said. “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.”

Nature News, though, called on Hal Levison, whose toy models have trouble creating Uranus and Neptune (8/06/2004), to do the reality check:

But Hal Levison, an astronomer who studies planetary dynamics at the Southwest Research Institute in Boulder, Colorado, says that the model is currently too simple to work. As the model is now configured, he says, the mid-size moons would inevitably accrete on to Titan rather than survive in isolation.

Perhaps he said this to help keep his colleagues in the planetary billiards hall employed, and to keep the game going (10/18/2012).

Warning: never buy a house from these folks.  They bang the materials against each other and hope something emerges.

The best part of this story is Eric Asphaug admitting that Saturn’s system looks young.  At the 15th anniversary of Cassini mission, he is well aware of the problems with the rings, Enceladus, Titan and other things that cannot fit into the standard 4.5 billion year timeline.  It’s a tacit admission that the evidence does not fit the standard story without a colossal ad hoc rescue device.