DENVER—“The Origin of Titan—So Big … So Alone.” That was the playful title of a talk given here yesterday at the annual meeting of the Division for Planetary Sciences. The gist? Saturn’s relatively huge moon Titan, which orbits unaccompanied by the usual retinue of similar-sized moons, started out as three or four standard-issue satellites of the ringed planet that ran amok, collided, and merged into one huge moon and a few scraps of debris.
Titan is “arguably the solar system’s most unusual satellite,” said the speaker, planetary dynamicist Douglas Hamilton of the University of Maryland, College Park. That’s because the standard model for producing satellites starts with a flat disk of primordial debris that eventually agglomerates into a regular system of several similar-size moons in roughly equally spaced orbits. This works fine for forming the nicely regular Jupiter satellite system, but Saturn’s is anything but regular. Titan is dominatingly big, having almost twice the mass of Earth’s moon and comprising 90% of the mass in orbit about Saturn. Titan is alone, orbiting in a million-kilometer gap bounded by tiny moons. And Titan’s orbit is odd: It is slightly elliptical rather than nearly circular and is tilted with respect to Saturn’s equator. With all those oddities, Hamilton said, “the biggest mystery is how it came to be in the first place.”
Hamilton suggested a solution to Titan’s mysterious origin in which the standard model started out fine at Saturn but later things went awry. Three or four regular satellites could have formed and started to drift outward under the influence of tides raised on the moons by Saturn’s gravitational pull, the same way Earth’s moon has been slowly drifting outward since it formed. But rather than slipping into a stable arrangement the way Jupiter’s four moons did, in Hamilton's scenario the orbits of Saturn’s initial moons became unstable and began to overlap and then the moons collided with one another. The collisions were relatively slow because the moons were all going in much the same direction, so the moons tended to merge rather than splatter as they hit.
The end result: one big moon alone in an orbital space once occupied by several smaller moons. Hamilton’s computer simulations suggest that the collisions would have left Titan with a bit of orbital tilt and elongation, as observed. And little Hyperion, an ice chunk orbiting beyond Titan with even more tilt and elongation, could be a splatter from one of the collisions.
“It’s very plausible,” says planetary physicist William McKinnon of Washington University in St. Louis, but he cautions that “it’s early days” for judging the new scenario. Among other things, he says, researchers need to check whether Hamilton’s tidal mechanism for driving the initial moons into unstable orbits would have worked so far from Saturn so long after they formed. And even Hamilton acknowledges he's not sure how he would “prove” that he is right.
McKinnon has one idea: If the Cassini spacecraft orbiting Saturn could pin down the nature of Titan’s interior, researchers would know whether it formed hot in violent collisions—which would have produced a rocky core—or it formed cold through the quiet agglomeration of primordial debris. But to gather the necessary evidence, a cash-strapped NASA would have to continue to fund Cassini for several more years.