Did Several Moons Collide to Make Saturn’s Titan?
“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.”