Today’s top, click-bait style headline comes to us from NASA’s Jet Propulsion Laboratory (JPL). They would like you to know that Saturn’s moon Titan is moving away from Saturn one hundred times faster than previously thought. Based on the questions we’re already getting on this story, this news has a lot of people worried about the fate of Titan. Well, folks, a hundred times a very small rate is still a pretty small rate, and we’d like you to know that while Titan is on the move, its eleven centimeters per year migration isn’t going to affect its fate any time soon. Titan is just going to keep on orbiting Saturn for billions of years to come.

Moon migration is nothing new. Our own moon is migrating away at a rate of 3.8 centimeters per year. At the most simplistic level, this is driven by the conservation of angular momentum and tidal forces. Since moons and planets aren’t perfect spheres, gravity pulls on the lumps and bumps on every world and tries to slow to lock their rotation, so that these worlds eventually end up with the same face showing to whatever they are orbiting, just like the same face of the moon always faces us. This is the first order of what is happening, but things get more complicated when we look at the details, and it’s those details that get us to the accelerated rate of the retreat we now see for Titan.

As I said, we knew Titan was migrating; we just had misunderstood how fast. Prior estimates were calculated, but this new research is based on analysis of data from Cassini. Specifically, scientists analyzed radio signals Cassini sent toward Earth during ten flybys of Titan that took place from 2006 to 2016. The interactions between the radio waves and Titan, when used in concert with optical images of Titan against star fields, gave the team two different approaches to measuring Titan’s position and motion.

This new result confirms a 2016 paper by CalTech’s Jim Fuller that revisited how inner and outer moons can migrate and determined that outer moons can migrate outward faster than more simplistic calculations predicted. 

With this new analysis of Titan’s observed motions, it’s possible to work backward to sort out the evolution of the Saturnian system. As we’ve brought up before, the exact age and history of Saturn’s rings is a matter of much debate, and it remains unclear how old or young or long-lasting the rings may be. This work is published in Nature Astronomy in a paper led by Valery Lainey.

With this new data on Titan, it appears that Titan started its life much closer to Saturn, and its location, and that of the rings as a whole, expanded much faster than previously thought. Maybe. We still don’t have a complete model, and we don’t have a time machine, so this is just one more clue that will have to be matched as we try to recreate the Saturnian system with software.

More Information

NASA’s JPL press release 

Caltech press release 

“Resonance Locking in Giant Planets Indicated by the Rapid Orbital Expansion of Titan,” Valéry Lainey et al., 2020 June, Nature Astronomy