Titan's Free-Floating Crust

Among the most anticipated results from NASA's Cassini mission was a chance to reveal the surface of Titan, Saturn's largest moon and arguably the most fascinating solid body in the solar system (besides Earth, of course).

Cassini's first good chance to peer through Titan's murky atmosphere using radar came in October 2004. Those initial radar images revealed a world brimming with geologic diversity:

But something strange came to light later on, after some of the same areas were viewed again on subsequent close passes: Titan's features weren't quite where they were supposed to be. In some cases they were nearly 20 miles (30 km) from their predicted locations.

Because Saturn has such a powerful gravitational grip on its moons, the mission team had assumed that Titan's spin axis is exactly perpendicular to its orbital plane and that the big moon rotates synchronously, completing exactly one rotation every time it goes around Saturn (15.945 days). But the radar data are telling a different story: Titan's spin axis has to be tipped slightly, just 0.3°, and its rotation needs to be ever-so-slightly faster than expected, by just 0.004%. Moreover, this slight rotational mismatch seems to be increasing — Titan is speeding up!

(It's amazing to me that the radar maps and spacecraft tracking are precise enough to see these discrepancies. But they're really real!)

One explanation might be that winds in Titan's massive atmosphere, which is much denser than Earth's, are slowly influencing the moon's rotation. Ordinarily this effect would be inconsequentially tiny. (For example, persistent seasonal winds in Earth's atmosphere change the length of our day by no more than a millisecond per year.)

But several years ago theorists surmised that Titan probably isn't solid throughout. Instead, they argue, this Mercury-size moon has a global layer of ammonia-infused water just below its water-ice crust. A global ocean would effectively decouple the crust from rotation of the deeper interior, allowing the outer shell to spin at its own rate and making it much more susceptible to external forces like surface winds.

In tomorrow's issue of Science, Ralph Lorenz (Applied Physics Laboratory) and eight others argue that an internal ocean is the best way to make sense of Titan's rotational quirks. "The bottom line," Lorenz told me via e-mail, is that "the rotation is changing, and the only way it could be changing is if the crust is decoupled."

Right now it's late winter in Titan's northern hemisphere, and the leading circulation model predicts that near-surface winds should be causing the spin to accelerate. That's what Cassini has observed. In a few years, once northern summer arrives, the spin rate should slow. Cassini should be able to track this reversal if can hold out until, say, 2011.

However, the model and Cassini's observations don't match exactly. Specifically, Titan seems to be about two years behind its predicted speed-up schedule. Perhaps the model needs tweaking, or other forces (such as Saturn's pull on a bulge in Titan's midsection) might be involved. Conceivably Titan's spin axis is wobbling, as has been suggested by French dynamicist Benoît Noyelles. It's even possible that Titan was spun up by a large impact in the recent past. But a strike that potent should only occur every 100 million years — and it would have punched a hole in the ice at least 100 km across!