Saturn's largest satellite, Titan, is drifting away from its planet 100 times faster than previously thought

Atmospheric moon taking a wander

Titan, the only known moon in the Solar System with an atmosphere, is drifting away from Saturn at a rate a hundred times faster than previous estimates, according to the latest research published in Nature Astronomy on Monday.

Previous studies suggested that Titan's orbit should be migrating only about 0.1 centimeters per year. But a team of astrophysicists led by the Jet Propulsion Laboratory at the California Institute of Technology (Caltech) have bumped that number up to 11 centimeters per year after analyzing data taken from NASA's now-defunct Cassini spacecraft.

What's driving Titan's migration is a little complicated. Firstly, planets and their satellites exert a gravitational pull on one another. On Earth, this causes tides, but also creates a torque that drags on the Earth's rotation and slows it down, but pushes the Moon forward and speeds it up. Over time, the Moon slowly edges away from us at about 3.8 centimeters per year.

A similar effect also occurs between Saturn and its satellite Titan. The tidal acceleration Saturn pushes onto Titan, however, is weaker and, as a result, scientists thought its moon must move away at a slower rate. But another process known as "resonance locking" is also at play.

"You can think of resonance locking like pumping your legs on a swing," Jim Fuller, co-author of the paper and a professor of theoretical astrophysics at Caltech, explained to The Register.

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"If you pump your legs at the same rate that you swing back and forth, you move higher and higher. In the Saturn system, the wobbling of Saturn is analogous to your motion on the swing set, and the leg pumping is analogous to the force produced by Titan. This causes Saturn to wobble with a large amplitude, and the resulting gravitational force causes Titan to spiral outwards."

Titan isn't the only Saturnian satellite to experience resonance locking; its other moons Mimas, Enceladus, Tethys, Dione, and Rhea probably go through the same process too. Don't worry, they probably won't escape from Saturn's gravitational grip entirely.

"We don't think Titan will escape, because eventually the resonance locking process won't be able to operate, just as you can't pump your legs hard enough to swing around the top of the swing set," said Fuller. "We don't yet know how far out Titan will migrate before this happens."

The researchers observed Titan's drift using two different instruments. One snapped images of background stars and estimated Titan's movements relative to the star's positions. The other tracked the gravitational influence of Titan by measuring Cassini's velocity.

"By using two completely different datasets, we obtained results that are in full agreement, and also in agreement with Jim Fuller's theory, which predicted a much faster migration of Titan," said Paolo Tortora, co-author of the study and a professor at the department of industrial engineering at the University of Bologna.

"We hope to use these results to understand the history of the Saturn moon system, and also to understand whether a similar process may operate in other systems, such as exoplanetary systems," Fuller concluded. ®

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