Saturn is orbited by 82 moons, and tidal friction within the giant planet causes the moons to migrate outwards. In a study published this week in the journal Nature Astronomy, researchers used two independent measurements obtained with NASA’s Cassini spacecraft to measure Titan’s orbital expansion rate. They found the hazy moon drifting a hundred times faster than previously thought — about 11 cm (4 inches) per year. The result suggests that Titan formed much closer to Saturn and has migrated outward to its current position.
This mosaic of Saturn and its largest moon Titan combines six images — two each of red, green and blue spectral filters — to create this natural color view. The images were obtained with Cassini’s wide-angle camera on May 6, 2012, at a distance of approximately 778,000 km (483,000 miles) from Titan. Image credit: NASA / JPL-Caltech / Space Science Institute.
Classical theories assumed that in systems such as Saturn’s, the outer moons like Titan migrated outward more slowly than moons closer in because they are farther from their host planet’s gravity.
In 2016, Dr. Jim Fuller of the Walter Burke Institute for Theoretical Physics and colleagues predicted that outer moons can migrate outward at a similar rate to inner moons because they become locked in a different kind of orbit pattern that links to the particular wobble of a planet and slings them outward.
“The new measurements imply that this kind of planet-moon interactions can be more prominent than prior expectations and that they can apply to many systems, such as other planetary moon systems, exoplanets and even binary star systems, where stars orbit each other,” said Dr. Fuller, co-author of the new Nature Astronomy paper.
To reach their results, the scientists mapped stars in the background of Cassini images and tracked Titan’s position.
They then compared them with an independent dataset: radio science data collected by Cassini.
During ten close flybys between 2006 and 2016, the spacecraft sent radio waves to Earth. Researchers studied how the signal’s frequency was changed by their interactions with their surroundings to estimate how Titan’s orbit evolved.
“By using two completely different datasets, we obtained results that are in full agreement, and also in agreement with our new theory, which predicted a much faster migration of Titan,” said co-author Dr. Paolo Tortora, a scientist at the University of Bologna.
The revised rate of Titan’s drift suggests that the moon started out much closer to Saturn, which would mean the whole system expanded more quickly than previously believed.
“This result brings an important new piece of the puzzle for the highly debated question of the age of the Saturn system and how its moons formed,” said lead author Dr. Valery Lainey, a researcher at NASA’s Jet Propulsion Laboratory and Paris Observatory.
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V. Lainey et al. Resonance locking in giant planets indicated by the rapid orbital expansion of Titan. Nat Astron, published online June 8, 2020; doi: 10.1038/s41550-020-1120-5
