Categories: AstronomySaturnTitan

Titan is Drifting Away from Saturn Surprisingly Quickly

Where did Saturn’s bizarro-moon Titan form? Did it form where it is now, or has it migrated? We have decades of data to look back on, so scientists should have some idea.

A new study based on all that data says that Titan is drifting away from Saturn more quickly than thought, and that has implications for where the moon initially formed.

Titan is a unique—and bizarre—moon in our Solar System. It’s the only moon with a thick atmosphere, and with liquid flowing on the surface. It’s also Saturn’s largest moon, and it’s actually larger than the planet Mercury.

The Voyager spacecraft gave us our first good look at Titan, but it couldn’t see through the thick, nitrogen-rich atmosphere. Then in 2004 the Cassini-Huygens mission was able to look at the surface with its more advanced instruments. The small Huygens lander gave us our first images from the surface of the extraordinary moon. More missions are in the conceptual phase, and one is planned: NASA’s Dragonfly mission will launch in 2026 and explore the surface.

Voyager 2 captured this image of Titan in 1981 from a distance of 2.3 million km (1.4 million miles.) It shows some detail in the clouds, but not much. Image Credit: NASA/JPL

But this new study doesn’t need any new missions or advanced instrumentation. It relies on the decades of data we already have to examine the question of Titan and its migration.

The study is titled “Resonance locking in giant planets indicated by the rapid orbital expansion of Titan.” The lead author is Valéry Lainey, formerly of JPL and now with the Paris Observatory. The study is published in the journal Nature.

The view of Titan from the descending Huygens spacecraft on January 14, 2005. Credit: ESA/NASA/JPL/University of Arizona.

This new study departs from previous studies, which showed that Titan formed roughly where it is now. Instead, it shows that Titan is drifting further and further from Saturn. And the rate at which it’s drifting is 100 times faster than expected. That result suggests that Titan has been migrating to its current orbital distance for 4.5 billion years.

The results also have implications for our understanding of Saturn’s rings, especially the ages of those rings, which have been the subject of much scientific debate lately.

“Most prior work had predicted that moons like Titan or Jupiter’s moon Callisto were formed at an orbital distance similar to where we see them now,” says Caltech’s Jim Fuller, assistant professor of theoretical astrophysics and co-author on the new paper. “This implies that the Saturnian moon system, and potentially its rings, have formed and evolved more dynamically than previously believed.”

Saturn’s rings and moons have been the subject of scientific debate. A 2019 study showed that the migration of Saturn’s moons has widened the Cassini Division in Saturn’s rings. Image Credit: Cassini, Dante, Baillié and Noyelles

Planets and moons can have pretty complex relationships. Sure, the much larger planet exerts a much larger gravitational force on the smaller moon. But moons like Titan—and Earth’s Moon—also have substantial mass. They exert their own gravitational pull on their planets. Just look at Earth’s tides, driven largely by the Moon.

On Earth, this pushing and pulling with the Moon also creates frictional stress inside the Earth. That in turn pulls the Moon forward in its orbit, and the end result is that the Moon is gradually moving away from the Earth. It’s much further away than it was in the past, during the age of the dinosaurs for example. The pushing and pulling between a planet and its moon is called tidal effects.

3.9 billion years ago, the Moon was much closer to Earth. It was about 21 Earth radii away, whereas today it is about 60 Earth radii away. Image Credit: Lunar Planetary Institute/David Kring

The speed of this process is positively glacial, though. The Moon moves further away from Earth at about 3.8 cm per year. Not much. At that rate, the Moon will be with the Earth until they’re both consumed by the Sun, when it goes into its red giant phase in about 5 or 6 billion years.

The relationship between Saturn and Titan is similar.

But since Saturn is a gas giant, whereas Earth is rocky, it’s not exactly the same. The frictional goings-on inside Saturn should be weaker, because of all that gas. Standard theories predict that Titan’s rate of migration away from Saturn should be only 0.1 cm per year, much less than that of the Earth’s Moon.

A top-down image of the orbits of some of Saturn’s larger moons, with Titan in red. The moons outside its orbit are (from the outside to the inside) Iapetus and Hyperion; those inside are Rhea, Dione, Tethys, Enceladus, and Mimas. Image Credit: By !Original: Rubble pileVector: Mysid. – Own work based on: Titan’s orbit.jpg., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4819125

This is where we come to the guts of this new study.

This study contains two separate methods of measuring Titan’s orbital expansion rate: astrometry and radiometry. Both of these methods made use of Cassini data. Two teams contributed to the study; one used astrometry, and the other used radiometry. The astrometry team used over 100 years of observational data going back to 1886, and continuing up to the end of the Cassini mission. The radiometric team used a model that took into account “all the relevant accelerations that affected the orbit of Titan and the trajectory of the Cassini spacecraft,” among other things.

“By using two completely independent data sets—astrometric and radiometric—and two different methods of analysis, we obtained results that are in full agreement,” says the study’s first author, Valéry Lainey formerly of JPL (which Caltech manages for NASA), now of Paris Observatory, PSL University. Lainey worked with the astrometry team.

The results of this study confirm the results of a 2016 study by co-author Jim Fuller. That study also showed that Titan’s migration rate is much faster than previously thought.

This figure from the study shows a possible evolutionary history of the orbital distance of six of Saturn’s moons as a function of time, for both a resonance-locking model (solid coloured lines) and a constant Q model (black dashed lines). The resonance-locking models are shaded by the effective tidal quality factor, Qef, at a given moment in time. For a deeper explanation, see the study. Image Credit: Lainey et al, 2020.

Fuller’s theory said that Titan squeezes Saturn at a particular frequency. That makes Saturn oscillate strongly. Think of a playground swing. If you swing your legs at the right time—or frequency—you push yourself higher. It’s called resonance locking.

Fuller said that the high amplitude of Saturn’s oscillation would actually dissipate a lot of that energy. In turn, that would drive Titan outward at a much greater rate than thought.

Resonance locking was not the only tidal dissipation mechanism this research looked, but it came out on top. “The rapid migration of Titan is unexpected for all tidal dissipation mechanisms,” the authors wrote, “except for resonance locking, which predicted the observed migration.” 

This new study found that Titan’s rate of migration away from Saturn is 11 cm (4.3 in) per year, 100 times faster than previous theories concluded.

Saturn’s moon Tethys, with its stark white icy surface, peeps out from behind the larger, hazy, colorful Titan in this view of the two moons obtained by NASA’s Cassini spacecraft. Saturn’s rings lie between the two. Credit: NASA/JPL-Caltech/Space Science Institute

This resonance locking model might apply to other systems of planets and moons as well. The authors write, “Resonance locking could operate in other moon systems, such as the Jovian system, where it might drive the outward migration of Io/Europa/Ganymede…”

In fact, it could apply to other solar systems and exoplanets. “Resonance locking can also act in stellar binaries and exoplanetary systems,” the authors write, though it’ll take more work to see how accurate it can be.

“The resonance locking theory can apply to many astrophysical systems. I’m now doing some theoretical work to see if the same physics can happen in binary star systems, or exoplanet systems,” says Fuller.

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Evan Gough

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