Titan is moving away from Saturn at a hundred times faster than we thought

Titan, Saturn's largest moon, moves away from its planet a hundred times more rapidly than we thought: 11 centimetres each year. This discovery, published on Nature Astronomy, not only suggests that the hypotheses about the birth and evolution of Saturn's system should now be modified, but it also challenges the traditional theory of tides, which has provided an explanation for moons' orbital migration for the last 50 years. A new theory has been put forward: "resonance locking" could now be used to study the evolution of other solar and extrasolar planetary systems as well as binary stars systems.

An international research team including four researchers from the University of Bologna carried out this study drawing from the data gathered during the Cassini-Huygens mission. The latter is a space mission that saw the cooperation of NASA, the European Space Agency (ESA) and the Italian Space Agency (ASI). Cassini spent more than 13 years orbiting Saturn and studying this planet's characteristics, its rings and its many moons. The mission ended in 2017 with a breath-taking dive into Saturn's atmosphere.

"This mission revolutionized our knowledge of Saturn and of the entire Solar System", says professor Paolo Tortora, head of the Radio Science and Planetary Exploration lab of the University of Bologna at the Forlì Campus. "This discovery adds a further puzzle piece to a long-debated jigsaw: the issue of the age of Saturn's moon system", adds Valéry Lainey, the principal investigator of the paper, now at the Observatoire de Paris (France), who carried out his research while at NASA's Jet Propulsion Laboratory (USA).

Titan and Saturn's moons

More than 80 moons orbiting Saturn – the second gas giant in our Solar System, after Jupiter – have been discovered to date and Titan is the largest one. Titan is larger than our Moon and even than Mercury, and it is the only satellite in the solar system with its own atmosphere, which is mainly composed of nitrogen. On Titan's surface we find liquid methane lakes with tributary rivers and rains, whereas below its surface there is an ocean of liquid water that may host a favourable environment for life.

While we know for a fact that Saturn stopped wondering 4.6 billion of years ago following the Big Bang, we do not fully grasp the complexities behind the system of moons orbiting it. This system includes Titan that is now 1.2 mln km away from Saturn. Calculating the speed to which a moon drifts away from its planet may be a way of providing an answer, because it would allow to trace back the moment of its formation.

Just as Moon's gravity causes ocean tides on planet Earth, the same does Titan on Saturn. Saturn's rotates on itself faster than Titan rotates around Saturn, for this reason the tidal peak is "preceding" Titan and is not directed towards it. Because of this misalignment, Titan receives a gravitational assist that moves it away from Saturn. This is similar to what happens with our Moon, which is drifting away from Earth 4 centimetres every year. In a very remote future in fact, we will see a smaller Moon. 

During the past 50 years, moon planetary migration was calculated using the theory of tides. According to this theory, in a system with many moons, like Saturn, the outer moons (like Titan) should drift away at a slower pace than those closer to the planet. However, data from Cassini published on Nature Astronomy portray a different picture. This new work shows that Titan actually moves away from Saturn 11 centimetres every year, essentially a hundred times faster than had been calculated using the theory of tides. 

A new theory finds confirmation

"Resonance locking" is the name of the new theory put forward to explain the result from Cassini mission. The theory was first suggested four years ago by California Institute of Technology and co-author of this study Jim Fuller. According to "Resonance locking", the outward migration of moons orbiting gas planets is caused by special resonances between the planet internal oscillation and the migration of moons. These resonances are able to lock moons during the planet's evolution and the energy created in this way causes moons to migrate faster than with classic tidal movements.

Titan's movements and the data from the Cassini mission confirm this new theory, which allowed an explanation for the rapid expansion of Saturn's moons system. Moreover, since "resonance locking" theory obtained validation, it can now be applied to the study of other planetary systems, such as Jupiter or even extrasolar planetary systems and binary stars systems.

The Cassini probe

How was it possible for researchers to calculate Titan's migration speed? This result was obtained through two completely independent methods. On the one hand, researchers used ground-based telescopes and pictures taken by the Cassini probe to accurately reconstruct the orbits of the main moons of Saturn. On the other hand, they used radio signals Cassini sent during its mission.

"Between 2006 and 2016, Cassini executed ten close flybys of Titan. We analysed the probe radio signal during these flybys and managed to measure this moon's orbital expansion with unprecedented accuracy", explains Luis Gomes Casajus, co-author of the study and researcher at the Radio Science and Planetary Exploration lab of the University of Bologna. "We were able to draw matching results from two different types of datasets. The so obtained results confirm Jim Fuller's theory about Titan's migration", adds Paolo Tortora.

Cassini's wonders do not cease even three years from its grand finale. "We will keep analysing the data from the Cassini mission because, I am sure, there are more sensational discoveries ahead", says Marco Zanoni who works at the Radio Science and Planetary Exploration lab and, since 2012, has worked also on the analysis and calibration of the Cassini probe. "The Cassini database is priceless and represents a goldmine for Italian researchers involved in this project for the comprehension of how complex planetary systems, like Jupiter or Saturn, work", adds Angelo Olivieri, scientific coordinator of Cassini-Huygens project for the Italian Space Agency, which provided crucial board devices for the mission.

The authors of the study

Valéry Lainey from NASA's Jet Propulsion Laboratory (USA) coordinated this study, which was then published in Nature Astronomy with the title "Resonance locking in giant planets indicated by the rapid orbital expansion of Titan".

Paolo Tortora, Luis Gomez Casajus, Marco Zannoni and Dario Modenini from the Department of Industrial Engineering participated in this study on behalf of the University of Bologna. Moreover, researchers from the California Institute of Technology (USA), from Queen Mary University of London (UK) as well as researchers from the Observatoire de Paris at PSL University (France) and from Jinan University (China) also took part in this research.