Titan’s fast escape


Titan, Saturn’s largest moon, moves away from its planet a hundred times faster than previously believed: about 11 centimeters each year. The discovery – published in the magazine Nature Astronomy – not only changes the assumptions made so far on the birth and evolution of the Saturn system, but questions the classic theory of tides, used for more than fifty years to study the orbital migration of the moons. Instead confirming a new theory – that of “resonance locking” – which could now be applied to study the evolution of other planetary systems, including extrasolar ones, and binary star systems.

An international team of researchers that includes four scholars from theUniversity of Bologna obtained these results starting from the analysis of the Cassini-Huygens mission data: a project born from the collaboration between NASA, European Space Agency (ESA) is Italian Space Agency (ASI). Cassini spent more than 13 years in orbit around Saturn to study its characteristics, its magnificent rings and its numerous moons, concluding the mission in 2017 with a spectacular dive into the atmosphere of the planet.

“Cassini was an extraordinary mission that revolutionized our knowledge of Saturn and the entire Solar System,” he says Paolo Tortora, professor at the University of Bologna and head of the Radio Science and Planetary Exploration Laboratory, active at the Forlì Campus of the Alma Mater. “This discovery is an important new piece to solve a long-debated problem: that of the age of the system of the moons of Saturn,” adds the first author of the paper Valéry Lainey, today at the Observatoire de Paris (France), who carried out the research while he was at the Jet Propulsion Laboratory of NASA (USA).

Titan and the moons of Saturn

To date, more than eighty moons have been discovered orbiting around Saturn, the second gas giant in our Solar System after Jupiter, but none come close to Titan’s size. Larger than our Moon and even the planet Mercury, Titan is the only satellite of the Solar System that has an atmosphere, mainly composed of nitrogen. On its surface there are lakes of liquid methane, fed by rivers and rain, and in the subsoil there is an ocean of liquid water, which could host favorable conditions for life.

While we know that Saturn was formed 4.6 billion years ago, following the Big Bang, it is less clear when the complex system of moons that orbit it around was born, including Titan which is now at 1.2 million miles away. One way to hypothesize an answer is to calculate the speed with which a moon moves away from the planet and thus goes backwards at the time of its formation.

Just as the gravitational pull of the Moon produces the tides we observe on Earth, Titan also produces tides on Saturn. Since Saturn rotates around its axis much faster than Titan rotates around it, the tidal peak on the planet is not directed precisely towards its satellite, but “precedes” it. Due to this misalignment, Titan receives a gravitational thrust that takes him away from Saturn. A similar situation occurs with our Moon, which is moving away from the Earth by 4 centimeters every year and therefore, in the distant future, it will appear in the sky smaller than we see it today.

For the past fifty years, the calculation of the orbital migration of the moons has been based on the classical theory of tides. According to this theory, in a system with many moons like that of Saturn, the outermost moons – Titan is one of them – should move more slowly than those closest to the planet. Cassini data presented on Nature Astronomy however, they tell us that this is not the case: Titan moves away from Saturn a hundred times faster than predicted by the classical theory of the tides, about 11 centimeters every year.

Confirmation of a new theory

However, there is another theory that can explain the results of the new study: that of “resonance locking”. Designed four years ago by Jim Fuller – researcher of the California Institute of Technology (USA) and co-author of the research -, this hypothesis predicts a rapid orbital migration of the moons of the gas planets caused by particular resonances between the oscillations of the internal structure of the planet and the orbital motion of the moons. These resonances can capture the moons during the evolution of the planet and therefore make them migrate faster than with the classic tide mechanism.

The discovery made about Titan’s movements is an important confirmation of this new theory and has made it possible to establish that Saturn’s system of moons has expanded more rapidly than previously thought. Not only that: having obtained this important confirmation, the “resonance locking” theory could now be applied to study the evolution of other planetary systems, for example that of Jupiter, and also of extrasolar planetary systems and binary star systems.

The role of the Cassini probe

But how did the researchers calculate Titan’s migration rate? The result was obtained with two completely independent methods. On the one hand, observations with terrestrial telescopes and photographic images taken by the Cassini spacecraft were used to accurately reconstruct the orbits of all the main moons of Saturn. On the other hand, the radio signals transmitted by Cassini during his mission were used.

“Between 2006 and 2016, Cassini built ten close flyovers of Titan: by analyzing the radio signal sent by the probe during these steps we were able to measure the moon’s orbital expansion very accurately,” he explains Luis Gomez Casajus, researcher at the University of Bologna at the Radio Science and Planetary Exploration Laboratory and co-author of the study. “Starting from two completely different types of data, we were thus able to obtain concordant results, which confirm what was predicted by Jim Fuller’s theory on the faster migration speed of Titan”, adds Paolo Tortora.

Despite having ended three years ago, the Cassini mission does not seem to have run out of surprises. “We will continue to analyze the data collected by Cassini for a long time, and I am sure that many other surprising discoveries await us,” he confirms Marco Zannoni, researcher from the same laboratory who has collaborated since 2012 in the calibration and data analysis activities of the Cassini probe. “The extremely valuable mission data archive represents a gold mine for the Italian researchers involved in this project, for the purpose of understanding complex planetary systems such as those of Saturn and Jupiter”, he adds Angelo Olivieri, scientific manager of the Cassini-Huygens project at the Italian Space Agency, who contributed to the mission with important on-board equipment.

The protagonists of the study

Coordinated by Valéry Lainey at NASA’s Jet Propulsion Laboratory (USA), the study was published in the journal 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 for the University of Bologna. Scholars from the California Institute of Technology (USA), Queen Mary University of London (United Kingdom), the Observatoire de Paris from the Université PSL (France) and Jinan University (China) also took part in the research.

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