The moon of Saturn Mimas, a snow plow in the rings of the planet

The world's second planet in terms of mass and size, Saturn is best known for its rings. These are divided by a broad band, the Cassini Division, whose formation was poorly understood until very recently. Today, researchers * from the CNRS, the Observatoire de Paris-PSL and the University of Franche-Comté have shown that Mimas, one of Saturn's moons, acted as a kind of hunting. -neige away, repelling the ice particles constituting the rings. The results are the result of two studies supported by the International Space Science Institute and CNES, the French Space Agency, published simultaneously in June 2019 in Monthly Notices from the Royal Astronomical Society.

Saturn's rings consist of ice particles whose orbital velocity increases as they get closer to the planet. The Cassini division is a broad dark band between the two most visible rings of Saturn, in which the particle density is considerably lower than that inside the rings. The researchers suspected a link between Mimas, one of Saturn's moons, and the group, as there is an area at the inner edge of the Division where particles gravitate around Saturn exactly twice as fast as Mimas. This phenomenon, called orbital resonance, spreads ice particles and produces a relatively narrow gap. Scientists from the CNRS, the Paris-PSL Observatory and the University of Franche-Comté have now shown that Mimas has recently moved closer to Saturn, making the Moon a sort of distant plow that has dug the f km width it has today. On the other hand, if Mimas' orbit moved outward, the particles would return to their original position, as if a snow plow were reversing and ceasing to push the snow, letting it spill out. again. Using numerical simulations, the researchers calculated that Mimas had to migrate 9,000 km inland in a few million years to fill the 4,500 km gap that is currently the Cassini division.

A natural satellite, such as the moon, tends to move away from the planet rather than closer to it. To be able to migrate inland, a moon must be able to lose energy, especially by heating up, which would cause the melting of its internal ice and the weakening of its external crust. However, the state of the surface of Mimas, which still bears the relatively old traces of meteorite impacts, does not correspond to such a scenario. The second hypothesis of the researchers, which remains to be confirmed, is that the heat loss was shared between Mimas and Enceladus, another of Saturn's moons, by orbital resonance. This would have caused the creation of the internal oceans detected by the Cassini probe beneath the surface of these two bodies.

Today, Mimas has begun to migrate to the outside. According to researchers' calculations, the Cassini division should take about 40 million years to close. Through these discoveries, scientists can consider the presence of holes in the rings of an exoplanet as a clue that it might have moons with oceans.

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* The researchers belong to the Institute of Celestial Mechanics and Calculus of Ephemeris (Observatory of Paris – PSL / CNRS), to the UTINAM Institute (CNRS / University of Franche-Comté), to the. Institute of Physics of the Globe of Paris (CNRS / University of Paris). / IPGP / IGN), Laboratory of Planetology and Geodynamics (University of Nantes / CNRS / University of Angers), Institute of Namur for Complex Systems (University of Namur) and the Jet Propulsion Laboratory (NASA).