The second largest planet in the solar system by mass and size, Saturn is best known for the rings that surround it. These are separated by a wide band, known as the Cassini division, but how this band formed is little understood. Researchers from the CNRS, the Observatoire de Paris – PSL and the University of Franche-Comté have now found that Mimas, one of Saturn’s satellites, acts as a remote snowplough pushing the ice particles that make up the rings. These results were obtained with the help of the International Space Science Institute and Cnes, and are the object of two studies published in the Monthly Notices of the Royal Astronomical Society.
Saturn’s rings are made of ice particles that rotate faster when they are close to the planet. The Cassini division is a wide black band, located between Saturn’s two most visible rings, in which the particle density is much lower than inside the rings. The researchers suspected that there was a link between Mimas, one of Saturn’s satellites and this band. Indeed, there is a place on the inner edge of the division where these particles rotate twice as fast as Mimas. This configuration, called resonance, pushes back the ice blocks to create a very thin hole.
The CNRS/Observatoire de Paris – PSL/University of Franche-Comté researchers have now shown that Mimas could have approached Saturn in the recent past, making the satellite a kind of remote snowplough that would have widened the initial hole to the 4500 km width we know of today. On the contrary, if Mimas had moved outwards from the orbit, the particles would have returned to their original position, as if the snowplough had backed up and stopped pushing the snow, allowing it to disperse again. Thanks to numerical simulations, the researchers calculated that Mimas had to migrate 9000 km inland in a few million years to open the 4500 km space.
Satellites, like the Moon, naturally tend to move away from their planets rather than towards them. In order to migrate inwards, however, the satellite must be able to evacuate energy by heating up, which should lead to the fusion of its internal ice and the weakening of its external crust. The state of the surface of Mimas, which still carries the stigma of relatively ancient meteorite impacts, does not support such a scenario. The researchers’ second hypothesis, which still needs to be confirmed, is that the heat dissipation was distributed between Mimas and Enceladus, another of Saturn’s satellites, through an orbital resonance that would have created the internal oceans that the Cassini probe detected below the surface of these astral bodies.
Today, Mimas has begun its outward migration. According to the researchers’ calculations, it should take about 40 million years for the Cassini division to close. The new work means that scientists could now consider the presence of divisions in the rings of exoplanets as clues that these exoplanets might harbour satellites with oceans.
The researchers responsible for this study are from: the Institut de mécanique céleste et de calcul des éphémérides (Observatoire de Paris – PSL / CNRS); the Institut UTINAM (CNRS / Université de Franche-Comté); the Institut de physique du globe de Paris (CNRS / Université de Paris / IPGP / IGN); the Laboratoire de planétologie et géodynamique (Université de Nantes / CNRS / Université d’Angers); the Namur institute for complex systems (Université de Namur); and the Jet Propulsion Laboratory (NASA)
Formation of the Cassini Division – I. Shaping the rings by Mimas inward migration
Kevin Baillié, Benoît Noyelles, Valéry Lainey, Sébastien Charnoz, Gabriel Tobie, Monthly Notices of the Royal Astronomical Society
Formation of the Cassini Division – II. Possible histories of Mimas and Enceladus
Benoît Noyelles, Kevin Baillié, Sébastien Charnoz, Valéry Lainey, Gabriel Tobie, Monthly Notices of the Royal Astronomical Society
Article source: CNRS press release in French, translated by LABNAUT