Saturn is well-known among the planets of our Solar System for its beautiful icy rings. Even from Earth we can see a 4500 km wide dark region, dividing the rings. This gap was discovered by Jean-Dominique Cassini in 1675, and is since known as the "Cassini Division". Some 340 years later, the mystery lingers over the origins of this ring-shaped hole. Though its location suggests an interaction with the Saturnian satellite Mimas, which is located outside the rings, the mechanism to form such a large structure remained largely unknown.
In the vicinity of the Cassini Division, the rings of Saturn are composed of icy particles
ranging from centimeters to possibly a few meters for largest ones. The closer to the planet these particles orbit, the faster. There is a distance such that the particles rotate exactly twice as fast as Mimas. This so-called “resonant” configuration removes the ice blocks from this specific orbit. If Mimas moves inward, this gap widens. On the other hand, an outward migration of the satellite would tend to carry this gap outward without widening it. Using semi-analytical models and numerical simulations, an international collaboration involving the Institute of Celestial Mechanics (IMCCE / Paris Observatory), the University of Franche-Comté, and the Institut de Physique du Globe showed that Mimas should have migrated inward over at least 9000 km in a few million years in order to clear the 4500 km of the Cassini Division, which are currently observed. A slower migration would have allowed the natural spread of particles from further away to replenish the Division at the same time it was dug. It is usually expected that such a satellite migrates outward rather than inward because of the tides it generates on the planet, in the same way that our Moon moves away from the Earth at the velocity of a few centimeters per year. An inward migration is only possible if the satellite heats strongly enough to dissipate the lost orbital energy. Such
heating could lead to melting some layers of ice that could form an ocean below the surface of the satellite, which could result in the appearance of surface geysers.
By simulating the gravitational interactions between the satellites on one hand, the resulting internal heating on the other, and finally the impact of these variations on the rings, these researchers have identified two extreme scenarios : one heating exclusively Mimas’ interior and the other one heating Enceladus instead, which would have then driven Mimas inward as the result of a resonant interaction. Taken independently, the first scenario struggles to explain the old age of the surface of Mimas while the second endangers the very survival of neighboring satellites. It is therefore possible that the Cassini Division results from a distribution of the heat between the two Mimas and Enceladus, but the history of these astonishing moons remains to be determined.
Our simulations also show that the outward migration of Mimas, which is currently observed, induces that the Cassini Division is currently filling up and should be closed in less than 40 million years. These times of opening and closing of the Cassini Division are consistent with the latest estimates of ring age (about 200 million years) and life expectancy (at least 170 million years).
Thus, the Cassini Division could be the signature of an episode of internal heating of the satellites sufficiently intense to form internal oceans. If one were to detect holes in rings around exoplanets, this would give clues to the presence of exomoons, i.e. satellites of these planets, and possible internal oceans.