Such assumptions, however, contradict a fact that make the Saturn rings exception: they consist of 90 to 95 percent of frozen water. Other celestial bodies are usually composed of at least half of silicates and metals - accordingly, the Saturn rings and the also extremely water-rich inner moons should have a higher proportion of these materials. The hypothesis of Robin Canup provides an explanation for all these objections: it is based on a Saturn companion the size of the Saturn moon Titan, which consisted in the core of silicates and iron, but around a lighter coat of ice carried. When crossing the Roche border, the tidal forces would have shaken and dragged on this layer of ice to such an extent that it was gradually rubbed off and the chunks of ice were distributed in a ring around Saturn. The heavy core, however, was attracted further by the planet and finally crashed into it.
According to the model, the ice rings initially had a good thousand times more mass than today. But after Canup's thesis, they subsequently expanded and lost material at the edges. This evolution may have been further assisted by meteorite collisions around the rings: it could have simultaneously ejected material from the rings while silicates and metals mingled into the rings. From the material drifting away from the rings, the inner moons could finally have formed like Tethys. Evidence for Canup's assumptions may soon be provided by the Cassini spacecraft, which is due to close at the end of its mission to determine the current mass and "fouling" of the Saturn rings before it will burn up in 2017 in the Saturnian atmosphere. If the measurement results match the mass calculated by Canup, this would be a valuable support of the hypothesis. The model could also help understand the formation of lunar and ring systems of other large planets.Robin Canup (Southwest Research Institute, Boulder): Nature, Vol. 468, No. 7326, p. 943, doi: 10.1038 / nature09661 dapd / science.de? Mascha Schacht advertisement