On the formation and stability of Uranus' rings

We discuss the possibility of outlining a global model of formation and evolution for the Uranian rings with the minimum allowance for arbitrary assumptions. A condition for the stability of the -ring against differential precession is derived and compared with the similar results by Goldreich and Tremaine (1979). We discuss the problem of the long-term stability of the rings against collisional broadening, showing that the assumption of a confinement due only to self-gravitation requires the presence of a large amount of mass in the rings (10²²–10²³ g for the -ring). This latter hypothesis seems difficult to accept both for observational and for theoretical reasons, so that we suggest that some other confining mechanism (even if more peculiar) should work for this system. Finally we describe a possible scenario for the formation of the rings based on the idea of the break-up at the Roche limit of a tidally evolved satellite: we assume a sequence of events which appears, at least qualitatively, adequate to account for the observational evidence and endowed with a low degree of arbitrarity.Paper presented at the European Workshop on Planetary Sciences, organised by the Laboratorio di Astrofisica Spaziale di Frascati, and held between April 23–27, 1979, at the Accademia Nazionale del Lincei in Rome, Italy.