The role of secondary resonances in the evolution of the Mimas-Tethys system

We have numerically investigated the role of the 200-year-long period, recently discovered, on the past tidal evolution of the Mimas-Tethys system through the present inclination-type resonance. We show that it has a deciding effect on the descriptions of the resonance motion, as considered up to now Several modes of evolution are found possible according to the value allowed for the present eccentricity of Tethys (which is very badly known). As a result, Mimas's inclination before capture may have been higher (up to 0.7 degrees) or lower (down to 0.03 degrees) than the value previously considered (0.42 degrees). Also, Tethys's eccentricity on capture may have been quite higher (approximate to 0.008 versus 0). Moreover, calculation of the probability of capture is found to be more complicated. We develop a method which takes chaos into account, and high probabilities of capture are found for quite small values of Tethys's eccentricity at capture time (e.g., a probability of 0.76 for an eccentricity of 8 x 10(-4)). (C) 1999 Academic Press.