Tidal effects on the evolution of Titan.

Abstract

Titan, the largest natural satellite with a significant free eccentricity, remains a mystery because of its obscuring atmosphere. Titan's thick stratospheric haze has prevented us from directly determining the nature of the surface or even Titan's rotation state. A satellite on an eccentric orbit experiences a zero net tidal torque when the rotation is slightly faster than synchronous, if there is no significant permanent asymmetry. Calculations indicate that Titan should be either synchronous with a period of 15.945 days or nonsynchronous with a period of 15.82 to 15.93 day. Titan's large free eccentricity should result in significant tidal dissipation. This can be used to constrain the existence and properties of both any hydrocarbon ocean on the surface and the satellite's internal structure. A volatile-poor, solid, differentiated interior model such as that given by Stevenson is found to be the most acceptable from the point of view of dissipation. Hydrodynamical-numerical ocean tide models are used to investigate the effects of changing ocean depth and land/ocean configurations. Ocean depths of more than 800 m have small enough dissipations to be allowed, depending on the model. The models allow some simple land configurations, most of which are shown to also be acceptable with respect to dissipation. Small, isolated seas are found to allow for much shallower liquids while maintaining low dissipations. Explanations of the current eccentricity other than that of primordial origin with subsequent low tidal dissipation are investigated, but are found to be of very low probability.