TIDAL DISSIPATION ON TITAN

Tidal dissipation may constrain the nature of the surface and interior of Titan. Provided the surface of Titan is covered by satellite-wide hydrocarbon seas and oceans, the solid body of Titan would have to respond not only to the external tidal disturbance potential but also to the loading and shifting weights of liquids on the surface. To estimate the tidal response of Titan's interior which is assumed to be differentiated into a crust-mantle-core structure, several endmember-type models have been considered, each of them constrained by Titan's mass and radius and consistent with either a volatile-rich or a volatile-poor evolution of the satellite. The dissipation rates as a consequence of the inelastic response of Titan's interior to body and loading tides have been computed from the imaginary parts of complex Love numbers and mass load coefficients, Additionally, a new analytical model of bottom frictional dissipation as a consequence of tidal currents in a global ocean is presented and is shown to compare well with a numerical model for both the radial and libration components of the ocean tide. In contrast to the solid volatile-poor interior structure, the volatile-rich interior contains an internal liquid ammonia-water region overlain by an icy shell, thereby mechanically decoupled from the deep interior. While the latter produces higher tidal dissipation rates than the former due to greater shell flexibility, the greater motion of the ocean floor accordingly induces smaller ocean currents, and hence much lower ocean dissipation rates for the volatile-rich scenario. The resultant total dissipation rates have been compared to those required to damp Titan's orbital eccentricity over the age of the Solar System. While the highly dissipative volatile-rich interior requires a time constant of orbit circularization of only about half this time period, the volatile-poor interior structure would allow restrictions on the minimum ocean depth and on ocean composition. Consequently, the volatile-rich scenario seems inconsistent with a primordial origin of the orbit eccentricity, This suggests Titan's interior may be rigid and that there is no global ocean. Alternatively, the eccentricity of the orbit may have a more recent origin. It is, however, also conceivable that the thermal history could have been governed by more rapid interior cooling in the past, thereby notably diminishing the amount of solid dissipation in the deep interior. (C) 1995 Academic Press, Inc.