Tides on Titan

Kurzfassung

In spite of its ice-rich bulk composition, Titan, Saturn's largest satellite, has many similarities to the early Earth, particularly in terms of atmospheric composition and organic chemistry. Titan is subject to time-variable radial and librational tides exerted by Saturn on the time-scale of its orbital period. The satellite's tidal response depends on its interior structure, thermal state, tidally effective rheology, and the frequency of tidal forcing. The tidally induced internal redistribution of mass results in variations of surface gravity, tilt, stress and strain, which could play a major role in triggering processes that are visible in Titan's geological surface record. We subdivide Titan's interior into four chemically homogeneous reservoirs and construct structural models that are required to satisfy the mean density, polar moment-of-inertia factor, and tidal potential Love number k2 as derived from Cassini gravity field data. It appears that outer ice shell thickness and the radial extent of the subsurface water-ammonia ocean are closely related to each other. Using corresponding linear combinations of the body tide Love-Shida numbers, we then calculate the expected amplitude patterns of tensile and maximum shear stresses at Titan's surface in response to the tidal forcing. We find peak-to-peak amplitudes of tide-induced surface displacement, tilt variation, and tidal stresses on the order of up to a few tens of meters, several arc seconds, and a few tens of kPa, respectively. Owing to the greater shell flexibility in the presence of an ocean, radial surface displacement is maximal over a tidal cycle, whereas tilt variation relative to the direction of gravity is minimal. The global pattern of maximum tidal distortion is broadly consistent with the zonal distribution of cryovolcanic candidate areas and lakes at high latitudes that may provide possible pathways for the release of methane in the satellite's atmosphere and are of great importance for basic conditions of habitability.