Dissipation of tidal energy in planetary satellites

Most natural satellites are in synchronous rotation and subject to tidal forces exerted by their primaries. The ellipsoidal shape of their surfaces was acquired early after formation and tidal despinning, when their deep interiors were sufficiently hot and deformable and hardly overlain by thin lithospheres. The nonspherical part of their present low-degree gravity fields is predominated by spin and tidal contributions, which are related to the radial mass distribution. In addition, tidally-induced, time-variable surface distortion and gravity variation due to tiny radial and librational tides occur along slightly eccentric orbits, thereby causing the dissipation of tidal energy. This has important consequences for the thermal state and orbital evolution of planetary satellites. Based on interior structure models and assumptions on tidally-effective rheological properties of planetary materials (i.e. ice, rock, internal ocean), we will address the tidal response of some active satellites in the outer solar system and examine the role of tidal heating for their past or present geologic activity.