Rotational stability of tidally deformed planetary bodies

[1] We consider the true polar wander (rotational variations driven by mass redistribution) of tidally deformed planetary bodies. The rotation pole of bodies without tidal deformation is stabilized by the component of the rotational bulge which retains a memory for prior rotational states, that is, a remnant rotational bulge. For planetary bodies with tidal deformation, the additional stabilizing effect of a remnant tidal bulge results in less permissive excursions of the rotation pole. The magnitude of the load driving reorientation is parameterized by Q, the ratio between the degree-2 gravitational potential of the load and the remnant rotational bulge. Reorientation is favored if the initial load longitude is close to 90 , that is, close to the center of the leading or trailing hemisphere. As an illustration of the new theory, we consider reorientation driven by internal loading on Saturn’s moon Enceladus. Small loads (jQj 1) are inconsistent with significant reorientation because of the small present-day angular separation between the load and the rotation axis. Larger loads (jQj 2) permit reorientations approaching 90 . Large reorientation scenarios are consistent with the present-day equatorial location of a geologically inferred ancient polar terrain.