Planets and moons reorient in space due to mass redistribution associated with various types of internal and external processes. While the equilibrium orientation of a tidally locked body is well understood, much less explored are the dynamics of the reorientation process. This is despite their importance for assessing whether enough time has passed for the equilibrium orientation to be reached, and for predicting the patterns of TPW-induced surface fractures (true polar wander, TPW, is used here for the motion of either the rotation or the tidal pole). Here we present a simple yet accurate method to compute the reorientation dynamics of a tidally locked body. The method is based on assuming that during the TPW the tidal and the rotation axes closely follow respectively the minor and the major axes of the total, time-evolving inertia tensor of the body.
Motivated by the presumed reorientation of Pluto, the use of our method is illustrated in several test examples. In particular, we analyze whether reorientation paths tend to be curved or straight when the load sign and the mass of the host body are varied. When tidal forcing is relatively small, the paths of negative anomalies (e.g. basins) towards the rotation pole are highly curved, while positive loads reach the sub- or anti-host point in a straightforward manner. Our results suggest that the Sputnik Planitia basin cannot be a negative anomaly at present day, and that the remnant figure of Pluto must have formed prior to the reorientation.
The situation is different for the icy satellites of Jupiter and Saturn. When the mass of the host body is relatively large, positive loads first move toward the center of the trailing or leading hemisphere, and reach the sub- or anti-host point only later, in a subsequent stage of TPW. The reorientation dynamics may have important consequences for the present location of some of the prominent features on the surfaces of icy moons. The custom written code LIOUSHELL that was used to perform the simulations is freely available on GitHub. V.P. and M.K. acknowledge support by the Czech Science Foundation through project No. 22-20388S.