Tidal-Rotational Dynamics of Solar System Worlds, From the Moon to Pluto

Abstract

The spins of planetary bodies are not stagnant; they evolve in response to both external and internal forces. One way a planet's spin can change is through true polar wander. True polar wander is the reorientation of a planetary body with respect to its angular momentum vector, and occurs when mass is redistributed within the body, changing its principal axes of inertia. True polar wander can literally reshape a world, and has important implications for a variety of processes—from the long-term stability of polar volatiles in the permanently shadowed regions of airless worlds like the Moon and Mercury, to the global tectonic patterns of icy worlds like Pluto. In this dissertation, we investigate three specific instances of planetary true polar wander, and their associated consequences. In Chapter 2 we investigate the classic problem of the Moon's dynamical figure. By considering the effects of a fossil figure supported by an elastic lithosphere, and the contribution of impact basins to the figure, we find that the lunar figure is consistent with the Moon's lithosphere freezing in when the Moon was much closer to the Earth, on a low eccentricity synchronous orbit. The South Pole-Aitken impact basin is the single largest perturbation to the Moon’s figure and resulted in tens of degrees of true polar wander after its formation. In Chapter 3 we continue our analyses of the lunar figure in light of the discovery of a lunar ”volatile" paleopole, preserved in the distribution of hydrogen near the Moon's poles. We find that the formation and evolution of the Procellarum KREEP Terrain significantly altered the Moon’s orientation, implying that some fraction of the Moon’s polar volatiles are ancient—predating the geologic activity within the Procellarum region. In Chapter 4 we investigate how the formation of the giant, basin-filling glacier, Sputnik Planitia reoriented Pluto. This reorientation is recorded in both the present- day location of Sputnik Planitia (near the Pluto-Charon tidal axis), and the tectonic record of Pluto. This reorientation likely reflects a coupling between Pluto’s volatile cycles and rotational dynamics, and may be active on other worlds with comparably large, mobile volatile reservoirs. Finally, in Chapter 5 we consider the broader context of these studies, and touch on future investigations of true polar wander on Mercury, Venus, Mars, Vesta, Ceres, and other worlds in our solar system.