IMPACT-DRIVEN TRUE POLAR WANDER OF THE MOON AND ITS IMPLICATIONS FOR THE LONG-TERM STABILITY OF POLAR VOLATILES

Large impacts can significantly alter the rotation states of planetary bodies. Impacts can change both the planet’s angular momentum and the planet’s inertia tensor through the formation of the impact basin and associated ejecta blanket. The combination of these effects results in complicated motions of both the spin axis and the planet with respect to the spin axis. The latter motion (the reorientation of the bulk planet with respect to the spin axis, which may remain fixed in inertial space) is known as true polar wander. Recent advances in our knowledge of the Moon’s interior structure from NASA’s LRO and GRAIL missions have enabled the first detailed dynamical studies of lunar polar wander (e.g. Keane & Matsuyama 2014, Garrick-Bethell et al. 2014, Siegler et al. 2016). While insightful, these past works focus on individual polar wander episodes. In this work, we present the first ever polar wander chronology for the Moon. This chronology is created from combining GRAIL-derived measurements of the perturbation of each large impact basin on the Moon’s inertia tensor and the lunar impact chronology. We find that each large lunar impact basin (D>100 km) reoriented the Moon by ~1°. The largest (and earliest) impact basin, South Pole-Aitken, resulted in ~15° of reorientation. Superimposed on these stochastic polar wander events is the comparatively gradual reorientation driven by the Moon’s interior thermal evolution and the volcanic activity underneath the Moon’s nearside Procellarum KREEP Terrane. Reorientation of the Moon has many important implications, including the spatial distribution of impact craters, global tectonic patterns, and the interpretation of paleomagnetic data. In this presentation, I will focus primarily on the implications of this polar wander chronology for the long-term stability of polar volatiles at the lunar poles. As the Moon reorients, the locations of permanently shadowed regions (and thus polar volatiles, like water ice) change location. We will present constraints for the age of lunar polar volatiles, which has implications for the nature of polar volatiles and the history of volatile delivery to the Earth-Moon system.