Reorientation of the Moon from its Bombardment History

The Moon preserves most of its ~4.5-billion-year surface bombardment history, visible through its present-day gravity field and topography datasets, recovered from the GRAIL [1] and LRO [2] missions, respectively. Any change to the distribution of mass, e.g., from a cratering event, influences the minimum (rotational) energy orientation of the Moon, causing a change in its secular orientation with respect to its rotational axis – a phenomenon known as True Polar Wander (TPW). Large craters, fewer in number, left large gravitational contributions (called anomalies), while smaller craters, more numerous, contributed to small individual gravitational anomalies. Tracking the incremental changes in deg-2 spherical harmonic components from these anomalies enables us to predict the secular orientation assuming a principal axis rotation. We present a new, independent, comprehensive lunar TPW analysis using gravity models of ~5200 craters and basins between 20 to 200 km in diameter [3,4] mapped directly from a high-resolution GRAIL gravity field solution [5].

Our results [6] show that small craters have a significant contribution to the deg-2 gravitational power of the Moon. All craters and basins in this diameter range (excluding the SPA basin) have contributed to nearly 25% of the present-day power of the Moon’s degree-2 gravitational figure. We present the time history of the changes in the lunar paleopole path (from impact events) dating from the post-South Pole Aitken (SPA) basin formation event (~4.25Ga) to the present-day [6]. Our TPW solutions show a consistent directional influence on the pole movement from small craters, primarily due to their non-uniform spatial distribution evident in the GRAIL gravity field and LOLA topography. We find that the Moon reoriented along the Earth-Moon tidal axis giving a post-SPA epoch paleopole within 10° of its present-day location. Our paleo-south pole at ~4.25Ga rests close to the rim of a large, degraded crater between the present-day Zeeman and Schrödinger craters at (80.4°S, 180°E). This reorientation implies that the current near-side Procellarum basin was ~10° higher in latitude than it is today, placing the Th-rich deposits in the Procellarum KREEP Terrane and its antipode closer to the poles with implications for the polar volatiles in the region.

References: [1] Zuber et al. (2013). Science, 339(6120), 668-671 [2] Smith et al. (2010). SSR., 150(1-4), 209-241 [3] Head et al. (2010). Science, 329(5998), 1504–1507 [4] Neumann et al. (2015). Science Adv., 1(9), e1500852 [5] Goossens et al. (2020). JGR: Pla., 125(2), 1-31; [6] Smith et al. PSJ (in revision).