Smith and 6 co-authors reported in September, 2022 that changes in mass distribution affect the gravitational figure and reorient a planetary body’s surface with respect to its rotational axis. The mass anomalies in the present-day lunar gravity field can reveal how the figure and pole position have evolved over the Moon’s history. By examining sequentially each individual crater and basin, working backward in time order through the catalog of nearly 5200 craters and basins between 1200 and 20 km in diameter, we investigate their contribution to the lunar gravitational figure and reconstruct the evolution of the pole
position by extracting their gravitational signatures from the present-day Moon.
The authors found that craters and basins in this diameter range, which excludes South Pole–Aitken, have contributed to nearly 25% of the present-day power from the Moon’s degree-2 gravitational figure and resulted in a total displacement of the Moon’s pole by ∼10° along the Earth–Moon tidal axis over the past ∼4.25 billion years. This also implies that the geographical location of the Moon’s rotational pole has not moved since ∼3.8 Ga by more than ∼2° in latitude owing to impacts, and this has implications for the long-term stability of volatiles in the polar regions.
The primary objective of this study was to assess the likely contribution from the thousands of small craters to the evolution of the Moon’s gravitational flattening and pole movement. The authors have estimated the polar wander of the Moon from the gravitational contribution of 5,197 craters and basins
with 1200 km > D ≥ 20 km as recorded in the present-day GRAIL gravity field. Our results show that the abundance and nonuniform distribution of 20–200 km craters make them a critical contributor to polar wander and thus that they should be considered in any planetary reorientation study. The method of
extracting gravity anomalies numerically, rather than by geophysical modeling, has enabled the inclusion and modeling of large numbers of smaller craters with a good accuracy.
The sequential treatment of each anomaly in age order results in an updated pole location after each subtraction and enables the derivation of an approximate pole path since the SPA impact.
Previous reconstructions of lunar polar wander using gravity field neglected the contribution of craters smaller than 200 km to the degree-2 gravity field of the Moon. When these smaller craters are included, we find the following:
1. A net displacement of the Moon’s pole by ∼10° in latitude in the general direction of the present-day Earth– Moon tidal axis with the paleopole coordinates post-SPA at ∼80° N/S, 0/180° E.
2. Most of this true polar wander occurs post-SPA in the Late Nectarian and Early Imbrian Period (pre-Orientale; Figure 8) and places the post-SPA south pole on the edge of the ∼140 km diameter De Forest crater located at 80.8° S, 191.2° E. The geographic location of this paleopole along the Earth–Moon tidal axis implies that the current near-side Procellarum region was ∼10° higher in latitude than it is today, placing the Th-rich deposits and their
antipode closer to the poles.
3. Since the Orientale impact, the pole has only moved ∼10% of its full latitude change since post-SPA and has passed through the Shackleton crater before arriving at its present-day location.
4. This suggests that the polar cold traps could have been relatively stable for polar volatile accumulation since the beginning of the Late Imbrian provided that there has been little change in the lunar obliquity during that time. This also implies that any volatiles released by post Orientale basin volcanism and reaching polar cold traps would be sequestered in any crater existing at that time in the current circumpolar region, including some currently at latitude ∼75° S.
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