South Pole Aitken Basin magnetic anomalies: Evidence for the true polar wander of Moon and a lunar dynamo reversal

摘要

The analysis of the Lunar Prospector magnetic data over South Pole Aitken basin reveals two categories of magnetic anomalies with distinctly different polarities. Modeling the anomalies in terms of vertical prism source bodies shows two separate true polar wander paths of the Moon. One is driven by the giant surface mass deficiency resulted from the formation of the South Pole Aitken basin and the other by the giant surface mass concentrations (mascons) formed inside the large impact basins, such as Serenitatis and Imbrium, and a vast volcanic deposit on Procellarum Terrane. A total of ~100° true polar wander path suggests that the source bodies are formed and magnetized during a long period. Moreover, the distinctly different polarities of the two categories of the anomalies support the idea that the core dynamo underwent a reversal or shutdown and started up with a different polarity, caused by the latter impacts. The impact-induced shock pressures of 10-20 GPa traveling inside the lunar core, the appreciable angular momentum transfer to the lunar mantle by the impacting bodies, and the impact-induced stratification of the core probably reversed the polarity of the core dynamo. We also model the north Crisium magnetic anomaly, extracted from the Lunar Prospector magnetic data. The paleomagnetic pole position obtained for the source body reveals that the north Crisium anomaly belongs to the first category of the anomalies. Plain Language Summary Using Lunar Prospector magnetic data of the Moon inside South Pole Aitken basin, we detect true polar wander of the Moon since the formation of the basin at ~4.1 Ga. The polar wander path shows two distinct parts, one after the basin formation but before the formation of the Imbrium basin at ~3.9 Ga and the other after the formation of the Imbrium basin. The total polar wander is about 100°, about 40° in the first period and 60° in the second. Tracing back the polar wander path, we obtained the original location of the South Pole Aitken basin at the time of its formation. There was also a core dynamo reversal probably caused by the impactor that created the Imbrium basin.