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

Abstract

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.

The Lunar Paleo‐Magnetosphere: Implications for the Accumulation of Polar Volatile Deposits

“…Here, we explore equatorial surface field values of 0.5, 2, and 5 μT and ultimately will show that paleomagnetic fields of these strengths have considerable effects on the solar wind H + surface flux. As for the orientation of the ancient dynamo, numerous recent studies have shown that the dynamo axis may not have been steady in time (Arkani-Hamed & Boutin, 2017;Baek et al, 2019;Nayak et al, 2017;Oliveira & Wieczorek, 2017;Takahashi & Tsunakawa, 2009). Hence, we model the field as a centered dipole oriented in the +Z SSE direction (parallel to the lunar spin axis) and also as a dipole perpendicular to the spin axis, in the +X/+Y SSE plane.…”

Strength, Depth, and Geometry of Magnetic Sources in the Crust of the Moon From Localized Power Spectrum Analysis

“…Though anorthositic rocks contain low abundances of metallic iron (e.g., Fuller & Cisowski, 1987), most of these regions are associated with low field strengths, especially when viewed in the magnetic field intensity plot of Figure 1 that uses a linear scale. Thermal evolution models predict that the highland crust would cool entirely below the Curie temperature of metallic iron (1038 K, Dunlop & Özdemir, 2015) within the first 100 Myr of lunar evolution (e.g., Arkani-Hamed & Boutin, 2017;Laneuville et al, 2017). Furthermore, most thermal evolution models predict that the core could have driven a dynamo powered by thermal convection during the first couple hundred million years (e.g., Evans et al, 2014;Konrad & Spohn, 1997;Laneuville et al, 2014;Scheinberg et al, 2015).…”

Reiner Gamma: A Magnetized Elliptical Disk on the Moon

“…The present consensus is that many, if not all, of the Moon's crustal magnetic anomalies formed in the presence of an ancient dynamo field (e.g., Arkani-Hamed & Boutin, 2017;Hood et al, 2017;Nayak et al, 2017;Oliveira & Wieczorek, 2017;Takahashi et al, 2014;Weiss & Tikoo, 2014;Wieczorek, 2018). However, the geologic formation mechanism for any of these anomalies has yet to be established.…”