Evidence for an Ancient Near‐Equatorial Lunar Dipole From Higher Precision Inversions of Crustal Magnetization

Abstract

Studies of lunar paleopoles have been used to make a variety of inferences about past episodes of true polar wander and the orientation of the ancient dynamo field. However, the large and variable uncertainties commonly reported for such studies make robust conclusions difficult. To make further progress, we used synthetic magnetic anomalies to assess a common method to estimate magnetization direction uncertainty. We find that with this method, magnetic anomalies with higher inclinations have systematically higher uncertainties than lower inclination anomalies. We call this effect inclination bias. A similar effect is found for declination, but it is weaker. We also find that this method often produces overly conservative uncertainty estimates. To avoid these effects, we use Monte Carlo methods to determine magnetization direction uncertainty. We apply our methods to five lunar magnetic anomalies with a wide range of reported magnetization directions and paleopole locations. We find that inclination bias partly explains the previously reported anomalously high and low direction uncertainties for two of these anomalies: Reiner Gamma and Airy. Our more robust uncertainties allow us to conclude that four paleopoles are located near the equator. Such low latitudes cannot be explained by true polar wander inferred from other independent datasets, such as the lunar gravity field and the polar hydrogen distribution. This in turn implies that the dynamo axis was once offset from the spin axis.

The palaeoinclination of the ancient lunar magnetic field from an Apollo 17 basalt

“…Lunar hydrogen deposits, which may represent ancient polar ice deposits, are observed in two distinct, antipodal locations offset from the current spin poles, suggesting that the Moon may have experienced a significant degree of true polar wander 7 . Assuming a selenocentric axial dipole that may have reversed, our paleoinclination measurement predicts paleopole locations offset from the spin axis that are indistinguishable from several independent studies at ∼ 3.7 Ga ago 5,6,7,52 (Figure 4c). However, no subsequent true polar wander is also indistinguishable from our measured paleoinclination.…”

“…A multipolar magnetic field is consistent with the low lunar rotation rate, and would also be favored by dynamo sources closer to the surface (e.g., a basal magma ocean 27 ) given the r −(l+2) dependence of multipolar fields. Clustering of paleopoles near the poles and the equator has been observed by numerous crustal magnetic studies 10,51 and would be consistent with a predominantly quadrupolar dynamo or else a persistent non-axial dipole field 52 .…”

“…The grey shaded region represents < 10 • dipole tilt. North 3.7 Ga paleopole locations from independent studies are shown in purple 5 , pink 7 , red 52 , orange 6 , green 51 and blue 55 .…”

Pinpointing the Mechanism of Magnetic Enhancement in Modern Soils Using High‐Resolution Magnetic Field Imaging

“…Further, we focused on the distribution in the ARM declination by applying average field value subtraction, which decreases the spread of the fitted direction in inclination (Figure 3c). Dipole inversions of declination are typically more accurate than that of inclination while, assuming isotropic sources, the variance in declination alone should provide full information on ARM reproducibility (Maxwell & Garrick‐Bethell, 2020).…”

Ultraviolet and magnetic perspectives at Reiner Gamma and the implications for solar wind weathering

“…This global magnetic field is estimated to have had a peak mean surface intensity of ~77 μT between 3.85 billion and 3.56 billion years ago (Ga) and declined to ~4 μT by 3.19 Ga, weakening further until around 2.5 Ga and completely ceasing to exist between ~0.80-1.92 Ga (Strauss et al, 2021;Mighani et al, 2020). Potential paleopoles have been calculated from MAs in many recent studies to establish a relative timeline of dynamo precession and lunar magnetism (e.g., Takahashi et al, 2014;Arkani-Hamed & Boutin, 2014;Oliveira & Wieczorek, 2017;Nayak et al, 2017;Ravat et al, 2020;Maxwell & Garrick-Bethell, 2020;Nichols et al, 2021). However, uncertainty remains due to disagreement on longevity of dynamo activity (e.g., Tarduno et al, 2021) and adding a further temporal complication, it has been noted that impact events that occur after the decline of a dynamo may demagnetize the surrounding crust (e.g., Halekas et al, 2002;Wieczorek, 2018).…”