Details

Autor(en) / Beteiligte

• Maxwell, R. E.
• Garrick‐Bethell, I.

Titel

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

Ist Teil von

• Journal of geophysical research. Planets, 2020-12, Vol.125 (12), p.n/a

Ort / Verlag

Washington: Blackwell Publishing Ltd

Erscheinungsjahr

2020

Link zum Volltext

Quelle

Wiley-Blackwell Journals

Beschreibungen/Notizen

• 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. Plain Language Summary Magnetic anomalies are large‐scale features (∼1–100's of kilometers) within the crusts of planetary bodies that occur when rocks are magnetized in the presence of a past geomagnetic field. Knowing the magnetization direction informs us about how the paleo‐geomagnetic field was generated, oriented, and evolved over time. However, there is no standard method of reporting the uncertainty of the best‐estimate direction. We have found that for one commonly used method of estimating uncertainty, there is a difference in uncertainties depending on what direction the rock is magnetized. We recommend a different method that the community can use to report uncertainties and present examples of this method on five lunar magnetic anomalies. We show that this method removes the difference in uncertainties based on magnetization direction. With a common method of citing uncertainty, it will be easier to compare future studies with one another, facilitating more robust claims about magnetic anomalies and paleomagnetic fields. We also show that the paleomagnetic field must have been pointing near the present‐day equator at some point in the Moon's history. This means that the magnetic field axis was once offset from the spin axis as determined by observations of gravity and polar hydrogen deposits. Key Points We describe a Monte Carlo method to obtain higher precision estimates of magnetization direction at crustal magnetic anomalies We find more robust evidence for near‐equatorial lunar paleopoles, implying an offset between the ancient dynamo dipole axis and spin axis