Exploring the origin of lunar magnetic anomalies through the Parker Inversion method

It is not yet understood whether the origin of the observed heterogeneous and weak lunar crustal magnetism is tied to a now extinct core-dynamo [1], to asteroid impacts [2, 3] or to a combination of both phenomena [4]. When using recent magnetic field maps (e.g., [5]) to study the magnetic sources, investigations to date have employed models relying on available geological and geophysical context, precluding the analysis of anomalies that are not correlated with known features. The Parker inversion method [6] overcomes these restrictions relying on a limited complexity of the magnetic sources by assuming unidirectional magnetization. It allows for the estimation of strength, location and direction of a set of surface dipoles that best fit the local set of magnetic data. We investigate the distribution of surface magnetization across the globe using Parker’s method independently of the specific geological or geophysical contexts, following the work of [7]. This approach enables the analysis of surface magnetic anomalies, ranging from those associated with impact craters (e.g., Moscoviense) or lunar swirls (e.g., Rimae Sirsalis) to the less-analysed polar regions. It captures varying morphologies, such as elongated (e.g., Hartwig), localized (e.g., Crozier), and more diffuse distributions (e.g., South Pole) of magnetized material. The overarching aim is to uncover the origin of magnetic anomalies and their significance for understanding lunar evolution.

Application of Parker’s method to isolated magnetic anomalies reveals a variety of magnetization distributions, reflecting the diversity of their morphologies and spatial patterns. Notably, a significant radial alignment of magnetized material related to the Imbrium basin suggests an ejecta origin for a number of near-side anomalies [2], for which the paleopole position is taken into consideration. We also see a clear correlation between the magnetization distribution and the antipodal regions of some large impact craters or basins, areas in which it is argued that the magnetic field could have been created or amplified by processes such as converging ejecta deposition, shock waves, and an ionized melt cloud from the impact [3, 4]. Finally, we recognize that the magnetized material of isolated and compact anomalies related to swirls aligns closely with the boundaries of these features [8], whereas large swirl structures show a poor correlation. This suggests the need for alternative analytical approaches for these regions.

Overall, our results reinforce previous hypotheses, in which large impacts played a pivotal role in shaping the morphology and distribution of lunar crustal magnetic sources.

 

References:

[1] Weiss B.P. and Tikoo S.M. (2014), Science (Vol. 346, Issue 6214)

[2] Hood L.L. et al. (2021), JGR Planets (Vol. 126, Issue 2)

[3] Hood & Artemieva (2008), Icarus (Vol. 193, Issue 2, pp. 485–502)

[4] Narrett et al. (2024), 55th LPSC

[5] Tsunakawa et al. (2015), JGR Planets (Vol. 120, Issue 6, pp. 1160–1185)

[6] Parker (1991), JGR Solid Earth (Vol. 96, Issue B10, pp. 16101–16112)

[7] Oliveira et al. (2024), JGR Planets (Vol. 129, Issue 2)

[8] Denevi et al. (2016), Icarus (Vol. 273, pp. 53–67)