Impactor material records the ancient lunar magnetic field in antipodal anomalies

Abstract

The Moon presently has no dynamo, but magnetic fields have been detected over numerous portions of its crust. Most of these regions are located antipodal to large basins, leading to the hypothesis that lunar rock ejected during basin-forming impacts accumulated at the basin antipode and recorded the ambient magnetic field. However, a major problem with this hypothesis is that lunar materials have low iron content and cannot become strongly magnetized. Here we simulate oblique impacts of 100-km-diameter impactors at high resolution and show that an ~700 m thick deposit of potentially iron-rich impactor material accumulates at the basin antipode. The material is shock-heated above the Curie temperature and therefore may efficiently record the ambient magnetic field after deposition. These results explain a substantial fraction of the Moon’s crustal magnetism, and are consistent with a dynamo field strength of at least several tens of microtesla during the basin-forming epoch.

Lunar Magnetic Anomalies and Polar Ice

“…The anomalies centered on the south pole are located adjacent to Schrödinger and the largest mass of strong anomalies in the north polar region is located near its antipode. The tendency for young lunar basins to have a concentration of anomalies at or near their antipodes can potentially be explained in terms of the impactor-added iron hypothesis, as suggested by recent numerical simulations (Wakita et al, 2021). However, the nature of lunar magnetic anomaly sources remains controversial with some authors suggesting igneous intrusion sources (e.g., Hemingway & Tikoo, 2018) and others arguing for sources in the deep crust (e.g., Wieczorek, 2018).…”

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

“…Multiple studies have aimed to determine the age of individual MAs by constraining the geologic context and potential source geometry, towards understanding when the MA may have been emplaced in the crust and the method of magnetization (e.g., Nicholas et al, 2007;Weiss & Tikoo, 2014;Wieczorek, 2018;Lee et al, 2019;Baek et al, 2019;Kelley & Garrick-Bethell, 2020). Two primary magnetization mechanisms have been proposed: 1) thermoremanent magnetization (TRM) during geologic processes (e.g., impacts and crystallization) within a period of active dynamo (e.g., Purucker et al, 2012;Wieczorek et al, 2012;Arkani-Hamed & Boutin, 2014;Hood & Spudis, 2016;Hemingway & Tikoo, 2018;Wakita et al, 2021), and 2) shock remanent magnetization (SRM) or pressure remanent magnetization (PRM) during impact or collision events that generated transient fields or amplified external fields in the immediate locality of the impact (e.g., Schultz and Srnka, 1980;Crawford & Schultz, 1999;Gattacceca et al, 2010;Bruck Syal and Schultz, 2015). Absent a dynamo, sources of magnetizing external fields have been proposed such as compressed fields associated with a cometary collision (Schultz and Srnka, 1980).…”

“…Moon via ejecta-related plasma and/or diffusion through the crust, resulting in magnetization at the antipode of the impact (e.g., Hood, 1987;Richmond et al, 2005;Hood & Artemieva, 2008). The methods described in 2) are influenced by both impactor and target composition, as well as impactor size, velocity, and angle of incidence, which could explain why not all MAs are antipodal or otherwise spatially correlated with craters or basins (Crawford, 2020;Wakita et al, 2021). However, recent work suggests that fields temporarily generated or amplified by antipodal impacts may be too weak and crustal circulation/diffusion too inefficient to singularly create the existing magnetic characteristics of impactantipodal MAs (Tikoo et al, 2015;Oran et al, 2020).…”

Possibility of Lunar Crustal Magmatism Producing Strong Crustal Magnetism

“…For example, some of the Moon's strongest anomalies, co‐located with the swirls at Gerasimovich, appear to be at the antipode of the Crisium impact basin (Kramer et al., 2011; Richmond et al., 2005). High‐resolution impact simulations have been used to investigate whether iron‐enriched ejecta can account for the presence of magnetic anomalies at basin antipodes and the radial alignment of anomalies (Hood & Artemieva, 2008; Wakita et al., 2021). These simulations reveal that ejecta tend to converge at the antipode of most lunar basin‐forming impacts, which could potentially explain a significant portion of the Moon's crustal magnetism (Hood & Artemieva, 2008; Wakita et al., 2021; Wieczorek et al., 2023).…”