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.
Highlights
The Moon presently has no dynamo, but magnetic fields have been detected over numerous portions of its crust
For moderately oblique impacts we find that antipodal ejecta is dominated by the impact materials, which can have high thermoremanent magnetization (TRM) susceptibility (XTRM) like the chondritic meteorite
The major difficulty with the antipodal ejecta hypothesis has been that all lunar materials so far discovered are not sufficiently magnetic to produce the fields observed at spacecraft altitude, for realistic values of the ancient magnetizing field
Summary
The Moon presently has no dynamo, but magnetic fields have been detected over numerous portions of its crust. We explore the hypothesis that antipodal ejecta contains b sufficient impactor material to explain the observed magnetization of anomalies antipodal to large basins, using high-resolution impact simulations. Focusing on the strongest anomalies within the antipode region, which are at the lunar swirls (Fig. 1a), we assume a magnetized layer thickness comparable to the horizontal length scale of the swirl pattern[19].
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