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The geologic origin of the Moon's crustal magnetic anomalies is unknown. Reiner Gamma is one of the most studied anomalies, and it is correlated with symmetric bright albedo markings known as swirls. Here we propose that its source magnetization arises from a uniformly magnetized elliptical disk, resulting from the melt sheet or floor deposits of an oblique impact crater. The magnetization was likely acquired in a dynamo field and may be as high as similar to 70 A/m, perhaps due to incorporation of impactor materials. The disk produces vertical fields at its edges that may channel solar wind flux to the surface, producing an elliptical dark region, while neighboring regions remain shielded and bright. Interestingly, the disk appears to be magnetized along its semiminor axis. Measurements of the low altitude magnetic field at Reiner Gamma would test these predictions and help answer additional questions about its interaction with the solar wind.

Plain Language Summary We propose that the source of the Moon's Reiner Gamma magnetic anomaly is a magnetized elliptical disk. The disk may be the melt sheet or floor deposits of an elliptical crater, produced by a highly oblique impact. The melt sheet incorporated highly magnetic material from the impactor and cooled in the presence of a dynamo field. This model can explain some of the elliptical dark and bright optical patterns observed on the soil at Reiner Gamma.