Evidence for Magnetized Basin Ejecta on the Moon From Observations and Modeling of Demagnetized Craters

Abstract

The formation of lunar crustal magnetic anomalies is not well understood, and most anomalies are not associated with any obvious geologic features. To investigate further, we studied lunar craters from 100 to 400 km in diameter (totaling 305 craters) that may have demagnetized the crust. We find that the four craters Chaplygin, Keeler, Gauss, and Fermi are highly likely to have demagnetized the crust, based on our statistical methods. We modeled the magnetic source of these craters as a simple hole in a thin magnetized plate, representing the destruction of a surficial magnetized layer (Hypothesis 1). Alternatively, we also simulated the impact demagnetization of deeper-seated magnetism in the crust by shock and temperature (Hypothesis 2). Some interior magnetization remains unexplained under both hypotheses, but the destruction of a pre-existing surficial layer of magnetized material is consistent with the location of the peak in each crater's magnetic field. We also find three of the craters are inversely correlated with remotely sensed iron, further supporting our interpretation that the craters demagnetized a surficial layer. The four craters are located on magnetized ejecta deposits from the South Pole-Aitken, Orientale, and Crisium basins. Hence, these four craters further support the hypothesis that large provinces of magnetized material on the Moon arise from hot impact ejecta that cooled in a dynamo field.