Magnetic Anomalies in Five Lunar Impact Basins: Implications for Impactor Trajectories and Inverse Modeling

Magnetic anomalies in lunar impact basins are especially important because the interiors of the basins cooled very slowly requiring a steady, long-lived magnetizing field to impart magnetization. They are the best orbital evidence for a former internal lunar dynamo field. In this study, five basins are selected that most confidently contain internal anomalies that are intrinsic to the basin, i.e., located within the inner basin rims and originating at the times of basin formation. Assuming that these internal anomalies are due to magnetization of impact melt containing iron from the impactor that created the basins, comparisons are made with previous numerical simulations to constrain the trajectories of the impactors. Results suggest that the five impactor trajectories were within roughly 45 degrees of being vertical and tended to lie on average parallel to the lunar equatorial plane and the ecliptic plane. Inverse modeling of the anomalies yields inferred directions of magnetization that are difficult to reconcile with the hypothesis that the lunar dynamo field originated in the small metallic core. Possible explanations include that the core dynamo behaved unlike the current terrestrial dynamo or that the magnetizing field was instead due to dynamo generation in an early shallow magma ocean. A recent large-scale map of the lunar crustal magnetic field is examined for the existence of magnetic anomalies internal to ringed impact basins. It is found that, of 25 basins with upper preNectarian and younger ages, 18 contain mapped internal anomalies with amplitudes of at least 1 nT at 30 km altitude. Of these, five are most confidently judged to contain intrinsic anomalies (i.e., anomalies located within the inner basin rims and originating at the times of basin formation): Crisium, Humboldtianum, Mendel-Rydberg, Moscoviense, and Nectaris. Comparing the anomaly distributions with previous numerical simulations of the impact of iron-rich planetesimals to form a large (SPA-sized) basin, inferences are drawn about the likely trajectories of the impactors. Specifically, results suggest that impactor trajectories for these basins were within ∼45° of being vertical and tended to lie on average parallel to the lunar equatorial plane and the ecliptic plane. Inverse modeling of anomalies within these basins yields inferred directions of magnetization that are difficult to reconcile with the axial centered dipole hypothesis for the geometry of the internal lunar dynamo field: Paleomagnetic pole positions are widely scattered and, in agreement with a recent independent study, the two main anomalies within Crisium yield significantly different directions of magnetization. NASA Lunar Data Analysis Program. Grant Number: 80NSSC18K1602 Peerreview