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

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.