Detailed study of the Mare Crisium northern magnetic anomaly

Abstract

AbstractLow‐altitude Lunar Prospector (LP) magnetometer data for Mare Crisium show two magnetic anomalies near its inner northern and southern edges. Because these features are located inside a basin, they were likely formed by slow cooling of the basin's melt, or the partially melted mantle, instead of by any impact‐related shock magnetization process. Therefore, they are important for assessing the nature of the ancient dynamo field that produced them. In this study we confine our attention to the simpler northern anomaly (CNA) and use low‐altitude (∼22 km) LP data to model its source body as a dipole and magnetized disks of different radii. We infer that the source is likely located ∼20–30 km from the surface and horizontally localized within a small region (<1° or <∼30 km radius). The surface field intensity calculated from the best fit dipole is in good agreement with that obtained from LP Electron Reflectometer data. Our magnetization directions are substantially different from two previous studies, largely due to using lower altitude data to perform our inversions. We also find a surprising sensitivity to small changes in source body latitude (∼1°). The magnetic paleopoles implied by our best fit models are distant from previous estimates by up to ∼50° of great circle arc and are substantially distant from the Moon's present rotation axis. Our results demonstrate how multiple altitude data sets must be used when estimating paleopoles and other properties of even the simplest of the Moon's magnetic anomalies.