Magnetic Anomalies Within the Crisium Basin: Magnetization Directions, Source Depths, and Ages

Abstract

Using Lunar Prospector magnetic field data, we identify four isolated anomalies (C NA , C SA1 , C SA1 , and C WA ) within the Crisium basin. We assume the sources of the anomalies to be buried point dipoles and find that the estimated depths and radial positions are well contained within Crisium's melt pool (∼250-km radii and ∼100-km depth). This implies that the anomalies recorded a thermoremanent magnetization in a dynamo field at ∼3.9 Ga. We also find that the anomalies can be classified into two groups (Groups 1 and 2) according to source depths and paleopoles. The sources of Group 1 (C NA and C SA1 ) are buried at ∼30 km. Estimated paleopoles are located at ∼45°N for C NA and near the equator (∼12°N) C SA1 , with remanence acquisition timescale of ∼5 Myr. The sources of Group 2 (C SA2 and C WA ) are more deeply buried at ∼55–70 km. Their paleopole positions are close to the present south pole, with a remanence acquisition timescale of ∼15–23 Myr. These variable paleopoles imply that the local field orientation shifted during formation of the anomalies. That is, the Moon may have experienced a ∼14-Myr-epoch of complex field geometries. These changes may have arisen from true polar wander driven by impact or internal density changes, or from equatorial dynamo orientations produced by anomalous core heat flux conditions. The long cooling timescales of the anomalies also suggest that they were not magnetized by transient fields produced by impact-induced stirring of the core.