Magnetic anomalies concentrated near and within Mercury’s impact basins: Early mapping and interpretation

Ninety-five low-altitude passes of MErcury Surface, Space ENvironment, GEochemistry, and Ranging magnetometer data from February, March, and April of 2015 have been applied to produce an approximate map of the crustal magnetic field at a constant altitude of 40 km covering latitudes of 35°–75°N and longitudes of 90°–270°E. Anomalies are concentrated near and within the Caloris impact basin. A smaller concentration occurs over and around Sobkou Planitia and an associated older large impact basin. The strongest anomalies are found within Caloris and are distributed in a semicircular arc that is roughly concentric with the basin rim. They imply the existence of a core dynamo at the time when Caloris formed (∼3.9 Gyr ago). Anomalies over high-reflectance volcanic plains are relatively weak while anomalies over low-reflectance material that has been reworked by impact processes are relatively strong. The latter characteristics are qualitatively consistent with the ejecta deposit model for anomaly sources.