Investigating Sources of Mercury's Crustal Magnetic Field: Further Mapping of MESSENGER Magnetometer Data

One hundred six low-altitude passes of magnetometer data from the last 2 months of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging mission have been applied to produce a map of the crustal magnetic field at a constant altitude of 40 km covering latitudes of 35–75^∘ N and longitudes of 270–90^∘ E. Some anomalies correlate significantly with impact basins/craters (e.g., Rustaveli and Vyasa), while other basins/craters have no obvious anomalies. A possible interpretation that is consistent with lunar evidence is that some impactors delivered more ferromagnetic Fe–Ni metal to the interior subsurfaces and ejecta fields of the craters/basins that they produced. The amount of metallic iron that could plausibly be delivered is limited by the diameter and mass of an impactor that would yield a crater with observed diameters (e.g., 200 km for Rustaveli). This in turn limits the maximum amplitude of anomalies that could be induced by impactor-added iron in the present-day Mercury global field to relatively low values. It is therefore concluded that if impactor-added iron is the source of the observed crater-associated anomalies, then they must be almost entirely a consequence of ancient remanent magnetization. A broad magnetic anomaly occurs over the northern rise, a topographically high region with an associated strong free air gravity anomaly. A possible interpretation of the latter anomaly is that an early major impact preconditioned the region for a later mantle uplift event.