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. Plain Language Summary The origin of crustal magnetic anomalies on heavily cratered, airless bodies like the Moon and Mercury remains uncertain. Mapping of MErcury Surface, Space ENvironment, GEochemistry, and Ranging magnetometer data at northern midlatitudes on Mercury shows that magnetic anomalies are concentrated near and within some impact basins and craters but not others. The association of anomalies with some craters but not others is consistent with the hypothesis that metallic iron in some impactors (either in an iron core or distributed in the body) was mixed into impact melt and ejecta which then became magnetized while cooling in a former core dynamo magnetic field. Other impactors containing little metallic iron (e.g., cometary nuclei) would have produced no magnetic anomalies. The amount of metallic iron that could plausibly have been delivered by impactors is too small to explain observed anomaly amplitudes as being induced by permeability in the present‐day M