The distribution and origin of Mercury's crustal magnetization

Mercury is the only planet other than Earth to possess both a global magnetic field that arises from a core dynamo, and a crustal magnetic field that arises from magnetized rocks. Crustal fields provide unique records of a planet's interior state and evolution because magnetizations depend on the magnetizing field and the magnetic minerals present. A major unresolved issue for Mercury is the relative contributions of present-day (induced) and ancient (remanent) magnetizations-both are expected but are difficult to separate. The former can elucidate variations in crustal iron content and/or magnetic mineral properties, the latter can constrain the dynamo history. Here, we use crustal field observations from the MErcury, Surface, Space ENvironment, GEochemistry and Ranging mission to develop a magnetization model for Mercury's northern latitudes. We use constraints on crustal thickness, magnetic mineralogy, iron content, and the present-day core field to assess the expected magnitudes of induced magnetizations. We find that these can fully explain the observed crustal fields at most places, and at some more strongly magnetized craters through impact-delivered iron. The strongest crustal fields, especially at Caloris, require a contribution from remanent magnetization and/or unusual magnetic mineral properties. Our results provide constraints on lateral variations in crustal iron content of both exogenic and endogenic origin. They also highlight a knowledge gap regarding the magnetic properties of Mercury's crust, specifically its magnetic susceptibility, that has critical implications for understanding whether even the strongest magnetizations can elucidate Mercury's dynamo history.