Mercuryʹs internal magnetic field: Constraints on large- and small-scale fields of crustal origin

MESSENGER and Mariner 10 observations of Mercuryʹs magnetic field suggest that small-scale crustal magnetic fields, if they exist, are at the limit of resolution. Large-scale crustal magnetic fields have also been suggested to exist at Mercury, originating from a relic of an internal dipole whose symmetry has been broken by latitudinal and longitudinal variations in surface temperature. If this large-scale magnetization is confined to a layer averaging 50 km in thickness, it must be magnetized with an intensity of at least 2.9 A/m. Fits to models constrained by such large-scale insolation variations do not reveal the predicted signal, and the absence of small-scale features attributable to remanence further weakens the case for large-scale magnetization. Our tests are hindered by the limited coverage to date and difficulty in isolating the internal magnetic field. We conclude that the case for large- and small-scale remanence on Mercury is weak, but further measurements by MESSENGER can decide the issue unequivocally. Across the terrestrial planets and the Moon, magnetization contrast and iron abundance in the crust show a positive correlation. This correlation suggests that crustal iron content plays a determining role in the strength of crustal magnetization.