Mercury's internal magnetic field: Constraints on fields of crustal origin

Observations of Mercury's internal magnetic field during MESSENGER's first flyby (M1) and the first and third flybys of Mariner 10 (M10-I, M10-III) suggest that small-scale crustal magnetic fields, if they exist, are at the limit of resolution. Small-scale crustal fields are most easily identified near closest approach (CA) as features with wavelengths comparable to, or larger than, the spacecraft altitude. One small feature (< 4 nT in magnitude) encountered near CA during MESSENGER's first flyby may be either a crustal magnetic field or a plasma pressure effect. By means of Parker's constrained optimization approach, with no assumptions on the direction of magnetization, we can place constraints on the product of magnetization and magnetized layer thickness from such observations. The second flyby (M2) will allow additional constraints to be placed on the presence of small-scale fields, and correlations will be possible among topographic profiles measured by the Mercury Laser Altimeter (MLA), features seen on MESSENGER and Mariner 10 images, and any variations in the internal field. This flyby will acquire the first images of the CA region of M10-III, which has been pivotal in establishing the dipolar character of Mercury's magnetic field. Our ability to isolate small-scale crustal magnetic fields has been hindered by the limited coverage to date, as well as the difficulty in isolating the internal field. Across the terrestrial planets and the Moon, minimum 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.