Mercury's magnetic field: a thermoelectric dynamo?

Abstract

Permanent magnetism and conventional dynamo theory are possible but problematic explanations for the magnitude of the Mercurian magnetic field. A new model is proposed in which thermoelectric currents driven by temperature differences at a bumpy core-mantle boundary are responsible for the (unobserved) toroidal field, and the helicity of convective motions in a thin outer core (thickness ∼10^2 km) induces the observed poloidal field from the toroidal field. The observed field of ∼3 × 10^(−7) T can be reproduced provided the electrical conductivity of Mercury's semiconducting mantle approaches 10^3 Ω^(−1) m^(−1). This model may be testable by future missions to Mercury because it predicts a more complicated field geometry than conventional dynamo theories. However, it is argued that polar wander may cause the core-mantle topography to migrate so that some aspects of the rotational symmetry may be reflected in the observed field.

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