Mercury\u2019s anomalous magnetic field caused by a symmetry-breaking self-regulating dynamo

Futoshi Takahashi,Hisayoshi Shimizu,Hideo Tsunakawa

doi:10.1038/s41467-018-08213-7

Abstract

The discovery of Mercury’s unusually axisymmetric, anomalously axially offset dipolar magnetic field reveals a new regime of planetary magnetic fields. The cause of the offset dipole remains to be resolved, although some exotic models have been proposed. Deciphering why Mercury has such an anomalous field is crucial not only for understanding the internal dynamics, evolutionary history and origin of the planet, but also for establishing the general dynamo theory. Here we present numerical dynamo models, where core convection is driven as thermo-compositional, double-diffusive convection surrounded by a thermally stably stratified layer. We show that the present models produce magnetic fields similar in morphology and strength to that of Mercury. The dynamo-generated fields act on the flow to force interaction between equatorially symmetric and antisymmetric components that results in north-south asymmetric helicity. This symmetry-breaking magnetic feedback causes the flow to generate and maintain Mercury’s axially offset dipolar field.

Highlights

The discovery of Mercury’s unusually axisymmetric, anomalously axially offset dipolar magnetic field reveals a new regime of planetary magnetic fields
Based on the co-density approach of treating thermal and compositional convection together[6], the weak field and its low secular variation can be explained by dynamo models incorporating a thermally stably stratified layer beneath the core mantle boundary (CMB), through which small-scale, high-frequency components deep inside the core are attenuated due to the skin effect[7,8]
The present models show that, without taking any speculative external forcing into account, a Mercury-like magnetic field could result from a spontaneous dynamo mechanism driven by thermocompositional convection, in which the compositional buoyancy is caused by the BU compositional process and thermal buoyancy mostly by the internal heating in the liquid core

Summary

Introduction

The discovery of Mercury’s unusually axisymmetric, anomalously axially offset dipolar magnetic field reveals a new regime of planetary magnetic fields. The dynamo-generated fields act on the flow to force interaction between equatorially symmetric and antisymmetric components that results in north-south asymmetric helicity. This symmetry-breaking magnetic feedback causes the flow to generate and maintain Mercury’s axially offset dipolar field. Based on the co-density approach of treating thermal and compositional convection together[6], the weak field and its low secular variation can be explained by dynamo models incorporating a thermally stably stratified layer beneath the core mantle boundary (CMB), through which small-scale, high-frequency components deep inside the core are attenuated due to the skin effect[7,8]. The combined effect on Mercury’s dynamo of double-diffusive convection and a core crystallization regime has not yet been explored

Methods

Results

Conclusion