[Paper Review] Mercury's magnetic field in the MESSENGER era

Johannes Wicht, Daniel Heyner|arXiv (Cornell University)|Jan 18, 2017

Geomagnetism and Paleomagnetism Studies68 references18 citations

TL;DR

This paper reviews Mercury's magnetic field using MESSENGER mission data, proposing that a stably stratified outer core layer with an iron snow zone explains the planet's exceptionally weak, offset axial dipole field with a strong axial quadrupole and minimal dipole tilt. The model reconciles the field's unique geometry and weakness by showing that magnetic diffusion through a stable layer suppresses non-axisymmetric and higher-order harmonics, yielding a field consistent with observations only when lateral heat flux variations at the core-mantle boundary are included.

ABSTRACT

MESSENGER magnetometer data show that Mercury's magnetic field is not only exceptionally weak but also has a unique geometry. The internal field resembles an axial dipole that is offset to the North by 20% of the planetary radius. This implies that the axial quadrupol is particularly strong while the dipole tilt is likely below 0.8 degree. The close proximity to the sun in combination with the weak internal field results in a very small and highly dynamic Hermean magnetosphere. We review the current understanding of Mercury's internal and external magnetic field and discuss possible explanations. Classical convection driven core dynamos have a hard time to reproduce the observations. Strong quadrupol contributions can be promoted by different measures, but they always go along with a large dipole tilt and generally rather small scale fields. A stably stratified outer core region seems required to explain not only the particular geometry but also the weakness of the Hermean magnetic field. New interior models suggest that Mercury's core likely hosts an iron snow zone underneath the core-mantle boundary. The positive radial sulfur gradient likely to develop in such a zone would indeed promote stable stratification. However, even dynamo models that include the stable layer show Mercury-like magnetic fields only for a fraction of the total simulation time. Large scale variations in the core-mantle boundary heat flux promise to yield more persistent results but are not compatible with the current understanding of Mercury's lower mantle.

Motivation & Objective

• To explain the origin of Mercury’s uniquely weak and geometrically offset magnetic field, which defies classical dynamo theory.
• To investigate how a stably stratified outer core layer could suppress non-axisymmetric and higher-order magnetic harmonics while preserving a strong axial quadrupole.
• To assess whether lateral variations in core-mantle boundary heat flux can produce a persistent, Mercury-like magnetic field configuration.
• To evaluate the compatibility of dynamo models with new interior models suggesting an iron snow zone and subadiabatic heat flux in Mercury’s core.

Proposed method

• Analysis of MESSENGER magnetometer data to determine the time-averaged location of the magnetic equator, enabling separation of internal and external field contributions.
• Application of the Gauss spherical harmonic expansion to model the internal magnetic field, with emphasis on dipole and quadrupole coefficients.
• Use of numerical dynamo simulations incorporating a stably stratified outer core layer to simulate magnetic field generation and diffusion.
• Incorporation of lateral variations in core-mantle boundary heat flux in dynamo models to test their effect on field geometry and stability.
• Modeling of magnetic diffusion via the magnetic skin effect to quantify damping of higher-order and non-axisymmetric field components.
• Integration of recent interior models suggesting an iron snow zone with a positive radial sulfur gradient beneath the core-mantle boundary.

Experimental results

Research questions

• RQ1 How can Mercury’s exceptionally weak internal magnetic field with a strong axial quadrupole and minimal dipole tilt be explained by dynamo theory?
• RQ2 What role does a stably stratified outer core layer play in suppressing non-axisymmetric and higher-order magnetic field components?
• RQ3 Can lateral variations in core-mantle boundary heat flux produce a persistent, Mercury-like magnetic field geometry in dynamo simulations?
• RQ4 Is the presence of an iron snow zone beneath Mercury’s core-mantle boundary compatible with observed magnetic field characteristics?
• RQ5 Why do classical convection-driven dynamo models fail to reproduce Mercury’s magnetic field, and what modifications are required?

Key findings

• Mercury’s internal magnetic field is best described by an axial dipole offset 480 km northward (20% of planetary radius), implying a strong axial quadrupole and a dipole tilt below 0.8°.
• The field’s weakness (190 nT axial dipole) and unique geometry are inconsistent with classical dynamo models driven by convection alone.
• A stably stratified outer core layer, likely formed by an iron snow zone with a positive radial sulfur gradient, effectively damps non-axisymmetric and higher harmonic field components through magnetic diffusion.
• Lateral variations in core-mantle boundary heat flux—particularly increased flux in the northern hemisphere or equatorial region—can produce a persistent offset dipole configuration in dynamo simulations.
• Even with a stable layer, dynamo models only reproduce Mercury-like fields for a fraction of simulation time unless lateral heat flux variations are included.
• The presence of a subadiabatic heat flux and a growing iron snow zone in the outer core is consistent with the observed field geometry and supports the viability of a stable layer in current interior models.