A silicate dynamo in the early Earth

Lars Stixrude,Roberto Scipioni,Michael P Desjarlais

doi:10.1038/s41467-020-14773-4

Abstract

The Earth’s magnetic field has operated for at least 3.4 billion years, yet how the ancient field was produced is still unknown. The core in the early Earth was surrounded by a molten silicate layer, a basal magma ocean that may have survived for more than one billion years. Here we use density functional theory-based molecular dynamics simulations to predict the electrical conductivity of silicate liquid at the conditions of the basal magma ocean: 100–140 GPa, and 4000–6000 K. We find that the electrical conductivity exceeds 10,000 S/m, more than 100 times that measured in silicate liquids at low pressure and temperature. The magnetic Reynolds number computed from our results exceeds the threshold for dynamo activity and the magnetic field strength is similar to that observed in the Archean paleomagnetic record. We therefore conclude that the Archean field was produced by the basal magma ocean.

Highlights

The Earth’s magnetic field has operated for at least 3.4 billion years, yet how the ancient field was produced is still unknown
Magnetohydrodynamic simulations, laboratory experiments, and studies of planetary bodies that host magnetic fields show that dynamo activity requires a sufficiently large magnetic Reynolds number Rm 1⁄4 μ0vLσ > 40, where μ0 is the magnetic susceptibility, v is the flow velocity, L is the depth of the layer, and σ is the electrical conductivity[6,7]
We combine our predictions of electrical conductivity with a model of the thermal evolution of the basal magma ocean to estimate the magnetic Reynolds number and the strength of the magnetic field generated by the silicate dynamo

Summary

Introduction

The Earth’s magnetic field has operated for at least 3.4 billion years, yet how the ancient field was produced is still unknown. The process by which the field is produced today: a dynamo hosted in the Earth’s metallic iron-rich liquid outer core is thought to be difficult to sustain in the early Earth because the core could not cool sufficiently rapidly[2,3,4]. For a silicate dynamo to operate the electrical conductivity must exceed 10,000 S/m, more than 100 times higher than the highest values measured in silicate liquids at low pressure and temperature[8,9], less than typical metallic conductivity (106 S/m for the Earth’s outer core). We combine our predictions of electrical conductivity with a model of the thermal evolution of the basal magma ocean to estimate the magnetic Reynolds number and the strength of the magnetic field generated by the silicate dynamo. We find that the magnetic Reynolds number exceeds 40 and that the field strength is in excellent agreement with paleointensity measurements

Methods

Results

Conclusion