A silicate dynamo in the early Earth

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.

Thermal Conductivity of Silicate Liquid Determined by Machine Learning Potentials

“…Previous ab initio studies have found that the Weidemann-Franz law is closely obeyed by oxide liquids at CMB conditions (Holmström et al, 2018). We therefore combine predictions of the electrical conductivity of a liquid of bulk silicate Earth composition (Stixrude et al, 2020) with the Wiedemann-Franz law to obtain k el = 1.1 W m −1 K −1 .…”

Electrical conductivity of warm dense silica from double-shock experiments

“…Certainly, pure SiO 2 magma oceans are unlikely as other elements such as Mg and Fe would enter the composition. Under such conditions, iron could very well stay mixed with silicates 20 increasing even more the conductivity 21 , 22 . The present experimental results thus confirm the ab initio simulation predictions 7 , 8 , 22 and support the idea that deep magma oceans are likely to produce a magnetic field in Super-Earths.…”

The Principal Hugoniot of Iron‐Bearing Olivine to 1465 GPa

“…This suggests that all silicates may become metallic at extreme temperature. Thus, these materials may generate planetary‐scale magnetic fields if basal magma oceans are sufficiently hot (e.g., Stixrude et al., 2020).…”

“…Experiments that can simulate the conditions of a large impact have thus far been restricted to end‐member oxide and silicate materials, including SiO 2 (Hicks et al., 2005, 2006; Knudson & Desjarlais, 2009; Kraus et al., 2012; Luo et al., 2003; Lyzenga & Ahrens, 1980; Lyzenga et al., 1983; Millot et al., 2015; Root et al., 2019), MgO (Bolis et al., 2016; Fat’yanov et al., 2018; McWilliams et al., 2012; Root et al., 2015), MgSiO 3 (Akins et al., 2004; Bolis et al., 2016; Deng et al., 2008; Fei et al., 2021; Fratanduono et al., 2018; Luo et al., 2004; Millot et al., 2020; Mosenfelder et al., 2009; Spaulding et al., 2012), and Mg 2 SiO 4 (Bolis et al., 2016; Davies et al., 2020; Jackson & Ahrens, 1979; Kim et al., 2021; Lyzenga & Ahrens, 1980; Mosenfelder et al., 2007; Root et al., 2018; Sekine et al., 2016; Syono et al., 1981; Watt & Ahrens, 1983). In natural silicates, iron (Fe) is incorporated in the crystal lattice by solid solution with magnesium (Mg) and is known to affect the solid‐state phase diagram as well as the electrical, thermo‐elastic, and optical properties of minerals and melts (Ohtani, 2009; Stixrude et al., 2020). Experiments on natural silicate compositions are rare (Furnish & Brown, 1986; Holland & Ahrens, 1997; Luo et al., 2004) but imperative to accurately model the formation of Earth and other terrestrial planets.…”

“…minerals and melts (Ohtani, 2009;Stixrude et al, 2020). Experiments on natural silicate compositions are rare (Furnish & Brown, 1986;Holland & Ahrens, 1997;Luo et al, 2004) but imperative to accurately model the formation of Earth and other terrestrial planets.…”