Viscosity of Fe‐Ni‐C Liquids up to Core Pressures and Implications for Dynamics of Planetary Cores

Abstract

The liquid cores of Earth and other terrestrial planets or moons are composed of Fe-Ni metal, alloyed with a range of potential light elements (LE). The dynamics of these liquid cores determines the formation and the duration of the planetary dynamo and magnetic field, which are crucial for planetary habitability. Knowledge on the viscosity of candidate Fe-Ni-LE alloys at high pressures and high temperatures (HP-HT) are required to understand and model the dynamics of these liquid cores. Additionally, viscosity is one of the key properties that determines the percolation velocity of iron alloy through silicate/oxide rocks, which is crucial for studying the time scale of core formation in Earth and other terrestrial planets. Carbon is among the top candidate light elements in Earth's and planetary cores due to its high cosmic abundance, siderophile nature and ubiquity in iron meteorites. The phase diagram and physical properties of Fe-(Ni)-C solids and liquids at HP-HT have been widely investigated (

Investigating metallic cores using experiments on the physical properties of liquid iron alloys

“…It is not clear from liquid structure measurements on pure iron whether a structural change exists Shen et al, 2004) and affects viscosity (Terasaki et al, 2002;Kono et al, 2015). Similarly, a structural transition has been reported for Fe-(Ni)-C (Shibazaki et al, 2015;Lai et al, 2017), but its effect on viscosity (and density) is not fully resolved (Terasaki et al, 2006;Sanloup et al, 2011;Shimoyama et al, 2013;Zhu et al, 2022). The correlation between structure and viscosity of molten iron alloys remains to be explored at pressure and temperature conditions relevant to planetary cores.…”

“…The viscosity of molten iron and alloys is measured using the falling or floating sphere viscometry technique at synchrotron facilities. A microsphere is placed either near the top of the sample for a falling sphere experiment (e.g., Terasaki et al, 2006) or near the bottom for a floating sphere experiment (e.g., Zhu et al, 2022). At the target pressure, the sample is rapidly heated to a temperature above the liquidus temperature of the sample, to ensure that the sample is fully molten and the sphere can fall or float freely, depending on the density contrast between the sphere and the molten alloy.…”

Viscosity Measurements at High Pressures: A Critical Appraisal of Corrections to Stokes' Law

“…Hence drag due to inertia may become more important for large d / D in low viscosity—high sphere‐falling‐velocity experiments. Such conditions may be relevant for depolymerized silicate melts (Cochain et al., 2017; Spice et al., 2015; Xie et al., 2020, 2021), molten metals (Dobson et al., 2000; LeBlanc & Secco, 1996; Rutter, Secco, Liu, et al., 2002; Rutter, Secco, Uchida, et al., 2002; Terasaki et al., 2001, 2002; Urakawa et al., 2001; Zhu et al., 2022), or volatile‐rich fluids (Abramson, 2007, 2009; Abramson & West‐Foyle, 2008; Audétat & Keppler, 2004; King et al., 1992).…”

“…The crystal experiences a negative buoyancy force when it is denser than the surrounding melt. The crystal may also float or rise when it is less dense than the melt (Secco, 1994;Zhu et al, 2022). Assuming the crystal is spherical, it will sink (or The sphere starts to sink by acceleration due to gravity.…”

“…The formation of an iron core would result in a magnetic geodynamo that shields the Earth from solar radiation. The modern geodynamo is driven by the convection of the liquid outer core and is influenced by the viscosity of the liquid alloy (Zhu et al., 2022).…”