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

Abstract

The viscosity of iron alloy liquids is the key for the core dynamo and core‐mantle differentiation of terrestrial bodies. Here we measured the viscosity of Fe‐Ni‐C liquids up to 7 GPa using the floating sphere viscometry method and up to 330 GPa using first‐principles calculations. We found a viscosity increase at ∼3–5 GPa, coincident with a structural transition in the liquids. After the transition, the viscosity reaches ∼14–27 mPa·s, a factor of 2–4 higher than that of Fe and Fe‐S liquids. Our computational results from 5 to 330 GPa also indicate a high viscosity of the Fe‐Ni‐C liquids. For a carbon‐rich core in large terrestrial body, the level of turbulence in the outer core would be lessened approaching the inner core boundary. It is also anticipated that Fe‐Ni‐C liquids would percolate in Earth’s deep silicate mantle at a much slower speed than Fe and Fe‐S liquids.Plain Language SummaryLiquid cores of Earth and other terrestrial planets are composed of Fe‐Ni metal with certain amounts of light elements (LE) such as H, C, O, Si, and S. Located at the center of the planets, these cores are under extremely high‐pressure high‐temperature conditions. The viscosity of Fe‐Ni‐LE liquids is the key to understanding the formation and life duration of core dynamo. In the present study, we used experimental and theoretical methods to study the viscosity of Fe‐Ni‐C liquid, a candidate liquid core component, at high pressures and temperatures. Our results show that the viscosity of Fe‐Ni‐C liquids first increase quickly at 3–5 GPa (GPa), and then increases slowly up to Earth’s core conditions at 330 GPa and 5530 K. The fast growth of viscosity concurs with a liquid structure transition at around 5 GPa, around the core pressure ranges of the Moon, Mercury, and several Jupiter’s satellites, and might result in difference in viscous forces in core dynamos between those cores with pressures above and below transition pressure. The viscosity of Fe‐Ni‐C liquids at Earth’s core conditions is higher than Fe and Fe‐S liquids from previous studies, which provides new clues for the operation of geodynamo in a carbon‐rich outer core.Key PointsExperimental and computational determinations of viscosity of Fe‐Ni‐C liquids up to core pressuresViscosities of Fe‐Ni‐C liquids increase by a factor of ∼2 at 3–5 GPa at which a liquid structural transition occursFe‐Ni‐C liquid has higher viscosity than Fe and Fe‐S liquid, causing difference in core dynamo and percolative core formation