Material properties of liquid iron at planetary core conditions

Grant, Sean Campbell
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The outer core of the Earth is composed primarily of liquid iron, and the inner core boundary is governed by the intersection of the melt line and the geotherm. Traditional static compression techniques have provided a wealth of information on the solid phase of iron, but struggle in the liquid phase, leaving this important phase relatively unexplored. We use dynamic compression to diagnose the high-pressure liquid state of iron by utilizing the shock-ramp capability at Sandia National Laboratories’ Z-Machine. This technique enables measurements of material states off the Hugoniot by initially shocking samples and subsequently driving a further, shockless compression. Planetary studies benefit greatly from isentropic, off-Hugoniot experiments since they cover states that are representative of the adiabatic profiles found in planets. We used this method to drive iron to pressure and temperature conditions similar to those of the Earth’s core, along an elevated-temperature isentrope in the liquid from 275 GPa to 400 GPa, providing valuable information near the melt line. We are also developing an ellipsometry diagnostic for these experiments to optically probe the conductivity of the sample. We derive the equation of state using a hybrid backward integration – forward Lagrangian technique on particle velocity traces to determine the pressure-density history of the sample. Our results are in excellent agreement with SESAME 92141, a previously published equation of state table. With our data and previous experimental data on liquid iron we derive new parameters for a Vinet-based equation of state. The table and our parameterized equation of state provide an updated means of modeling liquid iron cores in planetary interiors, to which we provide an example.