Thermodynamic properties of Fe-S alloys from molecular dynamics modeling: Implications for the lunar fluid core

• Thermodynamic properties of Fe-S liquids were simulated by molecular dynamics.
• The sound velocity in the lunar outer core is consistent with seismic models.
• The estimated density of the outer core does not match seismic models.
• A reasonable radius for the fluid outer core should be less than about 330 km.

Density and sound velocity of Fe-S liquids for the P-T parameters of the lunar core have not been constrained well. From the analysis of seismic wave travel time, Weber et al. (2011) proposed that the lunar core is composed of iron alloyed with ⩽6 wt% of light elements, such as S. A controversial issue in models of planetary core composition concerns whether Fe-S liquids under high pressure – temperature conditions provide sound velocity and density data, which match the seismic model. Here we report the results of molecular dynamics (MD) simulations of iron-sulfur alloys based on Embedded Atom Model (EAM). The results of calculations include caloric, thermal and elastic properties of Fe-S alloys at concentrations of sulfur 0–18 at.%, temperatures up to 2500 K and pressures up to 14 GPa. The effect of sulfur on the elastic properties of Fe-rich melts is most evident in the notably decreased density with added S content. In the MD simulation, the density and bulk modulus KT of liquid Fe-S decrease with increasing sulfur content, while the bulk modulus KS decreases as a whole but has some fluctuations with increasing sulfur content. The sound velocity increases with increasing pressure, but depends weakly on temperature and the concentration of sulfur. For a fluid Fe-S core of the Moon (∼5 GPa/2000 K) with 6–16 at.% S (3.5–10 wt%), the sound velocity and density may be estimated at the level of 4000 m s−1 and 6.25–7.0 g cm−3. Comparison of thermodynamic calculations with the results of interpretation of seismic observations shows good agreement of P-wave velocities in the liquid outer core, while the core density does not match the seismic models. At such concentrations of sulfur and a density by 20–35% higher than the model seismic density, a radius for the fluid outer core should be less than about 330 km found by Weber et al. because at the specified mass and moment of inertia values of the Moon an increase of the core density leads to a decrease of the core radius. These results are important for modeling the current state and composition of the lunar core and can be applied to models of large satellites’ core properties.