[S-IT18] Planetary cores: Structure, formation, and evolution

[SIT18-09] First-principle molecular dynamics simulations of Fe-S alloy melts under Mars core conditions

*Hang Zhang^1,2, Tao Liu², Zhicheng Jing² (1.Earthquake Research Institute, the University of Tokyo, 2.Department of Earth and Space Sciences, Southern University of Science and Technology)

Keywords:Mars core, First-principle molecular dynamics, sound velocity of Fe-S alloy melts, equation of state of Fe-S alloy melts

The physical properties of iron-sulfur alloy melts such as density, sound velocity, etc., are significant parameters to understanding the composition, structure, and physical state of the Mars core. A large gap, however, exists between previous experimental results for the Fe-S system at relatively low pressures (<20 GPa) and simulation results at much higher pressures (typically 100-300 GPa) that are more relevant to the Earth core. In this study, we performed first-principle molecular dynamics simulations on the Fe-S melt system with different sulfur contents in the pressure range of 0-80 GPa. The equation of state and properties such as bulk modulus, sound velocity, Grüneisen parameter, and atomic structures were obtained. We found that the properties of Fe-S melts are not very far from those described by ideal mixing models, and the deviation diminishes with increasing pressure. The Grüneisen parameter of Fe-S melts first increases with increasing pressure but decreases slowly after a specific pressure range. The sound velocity of Fe-S melts increases with increasing pressure, and after the same pressure range, the sound velocity curves of different sulfur contents will converge. Before this range, the increase of sulfur contents will decrease sound velocity, while after it, the effects will be reversed. Within this specific range, coordination numbers curves experience a change in trend, so there might be a transition in melt structure. The comparison between our simulation results and existing experimental data will provide constraints on the element composition of the Mars core. According to our calculation, most of the core is liquid, possibly with a small iron-nickel inner core. The liquid outer core may crystallize in a "snowing" manner and can trigger a new magnetic field in the future.