[S-CG48] Planetary interiors revealed by exploration, experiments, and simulations

[SCG48-02] Determination of liquid FeS density under Martian core conditions by X-ray diffraction measurements

*Fumiya Sakai¹, Kei Hirose^1,2, Shunpei Yokoo¹, Suyu Fu¹ (1.the University of Tokyo, 2.Earth-Life Science Institute)

Keywords:Martian core, FeS, liquid, X-ray diffraction, Diamond-anvil cell

NASA’s InSight spacecraft provided new insights into the Martian interior, constraining the core size to be 1650-1830 km with a mean density of 5.7-6.7 kg/cm³ based on seismic observations^[1][2]_. This shows a large density deficit (20-30%) compared with pure iron, which is attributed to its high concentrations of light elements such as sulfur^[1][2]. To account for the “low-density” core, 20-34 wt% S is required if it is a single light element. Such estimates are based on extrapolations from experimental results obtained at pressures^[3] (<10 GPa) lower than Martian core conditions (20-40 GPa) and limited temperatures, leading to significant uncertainty.

In this study, we performed melting experiments at 32-66 GPa and 2350-3200 K to determine the density of liquid FeS. High-pressure and high-temperature in-situ X-ray diffraction measurements were conducted at BL10XU, SPring-8. We used diffuse scattering signal from liquid FeS to determine its density by a recently-developed analytical procedure^[4].

The liquid FeS density we obtained deviates from the extrapolations of previous data^[3], in particular at relatively high temperatures. We determined the equation of state for liquid FeS by fitting Vinet and Anderson-Grüneisen thermal equations to the data. Our results suggest that the previous estimations for Martian core composition^[1][2] underestimate the sulfur concentration by 0.5-1.5 wt% S.

References.
[1] Stähler, S.C., et al. 2021. Science 373, 6553, 443-448.
[2] Khan, A., et al. 2023, Nature 622, 718-723.
[3] Xu, F., et al. 2021, EPSL 563, 116884.
[4] Kuwayama, Y., et al. 2020, PRL 124, 165701.