Pressure and Composition Effects on Sound Velocity and Density of Core-Forming Liquids: Implication to Core Compositions of Terrestrial Planets

A compositional variety of planetary cores provides insight into their core/mantle evolution and chemistry in the early solar system. To infer core composition from geophysical data, a precise knowledge of elastic properties of core-forming materials is of prime importance. Here, we measure the sound velocity and density of liquid Fe-Ni-S (17 and 30 at% S) and Fe-Ni-Si (29 and 38 at% Si) at high pressures and report the effects of pressure and composition on these properties. Our data show that the addition of sulfur to iron substantially reduces the sound velocity of the alloy and the bulk modulus in the conditions of this study, while adding silicon to iron increases its sound velocity but has almost no effect on the bulk modulus. Based on the obtained elastic properties combined with geodesy data, S or Si content in the core is estimated to 4.6 wt% S or 10.5 wt% Si for Mercury, 9.8 wt% S or 18.3 wt% Si for the Moon, and 32.4 wt% S or 30.3 wt% Si for Mars. In these core compositions, differences in sound velocity profiles between an Fe-Ni-S and Fe-Ni-Si core in Mercury are small, whereas for Mars and the Moon, the differences are substantially larger and could be detected by upcoming seismic sounding missions to those bodies. Plain Language Summary To estimate core compositions of terrestrial planets using geophysical data with high-pressure physical property of core-forming materials, we measure the sound velocity and density of liquid Fe-Ni-S and Fe-Ni-Si at high pressures. The effect of S and Si on elastic properties are quite different in the present conditions. Based on the obtained physical properties combined with geodesy data, S or Si content in the core of Mercury, Moon, and Mercury are estimated. In these core compositions, differences in sound velocity profiles between an Fe-Ni-S and Fe-Ni-Si core in Mars and the Moon are substantially large and could be detected by upcoming seismic sounding mission to Mars.