Local Structure and Density of Liquid Fe‐C‐S Alloys at Moon's Core Conditions. Issue 3 (8th March 2023)

• Record Type: Journal Article
• Authors: Zhao, Bin; Morard, Guillaume; Boulard, Eglantine; Boccato, Silvia; Siersch, Nicki C.; Rivoldini, Attilio; Guignot, Nicolas; Henry, Laura; King, Andrew; Zurkowski, Claire; Fei, Yingwei; Antonangeli, Daniele
• Is Part Of: Journal of geophysical research. Volume 128:Issue 3(2023)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2023-03-08
• Subjects: Moon's core -- Fe‐C‐S alloy -- density; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2022JE007577
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: The local structure and density of ternary Fe‐C‐S liquid alloys have been studied using a combination of in situ X‐ray diffraction and absorption experiments between 1 and 5 GPa and 1600–1900 K. The addition of up to 12 at% of carbon (C) to Fe‐S liquid alloys does not significantly modify the structure, which is largely controlled by the perturbation to the Fe‐Fe network induced by S atoms. The liquid density determined from diffraction and/or absorption techniques allows us to build a non‐ideal ternary mixing model as a function of pressure, temperature, and composition in terms of the content of alloying light elements. The composition of the Moon's core is addressed based on this thermodynamic model. Under the assumption of a homogeneous liquid core proposed by two recent Moon models, the sulfur content would be 27–36 wt% or 12–23 wt%, respectively, while the carbon content is mainly limited by the Fe‐C‐S miscibility gap, with an upper bound of 4.3 wt%. On the other hand, if the core is partially molten, the core temperature is necessarily lower than 1850 K estimated in the text, and the composition of both the inner and outer core would be controlled by aspects of the Fe‐C‐S phase diagram not yet sufficiently constrained. Plain Language Summary: Several geodetic and geochemical studies addressed the properties of the Moon's core, but its density remains poorly constrained. Core density is directly related to its chemical composition, which is crucial for better understanding Moon's origin and evolution. With carbon and sulfur being considered as two plausible light elements alloyed to iron to form the core, we selected a series of Fe‐C‐S compositions and studied the local structure and density of the corresponding liquids. The measured densities were integrated to build a thermodynamic model. Our results show that sulfur and carbon have a co‐effect on the density and sound velocity of the liquid alloys, with sulfur playing a much more significant role. Consequently, the sulfur content is relatively well constrained for a given density, which however largely differs from model to model. On the contrary, the constraints on C content remain loose because of the smaller effect of C on the density and the inadequate knowledge of the Fe‐C‐S ternary phase diagram. While specific C and S ranges can be pointed out under the hypothesis of a fully molten core, the compositional constraints on a partially molten core are limited by the lack of knowledge of Fe‐C‐S phase diagram at pertinent conditions. Key Points: We measured the local structure and density of liquid Fe‐C‐S alloys in the range of 1–5 GPa and 1600–1900 K A thermodynamic model based on asymmetric Margules formalism was built based on this data set We used this thermodynamic model to address three of the latest models of the Moon's core and to discuss the possible C and S content