Local Structure and Density of Liquid Fe‐C‐S Alloys at Moon's Core Conditions
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.
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