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arXiv · 2604.12222

Fe-H melting curve below 3 GPa: Implications for hydrogen in the lunar core

Jun Takeshita·Kei Hirose·Suyu Fu·Fumiya Sakai·Koutaro Hikosaka
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Abstract

It has been assumed that hydrogen is negligibly incorporated into core-forming metals below $\sim$3 GPa, and therefore the presence of hydrogen in iron cores of small terrestrial bodies including the moon has not been considered. Here we performed high-pressure melting experiments on the Fe-H system under H$_2$-saturated conditions, combined with synchrotron X-ray diffraction (XRD) measurements. Results demonstrate substantial depression of the Fe-H melting curve compared to that for Fe at 1.0-3.3 GPa, indicating that hydrogen is incorporated into liquid iron even at low pressures less than 1 GPa and the solubility is enhanced with increasing pressure. Based on the density of liquid Fe-H derived from diffuse scattering signal in XRD data, we found that the solubility of hydrogen in liquid iron is about 0.9 wt% at 3.6 GPa and likely enhanced to 1.2 wt% at 5 GPa corresponding to lunar core conditions. The 1.2 wt% H causes 9 % density reduction, which might fully explain the observed density deficit of the lunar core with respect to iron, depending on the density estimate from seismological data.

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Jun Takeshita, Kei Hirose, Suyu Fu, Fumiya Sakai, Koutaro Hikosaka. 2026-04-14. Fe-H melting curve below 3 GPa: Implications for hydrogen in the lunar core. https://arxiv.org/abs/2604.12222

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