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

TL;DR

This study reveals that hydrogen can be incorporated into liquid iron at pressures below 3 GPa, significantly affecting the density of lunar core material and challenging previous assumptions about hydrogen's role in small planetary cores.

Contribution

The paper provides experimental evidence of hydrogen solubility in liquid iron at low pressures and its implications for lunar core composition and density.

Findings

Hydrogen solubility in liquid iron is about 0.9 wt% at 3.6 GPa.

Hydrogen solubility increases to approximately 1.2 wt% at 5 GPa.

Hydrogen presence can explain the lunar core's density deficit.

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.