The center of the Moon is "missing" some weight, and it turns out hydrogen is the culprit.
April 16, 2026
Original Paper
Fe-H melting curve below 3 GPa: Implications for hydrogen in the lunar core
arXiv · 2604.12222
The Takeaway
We've long struggled to explain why the Moon's core is less dense than a ball of pure iron should be. This paper reveals that at the relatively low pressures found inside the Moon, hydrogen actually "dissolves" into liquid iron, lowering its melting point and making the core lighter. Everyone assumed hydrogen was too "flighty" to be trapped in a small planetary core, but it's actually a key ingredient. This means the Moon isn't just a dry rock; it has a "wet" heart. Understanding this helps us solve the mystery of how the Moon formed and what it’s actually made of deep inside.
From the 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