Hydrogen Made the Moon's Core Lighter ⚡ экспресс
Experiments on melting of the Fe–H system under hydrogen saturation and high pressures were performed with synchrotron X-ray diffraction. A significant depression of the Fe–H melting curve relative to pure iron was found at 1.0–3.3 GPa, indicating incorporation of hydrogen into liquid iron already below 1 GPa and increasing solubility with pressure. From diffuse X-ray scattering, the density of Fe–H melt was calculated: at 3.6 GPa it corresponds to a solubility of ~0.9 wt% H, and at 5 GPa (lunar core conditions) to ~1.2 wt%. Such hydrogen content provides a 9% density reduction and can fully explain the density deficit of the lunar core observed relative to pure iron, depending on seismic density estimates.
Previously it was thought that hydrogen barely mixes with liquid iron at low pressure, so it wasn't expected in the Moon's core. New experiments have flipped that view. Under the pressure conditions of the lunar interior, hydrogen readily seeps into the melt, changes the internal atomic order (entropy), and lowers the melting point — like a leavening agent making the metal more fluid and lighter.
At the pressure of the Moon's core (about 5 GPa), iron absorbs up to 1.2% hydrogen. That may seem like a drop in the ocean, but it's enough to reduce density by 9%. This is exactly the missing amount needed to explain data on the Moon's vibrations. The core resembles a soda: hydrogen, like invisible bubbles, makes it less dense than a solid metal.
This unexpected effect works beyond the Moon. The same impurity likely explains the anomalously light core of Mercury. The finding forces us to rethink the interiors of small planets, including distant exoplanets, where hydrogen could be hiding in iron cores, altering their properties.
🎯 The Moon isn't the only one: Mercury also has an unusually light core, and hydrogen might be hiding there too.