Hydrogen Made the Moon's Core Lighter ⚡ экспресс
It was previously believed that hydrogen does not enter the metallic cores of small planets like the Moon at pressures below ~3 GPa. However, melting experiments on the iron–hydrogen system under hydrogen saturation and synchrotron X-ray measurements showed the opposite: hydrogen dissolves in liquid iron even at pressures below 1 GPa, and its content increases with pressure. At lunar core conditions (~5 GPa), solubility reaches ~1.2 wt%, reducing density by 9%. This neatly explains the puzzling density deficit of the lunar core relative to pure iron, previously identified by seismologists.
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.