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

High-pressure experiments on the Fe-H system reveal that hydrogen is significantly soluble in liquid iron even at low pressures, with a solubility of approximately 1.2 wt% under lunar core conditions that could fully account for the observed density deficit of the Moon's core.

The Gist

Imagine the Moon as a giant, cooling chocolate truffle. Inside that truffle is a gooey, molten center (the core) made mostly of iron. For decades, scientists have been trying to figure out exactly what's mixed into that iron to make it lighter than pure iron, because the Moon's core is surprisingly "fluffy" for its size.

Usually, scientists thought the "lightening agents" were things like sulfur or carbon. But there was a secret ingredient they largely ignored: Hydrogen.

Why? Because the old rulebook said, "Hydrogen can't get into iron unless you squeeze it really, really hard (above 3 GPa)." Since the Moon's core isn't that deep, everyone assumed hydrogen was locked out, like a bouncer refusing entry to a VIP club.

This paper flips the script. The researchers, led by Takeshita and Hirose, decided to test that rulebook at lower pressures, closer to what the Moon actually experiences. Here is what they found, explained simply:

1. The "Melting Point" Surprise

Think of iron as a solid block of ice. To turn it into a liquid (magma), you need heat.

• The Old Belief: If you add hydrogen to iron at low pressures, it doesn't do much. The ice melts at the same temperature.
• The New Discovery: The researchers found that even at relatively low pressures (like the top of the Moon's core), adding hydrogen acts like salt on a winter sidewalk. It drastically lowers the temperature at which iron melts.
• The Result: Iron with hydrogen melts much earlier than pure iron. This means hydrogen is happily mixing into the liquid iron even at pressures we previously thought were too low.

2. The "Sponge" Effect

Imagine liquid iron as a sponge.

• Old View: The sponge is dry and hard to soak at low pressure. You need massive pressure to force the water (hydrogen) in.
• New View: The sponge is actually quite thirsty. As you squeeze the system (increase pressure), the sponge opens up and soaks up more hydrogen.
• The Numbers: At the bottom of the Moon's core, the liquid iron can hold about 1.2% hydrogen by weight. That might sound small, but in the world of planetary physics, that's a massive amount of "fluff."

3. Solving the "Missing Mass" Mystery

Here is the big picture:

• The Problem: The Moon's core is less dense than pure iron. It's too light. Scientists needed to find enough "light stuff" to explain this gap.
• The Solution: That 1.2% of hydrogen acts like a giant balloon inside the iron. It expands the volume without adding much weight, making the whole core less dense.
• The Verdict: The researchers calculated that this amount of hydrogen alone is enough to explain the Moon's core density (based on the most recent seismic data). It's like finding out the missing weight in a suitcase was just a few large, hollow balloons all along.

Why Does This Matter?

This changes how we see the Moon's history.

• The "Gas Blanket": When the Moon was young, it was covered in a thick atmosphere of gas from the collision that created it. This gas was full of water vapor.
• The "Soaking" Phase: As the Moon's surface was a global ocean of molten rock (a Magma Ocean), that water broke down into hydrogen. Because hydrogen loves iron, it sank down and got absorbed into the forming core, just like our "thirsty sponge."
• The Takeaway: The Moon's core isn't just iron and sulfur; it's likely an iron-hydrogen alloy. This suggests that small rocky bodies in our solar system might all have hydrogen-rich cores, changing our understanding of how planets form and evolve.

In a nutshell: The Moon's core is lighter than we thought because it's "watered down" with hydrogen. We just didn't realize the iron was thirsty enough to drink it up at such low pressures. This discovery turns the Moon's core from a simple iron ball into a complex, hydrogen-rich cocktail.

Technical Summary

1. Problem Statement

The internal structure of the Moon, particularly its core, presents a significant geophysical puzzle. Seismic and geodetic data indicate that the lunar core has a density 4% to 40% lower than that of pure iron. This "density deficit" necessitates the presence of light elements (such as Sulfur, Carbon, or Hydrogen) alloyed with the iron.

While Sulfur and Carbon are commonly considered, Hydrogen has historically been excluded from models of small terrestrial bodies (like the Moon) for two main reasons:

1. Thermodynamic Assumption: It was assumed that hydrogen is negligibly soluble in iron at pressures below ~3 GPa.
2. Phase Stability: Stoichiometric iron hydride (FeH) was thought to form only above ~3.5 GPa at room temperature.

Consequently, previous core-formation models assumed hydrogen was not incorporated into the cores of small rocky bodies. However, the lack of experimental data on the Fe-H melting curve below 3 GPa left a critical gap in understanding whether hydrogen could exist in the lunar core under its specific pressure-temperature (P-T) conditions.

2. Methodology

The authors conducted high-pressure and high-temperature experiments using a Laser-Heated Diamond Anvil Cell (DAC) combined with Synchrotron X-ray Diffraction (XRD) at the SPring-8 facility.

• Sample Preparation: A pure iron foil (~20 µm) was loaded into a Re gasket with KCl and a ruby ball (pressure markers). The sample chamber was loaded with supercritical hydrogen gas ($H_2$) under saturated conditions.
• Pressure-Temperature Range: Experiments were conducted at pressures ranging from 1.0 to 3.6 GPa and temperatures up to 1710 K.
• Detection of Melting:
  • XRD Peaks: Used to identify solid phases (specifically face-centered cubic, fcc, $FeH_x$).
  • Diffuse Scattering: The appearance of a diffuse scattering signal in XRD patterns was used as the primary indicator of the onset of melting (liquid phase).
• Density and Composition Determination:
  • Solid Phase: Hydrogen concentration ($x$ in $FeH_x$) in the solid phase was calculated based on the volume expansion of the fcc lattice relative to pure iron.
  • Liquid Phase: Since liquid quenches to a hydrogen-free bcc phase, direct chemical analysis was impossible. Instead, the authors calculated liquid density from the diffuse scattering signal using a method developed by Kuwayama et al. (2020). The hydrogen content in the liquid was then derived by comparing the liquid density to that of pure liquid iron, accounting for the volume expansion caused by hydrogen atoms ($\Delta V_H \approx 2.22 \, \text{Å}^3$).

3. Key Results

A. Melting Curve Depression

The study successfully mapped the Fe-H melting curve (solidus) from 1.0 to 3.3 GPa.

• Monotonic Depression: The melting temperature of the Fe-H system decreases monotonically as pressure increases from ambient to ~3 GPa.
• Magnitude: The depression relative to pure iron is significant:
  • ~240 K at 1 GPa
  • ~450 K at 2 GPa
  • ~580 K at 3 GPa
• Correction of Previous Models: This contradicts earlier hypotheses that suggested a sudden drop in melting temperature only above ~2 GPa due to a phase transition from body-centered cubic (bcc) to fcc. The results show that liquid Fe-H coexists with fcc $FeH_x$ and melting is depressed even at 1.0 GPa.

B. Hydrogen Solubility

The experiments demonstrated that hydrogen solubility in liquid iron increases with pressure even below 3 GPa.

• Concentration: The hydrogen mole fraction ($x$) in liquid iron increased from 0.14 at 1.0 GPa to 0.48 at 3.6 GPa.
• Extrapolation to Lunar Core: Based on the linear trend, the authors estimate the hydrogen solubility limit in liquid iron at lunar core conditions:
  • ~0.7 wt% H at 2.8 GPa (base of the Lunar Magma Ocean).
  • ~1.2 wt% H at 5 GPa (core-mantle boundary).

C. Density Implications

• A hydrogen content of 1.2 wt% causes a 9% reduction in the density of liquid iron.
• This density reduction aligns with the lower density estimates of the lunar core derived from recent seismic models (Kuskov et al., 2021: 6200–7000 kg/m³).
• While this amount of hydrogen alone may not fully explain the lower density estimates from earlier studies (Garcia et al., 2019; Viswanathan et al., 2019), it is sufficient to explain the deficit within the uncertainty of the Kuskov et al. model.

4. Discussion and Implications for the Lunar Core

The authors integrate their experimental data with planetary formation models to propose a new scenario for the lunar core:

1. Hydrogen Incorporation: During the formation of the Lunar Magma Ocean (LMO), the Moon was likely surrounded by a protolunar disk rich in water vapor. As the LMO crystallized, water could have dissociated, allowing hydrogen to partition into the segregating metallic core.
2. Partitioning: Using metal-silicate partition coefficients ($D_{H}^{metal/silicate}$) from recent studies (Tsutsumi et al., 2025), the authors calculate that if the LMO contained 300–1000 ppm $H_2O$, the resulting lunar core could contain 0.3–1.2 wt% hydrogen.
3. Primary Light Element:
   • Sulfur and Carbon: Previous studies suggest their solubility in solid iron at 5 GPa is limited (<0.5 wt% S, <1.5 wt% C). If the lunar core has a solid inner core, S and C cannot fully explain a density deficit in the solid phase.
   • Hydrogen: Unlike S and C, hydrogen is highly soluble in both liquid and solid iron at these pressures. The study suggests that hydrogen is likely a major light element in the lunar core, potentially explaining the density deficit of both the liquid outer core and the solid inner core.

5. Significance

• Paradigm Shift: This study overturns the long-standing assumption that hydrogen is insoluble in iron cores of small terrestrial bodies below 3 GPa.
• Revised Lunar Models: It provides a viable mechanism for the presence of significant hydrogen in the Moon's core, offering a unified explanation for the core's density deficit that does not rely solely on Sulfur or Carbon.
• Methodological Advance: The successful use of diffuse scattering to determine liquid density and composition in the Fe-H system at low pressures sets a precedent for future high-pressure metallurgy and planetary science experiments.
• Broader Context: These findings suggest that hydrogen may be a critical, yet previously overlooked, component in the cores of other small planetary bodies (e.g., Mercury, Mars, or exoplanets) where pressures are below 3 GPa.