The Fate of Hydrogen and Helium: From Planetary Embryos to Earth- and Neptune-like Worlds

Abstract

Hydrogen, helium, silicates, and iron are key building blocks of rocky and gas-rich planets, yet their chemical interactions remain poorly constrained. Using first-principles molecular dynamics and thermodynamic integration, we quantify hydrogen and helium partitioning between molten silicate mantles and metallic cores for Earth-to-Neptune-mass planets. Hydrogen becomes strongly siderophilic above $\sim$25 GPa but weakens beyond $\sim$200 GPa, whereas helium remains lithophilic yet increasingly soluble in metal with pressure. Incorporating these trends into coupled structure-chemistry models suggests that majority of hydrogen and helium reside in planetary interiors, not atmospheres, with abundances strongly depending on planet mass. Such volatile exchange may influence the redox states of secondary atmospheres, longevity of primordial envelopes, predicted CHNOPS abundances, and emergence of helium-enriched atmospheres, while He 1083 nm and H Lyman-$α$ lines provide potential probes of atmosphere-interior exchange. These findings link atomic-scale interactions to planetary-scale observables, providing new constraints on the origins of Earth-to-Neptune-sized worlds.

Figures (9)

• Figure 1: Partition coefficient for hydrogen between silicate and metallic melts, $D_{\mathrm{H}}^\mathrm{met/sil}$ as a function of temperature and pressure. Each point represents a unique $\{P, T\}$ condition. Circles denote thermodynamic-integration ab initio molecular dynamics (TI-AIMD) results from this study with $x_{\mathrm{H}}^{\mathrm{sil}} \lesssim 0.2$, while the square indicates a two-phase AIMD (2P-AIMD) calculation at $x_{\mathrm{H}}^{\mathrm{sil}} \sim 0.42$. Other symbols show experimental tagawa2021amalavergne2019aclesi2018aokuchi1997a and computational data li2020a from previous work. The horizontal gray line separates the siderophile (iron-loving) and lithophile (rock-loving) regimes, illustrating that hydrogen's strongly siderophilic nature at high temperatures and pressures.
• Figure 2: Partition coefficient for helium between silicate and metallic melts, $D_{\mathrm{He}}^\mathrm{met/sil}$ as a function of temperature and pressure. Each point represents a unique $\{P, T\}$ condition. As in Figure \ref{['fig:partition_coeff__H']}, circles denote thermodynamic-integration ab initio molecular dynamics (TI-AIMD) results from this study with $x_{\mathrm{He}}^{\mathrm{sil}} \lesssim 0.2$, while the square indicates a two-phase AIMD (2P-AIMD) calculation at $x_{\mathrm{He}}^{\mathrm{sil}} \sim 0.49$. Other symbols show experimental bouhifd2013amatsuda1993a and computational data li2022azhang2012axiong2021awang2022a from previous work. The horizontal gray line separates the siderophile (iron-loving) and lithophile (rock-loving) regimes, demonstrating that He remains largely lithophilic, with an increasing affinity towards iron at higher pressures.
• Figure 3: Composite figure illustrating the contrasting equilibrium partitioning of H and He as a function of planet mass. The right panels display the distribution of the total H (top) and He (bottom) budgets among atmosphere, mantle, and core. The central panel depicts the compositional makeup of these distinct phases. The left panel shows the H-to-He mass ratios across these phases, with the pink line indicating the Solar reference value. The results indicate that hydrogen is sequestered in planetary cores at low masses but shifts to the atmosphere for Neptune-like planets. In contrast, helium resides primarily in mantles, with the core becoming an increasingly important reservoir at higher masses.
• Figure 4: Illustration of atmosphere–mantle–core coupling across rocky embryos to gas-rich planets. Mantle convection acts as a planetary-scale “conveyor belt,” facilitating chemical communication between the atmosphere and the core. The efficiency of this volatile cycling---potentially probed by the He 1083 nm feature, especially when combined with H Lyman-$\alpha$---depends on the partition coefficients derived in this study. It governs the redistribution of H and He, the timescale of primordial atmosphere retention, aspects of redox balance in secondary atmospheres, and model predictions for the abundances of atmospheric CHNOPS species, ultimately shaping the H-to-He ratio in the atmospheres of Earth- to Neptune-like planets.
• Figure 5: Partition coefficient for hydrogen between silicate and metallic melts, $D_{\mathrm{H}}^{\mathrm{met/sil}}$, as a function of temperature, with pressure encoded by color. This figure complements Figure \ref{['fig:partition_coeff__H']} by highlighting the temperature dependence of $D_{\mathrm{H}}^{\mathrm{met/sil}}$ across a range of pressures. Each point corresponds to a unique ${P,T}$ condition. Circles denote thermodynamic-integration ab initio molecular dynamics (TI-AIMD) results from this study with $x_{\mathrm{H}}^{\mathrm{sil}} \lesssim 0.2$, and the square marks a two-phase AIMD (2P-AIMD) calculation at $x_{\mathrm{H}}^{\mathrm{sil}} \sim 0.42$. Other symbols represent experimental and computational data from previous work. The horizontal gray line separates siderophile (iron-loving) and lithophile (rock-loving) regimes, underscoring that hydrogen remains strongly siderophilic with only a weak dependence on temperature.