ABSTRACT

Diffusivity and solubility of gas molecules in crystalline water ice are fundamental parameters to understand physicochemical processes in various environments in space, icy bodies, planets, and the Earth. For instance, diffusion and solubility of methane in ice Ih could constrain the methane flux from sub-surface Martian ice, and could be used to discuss diffusive elemental fractionation of gases trapped in polar ice cores that could disturb the reconstruction of the Earth’s paleoatmosphere. The diffusion coefficient and the solubility of methane in ice Ih, however, have not yet been directly determined. In this study, we performed diffusive degassing experiments of helium and methane from ice Ih at 251–259 K to measure both the diffusion coefficient and solubility of methane in ice Ih. We first determined the diffusion coefficient and solubility of helium in ice Ih, and could successfully reproduce those reported in previous studies. We then determined the diffusion coefficient of methane in ice Ih, and the obtained diffusion coefficient of methane at 257 K was (5.2 ± 1.4) × 10⁻¹¹ m² s⁻¹. This is well consistent with that estimated by a molecular-dynamics simulation, where methane molecules do not diffuse through interstitial sites but by breaking hydrogen bonds between H₂O molecules. The methane solubility in ice Ih was estimated to be (7.2 ± 1.1) × 10⁻⁷ mol mol_H2O⁻¹ MPa⁻¹ at 257 K, which is smaller than those of He and Ne and could be due to a larger van der Waals radius of methane than those of light noble gases.