Diffusion of CH$_4$ in amorphous solid water

TL;DR

This study measures methane diffusion coefficients in amorphous solid water ice at low temperatures, showing how ice morphology influences diffusion rates and highlighting the scale-dependent applicability of Fick's law and Monte Carlo simulations.

Contribution

The paper provides new experimental diffusion coefficients for CH$_4$ in amorphous solid water and analyzes how ice structure affects diffusion, combining experiments with Monte Carlo modeling.

Findings

Diffusion coefficients range from 10$^{-12}$ to 10$^{-13}$ cm$^2$/s between 42 K and 60 K.

Diffusion varies by an order of magnitude depending on ice morphology.

Porosity and pore coalescence enhance diffusion within the ice.

Abstract

Context. The diffusion of volatile species on amorphous solid water ice affects the chemistry on dust grains in the interstellar medium as well as the trapping of gases enriching planetary atmospheres or present in cometary material. Aims. The aim of the work is to provide diffusion coefficients of CH$_4$ on amorphous solid water (ASW), and to understand how they are affected by the ASW structure. Methods. Ice mixtures of H$_2$O and CH$_4$ were grown in different conditions and the sublimation of CH$_4$ was monitored via infrared spectroscopy or via the mass loss of a cryogenic quartz crystal microbalance. Diffusion coefficients were obtained from the experimental data assuming the systems obey Fick's law of diffusion. Monte Carlo simulations modeled the different amorphous solid water ice structures investigated and were used to reproduce and interpret the experimental results. Results. Diffusion coefficients of methane on amorphous solid water have been measured to be between 10$^{-12}$ and 10$^{-13}$ cm$^2$ s$^{-1}$ for temperatures ranging between 42 K and 60 K. We showed that diffusion can differ by one order of magnitude depending on the morphology of amorphous solid water. The porosity within water ice, and the network created by pore coalescence, enhance the diffusion of species within the pores.The diffusion rates derived experimentally cannot be used in our Monte Carlo simulations to reproduce the measurements. Conclusions. We conclude that Fick's law can be used to describe diffusion at the macroscopic scale, while Monte Carlo simulations describe the microscopic scale where trapping of species in the ices (and their movement) is considered.