Molecular Mobility of Extraterrestrial Ices: Surface Diffusion in Astrochemistry and Planetary Science

TL;DR

This Perspective highlights diffusion of molecules on extraterrestrial ices as a critical driver of chemical evolution across interstellar and planetary environments. It surveys how diffusion is treated in astrochemical models, reviews experimental analogues and diffusion measurements, and surveys computational strategies—from MD and DFT to metadynamics and kinetic Monte Carlo—while emphasizing the role of surface complexity and low temperatures. The authors underscore substantial uncertainties in diffusion barriers, pre-exponential factors, and surface-structure effects, and advocate for interdisciplinary efforts incorporating new experiments and machine-learning approaches to constrain parameters. By improving diffusion parameterization, the work aims to enhance predictive power for astrochemical networks, linking molecular evolution from molecular clouds to planetary surfaces and informing origins of life and planetary habitability.

Abstract

Molecules are ubiquitous in space. They are necessary component in the creation of habitable planetary systems and can provide the basic building blocks of life. Solid-state processes are pivotal in the formation of molecules in space and surface diffusion in particular is a key driver of chemistry in extraterrestrial environments, such as the massive clouds in which stars and planets are formed and the icy objects within our Solar System. However, for many atoms and molecules quantitative theoretical and experimental information on diffusion, such as activation barriers, are lacking. This hinders us in unraveling chemical processes in space and determining how the chemical ingredients of planets and life are formed. In this article, an astrochemical perspective on diffusion is provided. Described are the relevant adsorbate-surface systems, the methods to model their chemical processes, and the computational and laboratory techniques to determine diffusion parameters, including the latest developments in the field. While much progress has been made, many astrochemically relevant systems remain unexplored. The complexity of ice surfaces, their temperature-dependent restructuring, and effects at low temperatures create unique challenges that demand innovative experimental approaches and theoretical frameworks. This intersection of astrochemistry and surface science offers fertile ground for physical chemists to apply their expertise. We invite the physical chemistry community to explore these systems, where precise diffusion parameters would dramatically advance our understanding of molecular evolution in space - from interstellar clouds to planetary surfaces - with implications on our understanding on the origins of life and planetary habitability.

Figures (4)

• Figure 1: Various examples of surface diffusion and related processes taking place in condensed phases in space. Each panel illustrates a distinct diffusion mechanism: Langmuir-Hinshelwood (top left) shows reactants diffusing before meeting; hot atom diffusion (top right) depicts energetic species moving rapidly across surfaces; non-diffusive reactions (bottom left) occur only requiring serendipitous proximity; and segregation (bottom right) shows separation of different molecular species in mixed ices. Image adapted from cuppen2024laboratory.
• Figure 2: The $^{13}$CO$_{2}$ asymmetric stretch vibrational mode observed in interstellar ice toward two different sources: HOPS153 (mixed/unsegregated ice) and IRAS20216 (segregated ice). The distinct spectral profiles reveal that segregation—a process driven by molecular diffusion—serves as a clear indicator of ice warming. Different molecular components (colored lines) contribute to the overall observed spectrum (black line). Data adapted from brunken2024jwst.
• Figure 3: Overview of experimentally determined diffusion energies for various atoms and molecules on different surfaces in K (left y-axis) and meV (right y-axis). The bar colour indicates the diffusing species, which are grouped in the categories ASW = Amorphous Solid Water, CO = carbon monoxide, and O-HOPG = oxygenated highly oriented pyrolytic graphite. We note that a relatively large amount of data is available for diffusion on water-ice, but data for carbonaceous, siliceous, and other ice surfaces is scarce, as the plot underlines. Various subdivision of the ice surfaces are made based on their porosity (non-porous, porous, compact), density (low, high), temperature, and depth of binding sites (deep, shallow), which drive the differences in diffusion barriers. Data is taken from M01manico2001laboratory (ASW), P05perets2005molecular (low and high density ASW), M08matar2008mobility (porous ASW), W10watanabe2010direct (shallow ASW), M13minissale2013quantum (ASW), M16minissale2016direct (compact ASW, O-HOPG), K18kimura2018measurements (8, 12, 15K CO), K20kouchi2020direct (porous ASW), F22furuya2022diffusion (compact ASW), M22miyazaki2022direct (ASW), T23tsuge2023surface (non-porous ASW).
• Figure 4: Result from a metadynamics simulation of the diffusion of a CO2 molecule over an ASW surface. The surface mesh indicates the van-der-Waals surface of the ASW surface while the color coding represents the interaction free energy of the CO2 with the surface at 30 K. The most stable locations coincide with the deeper areas on the surface. Diffusion pathways between sites avoid surface protrusions.