Methanol Formation in Hyperthermal Oxygen Collisions with Methane Clathrate Ice

Creators: Grayson, Robert W.¹; Giapis, Konstantinos P.¹; Goddard, William A.¹

1. California Institute of Technology

Abstract

The presence of small organic molecules at airless icy bodies may be significant for prebiotic chemistry, yet uncertainties remain about their origin. Here, we consider the role of hyperthermal reactive ions in modifying the organic inventory of ice. We employ molecular dynamics using the ReaxFF formalism to simulate bombardment of carbon-bearing ice by hyperthermal water group molecules (H_xO, x = 0–2) with kinetic energy between 2 and 58 eV. Methanol is the dominant closed-shell organic product for a CH₄ clathrate irradiated at low dose by atomic oxygen. It is produced at yields as high as 10%, primarily by a novel hot-atom reaction mechanism, while radiolysis makes a secondary contribution. At high irradiation doses (≳1.4 × 10¹⁵ cm^–2), the composition is driven toward greater carbon oxidation states with formaldehyde being favored over methanol production. Other water group impactors are less efficient at inducing chemistry in the ice, and alternate clathrate guest species (CO, CO₂) are very robust against hydrogenation.

Copyright and License

This publication is licensed under CC-BY-NC-ND 4.0.

Acknowledgement

We would like to thank Tom Nordheim (JHU APL), Zhen-Gang Wang (Caltech), and Mitchio Okumura (Caltech) for reviewing RWG’s dissertation, from part of which this manuscript is adapted. W.A.G. acknowledges support from JPL (Fund IA-1642096- Hypervelocity Sampling Across the Solar System) and from NSF (CBET 2311117). K.P.G. and R.W.G. acknowledge support from JPL (Fund IA-1653621- PDRDF: Plasma-surface interactions as a way to probe the composition of icy worlds).

Contributions

R.W.G. designed, executed, and analyzed simulations. K.P.G. and W.A.G. organized and supervised the work. All authors discussed results, contributed to manuscript writing, and approved the final version of the manuscript.

Supplemental Material

Supplementary figures, including visualizations of simulated surfaces, and force field parameters (PDF).

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