Methanol Formation in Hyperthermal Oxygen Collisions with Methane Clathrate Ice

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_<i>x</i>O, <i>x</i> = 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.