Abstract Methane-rich ices in the solar system and in interstellar environments undergo extensive chemical processing under cosmic-ray irradiation, leading to the formation of new species such as CHCH, C2H6, and C3H8. In Paper I, MNRAS 544, 855, 2025, we modeled the irradiation of CH4 ice with 15.7 MeV 16O5 + ions at 16 K using the PROCODA kinetic code, incorporating more than 1800 chemical reactions. In this work, we concentrate on the dominant chemical reaction responsible for the production and consumption of the hydrocarbons of interest. We find that C2H6 forms primarily through the reaction H2 + C2H5, while C3H8 originates mainly from H2 + C3H6. The radical CHCH is produced predominantly via the reaction C2H3 + R → H + CHCH. Radiation-driven processes dominate the early stages, while the post-relaxation chemistry is governed by neutral-neutral and hydrogenation reactions that control cumulative production. CHCH behaves as a short-lived intermediate due to its high effective consumption rates, with its destruction pathways shifting from CH4-dominated to H2-mediated channels over time. In contrast, C2H6 and C3H8 are more stable under energetic processing because of their comparatively lower consumption rates. Finally, effective rate coefficients (ERCs) and reaction enthalpies (ΔrH) are shown for each chemical reaction considered. Our results can provide constraints that can be directly incorporated into astrochemical models of cosmic-ray-processed ices.
Gerasimenko et al. (Thu,) studied this question.