Molecular Evolution of H ₂ O:O ₂ Ices at Different Temperatures in Simulated Space Environments. II. Evolution of Reaction Pathways

Abstract In this continuation of our study on the chemical evolution of H 2 O:O 2 ices (6:1 ratio) under analog cosmic-ray irradiation (0.8 MeV H + ) at 9, 50, and 100 K, we investigate changes in the reaction pathways that influence the formation and destruction of species. Using experimental data and the PROCODA code, we identify the most favorable reaction channels. In this work (1) we want to show for the selected species which responses are dominant (in consumption and production) in the chemical equilibrium (CE) phase, which is the stationary phase of astrophysical ices; (2) we show that the influence of temperature alters the dominance of some reactions; and (3) we show that the dominance of the responses varies with the time of exposure to radiation (dose) until reaching CE. We believe that this shift in dominance in chemical kinetics is associated with changes in the chemical environment of irradiated ices. Our results indicate that variations in the chemical environment, driven by differential exposure to ionizing radiation, govern the kinetics and dominant pathways of ice chemistry, highlighting the importance of the irradiation phase in shaping the chemical evolution of astrophysical ices. The findings provide insights into the chemical dynamics of astrophysical ices, establishing a connection between these ices and the composition of ices found on celestial bodies and in cold interstellar regions.