Molecular Evolution of H₂O:O₂ Ices at Different Temperatures in Simulated Space Environments. I. Chemical Kinetics and Equilibrium

Abstract We computationally investigated the chemical evolution of H 2 O:O 2 ices (6:1 ratio) under irradiation by cosmic-ray analogs (0.8 MeV H + ) at 9, 50, and 100 K to understand the implications the chemical evolution of O 2 -containing ices in space, such as the surface of the Moon, comets, outer solar system bodies such Europa and Enceladus, as well as Kuiper Belt objects, and cold regions of the interstellar medium (ISM). Using experimental data and the PROCODA code with 200 reactions coupled equations involving 12 species, we calculated physicochemical parameters such as effective rate coefficients (ERCs), chemical abundances, and desorption. Six species were observed experimentally (H 2 O, O 2 , HO 2 , H 2 O 2 , O 3 , and HO 3 ), while six were predicted but not observed in the experiments (H, H 2 , H 3 , O, OH, and H 3 O). Our findings highlight the influence of temperature on chemical equilibria and desorption yields, with certain reaction rates diminishing at 50 K. Among the results were the lists with the ERCs, and the reaction branching ratio obtained by best-fit models can be employed in astrochemical models. Curiously, we observe that the average ERCs for bimolecular collisions decrease by half as the ice temperature increases, varying from 5.8e-25 to 2.9e-25 cm 3 molecules −1 s −1 for the ices studied. These results enhance our understanding of the physical chemistry of astrophysical ices under ionizing radiation, providing valuable data for astrochemical models that assess the effects of cosmic radiation on the composition and stability of icy bodies in the solar system and denser and colder regions of the ISM.