2025MNRAS.542.2363G

During the star and planet formation, ultraviolet (UV) irradiation of ices on dust grain surfaces at low temperatures (${\sim} 10{\!-\!}$20 K) is considered an efficient pathway for interstellar complex organic molecules (iCOMs) in the inter- and circumstellar medium (ISM/CSM). In this work, a series of laboratory experiments was performed to investigate the UV photoprocessing of binary ice mixtures (CH$_4$:O$_2$) under the simulated astrophysical environments. A variety of newly formed products, e.g. CH$_2$O, CH$_3$OH, O$_3$, CO, CO$_2$, H$_2$O, H$_2$O$_2$, HCOOH, CH$_3$CH$_2$OH, and CH$_3$CHO, are observed and identified after UV irradiation. In addition, the photoproduction rates of new species and the photodestruction cross-sections of CH$_4$/CD$_4$ are also obtained. These experimental results provide key information for understanding the photochemical processing of CH$_4$- and O$_2$-containing ices under astronomically relevant conditions. Furthermore, the observation of some organic compounds (e.g. CH$_2$O and CH$_3$OH) and non-organic compounds (e.g. O$_3$ and H$_2$O) demonstrates that the ices of CH$_4$ and O$_2$ may play an essential role in the production of complex organic compounds and the evolution of circulated oxygen in space. Importantly, the formation reaction scheme of new species obtained here provides insight into the possible reaction pathways towards complex molecules observed from the ISM/CSM.