X-ray versus Ultraviolet Irradiation of Astrophysical Ice Analogs Leading to Formation of Complex Organic Molecules

In astrochemistry, complex organic molecules (COMs) are defined as species with at least one C atom and six or more atoms in total. More than 70 COMs were detected toward various interstellar and circumstellar regions. With the exception of methanol, CH₃OH, these COMs were only detected in the gas phase. But their sole existence challenges the scheme of gas-phase reactions and suggests a formation in ice covered dust grains or the ice-gas interface. Some COMs can be synthesized by surface reactions on dust, and other COMs seem to be indicative of energetic processing in the ice mediated by photons and cosmic rays. This work focuses on the formation of COMs in the ice driven by X-rays and ultraviolet photons. While the former should drive the chemistry in circumstellar environments of young stars, the latter start to dominate after about 100 Myr (million years) and are responsible for ice processing in interstellar dust. On the basis of our X-ray experiments of ice irradiation reported in the literature and new experiments on UV irradiation of ice samples of analog composition, a comparison of X-rays versus ultraviolet photon ice processing is presented here. Using infrared spectroscopy for the ice and mass spectrometry for the gas phase, pure/binary/ternary ice mixtures containing H₂O, CO, or NH₃ were studied. A wealth of COMs is made in the experimental simulations of ice analogs by both X-rays and UV photons. Among the COMs that result from ice photoprocessing are some of astrobiological interest that were confirmed in the UV irradiation of the H₂O:CO:NH₃ ice mixture, such as NH₂CH₂COOH (glycine), HCONH₂ (formamide), or CH₃CHO (acetaldehyde), similar to those detected during the Rosetta mission in comet 67P/Churyumov-Gerasimenko.