An experimental investigation into the interstellar sulfur chemistry

Abstract

The disparity between predicted sulfur abundances and identified reservoirs of sulfur in cold molecular clouds, also known as the sulfur depletion problem, has remained an ongoing debate for decades. Gas-phase formation mechanisms alone cannot account for the complexity or the observed number densities of the majority of the molecules observed in the interstellar medium (ISM); therefore, a detailed understanding of how these molecules are formed in interstellar environments is important. This thesis studies radiation-driven processes that can occur in interstellar analog ices of hydrogen sulfide (H2S)—the major sulfur-containing species observed in comets—linking the molecules observed in the ISM and planetary bodies in our solar system to a past in interstellar ices. Interstellar analog ices—pristine hydrogen sulfide (H2S) or mixtures of methane (CH4) and hydrogen sulfide (H2S)—were exposed to proxies of Galactic cosmic rays (GCRs) in the form of energetic electrons released in the GCR track in interstellar ices simulating typical cold molecular cloud lifetimes of a few 106 to 107 years. Here, sulfanes (H2Sn; n = 2-11) and cyclic octasulfur (S8) were identified as two possible sulfur inventories in the cold molecular clouds. The isomeric identification of octasulfur rings (S8) coincides with the recent identification of elementary sulfur in the carbonaceous asteroid (162173) Ryugu, thus providing compelling evidence on the link between sulfur in cold molecular clouds and in our Solar System. Abiotic formation pathways to the organosulfur molecules methanethiol (CH3SH), ethanethiol (C2H5SH), and dimethyl sulfide (CH3SCH3) were identified in the binary ices of methane–hydrogen sulfide (CH4−H2S), in contrast to the existing misconception of sole biotic origin of these molecules, with the awarded designation as biosignatures of life in exoplanets.