title: On the Astrochemistry of Sulphur Molecules in Interstellar and Solar System Ice Analogues
Creators: name: Mifsud, Duncan
Publisher: name: University of Kent
Year: 2023

description: Sulphur is the tenth most abundant element in the universe and is known to play many important roles in biochemical, geochemical, and atmospheric processes. However, the astrochemistry of sulphur presents many as yet unresolved problems. In the dense interstellar medium, for instance, there exists a notable paucity of sulphur in both the gas phase as well as within interstellar icy grain mantles, compared to the expected cosmic abundance of the element. Moreover, no conclusive detection of solid interstellar H2S has yet been made, despite the known efficiency of hydrogenation reactions on the surface of interstellar dust grains. Within the Solar System, solid SO2 is known to be a component of many icy bodies, including the Galilean moons of Jupiter and several comets, but the mechanism leading to its formation still largely eludes contemporary astronomers, chemists, and spectroscopists. This thesis probes a number of questions relating to sulphur ice astrochemistry in both interstellar and Solar System environments. It begins with a review of the current state of knowledge of extra-terrestrial sulphur chemistry, before delving into a broader discussion of the chemical characteristics of the cosmos and a brief history of astrochemistry as an independent field of research. A background to the relevant scientific concepts and principles routinely used in laboratory astrochemistry then follows, which includes detailed discussions on molecular structure and symmetry, spectroscopy (with an emphasis on measurements made in the mid-infrared spectral range), and radiation chemistry. A thorough description of a new experimental facility for ion and electron irradiation studies of astrophysical ice analogues based at the Institute for Nuclear Research (Atomki) is provided, along with various experiments performed to validate the set-up. The results of systematic studies on the mid-infrared absorption spectra of H2S and SO2 astrophysical ice analogues, both pure and mixed with H2O, are then presented and discussed in the context of their applicability to the detection of these sulphur-bearing species in different astrophysical environments. The results of the first ever systematic comparisons of the radiation chemistry and physics of the amorphous and crystalline phases of a number of pure astrophysical ice analogues, including some that bear sulphur atoms, are described in the subsequent chapter. The observed greater radiolytic decay rates of amorphous ices and the more rapid formation of molecular products as a result of their irradiation is discussed both in the context of the differences in the strength and extent of the intermolecular interactions between the amorphous and crystalline phases, as well as in light of the recent discoveries of complex organic molecules in astrophysical environments in which space radiationinduced amorphisation is thought to out-compete thermal crystallisation of ices. The results of experiments investigating the implantation of high-energy sulphur ions into oxygen-bearing ices as a potential mechanism towards the formation of icy SO2 on the surfaces of the Galilean moons of Jupiter are also presented, with the analysis of the data demonstrating that this is not likely to be a major contributor towards solid SO2 on these moons. This contrasts with the results obtained from experiments investigating the irradiation of oxygen-bearing ices deposited on top of elemental sulphur layers, which was shown to result in the formation of a number of volatile sulphur-bearing molecules such as SO2, CS2, and OCS. The thesis concludes with a discussion of potential directions for future work, as well as a number of suggestions on improving the present experimental set-up.