VUV photoabsorption of thermally processed carbon disulfide and ammonia ice mixtures - Implications for icy objects in the solar system

Many icy bodies in the solar system have been found to contain a rich mixture of simple molecules on their surfaces. Similarly, comets are now known to be a reservoir of molecules ranging from water to amides. The processing of planetary/cometary ices leads to the synthesis of more complex molecules some of which may be the harbingers of life. Carbon disulphide (CS₂) and ammonia (NH₃) are known to be present on many icy satellites and comets. Reactions involving CS₂ and NH₃ may lead to the formation of larger molecules that are stable under space conditions. In this paper we present temperature dependent VUV spectra of pure CS₂ in the ice phase, and of CS₂ and NH₃ ices deposited as (i) layered, and (ii) mixed ices at 10 K and warmed to higher temperatures until their sublimation. Pure CS₂ ice is found to have a broad absorption in the VUV region, which is unique for a small molecule in the ice phase. In layered and mixed ices, the molecules tend to affect the phase change and sublimation temperature of each other and also leave behind a form of CS₂-NH₃ complex after thermal annealing. This study of CS₂-NH₃ ice systems in layered and mixed configurations would support the detection of these species/complexes in mixed molecular ices analogous to that on planetary and cometary surfaces.