The lower atmosphere of Titan, Saturn’s largest moon, hosts a chemically rich environment where low-mass hydrocarbons and nitrogen-bearing molecules such as cyanides condense into mixed ices and aerosols at temperatures lower than 100 Kelvin. Such icy cryominerals may even include cocrystals made of two distinct molecular coformers in a precise arrangement, where recent studies have revealed multiple binary cocrystals viable under Titan conditions[1]. Further, these species undergo energetic processing from UV photons and Galactic Cosmic Ray particles that combined dissipate energy to surface level. Therefore, investigating the reactivity of species that comprise mixed aerosols and cocrystals is essential for understanding the chemical evolution of planetary bodies and to provide higher-order targets, potentially those of biological relevance, for future missions such as NASA’s upcoming Dragonfly mission.
In this talk, we present the outcomes of our recently completed Marsden funded research programme into the irradiation driven formation of nitrogen-bearing heterocycles from mixed ice phases and cocrystals[2]. Multiple techniques including FTIR spectroscopy, temperature-programmed desorption mass spectrometry, neutron scattering and x-ray diffraction, were used to characterise the structure and chemistry of Titan-relevant ices; supported by periodic-DFT calculations.
Spectroscopic data obtained from ices comprised of: diacetylene (C4H2), pyridine (C5H5N), the cocrystals between C5H5N and acetylene (C5H5N:C2H2) and C5H5N:C4H2 (a potential novel cocrystal) revealed phase-dependent features[3]. For example, a transition of the C5H5N:C2H2 system from the amorphous phase to its cocrystal structure coincided with altered IR band profiles to reflect enhanced molecular ordering and a narrowing distribution of intermolecular interactions. Next, when exposed to VUV irradiation, a phase-dependence in photoproduct formation was observed, suggesting cocrystal interactions and molecular packing acts to constrain molecular diffusion to instead favour recombination. In summary, these results provide reference data and new photochemical insights for Titan-relevant ices, demonstrating how crystallization can influence the stability and reactivity of organic cryominerals.