Energetic processing of pure and mixed hydrocarbon ices for application to Titan's atmosphere

Motivation. Polycyclic Aromatic Hydrocarbon (PAH) molecules are believed to be abundant and widespread on Titan. Cassini indeed detected large amounts of positive ions in the moon's ionosphere via mass spectrometry that have been identified as PAHs and PAH derivatives containing up to 24 carbon atoms [1]. Remote sensing infrared measurements have also suggested the presence of PAHs/PANHs up to 10-11 rings in the atmosphere [2,3]. They are considered key ingredients for the formation of the aerosol responsible for the orange haze layer and they can also end up closer to the surface, where they would condensate as it happens to benzene C₆H₆ and other hydrocarbons. What is the fate and role of condensed PAHs and hydrcarbons in Titan's lower atmosphere? Can they promote new chemistry?

Methodology. We used the AQUILA chamber at the ECRIS Facility at HUN-REN ATOMKI to investigate the energetic processing of pure ices phenanthrene (C₁₄H₁₀) and acetonitrile (CH₃CN) and their mixtures by 10 keV H+ (protons). Phenanthrene was chosen as a prototypical PAH molecule and acetonitrile has been detected by [4]   The ice samples were prepared in situ by depositing the molecule onto the cold substrate (20 K) placed in the AQUILA UHV chamber. The ices are then irradiated with proton fluences in the range [2.5 10¹²-1.5 10¹⁶] cm².  In the chamber the evolution of the ices is monitored by an FTIR spectrometer.

Results. The irradiation of phenanthrene ice as function of  proton fluences show a decrease of all the typical IR peaks of phenanthrene with simialr rates and no clear emergence of new features.  We interpreted this as the proton bombardment  leads to a compactification of the ices.  In the case of acetonitrile irradiation, we observe the formation of new strong peaks at around ~3130 cm^-1 and ~1645 cm^-1, which are consistent with the formation of HCN and H₃CHNH. Interestingly, the ice mixture C₁₄H₁₀:CH₃CN shows  the formation of the same peaks as for the acetonitrile ice only but their strength increases at a lower rate than for the pure sample.  This can be interpreted as if phenanthrene molecules within the mixture partially protects the acetonitrile from creating new species. Future analysis of the gas-phase QMS data taken at the same time as the irradiation will allow us to gain better insight in the processes happening within the ices. The result of this study has implication for our understanding of hydrocarbon ices evolution in Titan’s lower atmosphere.

References.

[1] Haythornthwaite et al 2021, Planet. Sci. J, 2, 26

[2] López-Puertas et al 2013, ApJ, 770 132

[3] Stikkelbroeck et al 2025, ID EPSC-DPS2025-1444

[4] Coustenis et al 2007, Icar., 189, 35

Acknowledgements. The authors gratefully acknowledge support from the Europlanet RI through the Transnational Access Project Grant no. 22-EPN3-053. The Europlanet RI has received funding from the European Union’s Horizon 2020 Research Innovation Program under grant agreement no. 871149.