Sputtered species during ion irradiation of adenine: Implications for Titan’s atmospheric chemistry

Cassini-Huygens detected complex molecules exceeding several thousand atomic mass units in Titan's atmosphere as well as energetic O+, OH+ and H2O+ ions in Saturn's magnetosphere. The heavy molecules are suggested to be nitrogenous polycyclic aromatic hydrocarbons (PANH). The energetic ions are thought to penetrate into Titan's atmosphere, potentially modifying the structure and chemistry of these macromolecules. This study investigates the composition of chemical species sputtered into the gas-phase once energetic ions impact said macromolecules. We have irradiated thin film samples of adenine (C5H5N5), a heterocyclic aromatic, at the HUN-REN Institute for Nuclear Research (ATOMKI) in Debrecen, Hungary. The experiments varied key parameters such as ion type (Neq+, Oq+, OHq+ and H2Oq+), energies (10 and 20 keV), temperatures (150 and 300 K) and fluences (up to 2 x 10^15 ions cm^-2). In-situ quadrupole mass spectrometry measured the residual gas of the experimental chamber, analysing the material that was sputtered into the gas phase during irradiation. The deconvolution of these mass spectra was done using a Monte-Carlo approach, revealing a plethora of compounds including HCN, N2, C2N2, C2H3N, C3HN, CH2N2, NH3, C2H2 and CH4. Larger species like C5H5N5, C4H4N4 and C3H3N3 were only detected at the on-set of the irradiation. The sputtering of hydrocarbons and nitriles highlights a potential feedback mechanism in Titan's atmosphere. The direct formation of CO and CO2 from PANH irradiation appears unlikely.