Ion irradiation of adenine: Implications for molecular growth in Titan’s atmosphere

The Cassini-Huygens mission revealed that energetic (1 – 1000 keV) water ions (OH_x⁺, x = 0,1,2), originating from Enceladus' geysers, precipitate in Titan's upper atmosphere where molecules reaching a mass-to-charge (m/z) of several thousand atomic mass units have been detected. These aerosol embryos, that were not expected at such high altitudes, have been attributed to polycyclic aromatic (nitrogen bearing) hydrocarbons (PANHs) that would result from the ionization and dissociation of the major atmospheric compounds, N₂ and CH₄ by solar photons [1].

A fraction of the Enceladus’ water ions must collide with Titan’s macromolecules but the impact on the atmospheric chemistry is unclear. Do the ions trigger the formation of more complex organic species, maybe including oxygen? Do they sputter some organic material back into the gas phase? Since aerosols sediment towards the surface, the formation of complex oxygenated molecules in the upper atmosphere would add a new dimension with strong exobiological implications to the carbon / nitrogen / hydrogen chemistry endogenous to Titan.

Titan’s photochemistry has inspired many to simulate these conditions in the laboratory [2]. The complex organic compounds resulting from photolysis or radiolysis of N₂/CH₄ mixtures have been deemed “tholins” and numerous studies have attempted to characterize their properties. Infrared spectroscopy has shown that the tholins chemical functions include primary and secondary amines, (iso)cyanides, carbodiimides, aliphatic and heteroaromatic groups. Exact mass Fourier transform mass spectrometry (FT-MS) has demonstrated that their constituent molecules extent to ~500 Da and have unsaturation levels consistent with high degrees of both cyclization and aromaticity, favoring PANH-type structures.

Despite the important literature on tholins, the chemical evolution of nitrogen-rich organics upon interaction with energetic particles has been largely unexplored. One study simulated in the laboratory how vacuum ultraviolet irradiation affects the tholins optical properties as probed by infrared spectroscopy [3]. This work provided evidence that photochemistry could deplete the sensitive primary and secondary amine functions while preserving nitrogen-bearing functionalities that are more strongly bound, such as tertiary amines, imines and nitriles. Adenine (C₅H₅N₅) is a simple heterocyclic aromatic molecule that has been identified in Titan’s tholins [4]. Infrared spectroscopy of adenine samples irradiated by photons, electrons or ions over a wide energy range shows its destruction as well as the formation a solid residue probably of macromolecular nature [5,6]. However, neither the molecular content of the organic residue or the sputtering into the gas phase have been investigated so far.

In this work, we have irradiated under ultra-high vacuum thin film samples of adenine. Adenine was deposited homogeneously in several hundred nm thin layers onto MgF₂ or ZnSe windows [10]. Irradiation experiments were performed either at the ARIBE beam line at GANIL (Caen, France), at the SIDONIE isotope separator (Orsay, France) or at the HUN-REN Institute for Nuclear research (Atomki) in Debrecen, Hungary [7,8,9]. Adenine samples were irradiated with either Ne^𝑞+, O^𝑞+, OH^𝑞+ or H₂O^𝑞+ at various energies from 10 to 70 keV, temperatures (150, 300 K), and fluences (up to 2x10¹⁵ ions.cm⁻²). Infrared absorption spectra of the adenine samples were obtained in situ during the irradiation with a FTIR spectrometer while the residual gas in the experimental chamber was measured with in-situ quadrupole mass spectrometry, analyzing the material that was sputtered into the gas phase during irradiation. The molecular content of the soluble phase of the irradiated samples was obtained with a very high-resolution mass spectrometer at the Institut de Planétologie et d’Astrophysique de Grenoble (France).

We show that the energetic ion irradiation of the samples leads to their destruction, through both chemical processes forming new species in the solid phase and sputtering into the gas phase. The macromolecules detected in the solid phase far exceed the molecular mass of adenine, reaching up to m/z 500. They show great chemical diversity and can be expressed as (HCN)_z-R families, where z can reach 17 and R can be C, H, N, NH, CH or CN. In total, nearly 100 individual families have been identified, 28 of which can be found in every irradiated sample. Their aromaticity equivalent is higher than that in other N rich samples such as Titan tholins and HCN-polymers, corresponding to polycyclic aromatic nitrogen-bearing hydrocarbons. A number of small adenine fragments, including N₂, nitriles and hydrocarbons, are sputtered into the gas phase, throughout the irradiations. Larger species like intact adenine are only detected at the on-set of the irradiations.

These experimental results suggest that ion deposition in Titan’s atmosphere may play a role in the rapid molecular growth occurring at high altitudes and should be considered in photochemical-microphysical models.

 

Acknowledgments

This work is supported by the Programme National de Planétologie (PNP), the French National Research Agency in the framework of the "Investissements d’avenir” program (ANR-15-IDEX-02) and the generic call for proposals (ANR-22-CE49-0017). The experiments were performed at the Grand Accélérateur National d’Ions Lourds (GANIL) by means of the CIRIL Interdisciplinary Platform, part of CIMAP laboratory, Caen, France. We acknowledge the fundings from ANR IGLIAS grant ANR-13-BS05-0004 of the French Agence Nationale de la Recherche. INGMAR is a IAS-IJCLab facility funded by the French Programme National de Planétologie (PNP), Faculté des Sciences d’Orsay, Université Paris-Sud (Attractivité 2012), P2IO LabEx (ANR-10-LABX-0038) in the framework Investissements d’Avenir (ANR-11-IDEX-0003-01). We acknowledge the funding from Europlanet 2024 RI (under the grant agreement No 871149).

 

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