Oxygen irradiation of diverse aromatics to paint a broad picture of sputtering, molecular growth and implantation in Titan’s aerosols

The Cassini-Huygens mission revealed great complexity in Titan’s atmosphere. Indeed, mass spectrometers onboard Cassini detected the presence of ions of unexpectedly high masses [1]. These macromolecular ions are understood as the precursors of the aerosols abundant at lower altitude with formation mechanisms likely driven by ion chemistry [2]. These precursor molecules probably are polycyclic aromatic (nitrogen bearing) hydrocarbons (PAHs & N-PAHs) identified by their C-H infrared emission signatures [3], compatible with the detected ion mass-to-charge ratios [4]. Chemistry in ionospheres is triggered by UV photons and energetic particles pervading the Solar System. But specific to Titan is its place within Saturn’s magnetosphere where oxygenated ions, sourced from the plumes of Enceladus, were detected. 10 to 100 keV oxygen ions can reach fluxes of ~10⁶ ions.cm^-2.s^-1 in Titan’s upper atmosphere [5]. Ions typically deposit on Titan between 1200 and 800 km in altitude and mainly loose energy until thermalization by interactions with N₂ [6]. Yet a fraction of these ions must interact directly with the organics and contribute to Titan’s complex chemistry. As ion irradiation is known to trigger sputtering, chemical growth and possibly implantation, what is the impact of these processes on Titan’s chemical budget?

Titan’s atmospheric chemistry has been historically investigated by subjecting N₂:CH₄ mixtures to representative energy sources triggering photolysis and radiolysis [7]. The resulting aerosol analogs, called “tholins”, are made of irregular polymeric structures with unsaturation levels indicative of N-PAHs, with infrared features of amines, (iso)cyanides, aliphatic and heteroaromatic groups. Tholins exposed to VUV [8] and plasma [9] irradiation showed erosion on the grains and non-uniform modification of chemical functions. Here, we used N-PAHs as simpler aerosol analogues to investigate and quantify distinct and competing processes triggered by ion irradiation. High resolution mass spectrometry analysis of O⁺-irradiated adenine (C₅H₅N₅) showed the formation of different families of (HCN)-like polymeric structures of condensed aromatics [10]. During irradiation, sputtering also occurs and expels small molecules to the gas phase, typically HCN, comparable to plasma-driven erosion [11]. But adenine is not fully representative of Titan aerosols as tholins show a range of N/C ratios from 1.5 to 0 [12]. To get a broader picture of oxygen irradiation on a range of representative molecules, we have conducted new experiments on adenine, adenine:chrysene mixtures, bathophenanthroline (C₂₄H₁₆N₂) and chrysene (C₁₈H₁₂) with N/C ratios of 1, ~0.2, 0.08 and 0 respectively.

Irradiation experiments were performed at the ARIBE beam line coupled to the IGLIAS chamber at GANIL (Caen, France) [13] and at the HUN-REN Institute for Nuclear research (Atomki) in Debrecen (Hungary) with the AQUILA chamber [14] and the Electron Cyclotron Resonance ion source [15]. We used oxygen ions at 10 and 20 keV (for ¹⁸O) and at 70 and 108 keV (for ¹⁶O) to irradiate samples at 150 or 300 K with maximum fluences of 2x10¹⁶ ions/cm². The experimental rationale varied depending on the process we aimed to quantify: single layers of hundreds of nanometers for sputtering and multiple layers for implantation. In-situ infrared spectroscopy and quadrupole mass spectrometry measurements are performed to track chemical changes and sputtering.

Infrared analysis shows the progressive destruction of the initial molecular film, associated to its intact sputtering and to radiolysis followed by sublimation of volatile species [16]. The appearance of new bands allows to identify and quantify abundant radiolytic products, associated with dehydrogenation processes. Samples irradiated with ¹⁸O ions were analyzed ex-situ with an 18T-FT-ICR mass spectrometer at the CARMeN Institute (Rouen, France) with a resolution allowing unambiguous detection of implanted ¹⁸O. The molecular content of the irradiated samples was analyzed by Laser Desorption Ionization, revealing high molecular complexity with m/z reaching 700.

We will present experimental results that provide insights into the heterogeneous processes in Titan’s upper atmosphere. By extracting sputtering yields and destruction cross sections, we provide input for photochemical-microphysical models of Titan’s complex atmosphere. By probing molecular growth through oxygen incorporation into C,H,N material, we investigate an added prebiotic interest for the aerosols sedimenting to the surface. This work can also have implications for outer solar system bodies like Triton, Pluto, Eris and Makemake where oxygen ions of the solar wind and galactic cosmic rays process ices and transient atmospheres of high hydrocarbon content.

Acknowledgments

This work is supported by 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) and ANR MIRRPLA grant (ANR-22-EXOR-0012) of the French Agence Nationale de la Recherche and Normandie Region (RIN 50/50). This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 871149. We acknowledge the funding from Europlanet’s Transnational Access Pilot programme 2025 (project code 25-EPN-P-5) and from Europlanet’s Transnational Access programme 2026 (project code 26-EPN-38). Access to the CNRS research infrastructure Infranalytics (FR2054) is gratefully acknowledged.

References

[1] F. J. Crary et al., PSS, 57, 1847-1856 (2009)

[2] P. Lavvas et al., Proc. Natl. Acad. Sci. U.S.A., 110, 2729-2734 (2013)

[3] M. López-Puertas et al., ApJ, 770, 132 (2013)

[4] R. P. Haythornthwaite et al., PSJ, 2, 26 (2021)

[5] T. E. Cravens et al., Geophysical Research Letters, 35, 3 (2008)

[6] S. Hörst et al., JGR, 113, E10 (2008)

[7] M. L. Cable et al., Chem Rev, 112, 1882 (2012)

[8] N. Carrasco et al., Nature Astronomy, 2, 489 (2018)

[9] A. Chatain et al., Icarus, 345, 113741 (2020)

[10] F. Matuszewski et al., Icarus, 445, 116865 (2026)

[11] A. Chatain et al., Icarus, 396, 115502 (2023)

[12] H. Imanaka et al., PNAS, 107, 12423-12428 (2010)

[13] B. Augé et al., Rev. Sci. Instrum., 89, 075105 (2018)

[14] R. Rácz et al., Rev. Sci. Instrum., 95, 095105 (2024)

[15] S. Biri et al., Eur. Phys. J. Plus, 136, 247 (2021)

[16] E. Dartois et al., A&A, 671, A156 (2023)