Titan cryomineralogy: pyridine-containing ices and their role in Titan's chemical evolution

Titan, Saturn's largest moon, offers a unique natural laboratory for exploring planetary and prebiotic chemistry. Its dense atmosphere, dominated by nitrogen and methane, undergoes extensive photochemistry and radiolysis driven by solar UV radiation, cosmic rays and energetic particles from Saturn's magnetosphere [1-3]. These processes generate a range of organic molecules, some of which can condense in the lower atmosphere, where they may aggregate into multi-component ice structures [4]. 

One consequence of the co-condensation is their assembly into co-crystals, materials with defined stoichiometry and molecular arrangement distinct from their pure components [4]. Co-crystals are of particular interest for their potential to further chemical complexity [5], potentially facilitating the formation of nitrogen-containing polycyclic aromatic hydrocarbons (NPAHs)—key species of astrobiological interest due to their stability and relevance to prebiotic chemistry. 

To date, nine co-crystals have been identified as possible cryominerals to be found on Titan’s surface [6]. Among them, a pyridine:acetylene (1:1) co-crystal has been identified as structurally stable under Titan-relevant temperature and pressure conditions [7]. Although pyridine has not been directly detected in Titan’s atmosphere, models suggest its formation via reactions between C₃N radicals and ethane [8], with estimated upper limits of 1.15 ppb above 300 km altitude [9]. Notably, pyridine⁺ ions have been shown to react exothermically with acetylene to yield NPAHs in the gas phase at cold temperatures [10]. Consequently, the pyridine:acetylene co-crystal is one of the key Titan cryominerals to be investigated for their potential to act as vessels for in situ reactions towards the formation of NPAHs. 

We investigated the reactivity of pyridine:acetylene ices (amorphous and co-crystalline) when exposed to vacuum ultraviolet (VUV) irradiation, performing analysis by a combined thin-film Infrared (IR) spectroscopy, temperature-programmed desorption (TPD) and quadrupole mass spectrometry (QMS) protocol. Our results show that VUV exposure leads to the formation of NPAHs and precursors from pyridine:actyelene ices; however, the extent of reactivity is strongly dependent on the ice phase. While amorphous and “dynamic” co-crystalline phases exhibited chemical activity, the fully stabilized co-crystal significantly reduced reactivity [11]. 

Building on this, we investigated the behavior of pyridine in binary mixtures with diacetylene, ethane, and acrylonitrile. IR and Raman spectroscopy, along with neutron and X-ray diffraction, were used to identify potential co-crystal formation, followed by energetic processing of thin-films using VUV or electron irradiation. In the case of mixed ices between pyridine and diacetylene, we found spectroscopic evidence of co-crystal formation, and irradiation results are consistent with the pyridine:acetylene co-crystal, indicating a lower degradation of pyridine when co-crystallized. In contrast, ethane and acrylonitrile did not co-crystallize with pyridine but instead induced a phase transition in crystalline pyridine. Among these systems, only the pyridine:acrylonitrile mixture yielded a new product (m/z = 102) upon irradiation with 5 keV electrons. 

The results indicate that co-crystallization or otherwise strong interactions with other Titan-relevant molecules may stabilize pyridine toward energetic processing, allowing its preservation and deposit on Titan’s surface, where further geophysical processing could drive molecular evolution. The induction of phase transitions instead of co-crystallization is a phenomenon that needs to be considered when searching for new co-crystals. 

References 

[1] He, C., Smith, M. A. (2014). Icarus, 238, 86-92. 

[2] Raulin, F., Brassé, C., Poch, O., Coll, P. (2012). Chem. Soc. Rev., 41, 5380. 

[3] Hörst, S. M. (2017). J. Geophys. Res. Planets. 122, 432-482. 

[4] Cable, M. L., Runčevski, T., Maynard-Casely, H. E., Vu, T. H., Hodyss, R. (2021). Acc. Chem. Res., 54, 3050-3059. 

[5] Gudipati, M., Jacovi, R., Couturier-Tamburelli, I., Lignell, A., Allen, M. (2013). Nat. Commun., 4, 1648. 

[6] Czaplinski, E. C., Vu, T. H., Maynard-Casely, H., Ennis, C., Cable, M. L., Malaska, M. J., Hodyss, R. (2025). ACS Earth Space Chem. 9, 253-264. 

[7] Czaplinski, E. C., Vu, T. H., Cable, M. L., Choukroun, M., Malaska, M. J., Hodyss, R. (2023). ACS Earth Space Chem. 7, 597-608. 

[8] Kranopolsky, V.A. (2009). Icarus, 201, 226-256. 

[9] Nixon, C. (2024). ACS Earth Space Chem. 8, 406-456. 

[10] Rap, D. B., Schrauwen, J. G. M., Marimuthu, A. N., Redlich, B., Brünken, S. (2022). Nat. Astron., 6, 1059-1067. 

[11] Lopes Cavalcante, L., Czaplinski, E. C., Maynard-Casely, H. E., Cable, M. L., Chaouche-Mechidal, N., Hodyss, R., Ennis, C. (2024). Phys. Chem. Chem. Phys., 26, 26842-26856. 

This work was supported by an AINSE Ltd. Postgraduate Research Award (PGRA) and the Marsden Fund Council from Government of New Zealand, managed by Royal Society Te Aparangi (Proposal: 21-UOO-123). We also acknowledge the Australian Centre of Neutron Scattering for provision of instrument through program proposal 13601. We thank New Zealand eScience Infrastructure (NeSI) for high performance computing resources (Project UOO03077). X-ray diffraction and Micro-Raman spectroscopy experiments were conducted at the Jet Propulsion Laboratory – NASA/Caltech as part of the JPL Visiting Student Research Program. We thank additional contributions from Prof. Brendan Kennedy (University of Sydney), Dr. Samuel Duyker (University of Sydney), Dr. Ellen Czaplinski (JPL) and Dr. Naila Chaouche (University of Otago).