Radiation-driven Destruction of N-heterocycles in H2O-ice mixtures

Simple nitrogen (N-) heterocycles are expected to be an abundant class of molecules within the interstellar medium (ISM).  Computational and laboratory studies have demonstrated previously that these molecules likely form during the polymerization of acetylene in the presence of hydrogen cyanide (Ricca et al. 2001, Hamid et al. 2014), a process expected to occur in the stellar outflows of carbon-rich AGB stars.  This production pathway is analogous to the formation of benzene, which has been detected in the presence of polyacetylenic chains (Cernicharo et al. 2001).   Additionally, laboratory studies have demonstrated that N-heterocycles readily form during the irradiation of icy materials.  Specifically, Materese et al. 2015 identified pyridine and isoquinoline in the room-temperature residues formed from photolyzed and warmed ices containing water, ammonia, and benzene or naphthalene.  More recently, Wang et al. 2024 identified pyrrole and indole in electron-irradiated acetylene and ammonia ice mixtures.

N-heterocycles are also present in meteoritic materials and in the samples returned from carbonaceous asteroids.  All five nucleobases (adenine, cytosine, guanine, thymine and uracil) and many other N-heterocycles were detected in extracts of an aggregate sample returned from asteroid Bennu by NASA’s OSIRIS-Rex (Glavin et al. 2025).  Many of these N-bearing species are critical compounds to terrestrial biology, and an extraterrestrial origin informs our understanding of prebiotic astrochemistry.  Simple N-heterocycles (pyridine, diazines, etc.) might be the prerequisite molecules for the formation of nucleobases present on the surfaces of small, outer Solar System objects.  However, despite the expectation of abundant N-heterocycles, these molecules have not been detected remotely in the ISM or the outer Solar System.  Several observational studies have provided upper limits on the abundances of pyridine and pyrimidine (Kuan et al. 2003, Cordiner et al. 2017).

A possible reason for the non-detections of N-heterocycles could be a difference in radiolytic stability compared to other aromatic compounds (e.g., benzene).  Peeters et al. 2005 previously explored this hypothesis demonstrating the photo-destruction of matrix-isolated pyridine, pyrimidine, and s-triazine and found that these molecules can be destroyed in diffuse regions of the ISM (higher UV fluxes), but the molecules should survive in dense molecular clouds, which can provide some UV shielding.  However, galactic cosmic rays (GCRs) readily penetrate dense interstellar clouds and continuously irradiate the surfaces of outer Solar System objects (e.g., TNOs), which likely contain primitive organic materials and possibly N-heterocycles.  Using facilities in NASA’s Cosmic Ice Laboratory and techniques described previously (e.g., Gerakines et al. 2022, Tribbett et al. 2024), we demonstrate the radiation-driven destruction of several simple N-heterocycles (pyridine, pyrimidine, pyridazine, pyrazine, and s-triazine) when embedded in a water-ice matrix.  We use infrared spectroscopy to quantify the radiolytic destruction of these molecules in water ices at 15 K and report their destruction rate constants and radiolytic half-lives.  We extrapolate these half-lives to the expected radiation doses received in interstellar dense molecular clouds (Moore et al. 2001) and on or within the surfaces of outer Solar System objects (Loeffler et al. 2020).  We also discuss the implications of our results in the context of the recent detections of N-heterocycles in the Bennu samples returned by OSIRIS-Rex.

 

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