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Titan’s equatorial dunes represent the most monumental surface structures in our Solar System, but the chemical composition of their dark organics remains a fundamental, unsolved enigma, with solid acetylene detected near the dunes implicated as a key feedstock. Here, we reveal in laboratory simulation experiments that aromatics such as benzene, naphthalene, and phenanthrene—prospective building blocks of the organic dune material—can be efficiently synthesized via galactic cosmic ray exposure of low-temperature acetylene ices on Titan’s surface, hence challenging conventional wisdom that aromatic hydrocarbons are formed solely in Titan’s atmosphere. These processes are also of critical importance in unraveling the origin and chemical composition of the dark surfaces of airless bodies in the outer Solar System, where hydrocarbon precipitation from the atmosphere cannot occur. This finding notably advances our understanding of the distribution of carbon throughout our Solar System such as on Kuiper belt objects like Makemake. 

Methylamine (CH 3 NH 2 ) and methanimine (CH 2 NH) represent essential building blocks in the formation of amino acids in interstellar and cometary ices. In our study, by exploiting isomer selective detection of the reaction products via photoionization coupled with reflectron time of flight mass spectrometry (Re-TOF-MS), we elucidate the formation of methanimine and ethylenediamine (NH 2 CH 2 CH 2 NH 2 ) in methylamine ices exposed to energetic electrons as a proxy for secondary electrons generated by energetic cosmic rays penetrating interstellar and cometary ices. Interestingly, the two products methanimine and ethylenediamine are isoelectronic to formaldehyde (H 2 CO) and ethylene glycol (HOCH 2 CH 2 OH), respectively. Their formation has been confirmed in interstellar ice analogs consisting of methanol (CH 3 OH) which is ioselectronic to methylamine. Both oxygen-bearing species formed in methanol have been detected in the interstellar medium (ISM), while for methanimine and ethylenediamine only methanimine has been identified so far. In comparison with the methanol ice products and our experimental findings, we predict that ethylenediamine should be detectable in these astronomical sources, where methylamine and methanimine are present.