doi:10.1038/s42004-024-01248-6engMerino, P.Martínez, L.Santoro, G.Martínez, J. I.Lauwaet, K.Accolla, M.Ruiz del Arbol, N.Sánchez-Sánchez, C.Martín-Jimenez, A.Otero, R.Piantek, M.Serrate, D.Lebrón-Aguilar, R.Quintanilla-López, J. E.Mendez, J.De Andres, P. L.Martín-Gago, J. A.n-Alkanes formed by methyl-methylene addition as a source of meteoritic aliphaticsART-2024-139429Aliphatics prevail in asteroids, comets, meteorites and other bodies in our solar system. They are also found in the interstellar and circumstellar media both in gas-phase and in dust grains. Among aliphatics, linear alkanes (n-CnH2n+2) are known to survive in carbonaceous chondrites in hundreds to thousands of parts per billion, encompassing sequences from CH4 to n-C31H64. Despite being systematically detected, the mechanism responsible for their formation in meteorites has yet to be identified. Based on advanced laboratory astrochemistry simulations, we propose a gas-phase synthesis mechanism for n-alkanes starting from carbon and hydrogen under conditions of temperature and pressure that mimic those found in carbon-rich circumstellar envelopes. We characterize the analogs generated in a customized sputter gas aggregation source using a combination of atomically precise scanning tunneling microscopy, non-contact atomic force microscopy and ex-situ gas chromatography-mass spectrometry. Within the formed carbon nanostructures, we identify the presence of n-alkanes with sizes ranging from n-C8H18 to n-C32H66. Ab-initio calculations of formation free energies, kinetic barriers, and kinetic chemical network modelling lead us to propose a gas-phase growth mechanism for the formation of large n-alkanes based on methyl-methylene addition (MMA). In this process, methylene serves as both a reagent and a catalyst for carbon chain growth. Our study provides evidence of an aliphatic gas-phase synthesis mechanism around evolved stars and provides a potential explanation for its presence in interstellar dust and meteorites.2024http://zaguan.unizar.es/record/14459410.1038/s42004-024-01248-6http://zaguan.unizar.es/record/144594oai:zaguan.unizar.es:144594info:eu-repo/grantAgreement/ES/AEI/PID2021-125309OA-I00info:eu-repo/grantAgreement/ES/AEI/TED2021-129416A-I00info:eu-repo/grantAgreement/EC/FP7/610256/EU/Gas and Dust from the Stars to the Laboratory: Exploring the NanoCosmos/NANOCOSMOSinfo:eu-repo/grantAgreement/ES/MCIU/RYC2018-024364-Iinfo:eu-repo/grantAgreement/ES/MICINN/AEI/CNS2022-135658info:eu-repo/grantAgreement/ES/MICINN/CEX2020-001039-Sinfo:eu-repo/grantAgreement/ES/MCINN/EUR2021-122006info:eu-repo/grantAgreement/ES/MICINN/FIS2016-77578-Rinfo:eu-repo/grantAgreement/ES/MICINN/FIS2016-77726-C3-1-Pinfo:eu-repo/grantAgreement/ES/MICINN/PGC2018-096047-B-I00info:eu-repo/grantAgreement/ES/MICINN/RYC2020-029800-Iinfo:eu-repo/grantAgreement/ES/MICINN/RYC2020-029810-Iinfo:eu-repo/grantAgreement/ES/MINECO/CEX2018-000805-Minfo:eu-repo/grantAgreement/ES/MINECO/MAT2017-85089-C2-1-Rinfo:eu-repo/grantAgreement/ES/MINECO/PID2020-113142RB-C21Communications Chemistry 7, 165 (2024), 10by-nc-ndhttps://creativecommons.org/licenses/by-nc-nd/4.0/deed.esinfo:eu-repo/semantics/openAccess