Production of linear alkanes via the solid-state hydrogenation of interstellar polyynes

Context . Highly unsaturated carbon chains, including polyynes, have been detected in many astronomical regions and planetary systems. With the success of the QUIJOTE survey of the Taurus Molecular Cloud-1 (TMC-1), the community has seen a “boom” in the number of detected carbon chains. On the other hand, the Rosetta mission revealed the release of fully saturated hydrocarbons, C 3 H 8 , C 4 H 10 , C 5 H 12 , and (under specific conditions) C 6 H 14 with C 7 H 16 , from the comet 67P/Churyumov-Gerasimenko. The detection of the latter two is attributed to dust-rich events. Similarly, the analysis of samples returned from asteroid Ryugu by Hayabusa2 mission indicates the presence of long saturated aliphatic chains in Ryugu’s organic matter. Aims . The surface chemistry of unsaturated carbon chains under conditions resembling those of molecular clouds can provide a possible link among these independent observations. However, laboratory-based investigations to validate such a chemistry is still lacking. In the present study, we aim to experimentally verify the formation of fully saturated hydrocarbons by the surface hydrogenation of C 2 n H 2 ( n > 1) polyynes under ultra-high vacuum conditions at 10 K. Methods . We undertook a two-step experimental technique. First, a thin layer of C 2 H 2 ice was irradiated by UV-photons (≥121 nm) to achieve a partial conversion of C 2 H 2 into larger polyynes: C 4 H 2 and C 6 H 2 . Afterwards, the obtained photoprocessed ice was exposed to H atoms to verify the formation of various saturated hydrocarbons. Results . In addition to C 2 H 6 , which was investigated previously, the formation of larger alkanes, including C 4 H 10 and (tentatively) C 6 H 14 , is confirmed by our study. A qualitative analysis of the obtained kinetic data indicates that hydrogenation of HCCH and HCCCCH triple bonds proceeds at comparable rates, given a surface temperature of 10 K. This can occur on the timescales typical for the dark cloud stage. A general pathway resulting in formation of other various aliphatic organic compounds by surface hydrogenation of N- and O-bearing polyynes is also proposed. We also discuss the astrobiological implications and the possibility of identifying alkanes with JWST.