Formation of Polyoxymethylenes in Extraterrestrial Ice Analogs of Formaldehyde Exposed to Ionizing Radiation

Abstract Polyoxymethylenes (POMs) have long been hypothesized as key organics in comets, as their decomposition products could account for the possible extended cometary formaldehyde (H 2 CO) source. However, POMs (X–(CH 2 O) n –Y; n > 2) are hitherto unobserved in deep space, and their possible formation mechanisms have still remained elusive. Here, we report on the laboratory formation of formaldehyde oligomers (H–(CH 2 O) n –H; n = 2−4) within extraterrestrial ice analogs of formaldehyde exposed to energetic electrons simulating secondary electrons generated by galactic cosmic rays passing through ices. The potential identification of these short-chain-length POMs, such as methoxymethanol (CH 3 OCH 2 OH), (methoxymethoxy)methanol (CH 3 OCH 2 OCH 2 OH), and (methoxymethoxymethoxy)methanol (CH 3 OCH 2 OCH 2 OCH 2 OH), is accomplished by utilizing the synchrotron vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry (SVUV-PI-ReTOF-MS) technique coupled with the temperature-programmed desorption method. A radical-induced formaldehyde polymerization mechanism is proposed to address the formation of POMs, with the competition between chain propagation and chain termination determining the chain length of the polymers and availability of reactive hydrogen atoms. The electron impact ionization of POMs as well as accompanied fragmentation of their cations are analyzed via a quadrupole mass spectrometer, with mass peaks compared with those collected by the Positive Ion Cluster Composition Analyzer instrument on board the Giotto spacecraft in the inner coma of comet 1P/Halley, and by the Ptolemy instrument on the comet 67P/Churyumov–Gerasimenko. Our findings might support the proposal that (small) POMs exist on comets and nominate hitherto unobserved POMs such as (methoxymethoxy)methanol (CH 3 OCH 2 OCH 2 OH) and (methoxymethoxymethoxy)methanol (CH 3 OCH 2 OCH 2 OCH 2 OH) for future astronomical searches.