Formation of Complex Organic Molecules (COMs) from Polycyclic Aromatic Hydrocarbons (PAHs): Implications for ISM IR Emission Plateaus and Solar System Organics Andrew L. Mattioda,* Gustavo A. Cruz-Diaz, Andrew Ging, Michael Barnhardt, Christiaan Boersma, Louis J. Allamandola, Todd Schneider, Jason Vaughn, Brandon Phillips, and Alessandra Ricca Cite This: ACS Earth Space Chem. 2020, 4, 2227-2245 Read Online ACCESS Metrics & More Article Recommendations ABSTRACT: The presence of complex organic molecules (COMs) in a variety of solar system objects has prompted various theories regarding their origins. This paper reports on a series of proton, electron, and UV radiation experiments, conducted singly and in combination, on polycyclic aromatic hydrocarbon (PAH) thin films to determine if PAH-related materials could be related to COMs. Fourier transform infrared (FTIR) and mass spectra reveal that the PAHs are fragmented and altered, producing new infrared features and a mass spectral pattern similar to those from small aliphatic and larger saturated hydrocarbon chain and ring systems. Mass spectra after proton and electron irradiation exhibit significant differences. The proton irradiation products appear to contain more oxygen-related species, possibly the result of higher H 2 O concentrations in the vacuum chamber, whereas electron irradiation generates a more abundant, larger mass organic species mass spectral pattern. Combined irradiation produces a superposition of the proton and electron results, with some subtle differences. Mass spectral patterns resulting from electron and combined irradiation compare favorably to Cassini ion neutral mass spectrometer (INMS) measurements of particles falling from Saturn’s inner rings into its upper atmosphere. FTIR results are compared to PAH emission from interstellar sources and absorption components in IR spectra observed toward low-mass young stellar objects (YSOs). Notably, some newly reported broad IR features appear very similar in structure to the plateaus underlying the astronomical 3.2-3.6, 6-9, and 10-15 μm emission plateaus and several of the absorption components found in spectra observed toward YSOs. The studies also indicate that the presence of other species, such as H 2 O, may significantly impact the radiation products. These results point to the fact that a top-down synthesis of solar system COMs is possible via irradiative processing of PAHs. KEYWORDS: ISM, PAHs, IR, mass spectra, irradiation, Saturn, complex organic materials (COMs) ■ INTRODUCTION Complex organic molecules (COMs), including prebiotic molecules like simple amino acids, have been found in many solar system objects (i.e., dust, comets, meteorites, moons, etc.); however, their origin remains a mystery. Currently, two major chemical evolutionary pathways have been proposed to explain the organic inventory of the solar system. The first pathway suggests an interstellar heritage whereby the prebiotic material found in the solar system formed from the erosion and functionalization of large, pre-solar carbon-bearing species. 1-4 The second pathway assumes that the origin of the solar system prebiotic matter is the result of the bottom-up synthesis of small Received: June 24, 2020 Revised: October 19, 2020 Accepted: October 26, 2020 Published: November 20, 2020 Article http://pubs.acs.org/journal/aesccq © 2020 American Chemical Society 2227 https://dx.doi.org/10.1021/acsearthspacechem.0c00165 ACS Earth Space Chem. 2020, 4, 2227-2245 Downloaded via AUCKLAND UNIV OF TECHNOLOGY on December 18, 2020 at 06:42:47 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.