Synthesizing Insoluble Organic Matter Precursor with Electron Irradiation on Simple Carbon-bearing Ices

The interstellar medium and many Solar System bodies such as comets, meteorites, interplanetary dust particles (IDPs) and Ultracarbonaceous Antarctic Micrometeorites (UCAMMs) contain vast reservoirs of organic molecules. Laboratory analyses on these bodies have revealed that a major portion of carbon is incorporated in Insoluble Organic Matter (IOM) – a carbonaceous cross-linked macromolecular network that is resistant to demineralizing acids and organic solvents. Further analyses on IOMs from different sources also shows it exhibits extreme D/H isotopic enrichment in IOM, hinting their origin in the cold interstellar medium, where ion-molecule reaction is able to facilitate deuterium incorporation. While many laboratory works have studied the formation of organic molecules, only a handful have been characterized as analogues or precursor of IOM. In addition, the formation of IOM itself is still a fundamental question, which further necessitates laboratory works to elucidate the conditions necessary for its formation.

We thus simulate the formation of IOM precursor by co-depositing astrophysically relevant gas mixtures at 10K and heavily irradiate the interstellar ice analogues with energetic electrons, a stand-in for cosmic rays. The resulting residue is characterized with Raman spectroscopy, focusing on the D (disordered) and G (graphitic) bands that are tell-tale signatures of IOM. We find that the transformation from ice to IOM precursor requires an intermediate step by first forming medium-complexity Soluble Organic Molecules and removing precursor ice. This then involves the reirradiation of the residue left after the volatile components including water are desorbed. We hypothesize that precursor ice, in particular H₂O, inhibits the growth of organic molecules to macromolecular sizes. These findings suggest that IOM formed in a heavily irradiated environment where precursors underwent an “ice-dry” cycle to remove volatile ice components.