A systematic IR and VUV spectroscopic investigation of ion, electron, and thermally processed ethanolamine ice

The recent detection of ethanolamine (EtA, HOCH₂CH₂NH₂), a key component of phospholipids, i.e. the building blocks of cell membranes, in the interstellar medium is in line with an exogenous origin of life-relevant molecules. However, the stability and survivability of EtA molecules under inter/circumstellar and Solar System conditions have yet to be demonstrated. Starting from the assumption that EtA mainly forms on interstellar ice grains, we have systematically exposed EtA, pure and mixed with amorphous water (H₂O) ice, to electron, ion, and thermal processing, representing ‘energetic’ mechanisms that are known to induce physicochemical changes within the ice material under controlled laboratory conditions. Using infrared (IR) spectroscopy we have found that heating of pure EtA ice causes a phase change from amorphous to crystalline at 180 K, and further temperature increase of the ice results in sublimation-induced losses until full desorption occurs at about 225 K. IR and vacuum ultraviolet (VUV) spectra of EtA-containing ices deposited and irradiated at 20 K with 1 keV electrons as well as IR spectra of H₂O:EtA mixed ice obtained after 1 MeV He⁺ ion irradiation have been collected at different doses. The main radiolysis products, including H₂O, CO, CO₂, NH₃, and CH₃OH, have been identified and their formation pathways are discussed. The measured column density of EtA is demonstrated to undergo exponential decay upon electron and ion bombardment. The half-life doses for electron and He⁺ ion irradiation of pure EtA and H₂O:EtA mixed ice are derived to range between 10.8 − 26.3 eV/16u. Extrapolating these results to space conditions, we conclude that EtA mixed in H₂O ice is more stable than in pure form and it should survive throughout the star and planet formation process.