Molecular Dynamics Simulations of Methane-Ammonia Ices Irradiated by Protons

Molecular dynamics (MD) calculations were carried out to simulate the solar wind irradiation, namely, H⁺, of methane-ammonia ices. To mimic a continuous ion bombardment of the ice, multiple impact cycles were performed on the ice target. Each impact cycle involved seven 0.829 keV H⁺ (total energy of 5.8 keV and a velocity of 400 km/s) impacting the surface for a duration of 0.5 ps, which was shown in previous work to be a sufficient time for any product resulting from H⁺ impacts of the ice to form and stabilize. At the end of each cycle, the ice was quenched to 15 K to prevent excessive heating and sublimation. The dominant radiolysis species formed in our simulations were those obtained from the reaction of methyl and amino radicals, namely, ethane, hydrazine, and methylamine. The formation of methylamine, the building block of the amino acid glycine, is in agreement with observations and previous irradiation experiments. Additional species resulting from progressive impact-mediated hydrogen loss of simple two-radical products, namely, ethyl, methanimine, aminomethyl, and diimine, were produced in significant quantities in our simulations and in previous irradiation experiments. Unsaturated molecules, such as vinyl, ethylene, and acetylene, were formed to a lesser extent by impact-mediated hydrogen loss. Larger product species, such as methanediamine, requiring the reaction of up to four radicalized ice molecules did form throughout the course of our simulations and were also obtained in previous irradiation experiments. Methanediamine is a precursor to nucleobases.