Solar Wind Irradiation of Methane and Methane-Water Ices: A Molecular Dynamics Approach

Molecular dynamics simulations were performed to characterize reaction products, resulting from solar wind irradiation, namely, H⁺, of methane and methane-water ices. In our approach, we used seven 0.829 keV H⁺ (total energy of 5.8 keV), with a velocity of 400 km/s, to hit the icy surface simultaneously, and we repeated this process multiple times to simulate continuous irradiation while quenching the ice to 15 K after each irradiation to prevent excessive heating and sublimation. Our simulations produced complex organic molecules previously obtained in laboratory experiments. For methane ice, molecules containing two carbons were predominant, with ethane and ethyl radicals being the most abundant, followed by ethylene, vinyl radical, and acetylene. Hydrocarbons containing three carbons (e.g., propane, propene, and propyl) were minor products, and only a few molecules containing four carbon atoms (e.g., iso-butene, 1-methylpropylidene, and 2-buten-2-yl) formed. Products that can be formed from the reaction of 1-3 impact fragmentation events, ethane, ethyl radical, and ethylene, monotonically increased over time, while products of 3 or more impact fragmentation events, vinyl, propane, and acetylene, formed over longer time scales. The number of methane complexes decreased over time. For a methane/water (1:1) ice mixture, most of the products consisted of methyl-water complexes, and their number increased with time. All the other oxygenated and nonoxygenated products formed in small amounts due to the water solvation of radicals. For a methane/water (4:1) ice mixture, the methyl-water complexes constituted 45% of the total products, with oxygenated and nonoxygenated products being formed in almost equal amounts. For methane-water ices, the proportions of alkanes, alkenes, and alkynes were very similar to those of pure methane. Dimethyl ether and ethanol formed for both 1:1 and 4:1 methane-water ices.