10 keV electron irradiation of methane ices at ocean world surface temperatures

The surface environmental conditions of many of the icy ocean worlds in the Solar System are outside of the thermal stability field of methane ice. However, mechanisms may exist that entrain or trap methane within water ice that arrives at the surface via plume eruptions or resurfacing events. At the surface, the ice grains may experience charged particle irradiation that could lead to the production of new complex hydrocarbons. We investigated the irradiation products of 12CH4 versus 13CH4, the temperature dependence of CH4 radiation processing and product stability, and the temperature stability of CH4 when mixed with H2O in a cryogenic vacuum environment. Significantly, methane ice experimentally encased in water ice was irradiated at 100 K in an ultra-high vacuum environment and produced similar radiation-formed secondary phases as pure methane ice irradiated at 20 K. The methane radiation products produced outside of the methane ice thermal stability field, while encased in a water ice film, were most notably 13CO2 and a candidate hydrocarbon feature. Evidence for complex hydrocarbons, such as C2H6, was observed during warming and sublimation of the sample. •The radiolysis of CH4 ices at ocean world environmental conditions was investigated.•CH4 ice inside H2O ice irradiated at 100 K created similar secondary phases as CH4 ice.•The temperature dependence of CH4 and CH4+H2O radiation processing was investigated.