Methane ices have been detected on ice-coated interstellar grains and on the surface of Kuiper belt objects. These ices are chemically altered by ionizing radiation in the form of energetic photons and charged particles, leading to complex organic molecules. Despite decades of research, the chemical makeup of these newly synthesized molecules has not been completely understood to date. Here, we present a novel application of reflectron time-of-flight mass spectrometry coupled to soft photoionization to probe the molecular formulas of the molecules formed upon interaction of ionizing radiation with simple methane and D4-methane ices. Our study depicts clear evidence of high-molecular-weight hydrocarbons of up to C22, among them alkanes, alkenes, and alkynes/dienes, with those product classes in italics identified for the first time on line and in situ. These studies are particular timely as they provide laboratory data of methane-processed ices, which can be compared to actual data from the New Horizons mission on route to Pluto.
Keywords: Kuiper belt objects; Pluto; high mass hydrocarbons; interstellar ice; ionizing radiation; methane.