Energetic Processing of N2:CH4 Ices Employing X-Rays and Swift Ions: Implications for Icy Bodies in the Outer Solar System

We experimentally address in this paper the physicochemical effects induced by ionizing photons (energies from 6 to 2000 eV) and swift heavy ions (15.7 MeV O-16(5+)) in the icy mixture containing N-2:CH4 (19:1) at 12 K and 19 K, respectively. The experiments simulate the effect of solar photons and X-rays, cosmic rays, and solar energetic particles (medium-mass ions) on the surface of icy bodies in the outer solar system, such as Triton, Titan, Pluto, and several other Kuiper Belt objects. The ice samples were analyzed by infrared spectroscopy (FTIR) at different fluences. From the energetic processing, the production of new molecules was observed. Among them, HCN, C2H4, C2H6, and N-3 have the highest production yield. Molecular half-lives of the species of interest were calculated and extrapolated to the astrophysical environment. The effective destruction yield (in molecules/ impact) of the parental species processed by the swift ions is up to six orders of magnitude higher than the value determined by employing X-rays. However, due to the differences between the fluxes of both ionizing radiation types in space, the half-lives of nitrogen and methane in the astrophysical scenarios addressed may have a huge variation. Photons dominate the chemical transformations at shorter distances from the Sun. Our results are a step toward a compilation of photochemical and radiolysis data that should allow the modeling of the abundance of astrophysical ices over long periods of time.