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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 16O5+) in the icy mixture containing N2: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 N3 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. [ABSTRACT FROM AUTHOR]

We use experimental mid-infrared optical constants and extended effective medium approximations to determine the porosity and the band strengths of multi-phase composite ices grown at 30 K. A set of porous H2O:CH4 ices are taken as a prototypical example. As a benchmark and proof of concept, the stoichiometry of the ice constituents is retreived with good accuracy from the refractive indices and the extinction coefficients of the reference binary ice mixtures with known compositions. Accurate band strengths are then calculated from experimental mid-infrared spectra of complex ices. We notice that the presence of pores has only a small effect on the overall band strengths, whereas a water dilution can considerably alter them. Different levels of porosity are observed depending on the abundance of methane used as a gas contaminant premixed with water prior to background deposition. The absorption profiles are also found to vary with deposition rate. To explain this, we use Monte Carlo simulations and we observe that the deposition rate strongly affects the pore size distribution as well as the ice morphology through reorganization processes. Extrapolated to genuine interstellar ices, the methodology presented in this paper can be used to evaluate the porosity and to quantify the relative abundances from observational data. [ABSTRACT FROM AUTHOR]