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Carbon monoxide is the second most abundant molecule on icy grains in the interstellar medium. It also exists on Pluto, Triton, comets, and possibly in other icy bodies of the outer solar system like Kuiper Belt objects. With the intense radiation fields that permeate virtually all unprotected regions of space, carbon monoxide ices can be processed through energetic particle bombardment (planetary magnetospheric particles, solar wind, Galactic cosmic ray particles, and UV photons). In the present study we have investigated the effects by condensing a 1 m layer of carbon monoxide ice on a substrate at 10 K and irradiated the sample with energetic (keV) electrons. These simulate the energetic electrons trapped in magnetospheres of planets and reproduce the irradiation effects of typical Galactic cosmic ray particles. A series of new carbon-chain (C 3 , C 6 ) and carbon oxide species were observed including the linear isomers of C . A reaction model was proposed that outlines different reaction pathways to each of these products. Using this model, the kinetics of each route of reaction was quantified, and from this, the mechanisms and dynamics of the reactions can be understood. This work should aid in the astronomical detection of new molecular species in solar system ices as well as building up a comprehensive reaction model to describe the chemical inventory of ices on interstellar dust grains.

Pure methane ices (CH 4 ) were irradiated at 10 K with energetic electrons to mimic the energy transfer processes that occur in the track of the trajectories of MeV cosmic-ray particles. The experiments were monitored via an FTIR spectrometer (solid state) and a quadrupole mass spectrometer (gas phase). Combined with electronic structure calculations, this paper focuses on the identification of CH x (x ¼ 1Y4) and C 2 H x (x ¼ 2Y6) species and also investigates their formation pathways quantitatively. The primary reaction step is determined to be the cleavage of a carbonhydrogen bond of the methane molecule to form a methyl radical (CH 3 ) plus a hydrogen atom. Hydrogen atoms recombined to form molecular hydrogen, the sole species detected in the gas phase during the irradiation exposure. In the matrix two neighboring methyl radicals can recombine to form an internally excited ethane molecule (C 2 H 6 ), which either can be stabilized by the surrounding matrix or was found to decompose unimolecularly to the ethyl radical (C 2 H 5 ) plus atomic hydrogen and then to the ethylene molecule (C 2 H 4 ) plus molecular hydrogen. The initially synthesized ethane, ethyl, and ethylene molecules can be radiolyzed subsequently by the impinging electrons to yield the vinyl radical (C 2 H 3 ) and acetylene (C 2 H 2 ) as degradation products. Upon warming the ice sample after the irradiation, the new species are released into the gas phase, simulating the sublimation processes interstellar ices undergo during the hot core phase or comets approaching perihelion. Our investigations also aid the understanding of the synthesis of hydrocarbons likely to be formed in the aerosol particles and organic haze layers of hydrocarbon-rich atmospheres of planets and their moons such as Titan.

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