Radiation synthesis of carbon dioxide in ice-coated carbon: Implications for interstellar grains and icy moons

We report the synthesis of carbon dioxide on an amorphous carbon-13 substrate coated with amorphous water ice from irradiation with 100keV protons at 20K and 120K. The quantitative studies show that the CO₂ is dispersed in the ice; its column density increases with ion fluence to a maximum value (in 10¹⁵ moleculescm^-2) of 1 at 20K and 3 at 120K. The initial yield is 0.05 (0.1) CO₂ per incident H⁺ at 20 (120)K. The CO₂ destruction process, which limits the maximum column density, occurs with an effective cross section of 2.5 (4.1) × 10 ^-17cm² at 20 (120)K. We discuss radiation-induced oxidation by reactions of radicals in water with the carbon surface and demonstrate that these reactions can be a significant source of condensed carbon dioxide in interstellar grains and in icy satellites in the outer solar system.