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 100 keV protons at 20 K and 120 K. The quantitative studies show that the CO 2 is dispersed in the ice; its column density increases with ion fluence to a maximum value (in 10 15 molecules cm −2 ) of ∼1 at 20 K and ∼3 at 120 K. The initial yield is 0.05 (0.1) CO 2 per incident H + at 20 (120) K. The CO 2 destruction process, which limits the maximum column density, occurs with an effective cross section of ∼2.5 (4.1) × 10 −17 cm 2 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.