Formation of CO2 Driven by Photochemistry of Water Ice Mixed with Carbon Grains

Abstract

We present results on photochemistry of carbon-grains/water-ice mixtures at temperatures from 10 to 150 K. Such a temperature range corresponds to the physical conditions found in molecular clouds, hot cores and corinos, protostellar envelopes, and planet-forming and debris disks. We demonstrate that UV irradiation of carbon-grains/water-ice mixtures leads to the formation of CO2, which, beyond the desorption temperature of CO2 partly escapes into the gas phase, and partly remains trapped on the surface of grains. Thus, we present the first direct evidence of the efficient formation of CO2 on carbon surfaces covered by water ice at high temperatures (up to 150 K) leading to a conclusion that the known low-temperature formation route of CO2 remains valid at high temperatures as long as H2O is present on carbon grains. Moreover, we demonstrate an improved capability of the dust-surface/crystalline-water-ice interface (as compared to amorphous water ice) to trap CO2 in the solid state well above the CO2 desorption temperature. The high-temperature chemical pathway to CO2 may lead to the chemical erosion of carbonaceous grains in planet-forming disks, providing an alternative explanation of the loss of solid carbon in the innermost disk regions that resulted in the formation of carbon-poor Earth and other terrestrial planets in the solar system.