Formation of carbon dioxide by surface reactions on ices in the interstellar medium

The formation of carbon dioxide by surface reactions has been investigated experimentally in conditions close to those encountered in the interstellar medium. Carbon monoxide and oxygen atoms have been concurrently deposited on a copper substrate at 5 K. The formation and release in the gas phase of carbon dioxide have been monitored by a mass spectrometer during a programmed desorption. Our measured rates and energy barrier show that it is possible to make CO2 efficiently in ice mantles on grains without the intervention of energizing (UV or energetic particles) agents.