Experimental investigation of surface reactions in carbon monoxide and oxygen mixtures

During hypersonic entry into the CO2 atmosphere of Mars, competing exotherm ic chemical reactions may occur on a spacecraft heatshield surface. Two pos sible surface reactions are O + O --> O-2 and CO + O --> CO2. The relative importance of these reactions on quartz is investigated using a diffusion t ube side-arm reactor together with two-photon laser-induced fluorescence fo r both O and CO species detection. The experiments show 1) that the presenc e of CO in the gas phase does not significantly affect the oxygen recombina tion reaction on quartz and 2) that the gas-phase CO concentration is not s ignificantly altered by the presence of atomic oxygen. These results indica te that for our experimental conditions the dominant surface reaction on qu artz in oxygen-carbon monoxide mixtures is O + O --> O-2. Current heating c omputations for Martian entries assume CO oxidation to be fully catalytic. The resulting entry heating values are significantly higher than those comp uted using the assumption of fully catalytic oxygen recombination. The data presented here indicate that the assumption of fully catalytic CO oxidatio n may be overly conservative for heatshield sizing purposes.