Experimental investigation of surface reactions in carbon monoxide and oxygen mixtures

During hypersonic entry into the CO2 atmosphere of Mars, competing exothermic chemical reactions may occur on a spacecraft heatshield surface. Two possible surface reactions are O + O --> O-2 and CO + O --> CO2. The relative importance of these reactions on quartz is investigated using a diffusion tube side-arm reactor together with two-photon laser-induced fluorescence for both O and CO species detection. The experiments show 1) that the presence of CO in the gas phase does not significantly affect the oxygen recombination reaction on quartz and 2) that the gas-phase CO concentration is not significantly altered by the presence of atomic oxygen. These results indicate that for our experimental conditions the dominant surface reaction on quartz in oxygen-carbon monoxide mixtures is O + O --> O-2. Current heating computations for Martian entries assume CO oxidation to be fully catalytic. The resulting entry heating values are significantly higher than those computed using the assumption of fully catalytic oxygen recombination. The data presented here indicate that the assumption of fully catalytic CO oxidation may be overly conservative for heatshield sizing purposes.