KINETICS OF THE HETEROGENEOUS REACTION CO-]CO2 ON INORGANIC OXIDE ANDWATER ICE SURFACES - IMPLICATIONS FOR THE MARTIAN ATMOSPHERE(O)

The heterogeneous reaction CO + O --> CO2 on water ice and various ino rganic oxide (SiO2, Al2O3, Fe2O3, Fe3O4, CaO, MgO, and TiO2) surfaces has been investigated in order to evaluate the proposition that this h eterogeneous reaction may contribute to the CO, stability in the Marti an atmosphere. The reaction probabilities were measured at bath 196 K and 294 K by using a fast flow-tube reactor coupled to an electron-imp act ionization mass spectrometer. The atomic oxygen was generated by p assing a mixture of O-2 and He through a microwave discharge at a side -arm inlet and then allowed to react with CO over these substrates in the reactor. The reaction product, CO,, was monitored as a function of time at the downstream end of the flow-tube reactor. The measured rea ction probabilities were determined from the growth rates of CO2 signa l and were found to range from 1.6 x 10(-7) to 1.0 x 10(-5) on the bas is of the geometric area of the reactor. If we adopt a typical reactio n probability of 5 x 10(-7) for the heterogeneous CO + O reaction on M artian aerosols at 196 K, a simple calculation suggests that this mech anism plays a negligible role in CO2 recycling as compared to that of the homogeneous CO + OH reaction.