Kinetics of the heterogeneous reaction CO + O → CO 2 on inorganic oxide and water ice surfaces: Implications for the Martian atmosphere

The heterogeneous reaction CO + O → CO 2 on water ice and various inorganic oxide (SiO 2 , Al 2 O 3 , Fe 2 O 3 , Fe 3 O 4 , CaO, MgO, and TiO 2 ) surfaces has been investigated in order to evaluate the proposition that this heterogeneous reaction may contribute to the CO 2 stability in the Martian atmosphere. The reaction probabilities were measured at both 196 K and 294 K by using a fast flow‐tube reactor coupled to an electron‐impact ionization mass spectrometer. The atomic oxygen was generated by passing 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 2 , was monitored as a function of time at the downstream end of the flow‐tube reactor. The measured reaction probabilities were determined from the growth rates of CO 2 signal and were found to range from 1.6 × 10 −7 to 1.0 × 10 −5 on the basis of the geometric area of the reactor. If we adopt a typical reaction probability of 5 × 10 −7 for the heterogeneous CO + O reaction on Martian aerosols at 196 K, a simple calculation suggests that this mechanism plays a negligible role in CO 2 recycling as compared to that of the homogeneous CO + OH reaction.