A new hypothesis is proposed for recycling Martian CO to CO2. The same hypothesis can satisfactorily explain the recently observed depletion in CO in the middle atmosphere of Mars. The mechanism involves oxidation of carbon monoxide through heterogeneous chemistry in the presence of aerosols. It is further suggested that H2O ice aerosols in the atmosphere of Mars are particularly effective in this process. The thrust for suggesting this mechanism came from the extensive presence of aerosols in the Martian atmosphere detected by the Auguste-Spectrophotometer Interferometer experiment on the Phobos spacecraft, combined with similar results from earlier missions, the detection of relatively low CO mixing ratios in the low to middle atmosphere by the Infrared Spectrometer experiment on the Phobos spacecraft, and the fact that earlier proposed mechanisms for recycling CO2 require either unacceptably high values of the eddy diffusion coefficient or a high waster vapor abundance in the middle atmosphere of Mars. The mechanism proposed in this paper might have an analog in the Antarctic ozone hole probelm, and it points out a need for laboratory measurements of appropriate sticking coefficients and rate constants. ¿ American Geophysical Union 1990