STRATOSPHERIC DEPLETION OF CO ON TITAN

A deep stratospheric depletion of CO is found in Titan's atmosphere comparing millimetric and infrared observations [Marten et al., 1988; Lutz et al., 1983]. We suggest that adsorption of CO on aerosols in the region of Titan's atmosphere where methane is expected to condense may change the vertical distribution of CO below almost-equal-to 40 km. Aerosols slowly pervading through the methane condensation region could transport enough CO to generate the observed tropospheric enrichment provided the sticking coefficient epsilon (fraction of collisions yielding adsorption) is 6 10(-4), assuming the same eddy diffusion coefficient K as in the Earth troposphere. In the case of a lower mixing rate, generally favoured, smaller values of epsilon are allowed (down to almost-equal-to 10(-6)). Besides, the final radius of methane particles, just above the region where methane evaporates (below almost-equal-to 10 km), must be 100-mu-m within a factor of 10. The hypothesis presented in this paper, if it is confirmed by laboratory measurements, could be an indirect evidence for large amounts of CH4 ice in the Titanian atmosphere.