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 ≈ 40 km. Aerosols slowly pervading through the methane condensation region could transport enough CO to generate the observed tropospheric enrichment provided the sticking coefficient ε (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 ε are allowed (down to ≈ 10 −6 ). Besides, the final radius of methane particles, just above the region where methane evaporates (below ≈ 10 km), must be 100 μm 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 CH 4 ice in the Titanian atmosphere.