The CH4 density in the upper atmosphere of Titan

Previous modeling by Banaszkiewicz et al. (2000a,b) showed that the CH4 thermospheric mixing ratio on Titan could vary as much as 35-40% due to ion-neutral chemical reactions. A new vertical methane profile has been computed by simultaneously modifying the stratospheric methane mixing ratio and the K(z) previously considered by Lara et al. (1996) and Banaszkiewicz et al. (2000a,b). A satisfactory fit of the methane thermospheric abundance and stratospheric mixing ratio of other minor constituents is achieved by placing the homopause at similar to1000 km and increasing the met ne stratospheric mixing ratio (q(CH4)) up to 3.8%. The new prop Sed eddy diffusion coefficient steadily rises from 1 X 10(7) cm(2) s(-1) at 700 km to I X 10(10) cm(2) s(-1) at 1500 km, whereas the stratospheric values are in the range (4-20) x 10(3) cm(2) s(-1). Other likely ionization sources that can influence the methane distribution are (i) a metallic ion layer produced by micrometeoroid infall and (ii) frequent X-rays solar flares. Analysis of the effects of these ionization sources on the methane distribution indicates that, unlike previously assumed, CH4 can suffer considerable variations. These variations, although proved in this work, must be cautiously regarded since several assumptions have to be made on the rate of N-2 and CH4 ionization by the processes previously mentioned. Hence, these results are only indicative of methane sensitivity to ionospheric chemistry. (C) 2002 Elsevier Science (USA).