Methane escape from Titan's atmosphere

Measurements of the mole fractions of CH4 and Ar-40 by the Ion Neutral Mass Spectrometer on the Cassini orbiter are analyzed to determine the rate of vertical mixing in Titan's atmosphere and the escape flux of CH4. Analysis of the Ar-40 data indicates an eddy mixing rate of 2 - 5 x 10(7) cm(2) s(-1), an order of magnitude smaller than previously determined from analysis of the CH4 distribution. The eddy profile determined from the Ar-40 data implies that CH4 distribution is best explained by postulating that it is escaping the atmosphere at the diffusion limited rate of 2.5 - 3.0 x 10(9) cm(-2) s(-1), referred to the surface. This represents a significant loss of atmospheric CH4, smaller than but comparable to the photochemical destruction rate. The escape rate is much larger than predicted by the Jeans escape formula and vigorous nonthermal mechanisms are not apparent.