Methane escape from Titan's atmosphere

Abstract

Measurements of the mole fractions of CH 4 and 40 Ar 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 CH 4 . Analysis of the 40 Ar data indicates an eddy mixing rate of 2–5 × 10 7 cm 2 s −1 , an order of magnitude smaller than previously determined from analysis of the CH 4 distribution. The eddy profile determined from the 40 Ar data implies that CH 4 distribution is best explained by postulating that it is escaping the atmosphere at the diffusion limited rate of 2.5–3.0 × 10 9 cm −2 s −1 , referred to the surface. This represents a significant loss of atmospheric CH 4 , 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.