Methane escape from Titan's atmosphere

Abstract

Measurements of the mole fractions of CH₄ and ⁴⁰Ar 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 ⁴⁰Ar data indicates an eddy mixing rate of 2-5 × 10⁷ cm² s^-1, an order of magnitude smaller than previously determined from analysis of the CH₄ distribution. The eddy profile determined from the ⁴⁰Ar 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 × 10⁹ 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.