A non-monotonic eddy diffusivity profile of Titan's atmosphere revealed by Cassini observations

Abstract

Recent measurements from the limb-view soundings of Cassini/CIRS and the stellar occultations from Cassini/UVIS revealed the complete vertical profiles of minor species (e.g., C_2H_2 and C_2H_4) from 100 to 1000 km in the atmosphere of Titan. In this study, we developed an inversion technique to retrieve the eddy diffusion profile using C_2H_2 as a tracer species. The retrieved eddy profile features a low eddy diffusion zone near the altitude of the detached haze layer (~550 km), which could be a consequence of stabilization through aerosol heating. Photochemical modeling results using the retrieved eddy profile are in better agreement with the Cassini measurements than previous models. The underestimation of C_2H_4 in the stratosphere has been a long-standing problem in planetary photochemical modeling, and the new eddy diffusion profile does not solve this problem. In order to match the observations, we suggest a new expression for the rate coefficient of the key reaction, H + C_2H_4 + M⟶C_2H_5 + M. The new reaction rate coefficient is estimated to be ~10 times lower than that used by Moses et al. (2005)'s model, and should be validated in the laboratory and tested against the hydrocarbon chemistry of giant planets.

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