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Published December 2014 | public
Journal Article

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


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.

Additional Information

© 2013 Published by Elsevier Ltd. Received 15 October 2013; Accepted 23 October 2013; Available online 23 November 2013. This research was supported in part by the Cassini UVIS program via NASA grant JPL.1459109 to the California Institute of Technology. Y.L.Y., C.L. and X.Z. were supported in part by NASANNX09AB72G grant to the California Institute of Technology. Y.L.Y. was also supported in part by NAI via Dr. M. Allen of J.P.L. We thank Dr. M. Line for the discussion on the retrieval algorithm, Dr. P. Lavvas for giving us detailed aerosol parameters from his model and Dr. M. Gerstell for improving the paper.

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