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Published April 20, 2015 | Published
Journal Article Open

Vertical Distribution of C_3-hydrocarbons in the Stratosphere of Titan

Abstract

Motivated by the recent detection of propene (C_3H_6) in the atmosphere of Titan, we use a one-dimensional Titan photochemical model with an updated eddy diffusion profile to systematically study the vertical profiles of the stable species in the C_3-hydrocarbon family. We find that the stratospheric volume mixing ratio of propene (C_3H_6) peaks at 150 km with a value of 5 × 10^(−9), which is in good agreement with recent observations by the Composite Infrared Spectrometer on the Cassini spacecraft. Another important species that is currently missing from the hydrocarbon family in Titan's stratosphere is allene (CH_2CCH_2), an isomer of methylacetylene (CH_3C_2H). We predict that its mixing ratio in the stratosphere is about 10^(−9), which is on the margin of the detection limit. CH_2CCH_2 and CH_3C_2H equilibrate at a constant ratio in the stratosphere by hydrogen-exchanging reactions. Thus, by precisely measuring the ratio of CH_2CCH_2 to CH_3C_2H, the abundance of atomic hydrogen in the atmosphere can be inferred. No direct yield for the production of cyclopropane (c-C_3H_6) is available. From the discharge experiments of Navarro-González & Ramírez, the abundance of cyclopropane is estimated to be 100 times less than that of C_3H_6.

Additional Information

© 2015 The American Astronomical Society. Received 9 January 2015; accepted for publication 23 March 2015; published 15 April 2015. This research was supported in part by NASA grants NNX09AB72G and Cassini UVIS program grant JPL.1459109 to the California Institute of Technology. X.Z. was supported by the Bisgrove Scholar Program at the University of Arizona. Y.L.Y. was supported in part by the NAI on Titan Astrobiology.

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