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[40.26] Sinks for CH₃ Radicals in Planetary Atmospheres: Stabilization of Chemically Activated C₂H₆ at 150 K.

D. C. Tardy (Department of Chemistry, The University of Iowa, Iowa City, Iowa 52242)

The predicted column abundances for CH₃ radicals are larger than what is observed by the ISO satellite for Saturn and Neptune. Models used for those predictions were based on extrapolating rate coefficients (k) for the reaction of CH₃ + CH₃ that produces vibrationally excited C₂H₆. The extrapolations were necessary because of the experimental difficulty in reproducing the atmospheric conditions of Saturn and Neptune in the laboratory: 150 K and 10^-5 mbar with H₂ and He. Recently, k was measured at 155 K in a He bath (see NASA/GSFC group listed in the Acknowledgements listed below); these results reduce the discrepancy between the observed and predicted column abundances. This work was undertaken to test the assumption that the energy removed by He per collision in stabilizing C₂H₆ is comparable to that of H₂. If the collision 'time' increases with decreasing temperature (sticky collisions) the energy removed would increase and k would increase; this increase in k would reduce the column abundance discrepancy. Computer experiments which simulated the collisions of vibrationally excited C₂H₆ (~30,000 cm^-1 of internal energy) with He and H₂ at 300 and 150 K were performed by integrating Hamilton's equation of motion with statistically sampled initial conditions. Within the statistics of the calculations He and H₂ have similar energy transfer removal rates with no apparent temperature dependence. All of these calculated removal rates decrease as the excitation energy of C₂H₆ decreases from 30,000 to 15,000 cm^-1. The energy removal rates will be compared to experimental values and their importance on the column abundances of CH₃ will be discussed. Acknowledgements. This work was supported by the Iowa Space Consortium. Interactions with Louis J. Stief, Regina J. Cody and Paul N. Romani from the Laboratory for Extraterrestrial Physics, NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771 were beneficial for this work.

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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.