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Rate Constant for the Reaction CH3 + CH3 Yields C2H6 at T = 155 K and Model Calculation of the CH3 Abundance in the Atmospheres of Saturn and NeptuneThe column abundances of CH3 observed by the Infrared Space Observatory (ISO) satellite on Saturn and Neptune were lower than predicted by atmospheric photochemical models, especially for Saturn. It has been suggested that the models underestimated the loss of CH3 due to poor knowledge of the rate constant k of the CH3 + CH3 self-reaction at the low temperatures and pressures of these atmospheres. Motivated by this suggestion, we undertook a combined experimental and photochemical modeling study of the CH3 + CH3 reaction and its role in determining planetary CH3 abundances. In a discharge flow-mass spectrometer system, k was measured at T = 155 K and three pressures of He. The results in units of cu cm/molecule/s are k(0.6 Torr) = 6.82 x 10(exp -11), k(1.0 Torr) = 6.98 x 10(exp -11), and k(1.5 Torr) = 6.91 x 10(exp -11). Analytical expressions for k were derived that (1) are consistent with the present laboratory data at T = 155 K, our previous data at T = 202 K and 298 K, and those of other studies in He at T = 296-298 K and (2) have some theoretical basis to provide justification for extrapolation. The derived analytical expressions were then used in atmospheric photochemical models for both Saturn and Neptune. These model results reduced the disparity with observations of Saturn, but not with observations of Neptune. However, the disparity for Neptune is much smaller. The solution to the remaining excess CH3 prediction in the models relative to the ISO observations lies, to a large extent, elsewhere in the CH3 photochemistry or transport, not in the CH3 + CH3 rate.
Document ID
Document Type
Reprint (Version printed in journal)
External Source(s)
Cody, Regina J. (NASA Goddard Space Flight Center Greenbelt, MD, United States)
Romani, Paul N. (NASA Goddard Space Flight Center Greenbelt, MD, United States)
Nesbitt, Fred L. (Catholic Univ. of America Washington, DC, United States)
Iannone, Mark A. (Millersville Univ. PA, United States)
Tardy, Dwight C. (Iowa Univ. Iowa City, IA, United States)
Stief, Louis J. (NASA Goddard Space Flight Center Greenbelt, MD, United States)
Date Acquired
August 22, 2013
Publication Date
November 15, 2003
Publication Information
Publication: Journal of Geophysical Research: Planets
Volume: 108
Issue: E11
Subject Category
Lunar and Planetary Science and Exploration
Report/Patent Number
Distribution Limits