DPS 2001 meeting, November 2001
Session 22. Outer Planet Atmospheres II: Chemistry and Thermal Structure
Oral, Chairs: K. Rages, J. Moses, Wednesday, November 28, 2001, 3:00-4:30pm, Regency E

[Previous] | [Session 22] | [Next]

[22.08] Rate Constant for the CH3 Recombination Reaction at T = 155K: A Loss Process in Outer Planet Atmospheres

R.J. Cody, L.J. Stief (NASA/GSFC), F.L. Nesbitt (Coppin State College), M.A. Iannone (Millersville University)

The methyl free radical (CH3) has been observed in the atmospheres of Saturn, Neptune and recently by Cassini - CIRS in Jupiter. Atmospheric models predict much higher abundances of CH3 than observed, especially in the case of Saturn. A major, but not exclusive, cause of the disagreement has been identified as the underestimation of the loss process for CH3 via the self-reaction at the low temperatures and pressures prevailing in these atmospheric systems.

Although the reaction CH3 + CH3 + M arrow C2H6 + M has been extensively studied both theoretically and experimentally, the laboratory conditions have been, with only a few exceptions, higher temperatures and pressures than those present in the outer planet atmospheres or M=Ar rather than H2 or He as the bath gas. We are measuring the rate constant of this reaction under physical conditions more suitable to these atmospheres, i.e. P = 0.6-2.0 Torr of He and T < 298K. Our measurements are now extended to T = 155K. The experimental technique is discharge fast flow with mass spectrometric detection and monitoring of the CH3 decay. The methyl radical is generated via the fast reaction F + CH4 arrow CH3 + HF. The lowest temperature of our or any other previous research was T = 200K. To work at lower temperatures required a new flow tube design and a different cooling technique. Both of these changes were made successfully.

The experimental results at T = 155K and P = 0.6-1.5 Torr He will be presented. The results are shown to be invariant for CH3 concentrations in the range (4.5-10.8)x1012 radicals cm-3. The impact of these results upon photochemical models of planetary atmospheres will be discussed in the paper by P. Romani and D. Tardy to be presented at this meeting.

The Planetary Atmospheres Program of NASA Headquarters has provided the funding for this research.

[Previous] | [Session 22] | [Next]