Low-Temperature Hydrocarbon Photochemistry: CH3 + CH3 Recombination in Giant Planet Atmospheres

Details Low-Temperature Hydrocarbon Photochemistry: CH3 + CH3 Recombination in Giant Planet Atmospheres Low-Temperature Hydrocarbon Photochemistry: CH3 + CH3 Recombination in Giant Planet Atmospheres

Nov 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002nla..work..258s&link_type=abstract

NASA Laboratory Astrophysics Workshop, held May 1-3 2002 at NASA Ames Research Center, Moffett Field, CA 94035-1000. Publisher:

Astronomy and Astrophysics

Astrophysics

Scientific paper

Planetary emissions of the methyl radical CH3 were observed for the first time in 1998 on Saturn and Neptune by the ISO (Infrared Space Observatory) mission satellite. CH3 is produced by VUV photolysis of CH4 and is the key photochemical intermediate leading complex organic molecules on the giant planets and moons. The CH3 emissions from Saturn were unexpectedly weak. A suggested remedy is to increase the rate of the recombination reaction: CH3 + CH3 + H2 → C2H6 + H2 at 140 K to a value at least 10 times that measured at room temperature in rare gases, but within the range of disagreeing theoretical expressions at low temperature. We are performing laboratory experiments at low temperature and very low pressure. The experiments are supported by RRKM theoretical modeling that is calibrated using the extensive combustion literature. The distinction between ``high" and ``low" pressure is a significant one. In the so called ``low pressure limit" the rate of recombination is limited by the rate of stabilization or energy removal by the third body called ``M" (really H2), and the overall recombination rate coefficient is written as: krecomb(M→0) ~ k0[M] In the ``high pressure limit" the buffer gas pressure is sufficiently high to stabilize every collision complex, and the overall recombination rate coefficient becomes pressure independent: krecomb(M→infty) ~ kinfty Calculations will be presented that indicate that k0 rises with decreasing temperature much faster than does kinfty. These results mean that low temperature laboratory experiments need to be performed at quite low pressures, say 0.01 mbar or less in order to extrapolate to the 0.001 mbar and below characteristic of the relevant regions of the giant planet atmospheres. This is consistent with the recent work in Stief's laboratory [1,2], in which no pressure dependence was observed at 155 and 202 K for pressures from 0.6 to 2.6 mbar. Supported by the NSF Planetary Astronomy and NASA Planetary Atmospheres programs.

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