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B. Fegley, Jr. (Planetary Chemistry Laboratory, Dept. of Earth \& Planetary Sciences, Washington University, St. Louis, MO 63130)
The iron grain catalyzed conversion of carbon monoxide to methane has been studied in carbon monoxide - hydrogen gas mixtures with the solar C/H ratio. This was done to test theoretical predictions by Lewis, Prinn, and myself that CO is converted to methane in the hypothesized subnebulae around the Jovian planets during their formation. Different types of iron catalysts, including pure iron metal, iron meteorite metal, and ordinary chondrites have been used for the reactions. The experiments are done in a flow system and reproduce the total pressures, temperatures, and C/H ratios for Jovian subnebula models in the literature. The amounts of methane and water produced were measured by gas chromatography. The catalyst surface areas were measured by gas absorption. The results show that CO is efficiently converted to methane. The efficiency varies with the type of catalyst; iron metal is most active, then iron meteorite metal, then the ordinary chondrite. The measured methane to water ratios are unity, as expected for the reaction CO + 3H2 = CH4 + H2O. The area-normalized methane production for two natural catalysts (Gibeon iron meteorite and Gao ordinary chondrite) were used to model methane production as a function of iron grain size and time in the subnebulae. The results confirm the theoretical predictions that methane was formed in Jovian protoplanetary subnebulae. Second, the experiments show that abundant methane was present during Titan's formation with a methane/CO ratio in the Saturnian subnebula much less than unity. Third, methane formation in the lower pressure solar nebula (about 10,000 times lower pressure) is much less efficient. Acknowledgments. This work was supported by grants from the NASA Origins of Solar Systems and Planetary Atmospheres Program.