The role of Fischer-Tropsch catalysis in solar nebula chemistry

Fischer-Tropsch catalysis, the iron/nickel catalyzed conversion of CO and H(2) to hydrocarbons, would have been the only thermally-driven pathway available in the solar nebula to convert CO into other forms of carbon. A major issue in meteoritics is to determine the origin of meteoritic organics: are they mainly formed from CO in the solar nebula via a process such as Fischer-Tropsch, or are they derived from interstellar organics? In order to determine the role that Fischer-Tropsch catalysis may have played in the organic chemical evolution of the solar nebula, we have developed a kinetic model for this process. Our model results agree well with experimental data from several existing laboratory studies. In contrast, empirical rate equations, which have been derived from experimental rate data for a limited temperature (T) and pressure (P) range, are inconsistent with experimental rate data for higher T and lower P. We have applied our model to pressure and temperature profiles for the solar nebula, during the epoch in which meteorite parent bodies condensed and agglomerated. We find that, under nebular conditions, the conversion rate of CO to CH(4) does not simply increase with temperature as the empirically-derived equations suggest. Instead, our model results show that this process would have been most efficient in a fairly narrow region that coincides with the present position of the asteroid belt. Our results support the hypothesis that Fischer-Tropsch catalysis may have played a role in solar nebula chemistry by converting CO into less volatile materials that can be much more readily processed in the nebula and in parent bodies.