2015 Annual Science Report
NASA Goddard Space Flight Center Reporting | JAN 2015 – DEC 2015
Surface Mediated Reactions in the Primitive Solar Nebula
Hydrogen, carbon monoxide and nitrogen gases are abundant in the primitive solar nebula, as are silicate dust and metallic grains. These gases can react on such grain surfaces to produce an abundance of carbon-bearing products that include volatile hydrocarbons, amines, alcohols, aldehydes and acids as well as more complex, less volatile species such as carbon nanotubes. Refractory carbonaceous deposits catalyze additional surface reactions. Nebular environments span a large range in time, temperature, pressure, catalyst composition and secondary reactions. We are working to understand the rates and products of such reactions that could occur in nebular environments.
For the initial carbon (available as CO), we have investigated the fraction that forms gas-phase compounds and the fraction that deposits as a carbonaceous solid (the gas/solid branching ratio). We measure their ratio as a function of time and temperature for iron, magnetite and amorphous iron silicate smoke catalysts during surface mediated reactions, in an excess of hydrogen and in the presence of nitrogen. The gas/solid branching ratio varies from more than 99 percent for an amorphous iron silicate smoke at 673K to less than 40 percent for a magnetite catalyst at 873K. The CO not converted into solids primarily forms methane, ethane, water and carbon dioxide, as well as a very wide range of organic molecules with very low fractions. Surprisingly, carbon deposits do not form continuous coatings on the catalytic surfaces, but instead form “filamentous” structures with extremely high surface area per unit volume (Figure 1).
Surface mediated reactions in natural protostellar environments are much more complex than previously believed. The ratio of solid carbon deposited onto grain surfaces versus the volatiles released back into the gas phase depends strongly on temperature and time, on the properties of the initial catalyst (composition and surface area) and on the previous history of the catalyst, e.g., the run number, used as a proxy to track the relative amount of previously deposited carbon. Even more interesting, the surface area of all catalysts studied increased with time as carbon deposits formed on or even within grain surfaces. In addition, the order of the overall reaction appears to be a function of temperature. At lower temperatures, the reaction depends linearly on the concentration of CO, suggesting control by adsorption of CO at reactive sites on the catalyst. At higher temperatures the overall reaction rate approaches second order, indicating a more important role for the Boudouard reaction (disproportionation of CO into C + CO2). This reaction depends on both pressure (second order kinetics) and temperature. However, the CO concentration in protostellar nebulae is considerably less than that in our experiments, so such reactions might not become important in nebular environments until much higher temperatures are reached.
For the primitive solar nebula, surface mediated reactions might provide a solution for a problem that modern chemical models of nebular processes do not address; namely, the conversion of large quantities of CO and carbon dioxide (generated by high temperature reactions under oxidizing conditions) back into solid carbonaceous species that can be more easily incorporated into planetesimals. Planetesimal accretion could also be enhanced by the formation of fibrous, nanotube-like growth on the surfaces of larger refractory meteoritic components such as chondrules or CAIs.
Such surface coatings could dampen the energy of grain-grain collisions and significantly increase their sticking efficiency in the higher temperature inner nebula. Much more work is needed to unravel the effects of competing chemical processes on grain surfaces in order to model their possible impact on nebular chemistry and dynamics.