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Project Progress

Protostellar nebulae are oxygen rich, yet a significant quantity of organic matter is still evident in meteorites and in comets when one might predict that such materials should have reacted with the silicate dust to form large amounts of CO. We know that nearly all meteoritic material was processed through a series of high temperature events, producing Calcium Aluminum Inclusions (CAIs), chondrules, crystalline silicate dust and amorphous condensates. By analogy a similar proportion of the carbonaceous grains should also have experienced these same conditions, yet macromolecular carbonaceous coatings exist on grains in meteorite matrices and volatile organic molecules are observed in reasonable abundance in comets. We have proposed that surface mediated reactions on silicate dust grains could have played a major role in the production of both volatile and macromolecular carbonaceous materials in the nebular environment. This is based on recognition that both inward as well as outward circulation must occur in protostellar nebulae in order to explain the presence of crystalline silicate grains, and on recent experiments in our lab that demonstrate the resilience of surface mediated organic synthesis. We have talked about the basic model of nebular circulation as it relates to the production of crystalline mineral grains found in comets at several meetings. A summary of this general circulation model is shown below.

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Our recent experiments have demonstrated that almost any free surface will act to catalyze the Fischer-Tropsch-Type conversion of CO and molecular hydrogen to complex hydrocarbons. In addition, we have now discovered that one of the products of these reactions is a macromolecular carbon coating on the grain surface that itself acts as a catalyst to promote the formation of hydrocarbons containing nitrogen if the reaction occurs in the presence of N₂, H₂ and CO. Far from acting as a poison, these coatings can greatly enhance the reactivity of some grain species such as amorphous magnesium silicate or silica particles. The net result of this process should be a relatively uniform composition for the macromolecular carbonaceous materials deposited on meteoritic grains and a significant increase in the quantity of organic materials that may have been produced and then circulated throughout protostellar nebula compared to previous models.

We have also noted that Ed Anders and colleagues had discredited FTT type reactions as the source of meteoritic organics by noting that the products of such reactions were isotopically light whereas meteoritic carbon was isotopically heavy. However, because Anders only considered the volatile products of the FTT reactions in his analysis, and because the meteoritic organics are much more likely to consist of the macromolecular carbonaceous coatings left on grain surfaces, a logical conclusion would be to recognize that his previous analyses actually supports the hypothesis that surface-mediated reactions were primarily responsible for the production of organic materials in the Primitive Solar Nebula. In other words, if the volatile organics are made of isotopically light carbon, then the carbonaceous coatings that remain on the grain surfaces are likely to be isotopically heavy, just like measurements of the meteoritic carbon made by Anders.

We are continuing our studies of the initial composition of the volatile organic species produced on both amorphous iron-silicate and magnesium-silicate grains as a function of time, temperature and degree of previous reaction products deposited on the grains’ surfaces. We have noted that the quantity of volatile organic products decreases with increased surface coating, although the overall rate of reaction of the CO does not seem to be effected. This seems to imply that a much greater fraction of the CO is incorporated into the macromolecular carbonaceous grain coating as the surface area of the coating increases, and that the coating itself may not produce any volatile organic material at all but might simply produce a thicker organic coatings on the grains.

Laboratory synthesized calcium oxide and calcium hydroxide grains: A candidate to explain the 6.8 μm band Yuki Kimura and Joseph A. Nuth III
Astrochemistry Laboratory, Code 691, Solar System Exploration Division, NASA’s Goddard Space Flight Center, Greenbelt, MD 20771, USA

We demonstrate that CaO and Ca(OH)₂ are excellent candidates to explain the 6.8 μm feature, which is one of the most obscure features in young stellar objects. We discuss the condensation of CaO grains and the potential formation of a Ca(OH)₂ surface layer. The infrared spectra of these grains are compared with the spectra of fifteen young stellar objects. We note that CaO-rich grains are seen in all meteoritic CAIs (calcium-aluminum-rich inclusions) and that the 6.8 μm feature has only been observed in young stellar objects. Therefore, we consider CaO grains to be a much more plausible candidate to explain the 6.8 μm feature than the organic materials previously suggested in the literature and hypothesize that they are produced in the hot interiors of young stellar environments via distillation from pre-existing hibonite.