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Project Progress

Analytical studies of natural materials such as meteorite components, interplanetary dust particles and presolar grains, and telescopic observations of the solids in comets, protostellar nebulae, giant molecular clouds and the interstellar medium place important chemical and textural constraints on the processing of materials throughout the history of the solar system. Many of these same constraints may also apply to the materials in modern protostellar systems. Unfortunately, the exact processing history of any given specimen is unknown, as is its original chemical or mineralogical composition prior to a processing event. Natural samples are often chemically complex materials that may have been formed by a variety of processes and in a range of environments at different times, all of which could result in the same end product. On the other hand, remote sensing studies of natural materials often average out many of the more distinctive or unusual aspects of a sample population, and may be blind to some components; e.g., large (mm-scale) mineral grains observed in comets or protostars appear as gray bodies, void of distinctive spectral information and might appear the same as a collection of very small (10nm—50nm), loosely bound aggregates of mm-scale.

We make dirt. We believe that the materials that we make in the laboratory are simple analogs of much more complex natural systems (Nuth et al, 2000, 2002). Our typical experiments are designed to study a natural process, though the parameters of the process might need to be considerably different in some aspects than would be found in nature. Preliminary results from the STARDUST Mission seem to indicate that crystalline grains are present in Kuiper Belt comets, and this point is also supported by the initial results from the Deep Impact mission. These observations indicate that large-scale materials transport probably occurred in the solar nebula, as we have advocated for many years (Nuth, 1999, Hill et al., 2001) based on our previous laboratory experiments. Our nebular model is taken from Nuth (2001).

Large-scale transport in protostellar nebula is an extremely important factor in understanding the chemistry of such systems. Most previous chemical models (e.g. Kress and Tielens, 2001) are “one pass systems.” By that we mean that as material falls into the nebula from the Giant Molecular Cloud and begins to evolve due to temperature-pressure changes closer to the sun, all chemical changes must be completed before the material reaches the radius at which it becomes incorporated into a larger body. As an example, Kress and Tielens (2001) modeled the Fischer-Tropsch conversion of CO and H₂ into CH₄ as a gas parcel fell towards the sun in order to see if such processes might produce enough organic materials to match the carbon/silicon ratio found in meteorites by the time the parcel reached ~3A.U. Another example was the hypothesis that Giant Gaseous Protoplanets might have supplied comets with the large quantity of volatile organic molecules observed (Fegley, 1999) because they could not be formed at the very low temperatures and pressures of the outer nebula where the comets formed. Large-scale mixing and transport (e.g., Boss, 2006) eliminates such concerns by allowing the reactive synthesis of important materials to occur at appropriate places within the nebula, then transporting that material throughout the system to the observed reservoirs. Of course this results in a much more complicated dynamic and chemical system, but one that should be closer to reality.

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(From “How were the comets made? Joseph A. Nuth III, American Scientist 89, 230-237 [2001])

We have found that amorphous iron silicate smokes are extremely efficient Fischer-Tropsch type catalysts, although we have also observed most solid materials tend to mediate the reaction of H₂ + CO (+N₂) => complex organic materials. More recently we have found that as the complex macromolecular carbonaceous deposit builds up on the iron silicate grains, their activity decreases to a small extent, while as this material increases on the surface of much less reactive materials, their catalytic activity increases greatly. Combined with the idea that considerable transport occurs from the inner to the outer nebula, this implies that large amounts of organic materials could have been produced in the high pressure — high temperature regions of the inner nebula, then been transported to regions of planetesimal growth. Although this mechanism had previously been abandoned because Anders and colleagues (Hayatsu and Anders, 1981) found that FTT reactions produce isotopically light products while meteoritic organics are enriched in heavier isotopes of carbon, this argument is quite supportive of the idea that the macromolecular organic materials deposited on the grains (and expected to be isotopically heavy, especially compared to the volatile products measured by Anders) are actually the same organics that are incorporated into the meteorite parent body. We have therefore initiated detailed studies of the time and temperature dependent rates of FTT-like reactions on amorphous grains.