in molecular clouds. The more complex molecules produced by the ultraviolet 

 photolysis of the simple molecules in the mantles are more refractory than the 

 simple ices and therefore can better survive in the harsh environment of the 

 diffuse interstellar medium. These processes can be studied in the laboratory, 

 and preliminary results of such studies indicate the ultraviolet photolysis can be 

 important in the evolution of interstellar dust. Some observational support for 

 the presence of complex molecular-grain mantles in the diffuse interstellar 

 medium exists. 



Models for the formation of the dust are likewise controversial. The refrac- 

 tory grain materials, such as silicates and the carbonaceous grains, are presum- 

 ably formed at high temperatures (100-2000 K) in the carbon-rich or oxygen- 

 rich outflow from late-type red giants, supergiants, planetary nebulae, novae, 

 supernovae, and protostellar nebulae. The detection of large infrared excesses 

 supports the picture of grain condensation around late-type red giants, super- 

 giants, and planetary nebulae. No unambiguous observational evidence exists yet 

 for the condensation of dust around supernovae. The primary locations for the 

 condensation of grains composed of the biogenic elements is one of the ques- 

 tions that the exobiology community would like to answer. As discussed earlier, 

 the complex grain mantles are presumed to be formed after the condensation 

 process, when the grains have been cycled within molecular clouds in the inter- 

 stellar medium. 



Little is known about the actual dust condensation process in the outflow 

 from stellar objects. Observations typically lack the spatial resolution required to 

 resolve the condensation zone, while experiments and theory are hampered by 

 the lack of knowledge of the relevant physical conditions in these regions. 

 Condensation possibly occurs in thermodynamic equilibrium with the most 

 refractory material condensing into small nuclei onto which somewhat less 

 refractory materials condense out sequentially when cooling nuclei reach the 

 condensation temperature of those materials in the outflow. Alternatively, 

 condensation might take place under highly supersaturated conditions and all 

 materials might then condense out more or less simultaneously into an 

 amorphous material. Since grains offered the best opportunity for biogenic 

 molecules to become incorporated into the protosolar nebula, it is most impor- 

 tant to understand the composition and elemental inventory of grains in distinct 

 locales. Of particular interest is the incorporation of trace constituents, which 

 have been detected in meteorites (e.g., 26 AI and S-process Xe) into this Stardust. 

 These tracers may allow the nucleosynthetic history of the parent grains to be 

 reconstructed. 



Finally, another important characteristic of grains, their distribution, is also 

 highly dependent on the condensation process. In particular, is coagulation 

 important for the formation of large grains? Obviously, large fluffy conglomer- 

 ates of small particles may have quite different physical properties (surface area, 

 strength, extinction properties) than large homogeneous particles. 



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