accretion in the space environment. Rates of accretion could be determined as a 

 function of the chemical composition, physical structure, and relative velocities 

 of the grains. 



For example, the effects of grain rotation on accretion are unknown. Large 

 grains in interstellar space may be spun up nonthermally to angular speeds of 

 10 5 -10 8 rev/sec. What happens when submicrometer core-mantle particles that 

 are so rapidly spinning collide? Do they melt and stick or do they immediately 

 tear apart? Up to what limiting rotational frequencies will they predominantly 

 coagulate? One approach to answering this question would be to insert ice- 

 coated iron needles about 0.1 micrometer thick into a cooled microgravity 

 chamber, and spin them up with a high-frequency magnetic field. The chamber 

 walls would have to be maintained at low temperatures-preferably ~20 K. Light 

 scattering is an excellent method to then follow the process of aggregation by 

 measuring both the scattered intensity at several angles and the polarization of 

 the transmitted and the scattered light. 



In addition to growing by the passive accretion of gaseous species to its sur- 

 face, a dust grain can provide an active surface to catalyze reactions of species 

 sorbed to it or it can itself be changed by chemical reactions with sorbed gases. 

 Chemical reactions between gas and dust have been hypothesized to occur in 

 interstellar clouds and in the solar nebula to account for the organic matter 

 observed by radio astronomers in the clouds and by chemists in meteorites, 

 comets, and interplanetary dust. These ideas are often expressed in terms of a 

 Fischer-Tropsch-Type (FTT) synthesis in which the surfaces of silicate, metal, 

 or metal oxide grains suspended in interstellar clouds or the solar nebula provide 

 active sites for catalysis. Molecules of hydrogen, carbon monoxide, carbon 

 dioxide, and ammonia sorbed on the sites at temperatures from 300 to 600 K 

 may have been converted spontaneously to organic compounds and other car- 

 bonaceous phases. These products, in the case of the solar nebula, were subse- 

 quently retained on the grains and accreted into primitive planetesimals. Accord- 

 ing to the FTT synthesis scenario, interstellar molecules represent products of 

 nebular synthesis that were ejected into the surrounding medium during dissi- 

 pation of prestellar nebulae. 



Recent data from analyses of organic matter in meteorites and from labora- 

 tory FTT syntheses suggest, however, that the FTT processes cannot explain all 

 the observed molecular and isotopic characteristics of the natural products. 

 What they can account for remains to be clearly established, and additional 

 artificial syntheses may provide the clues, provided they are conducted under 

 conditions that may be related to the natural processes. 



All laboratory FTT reactions have been conducted at or near a total pressure 

 of 1 atm with a bed of catalysts. Under these conditions, grains contact each 

 other, chemical intermediates can migrate from catalyst sites on one grain to 

 those on others, and opportunities exist for a diverse chemistry. In the nebular 

 environment, the total pressure is 1 0~ 3 to 1 0~ 6 atm and dust is expected to com- 



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