The gases selected for release would depend on the theoretical parent- 

 daughter relationships that are being tested and could include carbon monoxide, 

 carbon dioxide, methane, acetylene, ammonia, hydrogen cyanide, and acetoni- 

 trilc, among others. While lacking a full complement of gases, ices, or other 

 grains, these experiments would necessarily be incomplete simulations. On the 

 other hand, ground-based experiments would be similarly limited; moreover, the- 

 simulations of the vacuum, volume, and time scales of the inner coma would be 

 impossible in the laboratory. 



Formation of a gas-laden ice ball may be achieved by expanding a mixture of 

 water vapor with various gases, through a nozzle, into a vacuum chamber to 

 produce a snow, which may then be consolidated by compaction to the desired 

 extent. Dust particles could be injected simultaneously and mixed with the 

 snow. Alternatively, quantities of water vapor and gases could be frozen on a 

 cold surface and continuously scraped from it, until a large enough quantity is 

 accumulated to make into the ~1-m (or larger) ice ball. An ice ball of this 

 size is calculated to last for about 10 s sec (about 28 hours) under full solar 

 heating. 



Ices could be prepared at various densities and at different temperatures 

 corresponding to amorphous or crystalline states. Recent laboratory experiments 

 indicate that, depending on the temperature of formation and thermal history of 

 the ice, gases may be occluded in amorphous or crystalline forms of ice; differ- 

 ent gas-release characteristics may prevail, depending on the ice phase, in some 

 cases accompanied by ice grain ejection. Preservation of such sites would be 

 essential for study. If formed at temperatures lower than 80 K, the ball should 

 be kept in a dewar cooled by liquid helium, itself enclosed in a dewar of liquid 

 nitrogen, until the time of release. 



In the cases where dusts of either silicate or carbonaceous composition or 

 both were embedded in ice, the choice of material to simulate the dust would 

 necessarily be model-dependent. Candidate materials for carbonaceous grains, 

 for instance, could be amorphous carbon, polymers of HCN, coal dust, and ter- 

 restrial kerogens, among others. 



Experiments with an ice ball could not simulate the heterogeneity of physical 

 and chemical composition or the irregularity in the distribution of surface-active 

 sites that were actually observed in comet Halley. Determining how simpler 

 model systems behave, however, would be a prerequisite for gaining understand- 

 ing of the more complex systems. 



Precursor laboratory simulations should be conducted in a large, ground- 

 based, high-vacuum chamber where an ~1-m ice ball could be formed and sub- 

 jected to heating by infrared or xenon arc lamps. 



When compared with the large body of observational data now available as a 

 result of studies of comet Halley, it is expected that observations made on an 

 artificial coma and tail produced by a gas-laden ice ball in Earth orbit, together 



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