relevant molecules forming one class, and simple molecules relevant to planetary 

 atmospheres and interstellar chemistry forming another. 



The reactions of energetic neutral oxygen atoms with large molecules is a 

 completely unexplored field, and it is quite likely that a new area of chemistry 

 exists. That the source of the Shuttle oxygen glow has not been determined by 

 an extensive literature search underscores the uniqueness of a continuous source 

 of energetic neutral oxygen atoms. 



5.3 An Artificial Comet 



Organic components of the gas and dust play a prominent part in many of 

 the new and exciting results from the study of comet Halley. Foremost among 

 these results were the discoveries of abundant, fine-grained dust composed of 

 dark, apparently carbonaceous, matter that mantles inactive regions and is 

 ejected from active regions of the comet surface; evidence for CO and C0 2 as 

 parent molecules, in addition to H 2 and HCN; and CN jets associated with the 

 emission of dust from the nucleus. Yet, unanswered questions remain regarding 

 parent-daughter molecular relationships and the structure and composition of 

 the dust rich in the biogenic elements. For instance: What parental species in 

 addition to HCN are needed to explain the abundance of the CN radical? What 

 is the role of dust in contributing to gaseous species in the coma? What proper- 

 ties predispose the grains to be retained as mantling material? What are parent 

 molecules for CS, C 2 , and C 3 ? What species are responsible for the 3.2- to 

 3.6-jum emission features suggestive of C-H bonds? 



Although these questions specifically address unknowns in the composition 

 and chemical evolution of comet Halley, the answers would apply to virtually 

 all comets. They reflect continuing uncertainties about the interstellar versus 

 nebular origin of these objects and their constituents, their relationships to other 

 primitive bodies in the solar system, and the contributions they may have made 

 to prebiotic evolution and the inventories of the biogenic elements and com- 

 pounds on Earth and the other terrestrial planets. The pertinence of all these 

 issues to exobiology continue to make the study of comets an area of high 

 priority. 



In 1984 the injection of barium particles into the solar wind on the Active 

 Magnetosphere Particle Tracer Explorers (AMPTE) mission was carried out as 

 an "artificial comet" experiment to elucidate aspects of the plasma interaction 

 with the comet. In the future, the placement and operation of platforms in space 

 will provide opportunities to conduct analogous simulation studies of other 

 facets of cometary phenomena involving more realistic materials. These studies 

 should be conducted to provide assessments of two interesting questions: the 

 origin of daughter molecules and ions from putative parent species in either the 

 gas or the dust emitted from comets, and the dynamical evolution of cometary 

 dust mantles, aggregates, and grains as a function of their physical properties and 

 chemical composition. 



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