Following the extensive studies of comet Halley, and in the interim period 

 before another major comet mission, the first problem can now be approached 

 only with models based on phenomena observable by optical and radio tech- 

 niques. In particular it is possible to differentiate and study the coma and tail 

 when the comet is at small heliocentric distances, while an unresolved asteroid- 

 like inactive nucleus is all that is accessible for comets at several astronomical 

 units from the Sun. 



The generally accepted cometary model of an icy conglomerate a few kilo- 

 meters in diameter containing both volatile and refractory components was first 

 proposed on the basis of an analysis of the "nongravitational" force perturba- 

 tions of the orbit of periodic comet Encke. These forces arise from the jet action 

 produced by the nonuniform vaporization of matter from the surface of a 

 rapidly rotating comet nucleus. Subsequent investigations have concluded that 

 the major volatile component controlling the vaporization must be water ice, 

 and recent ultraviolet spectra of a number of comets provide strong confirming 

 evidence for this idea. Not only is water ice confirmed as the dominant volatile 

 in the nucleus, but an initial composition of volatiles similar to what is found in 

 interstellar molecular clouds appears to be needed to account for the observed 

 abundances of visible radicals. In a different approach, which ignores the details 

 of the chemistry and simply counts the end-product atoms that are then assem- 

 bled into a hypothetical nucleus, water ice again emerges as the dominant 

 constituent. 



Unlike planetary atmospheres, which are gravitationally bound and exhibit 

 only relatively mild temporal variations, the atmosphere of a comet is a highly 

 transitory and variable phenomenon. The most rudimentary coma models 

 assume that the parent molecules sublimate at the surface of the nucleus and 

 flow radially outward, subject only to the solar particle and ultraviolet radiation 

 fields that progressively decompose them into their constituents atoms (or ions), 

 which continue to flow radially outward. Although rudimentary, this one- 

 dimensional picture serves as a basis for derived abundances of H, O, and OH in 

 the coma that appear to be consistent with an H 2 source that is at least an 

 order of magnitude more abundant than any other hydrogen-bearing molecule. 

 For comet Halley the spacecraft measurements verified this predominance of 

 water. 



The fundamental compositional differences between comets, as deduced from 

 ground-based observations, is the dust-to-gas ratio. This is related to the amount 

 of observed continuum radiation (sunlight reflected from solid grains) relative to 

 gas fluorescence produced by C 2 , C 3 , CN, etc., which is taken as a measure of 

 the total gas production rate. Among the species detectable in the visible (which 

 represent less than 1% of the total volatile component), there appears little 

 variation from comet to comet, provided that observations are compared at simi- 

 lar heliocentric distances. The abundance of these species relative to H 2 (or at 

 least to OH presumably derived from H 2 0) is also relatively constant from 



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