A quantitative inventory of organic compounds and their distribution with 

 altitude within the atmosphere may illuminate the specific production mecha- 

 nisms responsible for their existence. At the moment, such mechanisms are 

 largely matters of theoretical speculation. Detailed atmospheric observations will 

 also contribute to an understanding of the aerosols and surface materials that 

 have accumulated over the age of Titan. 



Suggestions for Further Reading 



Hunten, D. M.; Tomasko, M. G.; Flasar, F. M.; Samuelson, E. E.; Strobel, 

 D. F.; and Stevenson, D. J.: Titan. In Saturn, T. Gehrels and M. S. Mathews, 

 eds., University of Arizona Press, 1984. 



3.5 Comets 



Comets are especially interesting as frozen remnants of the primordial solar 

 system. As such, they might provide clues needed to understand the subsequent 

 evolution and differentiation of the planets and their satellites following the 

 initial condensation of the solar nebula. Was there something special about the 

 conditions in the early evolution of the solar nebula that created the terrestrial 

 planets at the right distance from the Sun so that at least one of them was able 

 to chemically evolve life? In addition, the comets themselves may have contrib- 

 uted substantially to the inventory of volatiles and organics available to the 

 primitive Earth. We can estimate the rate of cometary impacts early on from the 

 cratering histories of the Moon and Mars, but we need to know the abundance 

 and probable structure of the biogenic elements within the comets to know how 

 much they might have affected the earliest phases of abiotic chemical evolution 

 on Earth. Three of the basic questions to be answered are 



1 . What are the compositions and structures of comets? 



2. Where and how were they formed? 



3. Did they contribute volatiles and/or organics to Earth? 



While specialized missions that approach or penetrate particular comets may 

 yield the most precise chemical information, such missions sample only one 

 comet at a time. To understand the global characteristics of the early solar 

 nebula will require a large statistical sample of cometary properties. Such data 

 can be collected only by remote observational surveys. What is learned from 

 these observations may well influence the detailed design of individual future 

 cometary missions such as CRAF. There will necessarily be much indirect infer- 

 ence associated with remote cometary observations, but that does not reduce 

 their importance. 



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