Oro: Experimental Organic Cosmochemistry 465 



comets are supposed to contain large amounts of reactive carbon compounds 

 and because it is considered that their composition reflects approximately the 

 composition of the primordial solar nebula and protoplanets.'*'* Indeed, a 

 recent model for the protoplanets of the solar system,-''^ as suggested by Fowler, 

 is almost identical to a cometary model proposed some time ago by Whipple^^ 

 and recently revised by the same author.'*^ Cn the basis of this physical and 

 chemical similarity it is reasonable to assume that the chemical processes which 

 occur in comets by the action of solar radiation, when these bodies are at dis- 

 tances of less than 3 A.U. from the sun may have also occurred, but in a much 

 larger scale in the Earth protoplanet. 



Furthermore, it is possible that the conditions for organic synthesis were 

 quite favorable during the transformation of the gravitationally undiffer- 

 entiated protoplanet into the primitive planet. This would result from the 

 mixing of the reactive precursors of organic compounds with inorganic particles, 

 such as silicate and metallic grains, which could have acted as surface catalysts. 

 Due to the low density of the synthesized organic compounds, these compounds 

 would migrate toward the exterior of the planet during the process of gravita- 

 tional differentiation. The nonvolatile combinations, ionic or high molecular 

 weight compounds, would accumulate on the surface of Earth, whereas the 

 gases and the compounds volatile at the prevailing temperatures, would be 

 evaporated into the outer region of the solar system where comets originate 

 presently .^^ The difiicult problem of the escape of gases and volatile com- 

 pounds from primitive planetary atmospheres has been discussed mainly by 

 Suess'^ and Urey.-'-^" 



With regard to the composition of the model, it is known that the spectra of 

 comets show fluorescence emission bands corresponding to the molecules or 

 radicals CN, CH, CH. , C2 , C3 , NH, NHo , and OH, to the radical ions CH+, 

 0H+, C0+, N2+, and CO2+ and to the atoms of Fe, Ni, Cr, and other ele- 

 ments. '-'^^ ■■*"•*' These emission bands are observed in the heads or in the tails 

 of comets when these bodies are at less than 3 A.U. from the sun. The band 

 corresponding to the CN radical is generally the first emission band to appear 

 on the tails of comets during the travel of these bodies toward the sun, and it is 

 also the band with the largest degree of extension into the comets' heads fol- 

 lowed in intensity by the C2 (Swan) and C-i bands. 



The above compounds exist in the nuclei of comets either as frozen free 

 radicals,^--^^ or as "ices"'*'^-'*^ (or crystalline clathrate type hydrates^^) of mole- 

 cules, which are vaporized and dissociated into radicals by the solar radiation. 

 In general, it is considered that the parent molecules of CN, NH2 , and OH are 

 hydrogen, cyanide or cyanogen, ammonia, and water, respectively. The 

 parent molecules of the carbon radicals are supposed to be methane, acetylene, 

 and other hydrocarbons. Therefore, a simplified experimental model could be 

 made of hydrogen cyanide, ammonia, and water. A slightly more complex 

 model could contain, in addition, cyanogen, acetylene, carbon monoxide, carbon 

 suboxide, and other compounds. There are certain relations between this 

 model and the 2 atmospheric models which have been studied previously, 

 namely, the "primitive planetary atmosphere" model,-^ and the "volcanic 

 atmosphere" model. ■*'^'^' These models should not be considered as providing 



