47 



distance from the protosun seems to be acceptable. 

 Cooling of the hot gas in the inner nebula close to the 

 protosun led to condensation of solid mineral grains. 

 As interstellar gas and dust were drawn into the solar 

 nebula they would have been heated to varying 

 degrees, depending on their distance from the 

 protosun. 



Further condensation accompanied by aggregation of fine- 

 grained material yielded planetesimals ranging in size from kilometers 

 to tens of kilometers. Continued growth of these objects by accre- 

 tion led ultimately to the formation of the solid bodies of the solar 

 system. Formation of organic matter from gases containing carbon, 

 nitrogen, hydrogen, oxygen, and sulfur could have accompanied the 

 condensation processes; mineral grains could have provided surfaces 

 to catalyze the synthesis. As previously noted, in the outer regions of 

 the nebula, temperatures would have remained low (<-150° C). 

 There, low-temperature accretion of organic and inorganic material 

 into planetesimals could have taken place, allowing the preservation 

 of the ices and other volatile compounds originally in the interstellar 

 medium as well as the rare gases helium, argon, neon, krypton, and 

 xenon. Thus, from the solar nebula came the Sun, the planets and 

 their satellites, comets, meteorites, and asteroids. In general, it is 

 thought that material accreted in the inner solar system originated at 

 relatively high temperatures and was depleted in volatile substances, 

 while the materials that accreted in the outer solar system were 

 volatile-rich. The validity of this simple model has been and con- 

 tinues to be subjected to critical assessment; inevitably, as theory, 

 experiments, and observations progress the model will undergo 

 changes, perhaps so many that a new model will emerge. In the 

 meantime, it provides a useful framework within which to discuss 

 various aspects of inorganic and organic chemical evolution. 



COMETS AND METEORITES 



Comets occupy an especially interesting place within this 

 framework. They may have been a source of part of the atmospheres 

 of the terrestrial planets, and they are believed to have been the 



