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earthly experience. Although many scientists believe there is little 

 basis for the speculation that individual interstellar organic molecules 

 found their way, intact and unchanged, to the prebiotic Earth's sur- 

 face, there is growing interest in the possibility that interstellar 

 molecules and dust might be preserved intact in comets and, in 

 altered form, in the carbonaceous meteorites. 1 These possibilities 

 stem from the probability that all matter in the solar system origi- 

 nated from interstellar dust and molecules. To the extent that 

 comets and carbonaceous meteorites contributed mass to the early 

 Earth, interstellar organic compounds could have survived to take 

 part in subsequent chemical evolution. 



Just as biological evolution implies that all organisms on Earth 

 have a common ancestry, so chemical evolution implies that all 

 matter in the solar system had a common origin. Observations of 

 various stages of star formation and evolution in dense interstellar 

 clouds support the view that our Sun and solar system formed from 

 interstellar dust and gas. Serious scientific consideration is being 

 given to the following scenario: 



An interstellar cloud of dust and gas underwent gravi- 

 tational collapse, perhaps triggered by a shock wave 

 generated by a nearby supernova; thus began the 

 chemical evolution of the nascent solar system. Cloud 

 contraction led to the formation of the primordial 

 solar nebula, an enormous spinning disc of dust and 

 gas with the proto- (or newly forming) Sun at the 

 center. Heating, associated with gravitational contrac- 

 tion, produced a thermal gradient in the nebula, pos- 

 sibly with temperatures in excess of 1300° C, close to 

 the protosun. According to the model, the tempera- 

 tures were about 300° C at the present distance of 

 the Earth to the Sun, but remained low (-150° C) at 

 Jupiter's distance and beyond. While uncertainties 

 exist about the temperatures in the nebula, the quali- 

 tative trend of decreasing temperature with increasing 



Carbonaceous meteorites are so named because of their high concentra- 

 tion of carbon (up to 6%). The nature of this carbon will be discussed in detail 

 shortly. 



