51 



meteorites even provide a connection with nucleosynthetic events 

 that preceded the solar system, perhaps that triggered its formation. 

 With few exceptions, the ages of meteorites fall within the range of 

 4.6 ±0.1 b.y. Since these objects constitute the oldest datable 

 material now available, their study provides clues to the very early 

 history of chemical evolution in the solar system. Although some 

 uncertainty remains in identifying the source(s) of meteorites, there 

 appears to be agreement that most were derived from asteroidal 

 parent bodies, either in the main asteroid belt or those with Earth- 

 crossing orbits or both. Some meteorites may be fragments of the 

 inactive cores of ice-depleted short-period comets. According to a 

 current scenario for solar-system origin, meteorites and the bodies 

 from which they were derived (parent bodies) were formed as a 

 result of the condensation and early evolution of planetesimals from 

 the primordial solar nebula; these represent the building blocks from 

 which solid planets and moons were assembled. 



Because of turbulence and thermal and pressure gradients in the 

 nebula, solid material that condensed at widely different radial dis- 

 tances and, therefore, different physical and chemical environments, 

 could have been brought together and assembled into a common 

 body. In this context, protoasteroidal and protocometary bodies 

 may be viewed as components of a distribution of planetesimals 

 that accumulated increasing proportions of ice and other volatile-rich 

 phases. The accumulation of the diverse ingredients into parent 

 bodies, possibly resembling asteroids, would have been accompanied 

 by various processes which would have further influenced the chem- 

 istry, mineralogy, and structural features of the material and, to vary- 

 ing degrees, masked the features that would have been characteristic 

 of primary solar nebula condensates and of originally interstellar 

 material. Presumably, perturbation of a parent body, perhaps by 

 collision with another object, yielded fragments of the bodies, some 

 of which eventually fell under the influence of the Earth's gravita- 

 tional field. 



Meteorites can be placed in two general categories: (1) partially 

 to fully differentiated and (2) undifferentiated objects. Differen- 

 tiated meteorites exhibit strong chemical fractionation relative to 

 average solar-system composition as represented by the Sun; they 

 show clear evidence of having been derived from parent bodies that 

 have undergone processes analogous to planetary core formation 



