Not all of the gas and dust bound in the initial fragment that collapsed to 

 form the Sun was incorporated into the central star, and some that was may 

 subsequently have been liberated via a strong T Tauri wind during the Sun's 

 infancy. A certain unknown fraction of the collapsing cloud must have been 

 incorporated into a toroidal nebulosity orbiting the developing stellar core. 

 This solar nebula provided the material from which the solar system planets 

 were formed as well as an environment for chemical processing of interstellar 

 biogenic elements and compounds into both more and less complex forms. The 

 solar nebula flattened into a viscous accretion disk, bounded above and below by 

 shocks. Late-arriving interstellar materials experienced more or less traumatic 

 chemical processing, depending on the location at which they crossed these 

 shock fronts and the initial velocity of the particles falling onto the shocks. The 

 picture is particularly unclear with respect to dust grains; some grains coated 

 with organic mantles may have entered the solar nebula with their complement 

 of biogenic elements unaltered; some grains without complex organic mantles 

 may have grown them as a result of the chemistry induced by the added energies 

 during the encounter; or some (perhaps all) grains may have been sputtered away 

 and converted into their basic elemental components during the shock wave pas- 

 sage, only to condense anew within the early nebula itself. That grains were 

 present within the early nebuia seems undeniable. Dust is observed associated 

 with comets which presumably preserve the primitive nebular material ; whether 

 the grains are of nebular or presolar origin is unknown. Processing of and on the 

 grains did not cease with their inclusion in the solar nebula. Turbulence may 

 have cycled the nebular material between extremely diverse thermal locales on 

 a time scale that would have resulted in transformed, possibly enhanced, chem- 

 ical complexity. Aggregation would have caused material to settle into the mid- 

 plane of the accretion disk, and fragmentation of this plane would have led to 

 the incorporation of grains and macromolecules into kilometer or larger size 

 planetesimals where internal heating and collisions may have strongly metamor- 

 phosed the included biogenic elements and compounds. Aggregates of this size 

 would have been resistant to the subsequent dispersal of the solar nebula, and 

 would eventually have formed into the planets, satellites, asteroids, and comets. 

 Some of the nebular material, initially dispersed, might have condensed and 

 reentered the solar system as icy comets at a later date. 



Chemical processing in the early solar system, evidenced by certain elemental 

 fractionation patterns in primitive meteorites, could also potentially cause small 

 isotopic fractionations for some of the elements in those meteorites. Isotopic 

 variations are, in fact, observed in such meteorites, but in a significant number of 

 cases the nature and/or magnitude of those variations are incompatible with a 

 local, i.e., solar system, process and are therefore attributed to presolar process- 

 ing. That processing can be either nucleosynthetic or chemical. An example of 



23 



