The survival of presolar grains during the formation of the solar system

The presolar grains found in meteorites survived potentially destructive processes in the protosolar environment: the radiation field of the collapsing protosolar envelope; the protoplanetary disk formed by the collapse, known as the solar nebula; and the accretion shock through which material passed from the envelope to the disk. Theoretical models of these regimes, combined with experimentally determined destruction criteria, can be used, in principle, to put constraints on the physical conditions that prevailed prior to and during the formation of meteorite parent bodies. Preliminary studies, in which interstellar species are assumed to be destroyed at well-defined, critical temperatures, indicate that refractory species (e.g., silicates) survived envelope and shock to enter the nebula at or within about 1 AU of the Sun; volatile species such as water ice and simple organics retained interstellar characteristics only beyond several AU, the destruction distances being dependent on the protosolar accretion luminosity and, to a lesser extent, the precise density configuration of the protosolar envelope. Upon entering the nebula it is likely that even refractory grains were destroyed out to some distance in the terrestrial planet region, but subsequent nebular cooling and radial advection resulted in the survivors' incorporation into the meteorite parent bodies. Presolar volatiles would have been incorporated in comets formed at distances at and beyond the orbits of Uranus and Neptune. These preliminary conclusions should be tested by better theoretical models of the protosolar environment, a search for survival patterns among meteorite classes, and the application of rigorously defined destruction criteria which take into account the nonequilibrium character of surviving species.