Constraints on nebular dynamics and chemistry based on observations of annealed magnesium silicate grains in comets and in disks surrounding Herbig Ae/Be stars

Understanding dynamic conditions in the Solar Nebula is the key to predicti on of the material to be found in comets. We suggest that a dynamic, large- scale circulation pattern brings processed dust and gas from the inner nebu la back out into the region of cometesimal formation-extending possibly hun dreds of astronomical units (AU) from the sun-and that the composition of c omets is determined by a chemical reaction network closely coupled to the d ynamic transport of dust and gas in the system. This scenario is supported by laboratory studies of Mg silicates and the astronomical data for comets and for protoplanetary disks associated with young stars, which demonstrate that annealing of nebular silicates must occur in conjunction with a large -scale circulation. Mass recycling of dust should have a significant eff ec t on the chemical kinetics of the outer nebula by introducing reduced, gas- phase species produced in the higher temperature and pressure environment o f the inner nebula, along with freshly processed grains with "clean" cataly tic surfaces to the region of cometesimal formation. Because comets probabl y form throughout the lifetime of the Solar Nebula and processed (crystalli ne) grains are not immediately available for incorporation into the first g eneration of comets, an increasing fraction of dust incorporated into a gro wing comet should be crystalline olivine and this fraction can serve as a c rude chronometer of the relative ages of comets. The formation and evolutio n of key organic and biogenic molecules in comets are potentially of great consequence to astrobiology.