Our understanding of chemical processes in the primitive Solar Nebula and of processes common to nebulae surrounding many protostars, has increased considerably as more detailed models of nebular evolution have become available. Early models (52) simply assumed that very hot gas in the Solar Nebula cooled slowly enough to maintain thermodynamic equilibrium until at least the more refractory vapors had condensed. Later models (53) examined potential consequences of local to medium-scale turbulence that would naturally accompany any viscous accretion disk. Prinn and Fegley (54) demonstrated that even major gas phase species such as N2/NH3 could fail to achieve equilibrium because of low temperatures and slow chemical-reaction rates near the outer planets. More recent work (55) has demonstrated that, for mineral species, constraints in achieving equilibrium with gas in the nebula are even more severe. In particular, oxidation/reduction or hydration reactions expected to occur spontaneously at temperatures ranging from 500 K to 700 K might easily lack the time necessary to achieve equilibrium in a rapidly evolving Solar Nebula.
Observations of Herbig Ae/Be stars discussed above add to the difficulties of assuming that chemical equilibrium is ever achieved at any place or time in the evolution of protostellar nebulae. Most models assume that mass accretion onto the sun occurs in a predictable way: mass falls onto the accretion disk and travels more or less steadily inward. Even turbulent mixing scenarios (53) are primarily consistent with this picture, because mixing only really occurs between adjacent chemical regimes. Observation of a large population of annealed silicates in comets perturbed into star-grazing orbits following planetary encounters implies that mixing occurs on significant distance scales. Such scales (several AU to 
100 AU) far exceed those in the models (53). This is consistent with the constraint that thermal annealing at 
1,000 K must be followed by condensation of an ice mantle onto annealed grains before their aggregation into comets. In a simple-minded scenario, mixing occurs from an annulus within about 1 AU from the central star out to well beyond at least 5 to 10 AU. In reality, the mixing lengths are probably both much longer and shorter than this (Fig. 3).
Shu et al. (56, 57) suggested that grains and dust aggregates reaching the surface of the protosun, in contact with the accretion disk, could be hurled out along magnetic field lines to land in the region of chondrite formation. Grains reaching such temperatures as part of a large dust aggregate quickly melt and coalesce through surface tension (greatly reducing their exposed surface) and resist evaporation until transported to cooler environments. Individual 10- to 100-nm grains exposed to solar surface temperatures would rapidly vaporize. Even if such vapors later recondensed, the new silicates would be highly amorphous and their IR spectra would more resemble grains condensed around the majority of mass-losing Asymtotic Giant Branch (AGB) stars than grains from olivine-rich comets. It seems unlikely that the chondrule-forming mechanism of Shu et al. (57) is directly responsible for transporting individual submicron grains heated to more moderate temperatures (1,000 K) to the outer nebula.
Large-scale circulation patterns that are capable of transporting significant quantities of presolar silicates from hotter nebular environments to beyond the snow line must exist at some stage of nebular evolution. Because individual grains should be closely coupled to the gas, such circulation patterns would also transport an even larger mass of gas equilibrated at high temperatures (1,000 K) out beyond the giant planet formation region. The gas composition would be similar to that predicted for Giant Gaseous Protoplanetary Subnebulae (58, 59), but would probably be spread more uniformly around the accretion disk.
If there is a steady flow from the inner to the outer regions of the Solar Nebula, then the chemistry of the gas-phase would be dominated by chemical kinetics to a much greater degree than currently modeled (54, 55). In particular, circulation patterns could lead to freshly condensed and partially annealed grains (natural, catalytic surfaces) in the outer Solar System. This could greatly enhance rates of gas-grain reactions, such as the Fischer Tropsch-Type synthesis of methane and higher hydrocarbons from CO and H2 or the analogous conversion of molecular N2 and H2 to NH3 (61, **). Such catalytic syntheses would be over and above the NH3 and hydrocarbons produced in the inner Solar System and transported with the processed grains.
The dynamic circulation pattern suggested by the steady increase in the proportion of processed dust in comets around Herbig Ae and Be stars is not consistent with models of evolving stellar accretion disks. This inconsistency may be due more to the limits of computational techniques used to model angular momentum transport in disks than to any physical reason preventing such circulation. In fact, large-scale circulation cells moving material from the inner to the outer regions, both above and below an inward-flowing accretion disk, have been discussed by both Prinn (62) and Stevenson (63). Perhaps it is time to reconsider this topic, at least for its potential to accurately predict the chemistry of comets.
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