  Samples returned from comet 81P/Wild 2 by Stardust confirm that substantial
quantities of crystalline silicates were incorporated into the comet at
formation. We investigate the constraints that this observation places upon
protoplanetary disk physics, assuming that outward transport of particles
processed at high temperatures occurs via advection and turbulent diffusion in
an evolving disk. We also look for constraints on particle formation locations.
Our results are based upon 1D disk models that evolve with time under the
action of viscosity and photoevaporation, and track solid transport using an
ensemble of individual particle trajectories. We find that two classes of disk
model are consistent with the Stardust findings. One class features a high
particle diffusivity (a Schmidt number Sc < 1), which suffices to diffuse
particles up to 20 microns in size outward against the mean gas flow. For Sc >
1, such models are unlikely to be viable, and significant outward transport
requires that the particles of interest settle into a midplane layer that
experiences an outward gas flow. In either class of models, the mass of inner
disk material that reaches the outer disk is a strong function of the disk's
initial compactness. Hence, models of grain transport within steady-state disks
underestimate the efficiency of outward transport. Neither model results in
sustained outward transport of very large particles exceeding a mm in size. We
show that the transport efficiency generally falls off rapidly with time.
Hence, high-temperature material must be rapidly incorporated into icy bodies
to avoid fallback, and significant radial transport may only occur during the
initial phase of rapid disk evolution. It may also vary substantially between
disks depending upon their initial mass distributions. We discuss implications
for Spitzer observations of crystalline silicates in T Tauri disks.
