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The first challenge in the formation of both terrestrial planets and thecores of gas giants is the retention of grains in protoplanetary disks. In mostregions of these disks, gas attains sub-Keplerian speeds as a consequence of anegative pressure gradient. Hydrodynamic drag leads to orbital decay anddepletion of the solid material in the disk, with characteristic timescales asshort as only a few hundred years for meter-sized objects at 1 AU. In thispaper, we suggest a particle retention mechanism which promotes theaccumulation of grains and the formation of planetesimals near the watersublimation front or ``snow line.'' This model is based on the assumption that,in the regions most interesting for planet formation, the viscous evolution ofthe disk is due to turbulence driven by the magneto-rotational instability(MRI) in the surface layers of the disk. The depth to which MRI effectivelygenerates turbulence is a strong function of the grain size and abundance. Asharp increase in the grain-to-gas density ratio across the snow line reducesthe column depth of the active layer. As the disk evolves towards aquasi-steady-state, this change in the active layer creates a local maximum inradial distribution of the gas surface density and pressure, causing the gas torotate at super-Keplerian speed and halting the inward migration of grains.This senario presents a robust process for grain retention which may aid in theformation of proto-gas-giant cores preferentially near the snow line.

Grain Retention and Formation of Planetesimals near the Snow Line in MRI-driven Turbulent Protoplanetary Disks