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