  We outline a scenario which traces a direct path from freely-floating nebula
particles to the first 10-100km-sized bodies in the terrestrial planet region,
producing planetesimals which have properties matching those of primitive
meteorite parent bodies. We call this "primary accretion". The scenario draws
on elements of previous work, and introduces a new critical threshold for
planetesimal formation. We presume the nebula to be weakly turbulent, which
leads to dense concentrations of aerodynamically size-sorted particles having
properties like those observed in chondrites. The fractional volume of the
nebula occupied by these dense zones or clumps obeys a probability distribution
as a function of their density, and the densest concentrations have particle
mass density 100 times that of the gas. However, even these densest clumps are
prevented by gas pressure from undergoing gravitational instability in the
traditional sense (on a dynamical timescale). While in this state of arrested
development, they are susceptible to disruption by the ram pressure of the
differentially orbiting nebula gas. However, self-gravity can preserve
sufficiently large and dense clumps from ram pressure disruption, allowing
their entrained particles to sediment gently but inexorably towards their
centers, producing 10-100 km "sandpile" planetesimals. Localized radial
pressure fluctuations in the nebula, and interactions between differentially
moving dense clumps, will also play a role that must be allowed for in future
studies. The scenario is readily extended from meteorite parent bodies to
primary accretion throughout the solar system.
