Formation of icy planetesimals in a turbulent solar nebula

We have constructed a numerical simulation of the formation of water-ice planetesimals in the outer solar nebula which incorporates global turbulence, condensation and sublimation of H2O, and collisional accumulation. Global turbulence based on the Kolmogorov turbulence spectrum is imposed on a two-dimensional azimuthally symmetric laminar solar nebula model, In a single simulation, an individual particle of a given size and density is placed in the nebula on a Keplerian orbit; its orbit evolves due to gas drag forces while simultaneously its size changes due to both H2O condensation and sublimation and the accumulation of background H2O-ice particles as it sweeps through the nebula. With the inclusion of the gas-grain exchange and the grains' long-term orbital evolution over large radial and vertical ranges, our approach extends beyond previous investigations. Major results include: (1) Turbulence can concentrate small particles into preferred regions in the nebula and can prevent the rapid loss of such particles into the Sun. (2) The suspension of mm and sub-mm particles and the sedimentation of large particles in the direction normal to the disk plane may modify their reprocessing properties, opacity, and the spectral energy distribution, (3) Particles experience wide ranges of ambient conditions (e.g., temperature and density) as they are buffeted about the nebula by turbulence. They may undergo significant chemical and/or structural changes as a result. (4) For planetesimals to grow from smaller particles, collisional accumulation must be efficient and rapid. A high midplane concentration of icy particles strongly favors planetesimal growth from small grains in the giant planet region of the Solar System. (C) 2000 Academic Press.