Formation of icy planetesimals in a turbulent solar nebula

We have constructed a numerical simulation of the formation of water-ice pl anetesimals 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 s imulation, 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 w hile simultaneously its size changes due to both H2O condensation and subli mation and the accumulation of background H2O-ice particles as it sweeps th rough the nebula. With the inclusion of the gas-grain exchange and the grai ns' 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 th e 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 larg e particles in the direction normal to the disk plane may modify their repr ocessing properties, opacity, and the spectral energy distribution, (3) Particles experience wide ranges of ambient conditions (e.g., temperatu re and density) as they are buffeted about the nebula by turbulence. They m ay undergo significant chemical and/or structural changes as a result. (4) For planetesimals to grow from smaller particles, collisional accumulat ion must be efficient and rapid. A high midplane concentration of icy parti cles strongly favors planetesimal growth from small grains in the giant pla net region of the Solar System. (C) 2000 Academic Press.