Particle-Gas Dynamics and Primitive Meteorite Parent Bodies

Details Particle-Gas Dynamics and Primitive Meteorite Parent Bodies Particle-Gas Dynamics and Primitive Meteorite Parent Bodies

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..339c&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 339

Mathematics

Logic

1

Accretion, Chondrules, Nebular Midplane, Parent Bodies, Rims, Solar Nebula

Scientific paper

A major obstacle to understanding the accretion of primitive meteorite parent bodies has been the lack of a credible theoretical framework for the environment in which the earliest accretion occurs. Chondrules and inclusions in primitive meteorites are in the millimeter-centimeter size range and the earliest aggregates of these objects must have been in the centimeter-meter size range. For these sizes, particle-gas dynamics are difficult to model: the particles are neither micron-sized and firmly anchored to the gas, nor kilometer-sized planetesimals already fully decoupled from the gas. Significant feedback and strong coupling between the gas and particle phases must be dealt with in this intermediate size range [1]. We have previously reported preliminary results concerning the stage of planetary formation during which the particulate material has grown into centimeter-to-meter sized primordial aggregates [2]. During this stage, particles are able to settle toward the midplane into a layer of mass density comparable to or much greater than that of the gas. We now report more mature results [3]. Our numerical models rely on the Reynolds averaged NavierStokes equations for the gas and particles, and are fully viscous, turbulent, and compressible. Our turbulence modeling uses a Prandtl local shear parametrization, validated by laboratory experiments. We have developed a new model for particle diffusivity (in turbulence) involving the particle Schmidt number, which is a function of particle size and density. We have modeled a cool, quiescent nebula at 1 AU (280K) and 10 AU (90K), and a possible FU Orionis or early high temperature stage (1000K) at 1 AU. Our main results include: (a) rapid accretion of planetesimals by gravitationally unstable fragmentation on an orbital timescale (the "Goldreich-Ward instability") is unlikely to occur until objects have already accreted by some other process to the mass of the largest known meteorite samples, if at all [4]; (b) from "seeds" as small as ten meters, growth of 10-100 km planetesimals can proceed rapidly by drift-augmented accretion in the particle-rich midplane with orbital decay of about one percent for the growing planetesimals; (c) outward transport of vapor and small entrained chips can account for significant radial compositional and mineralogical mixing in primitive meteorite parent bodies. We also report our ongoing work concerning a stage that might well occur at a different place or time, in which the particulates are still primarily in the subcentimeter size range. Such small particles are unable to sediment to the nebula midplane significantly (unless the nebula turbulence is unrealistically small), and will probably be dispersed throughout most of the vertical scale height of the gaseous nebula. Nevertheless, strong evidence exists for aerodynamic sorting in meteorite structural elements of these sizes [5]. Recently, potentially important clumping effects have been shown to occur in turbulence, for a specific particle size range, in both experimental [6] and numerical [7] studies. It appears to us that chondrule-sized objects are in the size range most susceptible to such effects. We have adapted the code used by [7] and are now running full three-dimensional numerical simulations of turbulence with entrained particles of arbitrary size and mass loading ratio. We will present our most recent numerical modeling and discuss the implications for chondrule-size distributions and "accretion rims" [8]. References: [1] Nakagawa Y. et al. (1986) Icarus, 67, 375-390. Weidenschilling S. J. and Cuzzi J. N. (1993) In Protostars and Planets III, University of Arizona. [2] Cuzzi J. N. et al. (1991) Monterey Meteoritical Society Meeting. [3] Cuzzi J. N. et al. (1993) Icarus, submitted for special issue on planet formation. [4] Weidenschilling S. J. (1980) Icarus, 44, 172-189. [5] Dodd R. T. (1976) ESPL 30, 281-291; Skinner W. R. and Leenhouts J. M. (1991) Meteoritics, 26, 396; Skinner W. R. and Leenhouts J. M., LPSC XXIV, 1315 (also, this meeting). [6] Crowe C. T. et al. (1985) Part. Sci. and Tech., 3, 149-158. [7] Squires K. and Eaton J. (1990) Phys. Fluids A2, 1191-1203. [8] Metzler K. et al. (1992) GCA, 56, 2873-2897; also T. Bunch et al (this meeting).

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