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Turbulent Concentration of mm-Size Particles in the Protoplanetary Nebula: Scale-Dependent CascadesThe initial accretion of primitive bodies (here, asteroids in particular) from freely-floating nebula particles remains problematic. Traditional growth-by-sticking models encounter a formidable "meter-size barrier" (or even a mm-to-cm-size barrier) in turbulent nebulae, making the preconditions for so-called "streaming instabilities" difficult to achieve even for so-called "lucky" particles. Even if growth by sticking could somehow breach the meter size barrier, turbulent nebulae present further obstacles through the 1-10km size range. On the other hand, nonturbulent nebulae form large asteroids too quickly to explain long spreads in formation times, or the dearth of melted asteroids. Theoretical understanding of nebula turbulence is itself in flux; recent models of MRI (magnetically-driven) turbulence favor low-or- no-turbulence environments, but purely hydrodynamic turbulence is making a comeback, with two recently discovered mechanisms generating robust turbulence which do not rely on magnetic fields at all. An important clue regarding planetesimal formation is an apparent 100km diameter peak in the pre-depletion, pre-erosion mass distribution of asteroids; scenarios leading directly from independent nebula particulates to large objects of this size, which avoid the problematic m-km size range, could be called "leapfrog" scenarios. The leapfrog scenario we have studied in detail involves formation of dense clumps of aerodynamically selected, typically mm-size particles in turbulence, which can under certain conditions shrink inexorably on 100-1000 orbit timescales and form 10-100km diameter sandpile planetesimals. There is evidence that at least the ordinary chondrite parent bodies were initially composed entirely of a homogeneous mix of such particles. Thus, while they are arcane, turbulent concentration models acting directly on chondrule size particles are worthy of deeper study. The typical sizes of planetesimals and the rate of their formation can be estimated using a statistical model with properties inferred from large numerical simulations of turbulence. Nebula turbulence is described by its Reynolds number Re = (L/eta)(exp 4/3), where L = H alpha(exp 1/2) is the largest eddy scale, H is the nebula gas vertical scale height, alpha the turbulent viscosity parameter, and eta is the Kolmogorov or smallest scale in turbulence (typically about 1km), with eddy turnover time t(sub eta). In the nebula, Re is far larger than any numerical simulation can handle, so some physical arguments are needed to extend the results of numerical simulations to nebula conditions. In this paper, we report new physics to be incorporated into our statistical models.
Document ID
20160001790
Acquisition Source
Ames Research Center
Document Type
Conference Paper
Authors
Cuzzi, J. N.
(NASA Ames Research Center Moffett Field, CA United States)
Hartlep, T.
(Bay Area Environmental Research Inst. Moffett Field, CA, United States)
Date Acquired
February 12, 2016
Publication Date
March 16, 2015
Subject Category
Astrophysics
Report/Patent Number
ARC-E-DAA-TN20601
Meeting Information
Meeting: Lunar and Planetary Science Conference
Location: The Woodlands, TX
Country: United States
Start Date: March 16, 2015
End Date: March 20, 2015
Sponsors: Lunar and Planetary Inst.
Funding Number(s)
CONTRACT_GRANT: NNX14AB66A
WBS: WBS 811073.02.07.02.97
Distribution Limits
Public
Copyright
Public Use Permitted.
Keywords
mm-size particles
turbulent nebulae
nonturbulent nebulae
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