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SUMMARY:Primary accretion of large planetesimals from chondrule size parti
 cles - Cuzzi\, J (NASA Ames Research Center)
DTSTART:20090929T130000Z
DTEND:20090929T140000Z
UID:TALK20351@talks.cam.ac.uk
CONTACT:Mustapha Amrani
DESCRIPTION:Primary accretion is the process by which the first large obje
 cts formed from freely floating nebula particles. Several clues as to the 
 nature of this process are to be found in primitive meteorites and asteroi
 ds. The most primitive chondritic meteorites display a characteristic text
 ure: predominance of mm-sized\, once-molten chondrules\, metal grains\, an
 d refractory oxide particles\, each surrounded by fine-grained dust rims a
 nd all\nembedded in a granular matrix. The size distribution of the chondr
 ules in all classes of chondrite is quite narrow and nearly universal in s
 hape\, but with a mean size distinctive of each class. At least two entire
  chondrite classes are each thought to derive from only one or two planete
 simals\, roughly 100 km in size and originally composed largely of chondru
 les with very similar properties. This ubiquitous and unusual texture is s
 urely\ntelling us something important about primary accretion\, but there 
 is no explanation for it at present. Moreover\, the extended duration of m
 eteorite parent body formation as revealed in isotopic age-dating\, and th
 e scarcity of melted asteroids\, suggest that primary accretion went on fo
 r a long time. We have shown how well-sorted\, chondrule-sized mineral par
 ticles can be concentrated\, by orders of magnitude\, into dense zones in 
 weak nebula turbulence. This turbulent concentration explains the characte
 ristic size and size distribution of chondrules in a natural way. We devel
 oped a cascade model of the statistics of dense zones and their correlatio
 n with gas vorticity\, which incorporates the effects of particle mass loa
 ding on the gas and predicts the fractional volume of particle-rich zones 
 which can evolve directly into objects with some physical cohesiveness. We
  have derived threshold conditions (combinations of particle density\, clu
 mp lengthscale\, gas density\, and\nlocal vorticity) which allow dense clu
 mps to proceed to become actual planetesimals. Combination of these thresh
 olds with our cascade models recently led us to a method for predicting th
 e relative abundance of primary planetesimals as a function of mass - thei
 r birth function - and even their production rate. The predictions can be 
 extended easily from the asteroid belt to the Kuiper belt\; similar size p
 opulations are found to arise. A number of challenges remain in validating
  and solidifying this scenario. The key elements of the cascade model must
  be validated (or modified) using deeper inertial ranges\, further from th
 e dissipation scale. The settling of dense clumps in the vertical componen
 t of solar gravity increases the local density of chondrule-size component
 s\nin regions near the midplane\, and must be modeled. Finally\, the self-
 gravity of dense particle clumps in turbulence must be modeled to assess t
 heir stability and mutual interactions.
LOCATION:Seminar Room 1\, Newton Institute
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