Primary Accretion of Asteroids and Kuiper Belt Objects
Abstract
Primary accretion is the process by which the first large objects formed from freely- floating nebula particles. Several clues as to the nature of this process are to be found in primitive meteorites and asteroids. The most primitive chondritic meteorites display a characteristic texture: predominance of mm-sized, once-molten chondrules, metal grains, and refractory oxide particles, each surrounded by fine-grained dust rims and all embedded in a granular matrix. The size distribution of the chondrules in all classes of chondrite is quite narrow and nearly universal in shape, 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 planetesimals, roughly 100 km in radius and originally composed largely of chondrules with very similar properties. This ubiquitous and unusual texture is surely telling us something important about primary accretion, but there is no generally accepted explanation for it at present. Moreover, the extended duration of meteorite parent body formation as revealed in isotopic age-dating, and the scarcity of melted asteroids, suggest that primary accretion went on for a long time. In prior research we have shown how well-sorted, chondrule-sized mineral particles are concentrated, by orders of magnitude, into dense zones in weak nebula turbulence. This "turbulent concentration" explains the characteristic size and size distribution of chondrules in a natural way. We developed a "cascade model" of the statistics of dense zones and their correlation with gas vorticity. The model incorporates the effects of particle mass loading on the gas and predicts the fractional volume of particle-rich zones which can evolve directly into objects with some physical cohesiveness. We derived threshold conditions (combinations of particle density, clump lengthscale, gas density, and local vorticity) which allow dense clumps to proceed to become actual planetesimals. Combination of these thresholds with our cascade models recently led us to the relative abundance of primary planetesimals as a function of mass - their "initial mass function" - and a rough estimate of their production rate. The theory also makes testable predictions about variance of chondrule ages in a parent body. The model is applicable not only to asteroids, but also to Kuiper Belt Objects in the 16-30AU region. We propose to carry this work forward. To test and refine the scenario, we propose to (a) include the self-gravity of dense particle clumps in complete 3D turbulent models, to assess their stability and mutual interactions; (b) validate the key elements of the cascade model, including the scaling properties of the process; (c) study the settling of dense clumps in the vertical component of solar gravity, which increases the local density of chondrule-size components in regions near the midplane; (d) refine our timescale estimates for primary accretion; and (e) collaborate in new observational studies of meteoritical evidence for or against aerodynamical sorting.
- Publication:
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NASA OSS Proposal
- Pub Date:
- 2010
- Bibcode:
- 2010oss..prop..100C