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G. J. Consolmagno SJ (Specola Vaticana), S. J. Weidenschilling (Planetary Science Institute), D. T. Britt (University of Tennessee)
Chondrites are well-compacted stones with porosities generally less than 10%, mostly as post-lithification cracks. Pressure, temperature, and water lithify terrestrial rocks; but though all meteorites have experienced some metamorphism, most have not seen sufficient pressure, temperature or aqueous alteration to account for their lithfication. How did nebular dust become well-lithified meteorites?
Pressures of 1-10 GPa are needed to compress terrestrial sandstones; 1 GPa pressures are reached only at the center of a Ceres-sized body. However, relative impact velocities of 1 km/s, equivalent to orbits with eccentricity of circa 0.05 in the asteroid region, have an energy density equivalent to a GPa.
When particle velocities are controlled by nebular gas drag, the maximum impact velocity equals the deviation of the gas from Keplerian rotation. Millimeter-sized particles coupled to the gas impact on meters-sized or larger bodies at roughly 50 m/s, compressing underdense "fairy castle" structures, though hardly lithifying them. Such bodies would be coherent enough to participate in further accretion, however.
Jupiter forming in the nebula (perhaps also inducing shocks to form chondrules) can perturb a 100-km planetesimal, even if not near a major resonance, to eccentricities fluctuating from 0 to 0.1 (resonant bodies attain much higher e's). Ten-km bodies should attain eccentricities of 0.05, while smaller ones would be damped to low eccentricity until the gas dissipated.
Collisions of such perturbed bodies at speeds many times the target body's escape velocity would disrupt similar-sized bodies, while collisions with smaller impactors would allow the target to survive. A series of such impacts could produce lithified regions in a more porous unconsolidated matrix. Subsequent collisional disruptions would dissipate this matrix, but allow the lithified regions to survive to the present.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.