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E. Asphaug, C. Agnor, Q. Williams (UCSC), J.-M. Petit (CNRS), A. S. Rivkin (MIT)
If the present population of asteroids derive from material that survived intense mass depletion in the protoplanetary disk between Earth and Jupiter, then ~1 in 1000 bodies survived scattering, close encounters and mergers to become the ancestors of the present main belt and the precursors of meteorites.
The rate at which planetesimals accrete onto embryos is comparable to the rate at which planetesimals come inside the Roche limit of an embryo (Asphaug and Benz Icarus 1996). For bodies with >1/2 their mass in regolith and for gravity-dominated bodies of fairly low viscosity (\nulim <~\sqrt{\rm G} \rho3/2R2 ~1011 poise for R=100 km), periapsis <~0.5 Rroche results in removal of half the original mass. Partially molten silicate bodies would certainly disrupt, especially if water was abundant at this early phase. Tidal disruption induces pressure-release melting and brings core and mantle material into sudden close association across wide surface area. Silicate and iron mix as the core and deep mantle are brought abruptly (in the course of hours) to low pressure. Melts degas abruptly and induce turbulent mixing.
Dynamical calculations show that a few percent of the surviving primordial asteroids underwent catastrophic tidal disruption during encounters with transitory main-belt embryos, if a majority were either partially molten or rubble piles during the first ~3Ma. A number of primordial asteroids would have been ripped apart into one or more central cores almost devoid of mantle rock, flanked by core-free bodies, with implications for iron-silicate mixing, degasing, and petrogenesis. As opposed to impact, this shock-free disruption and mixing of parent materials can explain e.g. the angrites, the highly varying cooling rates within a single taxonomic type, and mantle-removal of classic M-type asteroids (Psyche and Kleopatra) without invoking an intense bombardment that should have removed Vesta's crust.
The author(s) of this abstract have provided an email address for comments about the abstract: asphaug@es.ucsc.edu
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.