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S. J. Weidenschilling (Planetary Science Institute)
The recent discovery of a 26Mg anomaly in a eucrite (Srinivasan et al. 1999, Science 284, 1348) shows that Vesta was heated by 26Al, and accreted within a few My of the formation of the solar system. The canonical initial 26Al abundance of 5 x 10-5 would have caused enough heating to erase Mg isotopic heterogeneity of minerals in planetesimals that grew larger than about 30 km diameter in the first My, and bodies larger than about 50 km would have melted (LaTourette and Wasserburg 1998, EPSL 158, 91). Nonetheless, primitive (non-igneous) material is abundant in the asteroid belt, and about 50% of CAIs preserve 26Mg anomalies, implying that much of the mass was preserved in small planetesimals until after the 26Al decayed. These seemingly contradictory constraints are consistent with runaway growth of planetesimals in the asteroid region. Runaway occurs for a minority of bodies while the mass distribution remains dominated by small bodies. Simulations with the PSI accretion code starting with 1.5 Earth masses of km-sized planetesimals between 2 and 3 AU produce Vesta-sized (D = >500 km) bodies within 1 My, while half the mass is still in bodies with D =< 15 km. It is not necessary to invoke inhibition of runaway by Jovian perturbations or early depletion of mass in the asteroid zone in order to explain thermal histories of meteorite parent bodies.