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E. Asphaug (UCSC), W.F. Bottke, Jr. (SwRI), A. Morbidelli, J.-M. Petit (CNRS/Nice)
Recent numerical and theoretical developments (e.g. Wetherill 1992; Chambers and Wetherill 1998) suggest that hundreds or thousands of Moon- to Mars-sized planetary embryos may have resided between ~0.5 and 4 AU during early solar system accretion, to be scattered by mutual encounters and resonant perturbations with Jupiter and Saturn. At the same time, we lack compelling scenarios leading to the origin of iron meteorites, believed to represent the cores from approximately 85 different primordial planetesimals (Kail et al. 1994). Are M-type asteroids such as Kleopatra the exposed cores of these parent bodies? Early solar system collisions have been called upon to excavate this iron (Haack et al. 1996), although numerical impact models (Asphaug 1997) have found this task difficult to achieve, particularly when it is required to occur many dozens of times, yet not a single time for asteroid Vesta.
One possibility, consistent with the unusual shape of Kleopatra, is tidal disassembly of collisionally weakened differentiated planetesimals by close encounters with primordial planetary embryos. Differentiation enhances the efficacy of tidal disassembly, which is probably already comparable (Asphaug and Benz 1996) to the efficacy of collisional disassembly, but only for bodies of very low strength. Tidal disassembly has the further advantage of stripping all material from a given isosurface, whereas collisions partition energy into both fast and slow debris, leaving behind a rock mantle.
To further explore this idea, in comparison with the efficacy of collisional breakup of differentiated planetesimals, we determine the minimal encounter distances between evolving asteroids and the embryos as modeled by Petit et al. (2000). We then directly simulate these tidal encounters using a smooth particle hydrocode (SPH; Benz and Asphaug 1995), and compare tidal encounters to collisional encounters using the same code.