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E. Asphaug (UCSC)
Any quantitative assessment of planetary evolution requires a detailed understanding of planetesimal response to collisional and tidal perturbation: the opening of flaws under tension, the friction of grain upon grain, mechanical disaggregation and compaction, and the thermodynamics of hypervelocity impact. These and related phenomena comprise the geophysics of small planetary bodies - those asteroids, comets and satellites ~0.1 to ~3 km diameter for which gravitational and mechanical forces are comparable. Sculpted by an interplay between accretion and disruption, their behavior frequently defies our Earth-based intuition. Laboratory and scaling analyses cannot alone predict the behavior of small bodies. Instead, those results are incorporated into sophisticated computer models which are tested against known outcomes and applied to observation. Consider the brief, spectacular appearance and demise of comet Shoemaker-Levy 9, which demonstrated two of the three disasters which may befall a comet. Exhaustive modeling shows that SL9's breakup requires a modest (~1.6 km) strengthless progenitor. Forward models invoking serendipitous timing of brittle fracture of a much larger (>10 km) comet cannot explain the scale invariances witnessed in crater chains on Ganymede and Callisto, nor the monomodal sizes and spacings of fragments. These are natural outcomes of ``rubble-pile" breakups. Collisional models prove it is easier to disrupt than to disperse any planetesimal larger than ~250 m diameter; rubble piles are therefore common. More detailed analyses of small body geophysics are afforded by the present epoch's menagerie of small bodies, imaged in detail by radar and spacecraft. Concurrent modeling of crater diameter, regolith distribution, and tectonism for a given specific impact allows for interpretations of interior structure reminiscent of the means by which seismologists probe the Earth's interior. The latest models running on the world's fastest supercomputers incorporate both fragmentation and self-gravitation. With these modern tools we can begin to assemble our solar system from its earliest beginnings.
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