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E. Asphaug, C. Agnor, Q. Williams (UCSC)
Major aspects of terrestrial planet formation are well explained by a late-stage episode where hundreds of Moon- to Mars-sized embryos accreted through giant impacts into four diverse worlds. But accretion is the exception to the rule, and recent modelling (Agnor and Asphaug, ApJ Letters 2004) shows that mass accumulation occurs in fewer than half of late-stage giant impacts. Typically, the unaccreted smaller embryo continues on, severely perturbed, dynamically shredded, and thermophysically altered by the episode, even in the absence of impact shock.
We find that grazing (non-impacting) interlopers experience profound tidal strains and shears that strip away an atmosphere and much of the mantle. Tidal-induced oscillatory global pressure unloading approaches 100% at times, throughout the deep interior, before attaining a final hydrostatic equilibrium greatly reduced by mass loss and induced rapid rotation.
Because a given embryo is likely to suffer one or more non-accretionary collisions before becoming an accreted body, our results indicate an important suite of processes for petrogenesis, melting, degassing and fractionation, the effects of which we shall argue are found in the planetary, meteoritic, and asteroid record today, particularly among the remnants of the primordial population which escaped final accretion onto a larger body.
This research, including Agnor and Asphaug (2004), is supported by NASA PG&G Small Bodies & Planetary Collisions, and by supercomputer time from the NSF-funded beowulf cluster upsand at UCSC.
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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, 36 #4
© 2004. The American Astronomical Soceity.