melting has been constrained by radiometric dating; geochemical evidence for core formation has been discovered; and models for its thermal evolution are being formulated. The thermal histories of ordinary chondrite parent bodies are also interesting. Most ordinary chondrites have been metamorphosed, and detailed thermal models for their parent asteroids have been constructed based on decay of short-lived radionuclides.21 Chondrite cooling histories, determined from nickel diffusion profiles in metal grains, suggest that many bodies were disrupted by impacts and subsequently reaccreted into “rubble piles.”22 Earth-approaching S asteroids, possibly representing fragments of bodies heated to varying degrees, may be especially instructive for understanding asteroid thermal evolution and accretionary structure. Many carbonaceous chondrites have suffered aqueous alteration, and the source of the fluids that caused alteration in C-type asteroids was probably ice, originally accreted along with rocky material and later melted by decay of short-lived radionuclides, electromagnetic induction heating, or impacts.23 Information on the maturity of asteroid regoliths and the duration of their exposure has also been gained from studies of meteorite regolith breccias.
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