The cooling histories, and in particular the core cooling rates, of differentiated asteroids are calculated, employing a variety of thermal conductivity structures in order to simulate the potential insulating effects of regolith and megaregolith (brecciated impact rubble) layers. The regolith is assumed to have thermal conductivity 0.005¿ the unbrecciated mantle conductivity, and its thickness, in our most typical model, is assumed to be 0.003¿ the parent body radius. The megaregolith, if present, is assumed to have thermal conductivity 0.1¿ that of unbrecciated mantle. We find that a combination of relatively thick megaregolith and regolith can potentially reduce the core cooling rate by more than a factor of 10 below the rate predicted by models with conventional thermal conductivity structure. For any given measured cooling rate, the radius implied for the parent asteroid can potentially be reduced by a factor of 5 from the radius predicted by models with conventional thermal conductivity structure. Realistically, the regolith and megaregolith thickness required to engender these extreme effects were probably seldom generated, except perhaps within the largest asteroids (where, even if generated, they would quickly be reduced by compaction and sintering).

Nonetheless, our results for models with plausible regolith and megaregolith thicknesses indicate that most of the iron meteorites and the pallasites probably formed in relativity small (R less than 50 km) parent bodies. We have parameterized our results so that effects of intermediate configurations can readily be calculated. The parameterization also facilitates evaluation of additional variables, e.g., the initial temperature, the concentration of long-lived radioactive heat sources, etc., albeit these variables are far less crucial than the thermal conductivity structure. Our results indicate that the insulation of even a thin regolith causes metallalographic cooling rates to be relatively uniform, and practically independent of depth, within a given asteroid. Stochastic differences in cratering (regolith production) histories may have engendered radically different cooling rates from asteroids of similar radius, or similar cooling rates from asteroids of different radius. ¿ American Geophysical Union 1990