Heliocentric Zoning of the Asteroid Belt by Aluminum-26 Heating

The dependence of asteroid spectral class (and inferred composition and thermal history) on heliocentric radius has been held to be the result of heating by a solar energy source, most likely electrical induction, during the formation of the planetary system. Such variations in thermal history can be more simply explained by the presence of different amounts of the radionuclide aluminum-26, whose decay products are observed in meteorites, in planetesimals. These differences occurred naturally as a function of the increasing amount of time required to aecrete objects farther from the sun, during which aluminum-26 decayed from its initial concentration in the solar nebula. Both theory and isotopic evidence suggest that increases in aecretion time across the asteroid belt are of order several half-lives of aluminum-26, which is sufficient to produce the inferred differences in thermal history.