ASTEROIDS AND METEORITES - THERMAL CONNECTIONS

Most meteorites have experienced melting (achondrites, irons), metamorphism (ordinary chondrites), or aqueous alteration (carbonaceous chondrites). All of these thermal processes probably result from decay of short-lived radionuclides, principally 26Al and 60Fe, because asteroids are too small to retain heat from slowly decaying isotopes. Thermal calculations for asteroid-sized bodies based on these heat sources produce the required peak temperatures, radiometric ages, and cooling rates. Models for differentiated asteroids are complex, because heat sources migrate into the newly formed core and crust during the calculation. Thermal simulations of ice-bearing bodies are also difficult, because ice melting moderates heat excursions so that aqueous alteration occurs at low temperatures. Particulate regoliths on asteroid surfaces also insulate the bodies. Most thermal models assume that the asteroid accreted instantaneously prior to heating; newly developed incremental accretion scenarios can model the heat budget during asteroid growth, which is more realistic for rapidly decaying radioisotopes. Thermal models provide geologic context for meteorites by allowing linkages with their asteroid parent bodies. These models predict hot asteroid interiors beneath cooler surface layers, unless the bodies have been fragmented by collisions. Asteroid spectroscopy reveals that many asteroids are rubble piles produced by fragmentation and gravitational reassembly, providing a way to sample entire objects. Spectroscopy also suggests that the asteroid belt exhibits a thermal structure that can be understood by considering variations in the times of asteroid accretion with distance from the sun, concurrent with decay of short-lived radionuclides.