Thermal structure of the asteroid belt

The heliocentric pattern of asteroid types, with thermally processed objects closer to the Sun (as inferred from spectra), persists despite subsequent dynamical stirring of asteroid orbits and ejection of bodies from the main belt. Differentiated objects appear to have formed earlier than chondritic bodies, and dynamical modeling suggests they may have accreted 

Iron meteorite cooling rates a study of nickel diffusion 

Cooling rates have important implications for the sizes of meteorite parent bodies. Because of its high thermal conductivity, a metallic core should have a uniform temperature, but its rate of cooling is controlled by outer silicate layers that act like insulation. Larger asteroids cool more slowly, and Haack et al. (1990) developed an approximate relationship between the radius of an asteroid R and the cooling rate CR of its metallic core  

Using this eguation, the calculated radii of most iron meteorite parent bodies are found to have been -10-100 km. 

Contours of peak temperature (K) in asteroids as functions of size (diameter) and heliocentric distance. Accretion times corresponding to various solar distances are given at the top of the figure. Shaded vertical bars mark major divisions in the asteroid belt based on spectral interpretations of asteroid thermal histories. Modified from Grimm and McSween (1993). 

---

These models provide an explanation for the thermal structure of the asteroid belt that is probably correct in principle but not in its details. The recognition that differentiated asteroids formed earlier than chondrites, perhaps within the terrestrial planet region, requires models in which asteroid accretion was initiated earlier than 2 Myr after CAI formation. 

---