Asteroid compositional rings: Clues to the compositions of primordial planetesimals in the middle solar system

The distribution of asteroid taxonomic types at distances between 2.1 and 5.3 AU is discussed. There are four major, overlapping but compositionally distinct rings of asteroids present within this range of heliocentric distance. The rings, within which 80% of each of 4 major taxonomic types (S, C, P, and D) fall, are centered at 2.6 (0.7), 2.9 (0.8), 3.4 (0.7), and 4.6 (1.5) AU respectively, where the numbers in parentheses are the ring widths in AU. The overall physical resemblence between the asteroid rings and planetary rings is poor; physically the asteroid belt more closely resembles a debris strewn satellite system. Once the mineralogy of these taxonomic types is established, and complications arising from postaccretionary metamorphism are dealt with, they may be used as probes of physical conditions in the early solar system. The identification of primordial planetesimals can be used in studying the siblings of the planetesimals responsible for the final stage of planetary accretion.