GEOLOGIC STUDIES OF ASTEROIDS: FAMILIES, METEORITES, TAXONOMY, AND SPECTRAL MATCHING

The presence and nature of collisionally-produced asteroid families provide important constraints on the processes involved in the disruptions of large (~100-1000 km diameter) planetesimals, on the collisional lifetime of asteroids as a function of size and composition, on the thermal history and internal compositional structure of their parent bodies, and on the rate of orbital diffusion in the asteroid belt. A dynamical asteroid family is a group of asteroids that follow similar orbits about the Sun. There are disagreements on both the total number of dynamical families and their memberships. In addition, physical studies of asteroids and meteorites suggest that there is a problem with both the number of families identified to date and the inferred homogeneity within these families. A genetic (i.e., "real" or "true") asteroid family is one in which the members were derived from a common parent body. Genetic families provide glimpses of the interiors of small planetary objects whose compositions, thermal evolutions, and geochemical processes were established by the ambient conditions present during the formation epoch of the solar system. Thus asteroids provide the only in situ record from the earliest part of solar system history for the mainbelt region. Historically, only tests based on taxonomy or spectral matching have been applied to assay asteroid family membership. These methods are useful for sorting and classifying the objects, but they are unable to demonstrate genetic relationships. The most reliable way of testing the reality of asteroid families is by deriving the mineralogy of individual members. Remote sensing techniques must be used since in situ testing or sample return is not possible in most cases. However, significant progress can be made by using meteorites as validity checks on asteroid compositional interpretations derived from remote sensing data. That is a primary goal of the Family Asteroid Compositional Evaluation Survey, which was established to fill gaps in existing family asteroid spectroscopic databases and to obtain new data on additional family members. Recent results from analyses of the FACES database will be presented.