Compositional structure in the asteroid belt : results of a spectroscopic survey

Abstract

Visible wavelength spectra have been obtained for 1189 main-belt asteroids during the second phase of the Small Main-belt Asteroid Spectroscopic Survey. These telescopic measurements were made using charge-coupled device (CCD) detectors, allowing for the targeting of smaller asteroids. A majority of the asteroids sampled have estimated diameters of 30 km or less. The SMASSII survey provides the largest internally-consistent sample of asteroid spectra ever obtained, and reveals a greater range of spectral diversity among asteroids than has been previously shown. This diversity may arise from a broad range of surface ages, where smaller (collisionally younger) asteroids may have less evolved or processed surfaces than do larger asteroids. Processes that affect the development of a regalia, and the distribution of regalia particle sizes may produce the observed variations in spectral features. The larger sample size, greater spectral resolution, and greater photometric precision of the SMASSII survey, compared with the most extensive previous survey, provide a basis for developing a new "feature-based" taxonomic classification system for asteroids. This new taxonomy builds on the robust, large-scale structure of existing taxonomies, and relies on the presence or absence of specific spectral features to define new class boundaries. The SMASSII measurements reveal that many of the previous taxonomic classes that appeared to be distinctly separate, are instead spanned by a nearly continuous transition of spectral properties. This continuum may be evidence of sampling a more complete range of mineralogies. This newly found continuum makes it difficult to define a unique taxonomic structure using classical multivariate techniques, requiring that subjective boundaries be defined in some cases. The scale-length in spectral variance observed in many dynamical asteroid families was used as a metric for constraining the class sizes in this new taxonomy. The resulting new taxonomy consists of twenty-six spectral classes. Members of the previously defined X-class (which could be subdivided only using albedo information) display sufficient variation in spectral features to allow subgroups to be distinguished without the use of albedo. A subset of 465 SMASSII asteroids were specifically targeted to test the genetic reality of small dynamical "families." These families likely result from the collisional disruption of larger parent asteroids, and are identified as clusters of objects having similar orbital parameters. The targeted region, encompassing the heliocentric distance range of 2.690 to 2.815 AU, contains 14 families that had been previously identified, based on numerous analyses of orbital distributions in this region of the;1 main belt. A newly developed multivariate technique that analyzes the combination of spectral characteristics and orbital parameters reveals that all 14 of these previously proposed families are distinct from the "background" population mt asteroids. This result implies that each of these families is likely to have been truly formed by a collisional event, providing strong evidence for an extensive collisional history within the asteroid belt. Cand S-type 2 asteroids appear equally capable of forming families. Each family is found to be relatively homogeneous in its spectral characteristics, allowing the boundaries of tile families, and potential interlopers, to be more precisely identified than was possible in previous dynamical analyses. The relative spectral homogeneity within each of these families provides little evidence for any to have been formed from a differentiated parent body. It remains a mystery as to why there is no clear evidence of a major family containing members from· different geologic units, derived from a differentiated parent asteroid. In addition to the 14 previously identified dynamical families, our analysis has revealed five additional associations of spectrally similar asteroids in this same small heliocentric range. These spectral clusters may represent the dispersed fragments of older collisionally derived families.