The fraction of hydrated C-complex asteroids in the asteroid belt from SDSS data

The Sloan Digital Sky Survey (SDSS) Moving Object Catalog contains spectrophotometric information for thousands of asteroids, presenting the opportunity to probe objects much fainter than are typically reached in spectroscopic surveys. Using two different approaches, it is estimated that 30 ± 5% of the C-complex asteroids in the SDSS have a 0.7-μm band, implying that roughly two-thirds will have a 3-μm absorption band and hydrated minerals based on correlations between those two absorptions found by Howell et al. (Howell, E.S. et al. [2011]. EPSC-DPS Joint Meeting, p. 637). In an effort to avoid confusion with formal taxonomies, I call these objects Ch ∼ in this work, with the C ∼ group defined as those without evidence of a 0.7-μm band. This fraction appears fairly stable with solar distance, although there is evidence it is higher in the middle asteroid belt (2.50–2.82 AU) than outside those bounds. In the size range covered by the SDSS dataset, the Ch ∼ fraction is most consistent with a flat distribution. Inclusion of the SMASS and S3OS2 datasets suggests an overall minimum in Ch ∼ fraction from H ∼ 12–14, though this distribution may be biased by the solar distance variation mentioned above.