  Spectrally blue (B-type) asteroids are rare, with the second discovered
asteroid Pallas being the largest and most famous example. We conducted a
focused, infrared spectroscopic survey of B-type asteroids to search for
water-related features in these objects. Our results show that the negative
optical spectral slope of some B-type asteroids is due to the presence of a
broad absorption band centered near 1.0 micron. The 1-micron band can be
matched in position and shape using magnetite (Fe3O4), which is an important
indicator of past aqueous alteration in the parent body. Furthermore, our
observations of B-type asteroid (335) Roberta in the 3-micron region reveal an
absorption feature centered at 2.9 micron, which is consistent with the
absorption due to phyllosilicates (another hydration product) observed in CI
chondrites. The new observations suggest that at least some B-type asteroids
are likely to have incorporated significant amounts of water ice and to have
experienced intensive aqueous alteration.
