Identification of magnetite in b-type asteroids

Abstract

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 /μm. the 1 /μm 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 /μm region reveal an absorption feature centered at 2.9 /μm, 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. © 2010. The american astronomical society. All rights reserved.