3-μm spectrophotometric survey of m- and e-class asteroids

Many asteroids show absorption features diagnostic of hydrated minerals in the 3-μm region of the spectrum. Reflectance studies in this region can determine the hydration state of surface minerals and, by inference, the thermal histories of bodies. We have made spectrophotometric observations of eight E-class, M-class, X-class, and P-class asteroids in December 1991 and eight E-class and M-class asteroids in August 1993 using the Infrared Telescope Facility on Mauna Kea, Hawaii. In 1991 we discovered a 3-μm absorption band on the E-class asteroid 44 Nysa and the P-class asteroid 50 Virginia, and found a possible absorption on the E-class asteroid 64 Angelina. We confirmed the presence of a 3-μm absorption band on 44 Nysa and on the M-class asteroid 92 Undina (T. D. Jones et al., 1990, Icarus 88, 172-192) and detected this feature on the M-class asteroid 201 Penelope in 1993. These data demonstrate that high-albedo and medium-albedo objects such as E- and M-class asteroids can be hydrated and need not be interpreted as primarily of igneous origin. We revise the interpreted mineralogy of some E- and M-class asteroids as a result.