The Nature of M-Class Asteroids from 3-μm Observations

We have spectrophotometrically observed twenty M-class asteroids in the 1.2- to 3.5-μm region, sixteen of them for the first time. When we combined these with earlier surveys by Rivkin et al. (1995) and Jones et al. (1990) for a total of 27 asteroids, we find that over 35% of them have absorption features at 3 μm diagnostic of water of hydration. This absorption feature is inconsistent with these M-class asteroids being primarily Fe-Ni metal, which would be expected if they were remnant cores of disrupted differentiated objects. We also find that large (D>65 km) M asteroids are very likely to be hydrated (75%), while small (D<65 km) M asteroids are very unlikely to be hydrated (10%). We believe the interpretation most consistent with all data (astronomical, meteoritical, and geochemical modeling) holds the large M-class asteroids to be relatively primitive material perhaps akin to enstatite chondrites or salt-rich carbonaceous chondrites, with the iron meteorites found on Earth perhaps deriving from the smaller, anhydrous M asteroids. In keeping with Rivkin et al. (1995), we place the hydrated M asteroids in the separate W class.