Observations of main-belt asteroids in the 3-micron region

Abstract

Many asteroids show absorption features diagnostic of hydrated minerals in the 3-μ m spectral region. Reflectance studies in this region can determine the hydration state of surface minerals, and by inference, the thermal histories of bodies. Observations of M-class asteroids from 1.25-3.5 μ m show that many of these asteroids have water of hydration, and those that do cannot be interpreted as the cores of differentiated parent bodies. Because of this, it is suggested that the hydrated M asteroids should be split off into their own class–the W class. Simple spectral mixing models of these asteroids show they are consistent with enstatite chondritic material mixed with talc, suggesting the W asteroids may be the result of aqueous alteration of enstatite chondrites, though other models may also hold merit. The E asteroids are also found to have hydrated members, inconsistent with their interpretation as purely igneous bodies. A trend for large E and M asteroids to be hydrated is found. A compilation of S-class asteroid data at 3 μ m has been performed, supporting the finding that some S asteroids have spectra consistent with a mixture of ordinary chondrite and metal. There is some evidence for a trend altering the spectra of near-Earth asteroids to look like main-belt asteroids, but no simple trend can also include the ordinary chondrite meteorites. Variation in asteroids at 3 μ m was studied, and while no clear evidence of rotational variation is found, there is circumstantial evidence for latitudinal variation on several asteroids, perhaps as interior layers of an aqueously altered body are excavated. Finally, high-resolution studies of C-class asteroids were performed. A finding that 1 Ceres’ spectrum matches that of an ammoniated phyllosilicate is supported over an extended wavelength range. Observations of other CBG-class asteroids find no ammoniated minerals. The CBG-class asteroids, other than Ceres, all share very similar spectra, suggesting similar hydrated minerals on their surfaces.