IMPROVING M-ASTEROID SPECTRAL INTERPRETATIONS VIA THE DETECTION OF WEAK ABSORPTION FEATURES

The M-asteroids are a group of taxonomically-defined objects that are compositionally ambiguous due to the lack of obvious absorption features in the visible- and near-infrared spectral regions. Traditional interpretations suggest affinities with the enstatite chondrites and NiFe meteorites based on the featureless spectra and moderate albedos of these meteoritic analogues. Jones et al. (1990), Rivkin (1995) and Rivkin et al. (2000) introduced a third possibility by suggesting the presence of phyllosilicate minerals on the surfaces of several M-asteroids. In addition, Hardersen (2003) and Hardersen et al. (2002) recently reported the first detection of weak, mafic silicate features present on the surfaces of four M-asteroids.

Although mineralogic diversity should be expected within an asteroid taxonomic group (Gaffey et al. 1993), recent interpretations have widely different implications for the histories of particular asteroids. For example, if an M-asteroid is interpreted as consisting of phyllosilicate minerals, this suggests a relatively low-temperature thermal history promoting aqueous alteration processes. If a NiFe metal and/or mafic silicate surface composition is suggested, then this often implies a higher-temperature environment and a more thermally-evolved asteroid.

Initial work by Hardersen (2003) and Hardersen et al. (2002) reveal the presence of weak, ~2-4% deep spectral absorption features in the ~0.9-micron region for asteroids 69 Hesperia, 110 Lydia, 201 Penelope and 216 Kleopatra. These authors interpret these asteroids as consisting of minor Fe- and Ca-poor orthopyroxenes surrounding the metallic cores of differentiated, disrupted parent bodies.

Full-rotational, near-infrared spectral observations of 16 Psyche, 125 Liberatrix and 136 Austria were obtained on March 22nd and 23rd, 2002, at the NASA Infrared Telescope Facility. Spectra for each asteroid will be examined for the presence or absence of any weak absorption features. Interpretations for each asteroid will be discussed and compared with the interpretations of previous workers.