Spectroscopic survey of M-type asteroids

Abstract

M-type asteroids, as defined in the Tholen taxonomy (Tholen, D.J. [1984]. Asteroid Taxonomy from Cluster Analysis of Photometry. Ph.D. Dissertation, University of Arizona, Tucson), are medium albedo bodies supposed to have a metallic composition and to be the progenitors both of differentiated iron–nickel meteorites and enstatite chondrites. We carried out a spectroscopic survey in the visible and near infrared wavelength range (0.4–2.5 μm) of 30 asteroids chosen from the population of asteroids initially classified as Tholen M-types, aiming to investigate their surface composition. The data were obtained during several observing runs during the years 2004–2007 at the TNG, NTT, and IRTF telescopes. We computed the spectral slopes in several wavelength ranges for each observed asteroid, and we searched for diagnostic spectral features. We confirm a large variety of spectral behaviors for these objects as their spectra are extended into the near infrared, including the identification of weak absorption bands, mainly of the 0.9 μm band tentatively attributed to orthopyroxene, and of the 0.43 μm band that may be associated to chlorites and Mg-rich serpentines or pyroxene minerals such us pigeonite or augite. A comparison with previously published data indicates that the surfaces of several asteroids belonging to the M-class may vary significantly. We attempt to constrain the asteroid surface compositions of our sample by looking for meteorite spectral analogs in the RELAB database and by modeling with geographical mixtures of selected meteorites/minerals. We confirm that iron meteorites, pallasites, and enstatite chondrites are the best matches to most objects in our sample, as suggested for M-type asteroids. For 22 Kalliope, we demonstrate that a synthetic mixture obtained enriching a pallasite meteorite with small amounts (1–2%) of silicates well reproduce the spectral behavior including the observed 0.9 μm feature. The presence of subtle absorption features on several asteroids confirms that not all objects defined by the Tholen M-class have a pure metallic composition. A statistical analysis of spectral slope distribution vs. orbital parameters shows that our sample originally defined as Tholen M-types tend to be dark in albedo and red in slope for increasing value of the semi-major axis. However, we note that our sample is statistically limited by our number of objects (30) and slightly varying results are found for different subsets. If confirmed, the albedo and slope trends could be due to a difference in composition of objects belonging to the outer main belt, or alternatively to a combination of surface composition, grain size and space weathering effects.