Mid-ir Spectroscopy of M Asteroids with the Spitzer Irs: Preliminary Results

Introduction: We are conducting a survey of the emission spectra of 27 class M asteroids using the Spitzer Infrared Spec-trgraph (IRS; 5.2–38 µm). Although the visible and near-IR spectra of these asteroids are nearly featureless, ten of these asteroids are now known to have hydration features at 3 µm that are absent in the spectra of 15 others [1]. High S/N spectroscopy of these asteroids in the mid-infrared is likely to reveal key thermophysical and compositional information not available in the near-infrared. The M-asteroid taxon as originally designated [2, 3] comprised asteroids of moderate albedo that lacked mineral absorption features in the visible and near-infrared (0.3–1.1 µm). In the absence of direct spectral evidence of composition, the nature of M asteroids has remained uncertain. In one interpretation [e.g. 4] they have been thought to be the asteroid analogues to nickel-iron meteorites, in which case they would be fragments of the cores of one or more parent bodies which must have undergone extensive igneous differentiation before being catastrophically disrupted. Alternatively [5] they have been seen as analogues of the highly reduced enstatite (" E ") chondrites, whose parent bodies could not have been differentiated or indeed melted at all. Previous Evidence for Metallic and Non-Metallic M Asteroids: Radar observations have provided the most compelling direct evidence for the metallic compositions of at least some M asteroids. Very high radar albedos [6, 7] have been observed for various M asteroids, including 16 Psyche, 216 Kleopatra, and several near-Earth objects, consistent with high metal concentrations. The existence of nickel-iron meteorites also implies that their parent bodies must be somewhere among the small bodies of the solar system. Laboratory studies of iron meteorites [8] suggest that at least 70 distinct metallic parent bodies are represented in the meteorite collection. Radar observations of at least one other M asteroid [21 Lute-tia; 9]) suggest that its composition is non-metallic. A non-metallic nature for 21 Lutetia and various other M asteroids is also supported by observations of 3-µm absorptions which are characteristic of hydrated silicate minerals. These were first detected in the spectra of the M asteroids 55 Pandora and 92 Undina by Jones et al. [10]. These findings were confirmed by Rivkin et al. [1, 11], who surveyed the 3-µm spectra of 27 M asteroids and found hydration features on Pandora, Undina, Lutetia, and seven other M asteroids. On the basis of these observations, they suggested …