The composition of M-type asteroids: Synthesis of spectroscopic and radar observations M.E. Ockert-Bell a, * ,1 , B.E. Clark a,1 , M.K. Shepard b,2 , R.A. Isaacs a , E.A. Cloutis c , S. Fornasier d , S.J. Bus e a Department of Physics, Ithaca College, Ithaca, NY 14853, United States b Department of Geography and Geosciences, Bloomsburg University, Bloomsburg, PA 17815, United States c Department of Geography, University of Winnipeg, Winnipeg, MB, Canada R3B 2E9 d LESIA, Observatoire de Paris, 5 Place Jules Janssen, F-92195 Meudon Principal Cedex, France e Institute for Astronomy, 2680 Woodlawn Dr., Honolulu, HI 96822, United States article info Article history: Received 2 April 2010 Revised 3 August 2010 Accepted 3 August 2010 Available online 8 August 2010 Keywords: Asteroids Asteroids, Composition Asteroids, Surfaces abstract We have conducted a radar-driven observational campaign of 22 main-belt asteroids (MBAs) focused on Bus–DeMeo Xc- and Xk-type objects (Tholen X and M class asteroids) using the Arecibo radar and NASA Infrared Telescope Facilities (IRTF). Sixteen of our targets were near-simultaneously observed with radar and those observations are described in a companion paper (Shepard, M.K., and 19 colleagues [2010]. Ica- rus, in press). We find that most of the highest metal-content asteroids, as suggested by radar, tend to exhibit silicate absorption features at both 0.9 and 1.9 lm, and the lowest metal-content asteroids tend to exhibit either no bands or only the 0.9 lm band. Eleven of the asteroids were observed at several rota- tional longitudes in the near-infrared and significant variations in continuum slope were found for nine in the spectral regions 1.1–1.45 lm and 1.6–2.3 lm. We utilized visible wavelength data (Bus, S.J., Binzel, R.P. [2002b]. Icarus 158, 146–177; Fornasier, S., Clark, B.E., Dotto, E., Migliorini, A., Ockert-Bell, M., Barucci, M.A. [2010]. Icarus 210, 655–673.) for a more complete compositional analysis of our targets. Compositional evidence is derived from our target asteroid spectra using two different methods: (1) a v 2 search for spectral matches in the RELAB database, and (2) parametric comparisons with meteorites. This paper synthesizes the results of the RELAB search and the parametric comparisons with composi- tional suggestions based on radar observations. We find that for six of the seven asteroids with the high- est iron abundances, our spectral results are consistent with the radar evidence (16 Psyche, 216 Kleopatra, 347 Pariana, 758 Mancunia, 779 Nina, and 785 Zwetana). Three of the seven asteroids with the lowest metal abundances, our spectral results are consistent with the radar evidence (21 Lutetia, 135 Hertha, 497 Iva). The remaining seven asteroids (22 Kalliope, 97 Klotho, 110 Lydia, 129 Antigone, 224 Oceana, 678 Fredegundis, and 771 Libera) have ambiguous compositional interpretations when com- paring the spectral analogs to the radar analogs. The number of objects with ambiguous results from this multi-wavelength survey using visible, near-infrared, and radar wavelengths indicates that perhaps a third diagnostic wavelength region (such as the mid-infrared around 2–4 lm, the mid-infrared around 10–15 lm, and/or the ultraviolet around 0.2–0.4 lm) should be explored to resolve the discrepancies. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction Main-belt asteroids (MBAs) are critical for testing and modify- ing formation models of the Solar System. X-complex asteroids (Tholen, 1984; Bus and Binzel, 2002b; Mothé-Diniz et al., 2003) play a fundamental role in the investigations of early formation theories because they represent about 20% of inner main-belt asteroids and because they potentially include asteroid metallic cores (cf. Bell et al., 1989). Measurements in the near-infrared wavelengths (0.8–4.2 lm) are useful for compositional information such as absorption band center positions and depths (indicators of silicate mineralogy), continuum slope (indicator of spectral redness due to optical alter- ation, organics, or presence of metal) (Chapman and Gaffey, 1979; Clark et al., 2004a,b), and 3-lm features (indicator of hydrated mineral abundances (Rivkin et al., 1995, 2000)). Recent work in the visible and near-infrared wavelength ranges indicates that the X-complex asteroids are spectrally diverse be- yond the sub-groups (E, M, P) suggested by Tholen on the basis of albedo (Clark et al., 2004b). One Tholen X-complex subgroup, 0019-1035/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2010.08.002 * Corresponding author. Address: 62 Buck Rd., Lansing, NY 14882, United States. Fax: +1 607 274 1773. E-mail address: mbell5@twcny.rr.com (M.E. Ockert-Bell). 1 Guest observer at NASA Infrared Telescope Facility. 2 Guest observer at Arecibo Observatory. Icarus 210 (2010) 674–692 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus