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Spectroscopic Survey of M-Type Asteroids S Spectroscopic survey of M-type asteroids S. Fornasier, B.E. Clark, E. Dotto, A. Migliorini, M. Ockert-Bell, M.A. Barucci To cite this version: S. Fornasier, B.E. Clark, E. Dotto, A. Migliorini, M. Ockert-Bell, et al.. Spectroscopic survey of M- type asteroids. Icarus, Elsevier, 2010, 210 (2), pp.655. 10.1016/j.icarus.2010.07.001. hal-00693813 HAL Id: hal-00693813 https://hal.archives-ouvertes.fr/hal-00693813 Submitted on 3 May 2012 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Accepted Manuscript Spectroscopic survey of M–type asteroids S. Fornasier, B.E. Clark, E. Dotto, A. Migliorini, M. Ockert-Bell, M.A. Barucci PII: S0019-1035(10)00273-3 DOI: 10.1016/j.icarus.2010.07.001 Reference: YICAR 9497 To appear in: Icarus Received Date: 11 September 2009 Revised Date: 1 July 2010 Accepted Date: 1 July 2010 Please cite this article as: Fornasier, S., Clark, B.E., Dotto, E., Migliorini, A., Ockert-Bell, M., Barucci, M.A., Spectroscopic survey of M–type asteroids, Icarus (2010), doi: 10.1016/j.icarus.2010.07.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. ACCEPTED MANUSCRIPT Spectroscopic survey of M–type asteroids1 S. Fornasier1,2, B. E. Clark3,1, E. Dotto4, A. Migliorini5, M. Ockert-Bell3, M.A. Barucci1 1 LESIA, Observatoire de Paris, 5 Place Jules Janssen, F-92195 Meudon Principal Cedex, France 2 Universit´e de Paris 7 Denis Diderot, France 3 Department of Physics, Ithaca College, Ithaca, NY 14850, USA 4 INAF, Osservatorio Astronomico di Roma, via Frascati 33, I-00040 Monteporzio Catone (Roma), Italy 5 IASF-INAF, via del Fosso del Cavaliere 100, 00133 Roma, Italy Submitted to Icarus: September 2009 e-mail: [email protected]; fax: +33145077144; phone: +33145077746 Manuscript pages: 60; Figures: 10; Tables: 6 1Based on observations carried out at the European Southern Observatory (ESO), La Silla, Chile, ESO proposals 073.C-0622, 074.C-0049 and 078.C-0115 and at the Telescopio Nazionale Galileo, Canary Islands, Spain. Preprint submitted to Icarus July 9, 2010 ACCEPTED MANUSCRIPT Running head: Investigation of M–type asteroids Send correspondence to: Sonia Fornasier LESIA-Observatoire de Paris Batiment 17 5, Place Jules Janssen 92195 Meudon Cedex France e-mail: [email protected] fax: +33145077144 phone: +33145077746 2 ACCEPTED MANUSCRIPT Spectroscopic survey of M–type asteroids1 S. Fornasier1,2, B. E. Clark3,1, E. Dotto4, A. Migliorini5, M. Ockert-Bell3, M.A. Barucci1 Abstract M-type asteroids, as defined in the Tholen taxonomy (Tholen, 1984), 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 spectro- scopic survey in the visible and near infrared wavelength range (0.4-2.5 µm) of 30 as- teroids 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 behav- iors 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 chlo- rites 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 1Based on observations carried out at the European Southern Observatory (ESO), La Silla, Chile, ESO proposals 073.C-0622, 074.C-0049 and 078.C-0115 and at the Telescopio Nazionale Galileo, Canary Islands, Spain. Preprint submitted to Icarus July 9, 2010 ACCEPTED MANUSCRIPT for meteorite spectral analogues in the RELAB database and by modelling with geograph- ical 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 syntetic mixture ob- tained enriching a pallasite meteorite with small amounts (1-2%) of silicates well reproduce the spectral behaviour 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 versus 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. Key words: Asteroids, Spectroscopy, Meteorites 4 ACCEPTED MANUSCRIPT 1. Introduction Within the taxonomic system of Tholen (1984), M asteroids have feature- less spectra and red spectral slope over the visible wavelength range (0.34 – 1.04 µm) and medium visual albedos (0.1 – 0.3). Tholen and Barucci (1989) identify approximately 40 M-type asteroids using the Tholen (1984) class de- finition. The M-types are presumed to be the progenitors both of differentiated iron-nickel meteorites and enstatite chondrites (Gaffey 1976; Cloutis et al. 1990; Gaffey et al. 1993). Some authors have proposed alternative meteorite linkages, such as CH, CB (Bencubbinite) (Hardersen et al. 2005), or other carbonaceous chondrites (Vilas 1994). M-class albedos generally match those of nickel-iron meteorites, leading some to propose that these asteroids are the collisionally- produced fragments of differentiated metal cores (Bell et al. 1989). This interpre- tation requires parent bodies heated to at least 2000 ◦C to produce iron meteorites (Taylor, 1992). However, based on recent findings, these interpretations are be- ing challenged. Polarimetric measurements (Lupisko & Belskaya 1989) and radar observations (Ostro et al. 1991; 2000; Magri et al. 2007; Shepard et al. 2008) of selected M-type asteroids have revealed a large variety of surface properties that are in some cases inconsistent with only a metallic composition. Shepard et al. (2008), in a radar survey of 14 M-type asteroids, find that only four objects (16 Psyche, 216 Kleopatra, 758 Mancunia, and 785 Zwetana) have radar albedos high enough to be dominantly metallic. The first estimations of the bulk density of M-asteroids 16 Psyche (1.4–1.8 g/cm3) and 22 Kalliope (2.4 g/cm3) gave values surprisingly lower than expected (Viateau 2000; Kocheva 2003; Britt et al. 2002; Margot & Brown 2003). These low den- sities are not consistent with predominantly metal composition, and require very 5 ACCEPTED MANUSCRIPT high values for the bulk porosity. However, these values have been disputed and recently Shepard et al. (2008) estimates a very high bulk density for 16 Psy- che (7.6±3.0 g/cm3), more consistent with a metal composition. Descamps et al. (2008) find a bulk density of 3.35±0.33 g/cm3 for 22 Kalliope, higher than the first estimation but still implying a high bulk porosity. Of note is the evidence for hydrated minerals on the surfaces of some M- type asteroids (Jones et al. 1990; Rivkin et al. 1995; 2000; 2002), inferred from the identification of an absorption feature around 3 µm typically due to the first overtone of H2O and to OH vibrational fundamentals in hydrated silicates. Rivkin et al. (1995; 2000) proposed splitting the M-types into wet W-class asteroids (those which show the 3 µm band), and dry M-class asteroids (those which do not show the 3 µm band). If the 3 µm band on M-type asteroids is due to hydrated minerals, then these objects might not be anhydrous and igneous as previously believed and the thermal scenarios for the inner main belt may need a revision (e.g. Bell et al. 1989). Gaffey et al. (2002) propose alternative explanations for the 3 µm band: materials normally considered to be anhydrous containing structural OH; troilite, an anhydrous mineral that shows a feature at 3 µm, or xenolithic hydrous meteorite components on asteroid surfaces from impacts and solar wind implanted H. Recent spectroscopic observations in the visible and near infrared region show that the spectra of M asteroids are not uniformly featureless as previously be- lieved. In addition to the 3 µm band, faint absorption bands in the 0.9 µm re- gion have been identified on the surfaces of some M-type asteroids (Chapman & Gaffey, 1979; Hardersen et al.
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