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Spectroscopy of K-Complex Asteroids: Parent Bodies of Carbonaceous Meteorites? ∗ Beth Ellen Clark A, ,1, Maureen E

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Spectroscopy of K-Complex Asteroids: Parent Bodies of Carbonaceous Meteorites? ∗ Beth Ellen Clark A, ,1, Maureen E Icarus 202 (2009) 119–133 Contents lists available at ScienceDirect Icarus www.elsevier.com/locate/icarus Spectroscopy of K-complex asteroids: Parent bodies of carbonaceous meteorites? ∗ Beth Ellen Clark a, ,1, Maureen E. Ockert-Bell a,EdA.Cloutisb,DavidNesvornyc, Thais Mothé-Diniz d,SchelteJ.Buse a Department of Physics, Ithaca College, Ithaca, NY 14850, USA b Department of Geography, University of Winnipeg, Winnipeg, MB, R3B 2E9, Manitoba, Canada c Department of Space Sciences, Southwest Research Institute, 1050 Walnut Street 300, Boulder, CO 80302, USA d Universidade Federal do Rio de Janeiro, Observatório do Valongo, Ladeira Pedro Antônio, 43 CEP 20080-090, Rio de Janeiro, Brazil e University of Hawaii, Institute for Astronomy, 640 North A‘ohoku Place, 209, Hilo, HI 96720-2700, USA article info abstract Article history: This is the first focused study of non-Eos K asteroids. We have observed a total of 30 K-complex objects Received 27 November 2007 (12 K-2 Sk- and 13 Xk-type asteroids (from the Bus taxonomy), plus 3 K-candidates from previous work) Revised 23 January 2009 and we present an analysis of their spectral properties from 0.4 to 2.5 μm. We targeted these asteroids Accepted 3 February 2009 because their previous observations are spectrally similar enough to suggest a possible compositional Available online 14 March 2009 relationship. All objects have exhibited spectral redness in the visible wavelengths and minor absorptions Keywords: near 1 micron. If, as suggested, K-complex asteroids (including K, Xk, and Sk) are the parent bodies Asteroids of carbonaceous meteorites, knowledge of K-asteroid properties and distribution is essential to our Asteroids, composition understanding of the cosmochemical importance of some of the most primitive meteorite materials in our Asteroids, surfaces collection. This paper presents initial results of our analysis of telescopic data, with supporting analysis of laboratory measurements of meteorite analogs. Our results indicate that K-complex asteroids are distinct from other main belt asteroid types (S, B, C, F, and G). They do not appear to be a subset of these other types. K asteroids nearly span the range of band center positions and geometric albedos exhibited by the carbonaceouschondrites(CO,CM,CV,CH,CK,CR,andCI).WefindthatB-,C-,F-andG-typeasteroids tend to be darker than meteorites, and can have band centers longer than any of the chondrites measured here. This could indicate that K-complex asteroids are better spectral analogues for the majority of our carbonaceous meteorites than the traditional B-, C-, F- and G-matches suggested in the literature. This paper present first results of our ongoing survey to determine K-type mineralogy, meteorite linkages, and significance to the geology of the asteroid regions. © 2009 Elsevier Inc. All rights reserved. 1. Introduction be compositionally linked. Our results include (1) spectroscopic characterization from 0.4 to 2.5 μm; (2) comparison of our targets Our goal in asteroid spectroscopic studies is to determine spe- to the original K-type, 221 Eos, and its family; (3) comparison of K- cific links between classes of meteorites and their asteroid parent complex objects to S-, C-, B-, G- and F-type asteroids; (4) compari- bodies. Establishing these links is necessary in order to use mete- son of our targets to a library of carbonaceous chondrite meteorite orites to understand the chemical and physical conditions which spectra; and (5) a discussion of the implications of the findings of prevailed in the asteroid regions during the formation of the So- this study to the geology of the asteroid regions. This is a prelimi- lar System. Toward this end, we compare spectral properties of the nary report of an ongoing survey of the K-complex. asteroids to those of meteorites and mineral separates in order to determine the chemical and mineralogical structure of the asteroid 2. Background regions. In this paper, we assemble, coordinate, and analyze the avail- 2.1. Definition of K-complex main-belt asteroids able visible and near-infrared wavelength spectral data of K-com- plex asteroids. Sk- and Xk-class spectra strongly resemble K-class The most diagnostically useful wavelength region for asteroid- asteroid spectra, and are included in our study because they may meteorite studies has been from 0.3 to 3.5 μm, and a large body of work exists on the spectroscopic links between mete- orites and asteroids (e.g. Johnson and Fanale, 1973; Gaffey, 1976; * Corresponding author. Fax: +1 607 274 1773. E-mail address: [email protected] (B.E. Clark). Bell et al., 1989; Pieters and McFadden, 1994; Rivkin et al., 2000; 1 Guest observer at NASA Infrared Telescope Facility and currently visiting as- Gaffey et al., 2002; Burbine et al., 2002; Clark et al., 1995, 2004; tronomer at the Paris Observatory. Lazzaro et al., 2004). Tholen (1984) produced a widely used as- 0019-1035/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2009.02.027 120 B.E. Clark et al. / Icarus 202 (2009) 119–133 information content of asteroid spectral studies. Bell found a new class of objects that have C-type spectra (flat continuum slopes) in the near-infrared (0.8–2.5 μm) and S-type spectra (strong sil- icate absorptions at 0.9–1.0 microns) in the visible wavelengths (0.3–0.9 μm). Asteroid 221 Eos and its family member objects were the archetypes, and the K-class was born. (The letter “K” was cho- sen because it lies midway between “C” and “S”.) The Eos fam- ily asteroids have similar orbital and spectral properties, and are probably the fragments of a catastrophic disruption event (e.g. see Vokrouhlicky et al., 2006). Bus and Binzel (2002a, 2002b) used visible wavelength observations of several Eos family members to define the boundary of the K-class in their spectral feature- based taxonomy. The Sk and Xk-classes were first added by Bus and Binzel in 2002—these asteroids tend to fill gaps in the visi- ble wavelength spectral continuum between S-types and C-types (Fig. 1). Most of the K-type studies performed previously have fo- cused on the Eos asteroid family (Gradie, 1978; Binzel, 1988; Bell, 1989; Granahan et al., 1993; Xu et al., 1995; Veeder et al., 1995; Doressoundiram et al., 1998; Mothé-Diniz and Carvano, 2005; Fig. 1. 26 Asteroid taxonomic classes from analysis of visible wavelength spectra. The average spectra are plotted with constant horizontal and vertical scaling and Vokrouhlicky et al., 2006; Mothé-Diniz et al., 2008). Since asteroid are arranged in a way that approximates the relative position of each class in the families are believed to be fragments of an original parent body, primary spectral component plane defined by principal component 2 (PC2’) and family objects must be considered when estimating the relative slope (reproduced with permission from Bus and Binzel, 2002b). abundance distributions of asteroids in the main belt. According to Nesvorny et al. (2005) there are more than 4000 main belt aster- teroid taxonomy system from cluster analysis of the Eight Color oids belonging to the Eos family, but very few have been observed Asteroid Survey data (589 objects in eight broad bands: 0.337, spectroscopically. There are at least 56 K-complex asteroids that 0.359, 0.437, 0.550, 0.701, 0.853, 0.948, and 1.041 μm). Bus and have been spectrally observed in the visible wavelengths and are Binzel (2002a; 2002b) extended taxonomic classification to 1447 not part of the Eos dynamical family. Those are our targets. small main belt asteroids observed with CCD spectrographs in 187 We have published (Clark et al., 1995)alowresolutionasteroid channels from 0.435 to 0.925 μm (Fig. 1). spectral survey conducted using the Seven Color Asteroid infrared While the Bus and Binzel taxonomy is consistent with Tholen’s filterSystem(SCAS).Forthissurvey126objectswereobserved, for most of the major classes, there are differences in the details concentrating on the smaller main belt asteroids of the S- and of minor asteroid classes between the two taxonomies. Specifically, M-classes. One of the unexpected results of the SCAS survey was for the Bus taxonomy: (1) K maps to a subset of the Tholen S class the discovery that, among the Tholen classified (1984) main-belt and includes a couple of Tholen T and I class objects; (2) Sk maps S-type asteroids of the 50-km size range, 10% of the population to a subset of the Tholen S class; and (3) Xk objects are a combi- looked like K-types in the IR. However, when Bus and Binzel re- nation of Tholen T, C, X, M, P, and E objects. We note that Tholen’s classified the Tholen S-types, and began observing Bus S-types in wavelength range was longer than Bus and Binzel’s, but the spec- the IR, the fraction appearing to be K-types dropped to much less tral resolution of Tholen’s survey was lower. In their taxonomy, Bus than 10%. and Binzel merged Tholen’s B, C, F, and G classes, and created new classes Ch, Cg, Cb, and Cgh, in order to account for the differences 2.2. K-type asteroid–meteorite linkages in spectral range and resolution between the two surveys. The 52-Color Survey (Bell et al., 1988) of 102 asteroids in the in- Bell (1988) compared the 52-color data of 221 Eos with car- frared wavelength range of 0.8 to 2.5 μm significantly extended the bonaceous chondrites and found a resemblance between Eos and Table 1 Standard stars used in K-complex asteroid spectral data reduction. UT date Standard star Asteroids observed 2003 Aug 16 L107-684 L112-1333 L115-271 1103 Sequoia, 2100 Ra Shalom 2003 Aug 18 L107-684 L112-1333 L115-271
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