Icarus 202 (2009) 119–133

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Spectroscopy of K-complex asteroids: Parent bodies of carbonaceous meteorites?

Beth Ellen Clark a,∗,1, Maureen E. Ockert-Bell a, Ed A. Cloutis b, David Nesvorny c, Thais Mothé-Diniz d, Schelte J. Bus ea Department of Physics, Ithaca College, Ithaca, NY 14850, USAb Department of Geography, University of Winnipeg, Winnipeg, MB, R3B 2E9, Manitoba, Canadac Department of Space Sciences, Southwest Research Institute, 1050 Walnut Street 300, Boulder, CO 80302, USAd Universidade Federal do Rio de Janeiro, Observatório do Valongo, Ladeira Pedro Antônio, 43 CEP 20080-090, Rio de Janeiro, Brazile University of Hawaii, Institute for Astronomy, 640 North A‘ohoku Place, 209, Hilo, HI 96720-2700, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 27 November 2007Revised 23 January 2009Accepted 3 February 2009Available online 14 March 2009

Keywords:AsteroidsAsteroids, compositionAsteroids, surfaces

This is the first focused study of non-Eos K asteroids. We have observed a total of 30 K-complex objects(12 K-2 Sk- and 13 Xk-type asteroids (from the Bus taxonomy), plus 3 K-candidates from previous work)and we present an analysis of their spectral properties from 0.4 to 2.5 μm. We targeted these asteroidsbecause their previous observations are spectrally similar enough to suggest a possible compositionalrelationship. All objects have exhibited spectral redness in the visible wavelengths and minor absorptionsnear 1 micron. If, as suggested, K-complex asteroids (including K, Xk, and Sk) are the parent bodiesof carbonaceous meteorites, knowledge of K-asteroid properties and distribution is essential to ourunderstanding of the cosmochemical importance of some of the most primitive meteorite materials in ourcollection. This paper presents initial results of our analysis of telescopic data, with supporting analysis oflaboratory measurements of meteorite analogs. Our results indicate that K-complex asteroids are distinctfrom other main belt asteroid types (S, B, C, F, and G). They do not appear to be a subset of these othertypes. K asteroids nearly span the range of band center positions and geometric albedos exhibited by thecarbonaceous chondrites (CO, CM, CV, CH, CK, CR, and CI). We find that B-, C-, F- and G-type asteroidstend to be darker than meteorites, and can have band centers longer than any of the chondrites measuredhere. This could indicate that K-complex asteroids are better spectral analogues for the majority of ourcarbonaceous meteorites than the traditional B-, C-, F- and G-matches suggested in the literature. Thispaper present first results of our ongoing survey to determine K-type mineralogy, meteorite linkages, andsignificance to the geology of the asteroid regions.

© 2009 Elsevier Inc. All rights reserved.

1. Introduction

Our goal in asteroid spectroscopic studies is to determine spe-cific links between classes of meteorites and their asteroid parentbodies. Establishing these links is necessary in order to use mete-orites to understand the chemical and physical conditions whichprevailed in the asteroid regions during the formation of the So-lar System. Toward this end, we compare spectral properties of theasteroids to those of meteorites and mineral separates in order todetermine the chemical and mineralogical structure of the asteroidregions.

In this paper, we assemble, coordinate, and analyze the avail-able visible and near-infrared wavelength spectral data of K-com-plex asteroids. Sk- and Xk-class spectra strongly resemble K-classasteroid spectra, and are included in our study because they may

* Corresponding author. Fax: +1 607 274 1773.E-mail address: bclark@ithaca.edu (B.E. Clark).

1 Guest observer at NASA Infrared Telescope Facility and currently visiting as-tronomer at the Paris Observatory.

0019-1035/$ – see front matter © 2009 Elsevier Inc. All rights reserved.doi:10.1016/j.icarus.2009.02.027

be compositionally linked. Our results include (1) spectroscopiccharacterization from 0.4 to 2.5 μm; (2) comparison of our targetsto the original K-type, 221 Eos, and its family; (3) comparison of K-complex objects to S-, C-, B-, G- and F-type asteroids; (4) compari-son of our targets to a library of carbonaceous chondrite meteoritespectra; and (5) a discussion of the implications of the findings ofthis study to the geology of the asteroid regions. This is a prelimi-nary report of an ongoing survey of the K-complex.

2. Background

2.1. Definition of K-complex main-belt asteroids

The most diagnostically useful wavelength region for asteroid-meteorite studies has been from 0.3 to 3.5 μm, and a largebody of work exists on the spectroscopic links between mete-orites and asteroids (e.g. Johnson and Fanale, 1973; Gaffey, 1976;Bell et al., 1989; Pieters and McFadden, 1994; Rivkin et al., 2000;Gaffey et al., 2002; Burbine et al., 2002; Clark et al., 1995, 2004;Lazzaro et al., 2004). Tholen (1984) produced a widely used as-

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120 B.E. Clark et al. / Icarus 202 (2009) 119–133

Fig. 1. 26 Asteroid taxonomic classes from analysis of visible wavelength spectra.The average spectra are plotted with constant horizontal and vertical scaling andare arranged in a way that approximates the relative position of each class in theprimary spectral component plane defined by principal component 2 (PC2’) andslope (reproduced with permission from Bus and Binzel, 2002b).

teroid taxonomy system from cluster analysis of the Eight ColorAsteroid Survey data (589 objects in eight broad bands: 0.337,0.359, 0.437, 0.550, 0.701, 0.853, 0.948, and 1.041 μm). Bus andBinzel (2002a; 2002b) extended taxonomic classification to 1447small main belt asteroids observed with CCD spectrographs in 187channels from 0.435 to 0.925 μm (Fig. 1).

While the Bus and Binzel taxonomy is consistent with Tholen’sfor most of the major classes, there are differences in the detailsof minor asteroid classes between the two taxonomies. Specifically,for the Bus taxonomy: (1) K maps to a subset of the Tholen S classand includes a couple of Tholen T and I class objects; (2) Sk mapsto a subset of the Tholen S class; and (3) Xk objects are a combi-nation of Tholen T, C, X, M, P, and E objects. We note that Tholen’swavelength range was longer than Bus and Binzel’s, but the spec-tral resolution of Tholen’s survey was lower. In their taxonomy, Busand Binzel merged Tholen’s B, C, F, and G classes, and created newclasses Ch, Cg, Cb, and Cgh, in order to account for the differencesin spectral range and resolution between the two surveys.

The 52-Color Survey (Bell et al., 1988) of 102 asteroids in the in-frared wavelength range of 0.8 to 2.5 μm significantly extended the

information content of asteroid spectral studies. Bell found a newclass 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 werethe 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 areprobably the fragments of a catastrophic disruption event (e.g. seeVokrouhlicky et al., 2006). Bus and Binzel (2002a, 2002b) usedvisible wavelength observations of several Eos family membersto define the boundary of the K-class in their spectral feature-based taxonomy. The Sk and Xk-classes were first added by Busand 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;Vokrouhlicky et al., 2006; Mothé-Diniz et al., 2008). Since asteroidfamilies are believed to be fragments of an original parent body,family objects must be considered when estimating the relativeabundance distributions of asteroids in the main belt. According toNesvorny et al. (2005) there are more than 4000 main belt aster-oids belonging to the Eos family, but very few have been observedspectroscopically. There are at least 56 K-complex asteroids thathave been spectrally observed in the visible wavelengths and arenot part of the Eos dynamical family. Those are our targets.

We have published (Clark et al., 1995) a low resolution asteroidspectral survey conducted using the Seven Color Asteroid infraredfilter System (SCAS). For this survey 126 objects were observed,concentrating on the smaller main belt asteroids of the S- andM-classes. One of the unexpected results of the SCAS survey wasthe discovery that, among the Tholen classified (1984) main-beltS-type asteroids of the 50-km size range, 10% of the populationlooked like K-types in the IR. However, when Bus and Binzel re-classified the Tholen S-types, and began observing Bus S-types inthe IR, the fraction appearing to be K-types dropped to much lessthan 10%.

2.2. K-type asteroid–meteorite linkages

Bell (1988) compared the 52-color data of 221 Eos with car-bonaceous chondrites and found a resemblance between Eos and

Table 1Standard 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 Shalom2003 Aug 18 L107-684 L112-1333 L115-271 2100 Ra Shalom2004 Jul 12 L107-684 L110-361, 16CygB 173 Ino2005 Mar 16 L102-108 L105-56 757 Portlandia2005 Sep 17 64Hyades L93-101 L110-361 L112-1333 L115-271 441 Bathilde, 547 Praxedis, 559 Nanon2005 Sep 20 64Hyades L93-101 L110-361 L112-1333 L115-271 417 Suevia, 441 Bathilde, 547 Praxedis, 559 Nanon2005 Nov 15 64Hyades L93-101 L102-1081 L115-271 173 Ino, 397 Vienna, 417 Suevia, 599 Luisa2005 Nov 16 64Hyades L93-101 L115-271 397 Vienna, 417 Suevia, 686 Gersuind2006 Aug 12 L107-684 L110-361, L112-1333 2100 Ra Shalom2006 Aug 13 16CygB L112-1333 2100 Ra Shalom2006 Oct 02 64Hyades L93-101 441 Bathilde2006 Oct 05 L93-101 441 Bathilde2006 Oct 06 64Hyades 233 Asterope2006 Dec 17 64Hyades L97-249 L102-1081 221 Eos, 661 Cloelia, 742 Edisonia, 1545 Thernoe2006 Dec 19 64Hyades L97-249 L102-1081 221 Eos, 661 Cloelia, 742 Edisonia, 1545 Thernoe, 1903 Adzhimushka2007 Dec 21 64Hyades L97-249 L102-1081 679 Pax, 973 Aralia, 75 Eurydike2008 Mar 11 64Hyades L97-249 L102-1081 179 Klytaemnestra2008 Mar 12 64Hyades L97-249 L102-1081 75 Eurydike, 186 Celuta, 2606 Odessa2008 Jul 15 L105-56 L112-1333 L115-271 43 Ariadne, 250 Bettina, 332 Siri, 179 Klytaemnestra, 186 Celuta, 2606 Odessa

The “Standard star” names that have the prefix “L” are Landolt stars.

Spectroscopy of K-complex asteroids 121

Table 2aSummary of the properties of the K, K-candidate, and Sk asteroids observed.

Name Bustype

Bus–DeMeotype

Tholentype

D(km)

a(AU)

pv VIS(±4%)

NIR1(±10%)

NIR2(±25%)

Continuumslope

Bandcenter

Banddepth

43 Ariadne Sk Sq S 66 2.20 0.27 0.77 0.53 0.12 0.22 ± 0.00 1.00 ± 0.00 0.14 ± 0.0089 Julia K K S 152 2.55 0.18 0.98 0.46 0.11 0.21 ± 0.00 1.04 ± 0.00 0.09 ± 0.00179 Klytaemnestra Sk S S 78 2.97 0.16 0.81 0.20 0.00 0.16 ± 0.02 0.91 ± 0.01 0.10 ± 0.00186 Celuta K K S 50 2.36 0.19 0.75 0.36 0.00 0.16 ± 0.02 1.02 ± 0.01 0.08 ± 0.00221 Eosa K K S 104 3.01 0.14 0.70 0.16 0.00 −0.01 ± 0.00 1.03 ± 0.00 0.08 ± 0.01233 Asterope K Xk T 103 2.66 0.09 0.73 0.30 0.14 0.20 ± 0.02 1.03 ± 0.06 0.03 ± 0.01397 Vienna K L S 43 2.63 0.18 1.08 0.00 −0.12 0.07 ± 0.02 0.96 ± 0.02 0.03 ± 0.00402 Chloe K L S 54 2.56 0.15 0.71 0.13 0.00 0.10 ± 0.01 0.96 ± 0.01 0.03 ± 0.00472 Roma – S S 47 2.54 0.21 1.85 0.26 0.07 0.14 ± 0.00 0.91 ± 0.01 0.09 ± 0.00599 Luisab K L S 65 2.77 0.14 0.87 0.05 −0.06 0.02 ± 0.01 0.99 ± 0.02 0.03 ± 0.00661 Cloeliaa K K S 48 3.01 0.11 0.76 0.15 0.00 −0.00 ± 0.01 1.02 ± 0.01 0.07 ± 0.00679 Pax K L I 52 2.59 0.17 0.89 0.20 0.00 0.24 ± 0.01 0.89 ± 0.01 0.02 ± 0.01686 Gersuind – L S 41 2.59 0.14 0.33 0.12 0.00 0.16 ± 0.00 1.02 ± 0.02 0.02 ± 0.00742 Edisoniaa K K S 46 3.01 0.13 0.67 0.13 0.00 0.05 ± 0.01 1.04 ± 0.03 0.07 ± 0.001545 Thernoe K L – 19 2.77 0.10 0.66 0.00 −0.09 0.05 ± 0.01 0.92 ± 0.01 0.02 ± 0.001903 Adzhimushkaja K K – 37 3.00 0.08 0.83 0.12 0.00 −0.00 ± 0.00 1.04 ± 0.01 0.05 ± 0.002100 Ra Shalomc Xc B C 3.4 0.083 0.06 0.47 0.00 0.00 −0.08 ± 0.03 1.03 ± 0.04 0.03 ± 0.002100 Ra Shalomc Xc B C 3.4 0.083 0.06 0.28 −0.10 −0.06 −0.01 ± 0.00 0.98 ± 0.03 0.03 ± 0.00Values are from the JPL Horizons database (ssd.jpl.nasa.gov): D is the asteroid diameter, a is the semi-major axis of the asteroid and pv is the geometric albedo. Visiblespectrum slope (VIS) is measured as rise over run in reflectance from 0.47 to 0.75 μm, the near-infrared slope (NIR1) is measured between 1.1 and 1.6 μm, and NIR2 slope ismeasured between 1.7 and 2.49 μm (errors reflect changes in boundary points). Continuum slope is measured across the 1-μm band from about 0.75 to 1.5 μm. All slopesare given in units of %/μm. Band depth is measured as percentage of reflectance from the continuum slope (∼0.75 to 1.5 μm) to the Band center as outlined by Clark andRoush (1984). Bus–DeMeo taxonomy classification is from http://smass.mit.edu/busdemeoclass.html.

a EOS family.b Watsonia family (Sunshine et al., 2008).c Values given for longitudes 1 and 2 respectively, as defined in Shepard et al. (2008).

Table 2bSummary of the properties of the Xk asteroids observed.

Name Bustype

Bus–DeMeotype

Tholentype

D(km)

a(AU)

pv VIS(±4%)

NIR1(±10%)

NIR2(±25%)

Continuumslope (%)

Band center(μm)

Banddepth

75 Eurydike Xk K M 56 2.67 0.15 0.54 0.36 0.00 0.24 ± 0.01 1.07 ± 0.02 0.05 ± 0.0099 Dike Xk K C 69 2.66 0.06 0.38 0.04 0.00 0.06 ± 0.01 0.93 ± 0.03 0.01 ± 0.00114 Kassandra Xk Xk T 100 2.68 0.09 0.69 0.27 0.09 0.19 ± 0.01 1.03 ± 0.01 0.03 ± 0.00173 Ino Xk Xk C 154 2.74 0.06 0.47 0.10 0.07 0.06 ± 0.01 1.09 ± 0.03 0.02 ± 0.00250 Bettina Xk Xk M 80 3.15 0.26 0.63 0.24 0.19 0.30 ± 0.02 0.90 ± 0.01 0.04 ± 0.00417 Suevia Xk Xk X 41 2.8 0.20 0.58 0.27 0.11 0.20 ± 0.01 0.97 ± 0.01 0.04 ± 0.00441 Bathiilde Xk Xc M 70 2.81 0.14 0.68 0.12 0.05 – – –547 Praxedis Xk Xk XD 70 2.77 0.06 0.66 0.21 0.09 0.25 ± 0.01 0.89 ± 0.01 0.02 ± 0.00559 Nanon Xk Xk C 80 2.71 0.05 0.59 0.14 0.11 0.13 ± 0.01 0.95 ± 0.02 0.01 ± 0.00757 Portlandia Xk Xk XF 32 2.37 0.14 0.49 0.17 0.05 0.19 ± 0.01 0.89 ± 0.03 0.01 ± 0.00973 Aralia Xk T – 52 3.21 0.10 0.49 0.12 0.19 0.11 ± 0.00 0.90 ± 0.01 0.02 ± 0.001103 Sequoia Xk Xk E – 1.93 0.48 0.43 0.02 0.00 0.14 ± 0.02 0.89 ± 0.01 0.02 ± 0.002606 Odessa Xk Xk – – 2.77 – 0.67 0.11 0.15 0.20 ± 0.04 0.95 ± 0.04 0.03 ± 0.01See notes for Table 2a.

the CV and/or CO meteorites. Hiroi et al. (1994) showed that theCV3 meteorite Allende could be a spectral match to the 52-colordata of 221 Eos, provided that the asteroid had undergone somedegree of low-grade heating. Burbine et al. (2001) examined spec-tra from 0.44 to 1.65 microns for three Eos family K-types (221 Eos,599 Luisa, and 653 Berenike), and found that Eos and Berenikewere spectral analogs to a CO3 chondrite, and that Luisa wasa spectral analog to a CV3 chondrite. Shepard et al. (2008) de-scribe observations of a single near-Earth asteroid, 2100 Ra Shalom,across multiple wavelengths, including radar. They show spectralsimilarities between Ra Shalom and a CV3 chondrite, Grosnaja.While it is not related to Eos, they place Ra-Shalom in the K-class.Very recently, Mothé-Diniz et al. (2008) performed a mineralogicalanalysis of spectra (0.45 to 2.45 microns) of 30 different Eos familymembers. The most direct analog these workers found for the EosK-types were the R-chondrites, with the CK-chondrites finishing aclose second. However, their mixing analyses suggest a composi-tion dominated by forsteritic olivine with minor orthopyroxene,such as would be expected from the partial differentiation of aparent-body with original composition similar to ordinary chon-drites.

In summary, most previous work has focused on the Eos familyK-types, and there has been general agreement that CV, CO, and/orCK chondrites are good analogs, however there is some disagree-ment. To date, no meteorite spectral analogs have been suggestedfor the Xk or Sk asteroids.

3. Observations and methods

Our observations were conducted at the Mauna Kea Observa-tory 3.0 m NASA Infrared Telescope Facility (IRTF) in Hawaii. Weused the SpeX instrument, equipped with a cooled grating and anInSb array (1024 × 1024) spectrograph at wavelengths from 0.82to 2.49 μm. (Rayner et al., 2003). Spectra were recorded with aslit oriented in the north–south direction and opened to 0.8 arc-sec. A dichroic lens reducing the signal below 0.8 μm was used forall observations.

Following normal data reduction procedures of flat-fielding, skysubtraction, spectrum extraction, and wavelength calibration, eachspectrum was fitted with the ATRAN atmospheric model for tel-luric absorption features (Lord, 1992; Bus et al., 2003; Sunshine etal., 2004). This procedure required an initial estimate of precip-

http://smass.mit.edu/busdemeoclass.html

122 B.E. Clark et al. / Icarus 202 (2009) 119–133

(a)

Fig. 2. (a) Rotationally averaged spectra of K-, Sk- and K-candidate (S and Xc) asteroids. When possible, the new observations at 0.8 to 2.5 microns reported here havebeen spliced together with the visible wavelength observations (0.4 to 0.9 microns) of Bus and Binzel (2002b). Asteroids 472 and 686 are presented with their Tholenclassification. All other asteroids are shown with their Bus classifications. (b) Rotationally averaged spectra of Xk asteroids. When possible, the new observations at 0.8 to2.5 microns reported here have been spliced together with the visible wavelength observations (0.4 to 0.9 microns) of Bus and Binzel (2002b).

itable water in the atmospheric optical path using the zenith anglefor the observation and the known τ -values (average atmosphericwater) for Mauna Kea. This initial guess was iterated until the bestfit between predicted and observed telluric band shapes was ob-tained, and an atmospheric model spectrum was generated (Buset al., 2003). Following this, each asteroid spectrum was dividedby the atmospheric model and then ratioed to each star spectrum,similarly reduced, before normalization at 1.2 μm. The final spec-tra we report are averages of 3–5 asteroid/star ratios, calculated tominimize variations due to standard star and sky variability. Usu-

ally, 2–5 different standard stars were observed on any given nightat the telescope (Table 1). We used only “solar” standard stars. Inaddition, 1–3 observations were obtained of each different stan-dard star.

Tables 2a and 2b give a summary of the 30 K-complex aster-oids that were observed in the near-infrared for this program. Ofnote are suggested new taxonomic designations (in column three)for each object based on the Bus–DeMeo taxonomy (DeMeo et al.,2009). Several objects are worth special mention. 2100 Ra Shalomwas heavily observed across multiple wavelengths and was deter-

Spectroscopy of K-complex asteroids 123

(b)

Fig. 2. (continued)

mined to be most closely linked with CV meteorites by Shepard etal. (2008). We include it here because Shepard et al. (2008) pro-pose that it be designated a K-type asteroid. 1103 Sequoia has apublished IRAS albedo measurement (0.48) that places it solidlywithin the E-asteroid class (see Clark et al., 2004), however itsvisible wavelength properties place it in the Xk class. We includeit here for this reason, however we caution that if the albedo iscorrect then it cannot be associated with dark asteroids and me-teorites. 686 Gersuind is included because it was identified as aK-candidate in the SCAS survey (Clark et al., 1995), and its near-infrared spectral properties are similar to the Eos family members.However, the only visible wavelength-based taxonomic designationcalls it an S-type (Tholen, 1984). 472 Roma was found by Mothé-Diniz et al. (2008) to be a non-Eos K-candidate, so we include it inour analysis. 661 Cloelia and 742 Edisonia are both members of theEos family and are included here to show the near-infrared spec-

tral properties of family members for comparison to non-familymembers.

Fig. 2 presents our average K-complex spectra. Where possible,visible wavelength data from the Small Main Belt Asteroid Spec-troscopic Survey (SMASS) or Eight Color Asteroid Survey (ECAS)were added (Chapman and Gaffey, 1979; Zellner et al., 1985;Bus and Binzel, 2002b).

We measured and recorded several characteristics of the com-bined spectra (Tables 2a and 2b). The continuum slope wasmeasured as rise over run of the least-squares linear fit to thedata (change in normalized reflectance divided by change inwavelength—units are μm−1). Continuum slope was measured forfour wavelength regions; visible (VIS) from 0.45 to 0.7 μm, near-infrared (NIR1) from 1.1 to 1.6 μm, NIR2 from 1.7 to 2.45 μm, andacross the 1-micron band from 0.75 to 1.5 μm (Ockert-Bell et al.,2008). Each wavelength region reveals an important aspect of the

124 B.E. Clark et al. / Icarus 202 (2009) 119–133

(a)

Fig. 3. (a) This figure illustrates the subjective nature of data splicing between vis-ible and infrared wavelengths, and the subsequent uncertainty in measured bandparameters. Asteroid 661 Cloelia represents perhaps a worst-case-scenario. Panel (i)shows the SMASS visible and SpeX near-infrared data spliced at 0.83 microns (datalongward from SMASS and shortward from SpeX are truncated). Panel (iv), similarto (i) shows splicing at 0.91 microns. While these subjective differences in choice ofsplicing wavelength do not have a strong effect on band center (1.01 microns in (i)and 1.02 microns in (iv)), the band depth varies strongly (44%), from 0.044 to 0.069.(b) This figure illustrates the subjective of the continuum slope across the 1 μmband infrared wavelengths. Asteroid 661 Cloelia represents presents a clear exam-ple of slope variation due to the selection of points close to peaks in the spectra. Inthis case the slopes varied from 0.025 to 0.040.

(b)

Fig. 3. (continued)

spectrum. NIR1 is a function of the depth of the 1 μm absorp-tion band. NIR2 shows the amount of reddening of the spectrum,which may be related to space weathering or the presence of metalwithin the asteroid regolith (Ockert-Bell et al., 2008).

The meteorite absolute reflectance at 0.55 μm is assumed to beroughly comparable to asteroidal geometric albedo. We considerboth measurements to be indicative of the overall spectral “bright-ness” of the compositional material. Clark et al. (2001, 2002) andFanale et al. (1992) discuss the strength and viability of this as-sumption, especially in the context of disk-resolved observationsof an asteroid regolith.

Because it was critical to the identification of the band cen-ter, we have been careful with our technique of splicing the vis-ible and near-infrared spectra. Fig. 3a illustrates our method. Webegan by overlapping the “raw” spectra, merging the two wave-length regions at the common wavelengths. In several cases, thisprocess results in two “tails” of mismatch at the extreme wave-length ends of each region. Should we just chop these off? If wedo that—we can introduce possibly spurious features or shouldersto our combined spectra. Instead, we smooth the spectra, find theoverlap point(s), merge the two regions, and then delete excesswavelength coverage. This process smooths out any shoulders thatmight be introduced in the splicing, however it cannot completelyprevent shoulders because the slopes where the data overlap candiffer strongly. This splicing of the data from two different detec-tors is the most important source of error in the calculation ofband parameters. Band center positions were measured after con-tinuum (∼0.75–1.5 μm) removal from the top of the continuum tothe center of a third, fourth, or fifth order polynomial fit to theband.

To estimate the uncertainties in our band parameters we madethe same spectral measurement three times. Fig. 3b shows thatbecause there is significant scatter in the data at the peaks inreflectance on either side of the 1-micron band, there is some sub-jective choice in fitting a tangent line to approximate the spectralcontinuum. Fig. 3b shows three example tangent lines that could,arguably, be used. So, picking the tangent points at slightly dif-ferent wavelengths each time, we calculated the continuum slopeacross the 1-micron band in three different fits. We used eachcontinuum fit to divide the spectrum by the slope, then we fitthe absorption band with a third, fourth, or fifth-order polynomial,whichever resulted in the lowest residuals. This resulted in threedifferent measurements of each parameter. To estimate the uncer-tainties, we took the average, and used the range to represent theuncertainty of the measurement.

To set our targets in their asteroid context we have comparedtheir spectral properties with those of the Tholen S, C, B, G, andF-type asteroids (Tables 3a and 3b and Figs. 4a–4c), using visiblewavelength data from SMASS (Bus and Binzel, 2002b) spliced with

Spectroscopy of K-complex asteroids 125

Table 3aSummary of the properties of the Tholen B, C, F and G asteroid comparison objects.

Bustype

Bus–DeMeotype

Tholentype

pv VIS(±27%)

NIR1(±14%)

NIR2(±28%)

Continuumslope

Bandcenter

Banddepth

2 Pallas B B B 0.16 −0.03 −0.05 −0.04 −0.08 ± 0.00 1.53 ± 0.03 0.05 ± 0.0062 Erato Ch B BU 0.06 −0.31 0.02 0.10 −0.01 ± 0.00 1.28 ± 0.00 0.10 ± 0.00261 Prymno X X B 0.11 0.30 0.19 0.13 0.24 ± 0.01 0.92 ± 0.01 0.02 ± 0.00379 Huenna C – B 0.06 −0.02 0.16 0.18 0.11 ± 0.01 0.95 ± 0.02 0.04 ± 0.01431 Nephele B C B 0.06 −0.19 −0.02 0.23 0.03 ± 0.01 1.2 ± 0.2 0.11 ± 0.0210 Hygiea C C C 0.07 0.00 0.09 0.11 0.04 ± 0.01 1.25 ± 0.02 0.05 ± 0.0131 Euphrosyne Cb Cb C 0.05 0.05 0.17 0.18 0.12 ± 0.01 1.26 ± 0.04 0.03 ± 0.0041 Daphne Ch Xk C 0.08 −0.02 0.11 0.01 0.09 ± 0.01 0.82 ± 0.00 0.03 ± 0.0099 Dike Xk K C 0.06 0.38 0.04 −0.02 0.06 ± 0.01 0.93 ± 0.03 0.01 ± 0.00143 Adria Xc C C 0.05 0.26 0.12 0.11 0.09 ± 0.01 1.24 ± 0.03 0.04 ± 0.00144 Vibilia Ch Xk C 0.06 −0.24 0.15 0.05 0.14 ± 0.01 0.78 ± 0.01 0.05 ± 0.01173 Ino Xk Xk C 0.06 0.47 0.10 0.08 0.06 ± 0.01 1.09 ± 0.03 0.02 ± 0.00209 Dido Xc Xk C 0.03 0.42 0.27 0.21 0.16 ± 0.01 1.11 ± 0.03 0.08 ± 0.00304 Olga Xc Xk C 0.05 0.30 0.08 0.11 0.05 ± 0.00 1.24 ± 0.01 0.04 ± 0.00356 Liguria Xk C 0.05 0.39 0.11 0.03 0.07 ± 0.00 0.94 ± 0.01 0.04 ± 0.00375 Ursula Xc C C 0.04 0.36 0.16 0.21 0.14 ± 0.01 1.18 ± 0.06 0.03 ± 0.01511 Davida C C C 0.05 0.16 0.18 0.11 0.11 ± 0.00 1.36 ± 0.02 0.06 ± 0.00559 Nanon Xk Xk C 0.05 0.59 0.14 0.09 0.13 ± 0.01 0.95 ± 0.02 0.01 ± 0.00712 Bolivinia X Xc C 0.05 0.28 0.10 0.04 – – –2100 Ra Shalom Xc B C 0.06 0.47 −0.01 −0.01 −0.08 ± 0.03 1.03 ± 0.04 0.03 ± 0.00213 Lilaea B B F 0.09 −0.07 −0.09 −0.03 0.00 ± 0.01 0.00 ± 0.02 0.00 ± 0.01335 Roberta B C FP 0.06 −0.07 0.11 0.09 0.05 ± 0.01 1.15 ± 0.05 0.04 ± 0.00419 Aurelia – C F 0.05 0.42 0.12 0.12 0.03 ± 0.01 1.19 ± 0.04 0.07 ± 0.03426 Hippo – B F 0.05 −0.04 −0.29 −0.03 −0.15 ± 0.01 1.70 ± 0.02 0.09 ± 0.00505 Cava – Xk FC 0.04 0.09 0.14 0.17 0.08 ± 0.04 1.1 ± 0.1 0.06 ± 0.02554 Peraga Ch Ch FC 0.05 0.01 0.21 −0.03 0.17 ± 0.02 0.69 ± 0.01 0.02 ± 0.00704 Interamnia B C F 0.07 −0.14 0.04 0.18 0.04 ± 0.01 1.19 ± 0.04 0.10 ± 0.011 Ceres C C G 0.09 0.10 0.03 0.05 0.01 ± 0.00 1.30 ± 0.04 0.03 ± 0.0013 Egeria Ch Ch G 0.08 −0.18 0.06 0.02 0.08 ± 0.02 0.70 ± 0.01 0.03 ± 0.0019 Fortuna Ch Ch G 0.04 −0.19 0.25 0.11 0.13 ± 0.02 0.74 ± 0.02 0.04 ± 0.00106 Dione Cgh Cgh G 0.09 −0.17 0.17 −0.03 0.09 ± 0.09 0.96 ± 0.02 0.05 ± 0.00130 Elektra Ch Ch G 0.08 −0.19 0.27 0.21 0.11 ± 0.03 0.98 ± 0.04 0.06 ± 0.01166 Rhodope Xe Xk GC – 0.33 0.22 0.08 0.20 ± 0.01 0.99 ± 0.02 0.02 ± 0.00See notes for Table 2a.

Table 3bSummary of the properties of the Tholen S asteroids.

Tholentype

Bustype

pv VIS(±13%)

NIR1(±18%)

NIR2(±22%)

Continuumslope

Bandcenter

Banddepth

42 Isis S(I) L 0.17 0.98 0.58 0.01 0.35 ± 0.06 1.01 ± 0.02 0.13 ± 0.01354 Eleonora S(I) Sl 0.19 1.02 0.44 0.05 0.52 ± 0.05 1.14 ± 0.00 0.18 ± 0.0139 Laetitia S(II) S 0.29 0.83 0.64 0.06 0.27 ± 0.02 1.01 ± 0.01 0.10 ± 0.0168 Leto S(II) – 0.23 0.73 0.28 0.07 0.24 ± 0.05 1.06 ± 0.01 0.13 ± 0.0263 Ausonia S(II–III) Sa 0.16 1.25 0.42 −0.02 0.48 ± 0.05 0.95 ± 0.02 0.12 ± 0.0115 Eunomia S(III) S 0.21 1.59 1.24 0.14 0.23 ± 0.01 1.02 ± 0.00 0.14 ± 0.01532 Herculina S(III) S 0.17 0.90 0.26 0.10 0.21 ± 0.03 0.99 ± 0.01 0.13 ± 0.01115 Thyra S(III–IV) S 0.27 1.16 0.58 0.12 0.18 ± 0.02 0.97 ± 0.01 0.11 ± 0.013 Juno S(IV) Sk 0.24 1.08 0.69 −0.04 0.15 ± 0.00 0.98 ± 0.00 0.12 ± 0.0027 Euterpe S(IV) S 0.16 1.67 0.91 0.21 0.22 ± 0.04 0.99 ± 0.01 0.13 ± 0.0118 Melpomene S(V) S 0.22 0.97 0.63 0.06 0.28 ± 0.02 0.94 ± 0.02 0.07 ± 0.00264 Libussa S(V) S 0.30 0.97 0.48 0.07 0.28 ± 0.02 0.94 ± 0.01 0.12 ± 0.0120 Massalia S(VI) S 0.21 1.40 0.63 0.02 0.16 ± 0.00 0.94 ± 0.00 0.12 ± 0.001036 Ganymed S(VI–VII) S 0.29 0.94 0.21 0.06 0.38 ± 0.03 0.93 ± 0.01 0.16 ± 0.00674 Rachele S(VII) S 0.20 1.09 0.51 0.10 0.17 ± 0.01 0.91 ± 0.00 0.10 ± 0.00See notes for Table 2a. Asteroid subclasses I–VII were defined by Gaffey et al. (1993).

infrared wavelength 52-Color data from Bell et al. (1988) and/ornew SpeX observations performed for this study. Some of our com-parison objects were also observed by Burbine et al. (2002) in theSMASSIR study, and those data are over-plotted when available.

To set our targets in their meteorite context we have com-pared their spectral properties with those of laboratory spectra ofanalog meteorites suggested in the literature (Table 4 and Fig. 5).Spectral parameters for the meteorites were obtained from RELABspectra measured using grain sizes less than 75 μm, as listed inTable 4. Note that both Tables 2a, 2b and 3a, 3b present all aster-oids used for this study with their taxonomic classifications fromboth the Bus and Binzel (2002b) system, and the Tholen (1984)system.

4. Results

We have searched for correlations, trends, and clusters amongthe spectral parameters we have measured to record the spectralcharacteristics of our targets and comparison objects. We find thatthe most useful spectral parameters for characterizing the varia-tion in our data are brightness, absorption band center wavelength,and the continuum slope in the visible wavelengths from 0.45 to0.7 μm (VIS), and the infrared wavelengths from 1.7 to 2.49 μm(NIR2). These parameters show clustering that indicates a spec-tral similarity between K-complex objects and carbonaceous me-teorites. None of the measured parameters show clustering thatcontradicts this affinity.

126 B.E. Clark et al. / Icarus 202 (2009) 119–133

(a)

Fig. 4. (a) Spectra of B, F, and G type asteroids. Visible wavelength spectra are from Bus and Binzel (2002b). Long wavelength spectra come variously from this study (darkstars), from the Bell et al. (1988) 52-Color Survey (plus signs), and from Burbine and Binzel (2002) (open triangles). (b) Spectra of C type asteroids. Visible wavelength spectraare from Bus and Binzel (2002b). Long wavelength spectra come variously from this study (dark stars), from the Bell et al. (1988) 52-Color Survey (plus signs), and fromBurbine and Binzel (2002) (open triangles). (c) Spectra of S type asteroids. Visible wavelength spectra are from Bus and Binzel (2002b). Long wavelength spectra come fromthe Bell et al. (1988) 52-Color Survey (plus signs), and from Burbine and Binzel (2002) (open triangles).

There are no interesting trends or clusters when band depth,continuum slope from 0.75 to 1.5 μm, or NIR1 slope is comparedbetween meteorites and asteroids. In these parameters, all objectsoverlap.

Although we do not show it, a comparison of visible slopeversus band center (or band depth) shows strong clustering ac-cording to taxonomy, as expected, because these parameters were

largely used to define the taxonomy (Bus and Binzel, 2002b;

Tholen, 1984).

Fig. 6a shows brightness versus NIR2 for the asteroids and me-

teorites, respectively. Fig. 6b shows the visible slope versus band

center position in wavelength for the asteroids and meteorites, re-

spectively.

Spectroscopy of K-complex asteroids 127

(b)

Fig. 4. (continued)

These figures begin to illustrate several points about K-complexasteroids:

1. K-complex asteroids are distinct from other main belt asteroidtypes S, B, C, F, and G. They do not appear to be a subset ofthese other types.

2. K-complex asteroid albedos span the range of 0.05 to 0.27. Car-bonaceous meteorite brightness spans the range 0.02 to 0.22,and B-, C-, F- and G-asteroid albedos span the range 0.03to 0.16. Assuming that geometric albedo and brightness at 0.55microns are roughly comparable metrics, we find better agree-ment between the K-complex asteroids and the carbonaceousmeteorites than between the B-, C-, F- and G-class objects and

the carbonaceous meteorites. The B-, C-, F- and G-class aster-oids tend to be darker than the meteorites.

3. There is a lot of overlap in NIR2 slope. K-complex asteroidstend to have less steeply sloped NIR2 values than C-, B-, F-and G-asteroids. Meteorites are intermediate between the two.

4. K-complex asteroids show a very large range in VIS slopes.Carbonaceous meteorites show a similar range of values, how-ever C-, B-, F- and G-asteroids tend to have slope values at thelower end of the range only.

5. K- and Xk-asteroids nearly span the range of band center po-sitions exhibited by the carbonaceous chondrites, whereas B-,C- and F-type asteroids show longer wavelength band centersthan any of the chondrites measured here.

128 B.E. Clark et al. / Icarus 202 (2009) 119–133

(c)

Fig. 4. (continued)

Taken together, these comparisons indicate that K-complex as-teroids are better spectral analogues for the carbonaceous me-teorites we’ve measured than the traditional B-, C-, F- and G-matches suggested in the literature (e.g. Gaffey et al., 2002;Burbine et al., 2002). These trends will either be verified or re-futed when our larger survey is completed.

In Fig. 7 we show a break-down of the meteorite and aster-oid classes in terms of band center near 1 μm. The 1 μm band issensitive to the olivine-pyroxene mineralogy of the material, andis perhaps the main mineralogically diagnostic spectral parameterwe have measured (Cloutis et al., 1986; Sunshine et al., 2004). Al-though we do not have enough representatives from the CV, CH,or CI meteorites classes to suggest any correlations or trends, wenote that K (including Sk and K-candidates) and Xk asteroids spanthe band center range of 0.9 to 1.2 microns, and that most mete-orite classes (except for the CM-class) fit within this range. The B-,C-, F- and G-class asteroids, however, have band centers rangingfrom 0.7 to around 1.7, and are not as easily seen to be similar tothe carbonaceous chondrites.

We have also performed a search for spectral matches betweenour target asteroids and our comparison meteorite data set. In this

search we created overlays of all possible matches and kept a tallyof the “good” matches, where “good” is defined as visible agree-ment in spectral shape, continuum slopes, and absorption banddepths. On the basis of this search, we find that K asteroids aremost consistent with CO and CK (and possibly CI and CV) mete-orites (Fig. 8, top). Xk asteroids, however, tend to show a “red”NIR1 continuum slope that is best matched with CM (and possiblyCR and CH) meteorites (Fig. 8, bottom).

5. Discussion

Previously, we estimated that roughly 10% of the smaller main-belt (Tholen class) S-types are actually K-types (Clark et al., 1995).This was an exciting possibility because S-types and K-types havesuch disparate meteoritic interpretations. We have also suggestedthat CV-CO meteorites are better candidates than OC meteoritesfor the precursor material of the differentiated S-types (Meibomand Clark, 1999). If so, then it could be inferred that carbonaceouschondritic material, as opposed to ordinary chondritic material,once dominated the main belt.

Spectroscopy of K-complex asteroids 129

Table 4Meteorite comparison data set.

RELAB # Sample Type R(0.55 μm)

VIS(±31%)

NIR1(±3%)

NIR2(±16%)

Continuumslope

Bandcenter

Banddepth

c1ph49 PCA91467 CH 0.10 2.68 0.26 0.12 0.29 ± 0.01 0.93 ± 0.00 0.03 ± 0.00mgp080a Orgueil CI 0.05 0.48 −0.01 −0.04 0.03 ± 0.00 0.89 ± 0.01 0.05 ± 0.00c1ph45 MET01149 CK3 0.19 0.30 0.36 −0.01 0.09 ± 0.00 1.09 ± 0.01 0.10 ± 0.00c1ph35 ALH85002 CK4 0.15 0.15 0.10 −0.03 −0.08 ± 0.01 1.10 ± 0.00 0.08 ± 0.00c1ph46 PCA91470 CK4 0.22 0.14 0.22 −0.03 −0.04 ± 0.00 1.10 ± 0.01 0.11 ± 0.00c1ph53 DAV92300 CK4 0.13 0.12 0.22 0.02 −0.03 ± 0.01 1.10 ± 0.01 0.11 ± 0.00c1ph47 EET83311 CK5 0.22 0.11 0.10 −0.07 −0.09 ± 0.01 1.12 ± 0.01 0.08 ± 0.00c1ph43 MET01070 CM1 0.05 −0.42 0.05 −0.05 −0.15 ± 0.01 0.73 ± 0.00 0.04 ± 0.00c1ph44 PCA02012 CM2 0.13 0.65 0.18 0.11 0.11 ± 0.00 1.02 ± 0.01 0.02 ± 0.00c1ph32 MET00639 CM2 0.03 −0.04 0.05 0.00 0.05 ± 0.02 0.69 ± 0.02 0.01 ± 0.00c1ph33 WIS91600 CM2 0.02 0.55 0.47 0.32 – – –c1ph51 QUE97077 CM2 0.05 −0.21 0.41 0.07 0.20 ± 0.04 0.76 ± 0.03 0.04 ± 0.00c1ph52 QUE99038 CM2 0.14 0.46 0.26 0.06 0.09 ± 0.00 1.01 ± 0.01 0.06 ± 0.00c1ph34 ALH82101 CO3 0.17 0.65 0.17 −0.05 0.00 ± 0.00 1.07 ± 0.00 0.05 ± 0.00c1ph42 FRO95002 CO3 0.15 0.50 0.12 −0.01 −0.00 ± 0.00 1.09 ± 0.01 0.05 ± 0.00c1ph50 MET00737 CO3 0.17 0.70 0.13 −0.02 −0.04 ± 0.00 1.04 ± 0.01 0.07 ± 0.00c1ph57 FRO99040 CO3 0.17 0.38 0.10 −0.03 −0.01 ± 0.00 1.10 ± 0.01 0.04 ± 0.00c1ph48 PCA91082 CR2 0.10 1.63 0.22 0.10 0.27 ± 0.01 0.92 ± 0.00 0.03 ± 0.00c1ph54 QUE99177 CR2 0.13 1.49 0.26 0.10 0.28 ± 0.01 0.94 ± 0.00 0.02 ± 0.00c1ph55 MET00426 CR2 0.10 1.66 0.17 0.07 0.19 ± 0.00 0.94 ± 0.00 0.023 ± 0.00c1ph56 MAC87320 CR2 0.10 1.34 0.22 0.12 0.25 ± 0.01 0.93 ± 0.00 0.02 ± 0.00c1ph41 QUE93744 CV3 0.12 0.51 0.02 −0.01 −0.03 ± 0.00 1.07 ± 0.01 0.02 ± 0.00R (0.55 μm) is the absolute reflectance at 0.55 μm for comparison to pv of asteroids. See notes for Table 2a.

a From Gaffey (1976).

The general mismatch between CC meteorites and their pre-sumed asteroid parent bodies (Tholen class C, B, G, and F) has beenknown for a long time (e.g. Britt et al., 1991; Hiroi et al., 1996;Pieters and McFadden, 1994; Burbine et al., 2002), and has gen-erally been attributed to slight differences between the way ameteorite is prepared for spectral measurements and the way anasteroid surface is affected by exposure to impacts and the spaceenvironment. Britt et al. (1991) compared asteroids and meteoritesusing a principal components analysis and found a systematic dif-ference between the CC meteorites and the Tholen classes C, B, Gand F. Fornasier et al. (1999) compare CM meteorites to C- andG-type asteroids, and Burbine et al. (2002) summarize the find-ings from the literature up to 2002. Hiroi et al. (1993, 1994, 1996)show that heated meteorites and several rare Antarctic meteoritesprovide remarkable spectral matches to asteroids in the Tholen C,B, G and F classes. The meteorites were experimentally heated inan oven to simulate thermal metamorphism of the minerals. Hiroisuggests that some of the larger asteroids may be the heated innerportions of once larger bodies and that common CI/CM meteoritesmay have come from the lost outer portions which escaped ex-tensive late-stage heating events. However, it is also possible thatK-complex asteroids will provide a satisfactory match to some ofthe CC meteorites without the need to invoke other processes.

We note that space weathering effects may also have affectedour spectral match search and parametric comparisons in the anal-ysis described above. Using statistical arguments, Lazzarin et al.(2006) have shown that space weathering probably occurs on allasteroid types. Although this is true, many types lack the highalbedo and strong spectral band contrasts that make weatheringeffects easily detectable (Clark et al., 2002). It is possible that theK-complex asteroids are carbon-rich, and it is known that opaquematerials mask the optical effects of space weathering products(Pieters et al., 2000). Some workers suggest that carbon-rich sur-faces alter to become spectrally red (Lazzarin et al., 2006), othershave found spectral redness to decrease (Nesvorny et al., 2005;Hiroi et al., 2004). Since there is no consensus at this point, wemust await possible future discoveries from space weathering sim-ulations, experiments, and laboratory study following a sample re-turn spacecraft mission.

In the course of the analysis of this dataset, we have encoun-tered many problems related to taxonomy. Some objects have onlybeen classified in one system and not the others, and each systemuses slightly different criteria for classification making it difficult tocompare across taxonomies. In this initial analysis of our ongoingsurvey, we have attempted to de-emphasize taxonomic classifica-tion for the purposes of meteorite comparisons, and we have at-tempted to be inclusive (lumping sub-groups together) rather thandivisive (following or creating new sub-groupings). Nevertheless,some problems remain. The K-complex includes part of the TholenS-type but also of the low albedo C-type (and dark types asso-ciated), so it is perhaps to be expected that the albedo range iswider. Albedo values lower than 0.08–0.09 for the K-complex arereferred to as Bus Xk asteroids which were once C- or D-types inthe Tholen classification. Albedo values higher than 0.20 are asso-ciated with Tholen S-, M-, or E-types. So, it seems that the albedorange of Bus K asteroids is between 0.09 and 0.19, similar to theCK and CO chondrites, while Tholen B–F–C–G-types have albedovalues closer to the CH, CI, and CM meteorites, with CR and CVmeteorites having albedos which could be consistent with bothgroups. As indicated by the very new Bus–DeMeo classificationsshown in the tables, several asteroids targeted for this survey willprobably fall out of the K-complex as work continues.

Vilas and Gaffey (1989) and Vilas et al. (1994) noted the pres-ence of a spectral feature at 0.7 μm for C-asteroids that can beattributed to the aqueous alteration of anhydrous silicates, in par-ticular the Fe2+ ⇒ Fe3+ charge transfer transition in oxidized ironfound in phyllosilicates. The feature was found in 44% of the 45spectra taken of C-asteroids. This absorption feature is also seenin some CM2 class meteorite laboratory spectra and in terrestrialphyllosilicates. Vilas and Gaffey suggest that the presence of thisabsorption feature implies that the C-asteroids and some CM2 me-teorites may have formed through the same aqueous alterationprocesses, which further implies that the C-asteroids may be a pos-sible source of some carbonaceous chondrite meteorites. Of the 22meteorite sample spectra we obtained from RELAB, the 0.7 micronfeature is detectable in one out of six CM2 spectra. The correla-tions between 0.7 and 3-micron studies of asteroids have provento be very effective in determining the aqueous alteration history

130 B.E. Clark et al. / Icarus 202 (2009) 119–133

(a)

(b)

Fig. 5. (a) Spectra of carbonaceous chondrite meteorites with band centers less than 1.0 μm. All samples were measured at a grain size less than 75 microns. (b) Spectra ofcarbonaceous chondrite meteorites with band centers longer than 1.0 μm. All samples were measured at a grain size less than 75 microns.

Spectroscopy of K-complex asteroids 131

(a) (b)

Fig. 6. (a) Brightness (geometric albedo for asteroids, reflectance at 0.55 microns for meteorites) versus the second near-infrared continuum slope (1.7–2.49 μm) for B, C, F, G,K, Xk, and S type asteroids as compared to Carbonaceous Chondrite (CC) meteorites. Brightness is a unitless quantity that measures the ratio of reflected to incident light, andNIR2 is in reflectance over wavelength (1/μm) units. Circles have been added to indicate major clusters. The red circle contains the S-class asteroids, the cyan circle containsmost of the B-, C-, F- and G-class asteroids, and the green circle contains most of the K-complex objects. Note that the black diamonds, representing the carbonaceouschondrite meteorites, fall mainly within the green circle (K-complex parameters). (b) Visible wavelength slope (0.47 to 0.75 μm) versus band center of the 1.0 μm band forB, C, F, G, K, Xk, and S type asteroids as compared to Carbonaceous Chondrite (CC) meteorites. VIS slope is in reflectance over wavelength (1/μm) units, and band center iswavelength in microns. Circles have been added to indicate major clusters. The red circle contains the S-class asteroids, the cyan circle contains most of the B-, C-, F- andG-class asteroids, and the green circle contains most of the K-complex objects. Note that the black diamonds, representing the carbonaceous chondrite meteorites, fall mainlynear or within the green circle (K-complex parameters), but several black diamonds to the lower left of the figure are distinct.

Fig. 7. Band center near 1 micron (in microns) is plotted for all meteorite and as-teroid classes examined here. Note that K (including Sk- and K-candidates) and Xkasteroids span the range of 0.9 to 1.2 microns, and that most meteorite classes (ex-cept for the CM-class) fit within this range. The B-, C-, F- and G-class asteroids,however, have band centers ranging from 0.7 to around 1.7. Note that there are notenough CV, CH, or CI meteorites to suggest any correlations or trends.

of asteroids (Rivkin et al., 2002). The 0.7 μm feature was not seenin the K-complex asteroids studied to date.

Asteroids 221 Eos, 661 Cloelia, and 742 Edisonia are dynamicalmembers of the Eos family (Mothé-Diniz et al., 2008). In fact, thesethree objects have very similar orbits located in the central part ofthe Eos family. Dynamical models that include the initial ejection

Fig. 8. (top) Scaled reflectance of 742 Edisonia (solid line) compared to CO3-typemeteorite c1ph42 (dashed line). (bottom) Scaled reflectance of 973 Aralia (solid line)compared to CM2-type meteorite c1ph44 (dashed line).

field of fragments indicate that the Eos family is approximately 1.3billion years old (Vokrouhlicky et al., 2006). This can be taken asthe approximate age of the surfaces of Eos family members.

None of the other targets in this study are part of any of theknown families. According to Bottke et al. (2005), the collisionallifetimes of most of our targets can be very long so that theymost likely date back to the pre-Late-Heavy-Bombardment epoch,more than 3.8 billion years ago. The three Eos family members are

132 B.E. Clark et al. / Icarus 202 (2009) 119–133

Table 5Asteroid orbital parameters.

Numberobjects

Avg a(AU)

Min a(AU)

Max a(AU)

BusXk 39 2.736 1.933 3.414K 33 2.779 1.392 3.030

TholenB 8 2.982 2.331 3.150C 139 2.866 2.197 3.885F 28 2.664 2.554 3.429G 9 2.883 2.362 3.187

Note: Values are from the JPL Horizons database (ssd.jpl.nasa.gov). Average semi-major axis is calculated for “pure” types (e.g. “C” only, not “Cgh” etc.). Average a forB asteroids does not include Chiron at 13.7 AU.

thus substantially younger than our other targets. This is interest-ing given the observation (by visual inspection of Fig. 2) that theEos family has deeper band depths near 1 micron than our othertargets. Other workers have suggested a trend between band depthand age for the S-type asteroids (e.g. Binzel et al., 2002).

In Table 5 we summarize some information regarding the or-bital parameters of our target asteroids. We include informationabout our comparison asteroid data sets. It is interesting to notethat K and Xk asteroids have an average heliocentric distance ofabout 2.7–2.8 AU, while the C, B, and G-class asteroids range fromabout 2.9–3.0 AU, and the S-class asteroids average distance is 2.5–2.7 AU.

6. Conclusion

We have observed a total of 30 K-complex objects (12 K, 2 Sk,and 13 Xk-type asteroids (from the Bus and Binzel taxonomy), plus3 K-candidates from previous work) and we have presented ananalysis of their spectral properties from 0.4 to 2.5 μm. Our anal-ysis suggests that carbonaceous chondrite meteorites are betterspectral analogs for the K-complex asteroids than for the previ-ously suggested C-, B-, G- and F-type asteroids. Of the 13 Xk aster-oids we targeted, three objects were previously classified C-typesin the Tholen system, and seven objects were previously classifiedas E, M, or X.

The K-complex asteroids we have examined orbit in the mainbelt at an average distance of 2.8 AU (when the Eos family andthe NEAs are not considered), but the largest class of low albedoasteroids (C-types) orbit at an average distance of 2.9 AU. So, ifcarbonaceous meteorites come primarily from K-complex asteroids,is it possible that the C-complex remains relatively unsampled bymeteorite delivery mechanisms to Earth?

The conclusions of this work have implications for models ofasteroid belt evolution and the dynamical delivery of meteoritesto the inner Solar System. Thus it is important to verify the me-teorite interpretations of our asteroid observations with furtherwork, preferably with observations covering the complete miner-alogically diagnostic wavelength region from 0.4 to 3.4 μm. It willbe especially important to target objects which are not Eos familymembers.

Acknowledgments

The Bus–DeMeo taxonomy classification is from the web toolby S.L. Slivan, developed at MIT with the support of NSF Grant0506716 and NASA Grant NAG5-12355. The authors also to ac-knowledge the excellent telescope operation of Bill Golisch andPaul Sears, and the excellent RELAB assistance of Takahiro Hiroi.Finally, we are grateful for the tough reviews by Jessica Sunshineand an anonymous reviewer.

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Spectroscopy of K-complex asteroids: Parent bodies of carbonaceous meteorites?IntroductionBackgroundDefinition of K-complex main-belt asteroidsK-type asteroid-meteorite linkages

Observations and methodsResultsDiscussionConclusionAcknowledgmentsReferences