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Icarus 202 (2009) 119–133

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Spectroscopy of K-complex : Parent bodies of carbonaceous ? ∗ 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 , 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- properties and distribution is essential to our Asteroids, composition understanding of the cosmochemical importance of some of the most primitive 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 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 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, , 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 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 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 (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 , 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 - 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 used in K-complex asteroid spectral data reduction.

UT date Standard Asteroids observed 2003 Aug 16 L107-684 L112-1333 L115-271 , 2100 Ra Shalom 2003 Aug 18 L107-684 L112-1333 L115-271 2100 Ra Shalom 2004 Jul 12 L107-684 L110-361, 16CygB 2005 Mar 16 L102-108 L105-56 2005 Sep 17 64Hyades L93-101 L110-361 L112-1333 L115-271 , , 2005 Sep 20 64Hyades L93-101 L110-361 L112-1333 L115-271 , 441 Bathilde, 547 Praxedis, 559 Nanon 2005 Nov 15 64Hyades L93-101 L102-1081 L115-271 173 Ino, , 417 Suevia, 2005 Nov 16 64Hyades L93-101 L115-271 397 Vienna, 417 Suevia, 2006 Aug 12 L107-684 L110-361, L112-1333 2100 Ra Shalom 2006 Aug 13 16CygB L112-1333 2100 Ra Shalom 2006 Oct 02 64Hyades L93-101 441 Bathilde 2006 Oct 05 L93-101 441 Bathilde 2006 Oct 06 64Hyades 2006 Dec 17 64Hyades L97-249 L102-1081 221 Eos, , 742 Edisonia, 1545 Thernoe 2006 Dec 19 64Hyades L97-249 L102-1081 221 Eos, 661 Cloelia, 742 Edisonia, 1545 Thernoe, 1903 Adzhimushka 2007 Dec 21 64Hyades L97-249 L102-1081 , 973 Aralia, 2008 Mar 11 64Hyades L97-249 L102-1081 2008 Mar 12 64Hyades L97-249 L102-1081 75 Eurydike, , 2606 Odessa 2008 Jul 15 L105-56 L112-1333 L115-271 , , , 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 2a Summary of the properties of the K, K-candidate, and Sk asteroids observed.

Name Bus Bus–DeMeo Tholen D a pv VIS NIR1 NIR2 Continuum Band Band type type type (km) (AU) (±4%) (±10%) (±25%) slope center depth 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 ± 0.00 KK S 152 2.55 0.18 0.98 0.46 0.11 0.21 ± 0.00 1.04 ± 0.00 0.09 ± 0.00 179 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 ± 0.00 186 Celuta KK S 50 2.36 0.19 0.75 0.36 0.00 0.16 ± 0.02 1.02 ± 0.01 0.08 ± 0.00 221 Eosa KK S 1043.01 0.14 0.70 0.16 0.00 −0.01 ± 0.00 1.03 ± 0.00 0.08 ± 0.01 233 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 ± 0.01 397 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 ± 0.00 402 Chloe KL S 54 2.56 0.15 0.71 0.13 0.00 0.10 ± 0.01 0.96 ± 0.01 0.03 ± 0.00 –S S 47 2.54 0.21 1.85 0.26 0.07 0.14 ± 0.00 0.91 ± 0.01 0.09 ± 0.00 599 Luisab KL S 652.77 0.14 0.87 0.05 −0.06 0.02 ± 0.01 0.99 ± 0.02 0.03 ± 0.00 661 Cloeliaa KK S 483.01 0.11 0.76 0.15 0.00 −0.00 ± 0.01 1.02 ± 0.01 0.07 ± 0.00 679 Pax KL I 52 2.59 0.17 0.89 0.20 0.00 0.24 ± 0.01 0.89 ± 0.01 0.02 ± 0.01 686 Gersuind –L S 41 2.59 0.14 0.33 0.12 0.00 0.16 ± 0.00 1.02 ± 0.02 0.02 ± 0.00 742 Edisoniaa KK S 463.01 0.13 0.67 0.13 0.00 0.05 ± 0.01 1.04 ± 0.03 0.07 ± 0.00 1545 Thernoe KL – 19 2.77 0.10 0.66 0.00 −0.09 0.05 ± 0.01 0.92 ± 0.01 0.02 ± 0.00 1903 Adzhimushkaja KK – 373.00 0.08 0.83 0.12 0.00 −0.00 ± 0.00 1.04 ± 0.01 0.05 ± 0.00 2100 Ra Shalomc Xc B C 3.40.083 0.06 0.47 0.00 0.00 −0.08 ± 0.03 1.03 ± 0.04 0.03 ± 0.00 2100 Ra Shalomc Xc B C 3.40.083 0.06 0.28 −0.10 −0.06 −0.01 ± 0.00 0.98 ± 0.03 0.03 ± 0.00

Values are from the JPL Horizons database (ssd.jpl..gov): D is the asteroid diameter, a is the semi-major axis of the asteroid and pv is the geometric . Visible spectrum 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 is measured 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 slopes are 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 and Roush (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 2b Summary of the properties of the Xk asteroids observed.

Name Bus Bus–DeMeo Tholen D a pv VIS NIR1 NIR2 Continuum Band center Band type type type (km) (AU) (±4%) (±10%) (±25%) slope (%) (μm) depth 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 ± 0.00 Xk K C 69 2.66 0.06 0.38 0.04 0.00 0.06 ± 0.01 0.93 ± 0.03 0.01 ± 0.00 Xk Xk T 100 2.68 0.09 0.69 0.27 0.09 0.19 ± 0.01 1.03 ± 0.01 0.03 ± 0.00 173 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 ± 0.00 250 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 ± 0.00 417 Suevia Xk Xk X 41 2.80.20 0.58 0.27 0.11 0.20 ± 0.01 0.97 ± 0.01 0.04 ± 0.00 441 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 ± 0.00 559 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 ± 0.00 757 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 ± 0.00 973 Aralia Xk T – 52 3.21 0.10 0.49 0.12 0.19 0.11 ± 0.00 0.90 ± 0.01 0.02 ± 0.00 1103 Sequoia Xk Xk E – 1.93 0.48 0.43 0.02 0.00 0.14 ± 0.02 0.89 ± 0.01 0.02 ± 0.00 2606 Odessa Xk Xk – – 2.77 – 0.67 0.11 0.15 0.20 ± 0.04 0.95 ± 0.04 0.03 ± 0.01

See notes for Table 2a. the CV and/or CO meteorites. Hiroi et al. (1994) showed that the In summary, most previous work has focused on the Eos family CV3 meteorite Allende could be a spectral match to the 52-color K-types, and there has been general agreement that CV, CO, and/or data of 221 Eos, provided that the asteroid had undergone some CK chondrites are good analogs, however there is some disagree- degree of low-grade heating. Burbine et al. (2001) examined spec- ment. To date, no meteorite spectral analogs have been suggested tra from 0.44 to 1.65 microns for three Eos family K-types (221 Eos, for the Xk or Sk asteroids. 599 Luisa, and ), and found that Eos and Berenike were spectral analogs to a CO3 chondrite, and that Luisa was 3. Observations and methods a spectral analog to a CV3 chondrite. Shepard et al. (2008) de- scribe observations of a single near- asteroid, 2100 Ra Shalom, Our observations were conducted at the Mauna Kea Observa- across multiple wavelengths, including radar. They show spectral tory 3.0 m NASA Infrared Telescope Facility (IRTF) in Hawaii. We similarities between Ra Shalom and a CV3 chondrite, Grosnaja. used the SpeX instrument, equipped with a cooled grating and an While it is not related to Eos, they place Ra-Shalom in the K-class. InSb array (1024 × 1024) spectrograph at wavelengths from 0.82 Very recently, Mothé-Diniz et al. (2008) performed a mineralogical to 2.49 μm. (Rayner et al., 2003). Spectra were recorded with a analysis of spectra (0.45 to 2.45 microns) of 30 different Eos family slit oriented in the north–south direction and opened to 0.8 arc- members. The most direct analog these workers found for the Eos sec. A dichroic lens reducing the signal below 0.8 μm was used for K-types were the R-chondrites, with the CK-chondrites finishing a all observations. close second. However, their mixing analyses suggest a composi- Following normal data reduction procedures of flat-fielding, sky tion dominated by forsteritic olivine with minor orthopyroxene, subtraction, spectrum extraction, and wavelength calibration, each such as would be expected from the partial differentiation of a spectrum was fitted with the ATRAN atmospheric model for tel- parent-body with original composition similar to ordinary chon- luric absorption features (Lord, 1992; Bus et al., 2003; Sunshine et drites. al., 2004). This procedure required an initial estimate of precip- 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 have been 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 Tholen classification. All other asteroids are shown with their Bus classifications. (b) Rotationally averaged spectra of Xk asteroids. When possible, the new observations at 0.8 to 2.5 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 angle ally, 2–5 different standard stars were observed on any given night for the observation and the known τ -values (average atmospheric at the telescope (Table 1). We used only “solar” standard stars. In water) for Mauna Kea. This initial guess was iterated until the best addition, 1–3 observations were obtained of each different stan- fit between predicted and observed telluric band shapes was ob- dard star. tained, and an atmospheric model spectrum was generated (Bus Tables 2a and 2b give a summary of the 30 K-complex aster- et al., 2003). Following this, each asteroid spectrum was divided oids that were observed in the near-infrared for this program. Of by the atmospheric model and then ratioed to each star spectrum, note are suggested new taxonomic designations (in column three) similarly reduced, before normalization at 1.2 μm. The final spec- for each object based on the Bus–DeMeo taxonomy (DeMeo et al., tra we report are averages of 3–5 asteroid/star ratios, calculated to 2009). Several objects are worth special mention. 2100 Ra Shalom minimize variations due to standard star and sky variability. Usu- was 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 et tral properties of family members for comparison to non-family al. (2008). We include it here because Shepard et al. (2008) pro- members. pose that it be designated a K-type asteroid. 1103 Sequoia has a Fig. 2 presents our average K-complex spectra. Where possible, published IRAS albedo measurement (0.48) that places it solidly visible wavelength data from the Small Main Belt Asteroid Spec- within the E-asteroid class (see Clark et al., 2004), however its troscopic Survey (SMASS) or Eight Color Asteroid Survey (ECAS) visible wavelength properties place it in the Xk class. We include were added (Chapman and Gaffey, 1979; Zellner et al., 1985; it here for this reason, however we caution that if the albedo is Bus and Binzel, 2002b). correct then it cannot be associated with dark asteroids and me- We measured and recorded several characteristics of the com- teorites. 686 Gersuind is included because it was identified as a bined spectra (Tables 2a and 2b). The continuum slope was K-candidate in the SCAS survey (Clark et al., 1995), and its near- measured as rise over run of the least-squares linear fit to the infrared spectral properties are similar to the Eos family members. data (change in normalized reflectance divided by change in − However, the only visible wavelength-based taxonomic designation wavelength—units are μm 1). Continuum slope was measured for calls it an S-type (Tholen, 1984). 472 Roma was found by Mothé- four wavelength regions; visible (VIS) from 0.45 to 0.7 μm, near- Diniz et al. (2008) to be a non-Eos K-candidate, so we include it in infrared (NIR1) from 1.1 to 1.6 μm, NIR2 from 1.7 to 2.45 μm, and our analysis. 661 Cloelia and 742 Edisonia are both members of the across the 1-micron band from 0.75 to 1.5 μm (Ockert-Bell et al., Eos family and are included here to show the near-infrared spec- 2008). Each wavelength region reveals an important aspect of the 124 B.E. Clark et al. / Icarus 202 (2009) 119–133

(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 or the presence of metal within the asteroid regolith (Ockert-Bell et al., 2008). The meteorite absolute reflectance at 0.55 μm is assumed to be roughly comparable to asteroidal . We consider both measurements to be indicative of the overall spectral “bright- ness” of the compositional material. Clark et al. (2001, 2002) and Fanale et al. (1992) discuss the strength and viability of this as- sumption, especially in the context of disk-resolved observations of 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. We began by overlapping the “raw” spectra, merging the two wave- length regions at the common wavelengths. In several cases, this process results in two “tails” of mismatch at the extreme wave- length ends of each region. Should we just chop these off? If we do that—we can introduce possibly spurious features or shoulders to our combined spectra. Instead, we smooth the spectra, find the overlap point(s), merge the two regions, and then delete excess wavelength coverage. This process smooths out any shoulders that might be introduced in the splicing, however it cannot completely prevent shoulders because the slopes where the data overlap can differ strongly. This splicing of the data from two different detec- tors is the most important source of error in the calculation of band parameters. Band center positions were measured after con- tinuum (∼0.75–1.5 μm) removal from the top of the continuum to the center of a third, fourth, or fifth order polynomial fit to the band. To estimate the uncertainties in our band parameters we made thesamespectralmeasurementthreetimes.Fig. 3b shows that because there is significant scatter in the data at the peaks in reflectance on either side of the 1-micron band, there is some sub- jective choice in fitting a tangent line to approximate the spectral continuum. Fig. 3b shows three example tangent lines that could, arguably, be used. So, picking the tangent points at slightly dif- (a) ferent wavelengths each time, we calculated the continuum slope across the 1-micron band in three different fits. We used each Fig. 3. (a) This figure illustrates the subjective nature of data splicing between vis- continuum fit to divide the spectrum by the slope, then we fit ible and infrared wavelengths, and the subsequent uncertainty in measured band the absorption band with a third, fourth, or fifth-order polynomial, parameters. 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 (data whichever resulted in the lowest residuals. This resulted in three longward from SMASS and shortward from SpeX are truncated). Panel (iv), similar different measurements of each parameter. To estimate the uncer- to (i) shows splicing at 0.91 microns. While these subjective differences in choice of tainties, we took the average, and used the range to represent the splicing wavelength do not have a strong effect on band center (1.01 microns in (i) uncertainty of the measurement. 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 μm To set our targets in their asteroid context we have compared band infrared wavelengths. Asteroid 661 Cloelia represents presents a clear exam- theirspectralpropertieswiththoseoftheTholenS,C,B,G,and ple of slope variation due to the selection of points close to peaks in the spectra. In F-type asteroids (Tables 3a and 3b and Figs. 4a–4c), using visible this case the slopes varied from 0.025 to 0.040. wavelength data from SMASS (Bus and Binzel, 2002b)splicedwith Spectroscopy of K-complex asteroids 125

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

Bus Bus–DeMeo Tholen pv VIS NIR1 NIR2 Continuum Band Band type type type (±27%) (±14%) (±28%) slope center depth BB B 0.16 −0.03 −0.05 −0.04 −0.08 ± 0.00 1.53 ± 0.03 0.05 ± 0.00 Ch B BU 0.06 −0.31 0.02 0.10 −0.01 ± 0.00 1.28 ± 0.00 0.10 ± 0.00 X X B 0.11 0.30 0.19 0.13 0.24 ± 0.01 0.92 ± 0.01 0.02 ± 0.00 C– B 0.06 −0.02 0.16 0.18 0.11 ± 0.01 0.95 ± 0.02 0.04 ± 0.01 BC B 0.06 −0.19 −0.02 0.23 0.03 ± 0.01 1.2 ± 0.20.11 ± 0.02 C C C 0.07 0.00 0.09 0.11 0.04 ± 0.01 1.25 ± 0.02 0.05 ± 0.01 Cb Cb C 0.05 0.05 0.17 0.18 0.12 ± 0.01 1.26 ± 0.04 0.03 ± 0.00 Ch Xk C 0.08 −0.02 0.11 0.01 0.09 ± 0.01 0.82 ± 0.00 0.03 ± 0.00 99 Dike Xk K C 0.06 0.38 0.04 −0.02 0.06 ± 0.01 0.93 ± 0.03 0.01 ± 0.00 Xc C C 0.05 0.26 0.12 0.11 0.09 ± 0.01 1.24 ± 0.03 0.04 ± 0.00 Ch Xk C 0.06 −0.24 0.15 0.05 0.14 ± 0.01 0.78 ± 0.01 0.05 ± 0.01 173 Ino Xk Xk C 0.06 0.47 0.10 0.08 0.06 ± 0.01 1.09 ± 0.03 0.02 ± 0.00 Xc Xk C 0.03 0.42 0.27 0.21 0.16 ± 0.01 1.11 ± 0.03 0.08 ± 0.00 Xc Xk C 0.05 0.30 0.08 0.11 0.05 ± 0.00 1.24 ± 0.01 0.04 ± 0.00 Xk C 0.05 0.39 0.11 0.03 0.07 ± 0.00 0.94 ± 0.01 0.04 ± 0.00 Xc C C 0.04 0.36 0.16 0.21 0.14 ± 0.01 1.18 ± 0.06 0.03 ± 0.01 C C C 0.05 0.16 0.18 0.11 0.11 ± 0.00 1.36 ± 0.02 0.06 ± 0.00 559 Nanon Xk Xk C 0.05 0.59 0.14 0.09 0.13 ± 0.01 0.95 ± 0.02 0.01 ± 0.00 712 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 ± 0.00 BB F 0.09 −0.07 −0.09 −0.03 0.00 ± 0.01 0.00 ± 0.02 0.00 ± 0.01 B C FP 0.06 −0.07 0.11 0.09 0.05 ± 0.01 1.15 ± 0.05 0.04 ± 0.00 –C F 0.05 0.42 0.12 0.12 0.03 ± 0.01 1.19 ± 0.04 0.07 ± 0.03 – B F 0.05 −0.04 −0.29 −0.03 −0.15 ± 0.01 1.70 ± 0.02 0.09 ± 0.00 – Xk FC 0.04 0.09 0.14 0.17 0.08 ± 0.04 1.1 ± 0.10.06 ± 0.02 Ch Ch FC 0.05 0.01 0.21 −0.03 0.17 ± 0.02 0.69 ± 0.01 0.02 ± 0.00 B C F 0.07 −0.14 0.04 0.18 0.04 ± 0.01 1.19 ± 0.04 0.10 ± 0.01 1 CC G 0.09 0.10 0.03 0.05 0.01 ± 0.00 1.30 ± 0.04 0.03 ± 0.00 Ch Ch G 0.08 −0.18 0.06 0.02 0.08 ± 0.02 0.70 ± 0.01 0.03 ± 0.00 Ch Ch G 0.04 −0.19 0.25 0.11 0.13 ± 0.02 0.74 ± 0.02 0.04 ± 0.00 106 Cgh Cgh G 0.09 −0.17 0.17 −0.03 0.09 ± 0.09 0.96 ± 0.02 0.05 ± 0.00 Ch Ch G 0.08 −0.19 0.27 0.21 0.11 ± 0.03 0.98 ± 0.04 0.06 ± 0.01 166 Xe Xk GC – 0.33 0.22 0.08 0.20 ± 0.01 0.99 ± 0.02 0.02 ± 0.00

See notes for Table 2a.

Table 3b Summary of the properties of the Tholen S asteroids.

Tholen Bus pv VIS NIR1 NIR2 Continuum Band Band type type (±13%) (±18%) (±22%) slope center depth S(I) L 0.17 0.98 0.58 0.01 0.35 ± 0.06 1.01 ± 0.02 0.13 ± 0.01 S(I) Sl 0.19 1.02 0.44 0.05 0.52 ± 0.05 1.14 ± 0.00 0.18 ± 0.01 S(II) S 0.29 0.83 0.64 0.06 0.27 ± 0.02 1.01 ± 0.01 0.10 ± 0.01 S(II) – 0.23 0.73 0.28 0.07 0.24 ± 0.05 1.06 ± 0.01 0.13 ± 0.02 S(II–III) Sa 0.16 1.25 0.42 −0.02 0.48 ± 0.05 0.95 ± 0.02 0.12 ± 0.01 S(III) S 0.21 1.59 1.24 0.14 0.23 ± 0.01 1.02 ± 0.00 0.14 ± 0.01 S(III) S 0.17 0.90 0.26 0.10 0.21 ± 0.03 0.99 ± 0.01 0.13 ± 0.01 S(III–IV) S 0.27 1.16 0.58 0.12 0.18 ± 0.02 0.97 ± 0.01 0.11 ± 0.01 3Juno S(IV) Sk 0.24 1.08 0.69 −0.04 0.15 ± 0.00 0.98 ± 0.00 0.12 ± 0.00 S(IV) S 0.16 1.67 0.91 0.21 0.22 ± 0.04 0.99 ± 0.01 0.13 ± 0.01 S(V) S 0.22 0.97 0.63 0.06 0.28 ± 0.02 0.94 ± 0.02 0.07 ± 0.00 S(V) S 0.30 0.97 0.48 0.07 0.28 ± 0.02 0.94 ± 0.01 0.12 ± 0.01 S(VI) S 0.21 1.40 0.63 0.02 0.16 ± 0.00 0.94 ± 0.00 0.12 ± 0.00 S(VI–VII) S 0.29 0.94 0.21 0.06 0.38 ± 0.03 0.93 ± 0.01 0.16 ± 0.00 S(VII) S 0.20 1.09 0.51 0.10 0.17 ± 0.01 0.91 ± 0.00 0.10 ± 0.00

See 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/or 4. Results new SpeX observations performed for this study. Some of our com- parison objects were also observed by Burbine et al. (2002) in the We have searched for correlations, trends, and clusters among SMASSIR study, and those data are over-plotted when available. the spectral parameters we have measured to record the spectral To set our targets in their meteorite context we have com- characteristics of our targets and comparison objects. We find that pared their spectral properties with those of laboratory spectra of the most useful spectral parameters for characterizing the varia- analog meteorites suggested in the literature (Table 4 and Fig. 5). tion in our data are brightness, absorption band center wavelength, Spectral parameters for the meteorites were obtained from RELAB and the continuum slope in the visible wavelengths from 0.45 to spectra measured using grain sizes less than 75 μm, as listed in 0.7 μm (VIS), and the infrared wavelengths from 1.7 to 2.49 μm Table 4. Note that both Tables 2a, 2b and 3a, 3b present all aster- (NIR2). These parameters show clustering that indicates a spec- oids used for this study with their taxonomic classifications from tral similarity between K-complex objects and carbonaceous me- both the Bus and Binzel (2002b) system, and the Tholen (1984) teorites. None of the measured parameters show clustering that system. contradicts 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 (dark stars), 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 spectra are 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 from Burbine and Binzel (2002) (open triangles). (c) Spectra of S type asteroids. Visible wavelength spectra are from Bus and Binzel (2002b). Long wavelength spectra come from the 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, largely used to define the taxonomy (Bus and Binzel, 2002b; continuum slope from 0.75 to 1.5 μm, or NIR1 slope is compared Tholen, 1984). between meteorites and asteroids. In these parameters, all objects Fig. 6a shows brightness versus NIR2 for the asteroids and me- overlap. teorites, respectively. Fig. 6b shows the visible slope versus band Although we do not show it, a comparison of visible slope versus band center (or band depth) shows strong clustering ac- center position in wavelength for the asteroids and meteorites, re- cording to taxonomy, as expected, because these parameters were spectively. Spectroscopy of K-complex asteroids 127

(b)

Fig. 4. (continued)

These figures begin to illustrate several points about K-complex the carbonaceous meteorites. The B-, C-, F- and G-class aster- asteroids: oids tend to be darker than the meteorites. 3. There is a lot of overlap in NIR2 slope. K-complex asteroids 1. K-complex asteroids are distinct from other main belt asteroid tend to have less steeply sloped NIR2 values than C-, B-, F- types S, B, C, F, and G. They do not appear to be a subset of and G-asteroids. Meteorites are intermediate between the two. these other types. 4. K-complex asteroids show a very large range in VIS slopes. 2. K-complex asteroid albedos span the range of 0.05 to 0.27. Car- Carbonaceous meteorites show a similar range of values, how- bonaceous meteorite brightness spans the range 0.02 to 0.22, ever C-, B-, F- and G-asteroids tend to have slope values at the and B-, C-, F- and G-asteroid albedos span the range 0.03 lower end of the range only. to 0.16. Assuming that geometric albedo and brightness at 0.55 5. K- and Xk-asteroids nearly span the range of band center po- microns are roughly comparable metrics, we find better agree- sitions exhibited by the carbonaceous chondrites, whereas B-, ment between the K-complex asteroids and the carbonaceous C- and F-type asteroids show longer wavelength band centers meteorites than between the B-, C-, F- and G-class objects and than 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- search we created overlays of all possible matches and kept a tally teroids are better spectral analogues for the carbonaceous me- of the “good” matches, where “good” is defined as visible agree- teorites we’ve measured than the traditional B-, C-, F- and G- ment in spectral shape, continuum slopes, and absorption band matches suggested in the literature (e.g. Gaffey et al., 2002; depths. On the basis of this search, we find that K asteroids are Burbine et al., 2002). These trends will either be verified or re- most consistent with CO and CK (and possibly CI and CV) mete- futed when our larger survey is completed. orites (Fig. 8, top). Xk asteroids, however, tend to show a “red” In Fig. 7 we show a break-down of the meteorite and aster- NIR1 continuum slope that is best matched with CM (and possibly oid classes in terms of band center near 1 μm. The 1 μm band is CR and CH) meteorites (Fig. 8,bottom). sensitive to the olivine-pyroxene mineralogy of the material, and is perhaps the main mineralogically diagnostic spectral parameter 5. Discussion we 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, we Previously, we estimated that roughly 10% of the smaller main- note that K (including Sk and K-candidates) and Xk asteroids span belt (Tholen class) S-types are actually K-types (Clark et al., 1995). the band center range of 0.9 to 1.2 microns, and that most mete- This was an exciting possibility because S-types and K-types have orite classes (except for the CM-class) fit within this range. The B-, such disparate meteoritic interpretations. We have also suggested C-, F- and G-class asteroids, however, have band centers ranging that CV-CO meteorites are better candidates than OC meteorites from 0.7 to around 1.7, and are not as easily seen to be similar to for the precursor material of the differentiated S-types (Meibom the carbonaceous chondrites. and Clark, 1999). If so, then it could be inferred that carbonaceous We have also performed a search for spectral matches between chondritic material, as opposed to ordinary chondritic material, our target asteroids and our comparison meteorite data set. In this once dominated the main belt. Spectroscopy of K-complex asteroids 129

Table 4 Meteorite comparison data set.

RELAB # Sample Type R VIS NIR1 NIR2 Continuum Band Band (0.55 μm) (±31%) (±3%) (±16%) slope center depth c1ph49 PCA91467 CH 0.10 2.68 0.26 0.12 0.29 ± 0.01 0.93 ± 0.00 0.03 ± 0.00 mgp080a Orgueil CI 0.05 0.48 −0.01 −0.04 0.03 ± 0.00 0.89 ± 0.01 0.05 ± 0.00 c1ph45 MET01149 CK3 0.19 0.30 0.36 −0.01 0.09 ± 0.00 1.09 ± 0.01 0.10 ± 0.00 c1ph35 ALH85002 CK4 0.15 0.15 0.10 −0.03 −0.08 ± 0.01 1.10 ± 0.00 0.08 ± 0.00 c1ph46 PCA91470 CK4 0.22 0.14 0.22 −0.03 −0.04 ± 0.00 1.10 ± 0.01 0.11 ± 0.00 c1ph53 DAV92300 CK4 0.13 0.12 0.22 0.02 −0.03 ± 0.01 1.10 ± 0.01 0.11 ± 0.00 c1ph47 EET83311 CK5 0.22 0.11 0.10 −0.07 −0.09 ± 0.01 1.12 ± 0.01 0.08 ± 0.00 c1ph43 MET01070 CM1 0.05 −0.42 0.05 −0.05 −0.15 ± 0.01 0.73 ± 0.00 0.04 ± 0.00 c1ph44 PCA02012 CM2 0.13 0.65 0.18 0.11 0.11 ± 0.00 1.02 ± 0.01 0.02 ± 0.00 c1ph32 MET00639 CM2 0.03 −0.04 0.05 0.00 0.05 ± 0.02 0.69 ± 0.02 0.01 ± 0.00 c1ph33 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 ± 0.00 c1ph52 QUE99038 CM2 0.14 0.46 0.26 0.06 0.09 ± 0.00 1.01 ± 0.01 0.06 ± 0.00 c1ph34 ALH82101 CO3 0.17 0.65 0.17 −0.05 0.00 ± 0.00 1.07 ± 0.00 0.05 ± 0.00 c1ph42 FRO95002 CO3 0.15 0.50 0.12 −0.01 −0.00 ± 0.00 1.09 ± 0.01 0.05 ± 0.00 c1ph50 MET00737 CO3 0.17 0.70 0.13 −0.02 −0.04 ± 0.00 1.04 ± 0.01 0.07 ± 0.00 c1ph57 FRO99040 CO3 0.17 0.38 0.10 −0.03 −0.01 ± 0.00 1.10 ± 0.01 0.04 ± 0.00 c1ph48 PCA91082 CR2 0.10 1.63 0.22 0.10 0.27 ± 0.01 0.92 ± 0.00 0.03 ± 0.00 c1ph54 QUE99177 CR2 0.13 1.49 0.26 0.10 0.28 ± 0.01 0.94 ± 0.00 0.02 ± 0.00 c1ph55 MET00426 CR2 0.10 1.66 0.17 0.07 0.19 ± 0.00 0.94 ± 0.00 0.023 ± 0.00 c1ph56 MAC87320 CR2 0.10 1.34 0.22 0.12 0.25 ± 0.01 0.93 ± 0.00 0.02 ± 0.00 c1ph41 QUE93744 CV3 0.12 0.51 0.02 −0.01 −0.03 ± 0.00 1.07 ± 0.01 0.02 ± 0.00

R (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- In the course of the analysis of this dataset, we have encoun- sumed asteroid parent bodies (Tholen class C, B, G, and F) has been tered many problems related to taxonomy. Some objects have only known for a long time (e.g. Britt et al., 1991; Hiroi et al., 1996; been classified in one system and not the others, and each system Pieters and McFadden, 1994; Burbine et al., 2002), and has gen- uses slightly different criteria for classification making it difficult to erally been attributed to slight differences between the way a compare across taxonomies. In this initial analysis of our ongoing meteorite is prepared for spectral measurements and the way an survey, we have attempted to de-emphasize taxonomic classifica- asteroid surface is affected by exposure to impacts and the space tion for the purposes of meteorite comparisons, and we have at- environment. Brittetal.(1991)compared asteroids and meteorites tempted to be inclusive (lumping sub-groups together) rather than using a principal components analysis and found a systematic dif- divisive (following or creating new sub-groupings). Nevertheless, ference between the CC meteorites and the Tholen classes C, B, G some problems remain. The K-complex includes part of the Tholen and F. Fornasier et al. (1999) compare CM meteorites to C- and S-type but also of the low albedo C-type (and dark types asso- G-type asteroids, and Burbine et al. (2002) summarize the find- ciated), so it is perhaps to be expected that the albedo range is ings from the literature up to 2002. Hiroi et al. (1993, 1994, 1996) wider. Albedo values lower than 0.08–0.09 for the K-complex are show that heated meteorites and several rare Antarctic meteorites referred to as Bus Xk asteroids which were once C- or D-types in provide remarkable spectral matches to asteroids in the Tholen C, the Tholen classification. Albedo values higher than 0.20 are asso- B, G and F classes. The meteorites were experimentally heated in ciated with Tholen S-, M-, or E-types. So, it seems that the albedo an oven to simulate thermal metamorphism of the minerals. Hiroi range of Bus K asteroids is between 0.09 and 0.19, similar to the suggests that some of the larger asteroids may be the heated inner CK and CO chondrites, while Tholen B–F–C–G-types have albedo portions of once larger bodies and that common CI/CM meteorites values closer to the CH, CI, and CM meteorites, with CR and CV may have come from the lost outer portions which escaped ex- meteorites having albedos which could be consistent with both tensive late-stage heating events. However, it is also possible that groups. As indicated by the very new Bus–DeMeo classifications K-complex asteroids will provide a satisfactory match to some of shown in the tables, several asteroids targeted for this survey will the CC meteorites without the need to invoke other processes. probably fall out of the K-complex as work continues. We note that space weathering effects may also have affected Vilas and Gaffey (1989) and Vilas et al. (1994) noted the pres- our spectral match search and parametric comparisons in the anal- ence of a spectral feature at 0.7 μm for C-asteroids that can be ysis described above. Using statistical arguments, Lazzarin et al. attributed to the aqueous alteration of anhydrous silicates, in par- + + (2006) have shown that space weathering probably occurs on all ticular the Fe2 ⇒ Fe3 charge transfer transition in oxidized iron asteroid types. Although this is true, many types lack the high found in phyllosilicates. The feature was found in 44% of the 45 albedo and strong spectral band contrasts that make weathering spectra taken of C-asteroids. This absorption feature is also seen effects easily detectable (Clark et al., 2002). It is possible that the in some CM2 class meteorite laboratory spectra and in terrestrial K-complex asteroids are -rich, and it is known that opaque phyllosilicates. Vilas and Gaffey suggest that the presence of this materials mask the optical effects of space weathering products absorption feature implies that the C-asteroids and some CM2 me- (Pieters et al., 2000). Some workers suggest that carbon-rich sur- teorites may have formed through the same aqueous alteration faces alter to become spectrally red (Lazzarin et al., 2006), others processes, which further implies that the C-asteroids may be a pos- have found spectral redness to decrease (Nesvorny et al., 2005; sible source of some meteorites. Of the 22 Hiroi et al., 2004). Since there is no consensus at this point, we meteorite sample spectra we obtained from RELAB, the 0.7 micron must await possible future discoveries from space weathering sim- feature is detectable in one out of six CM2 spectra. The correla- ulations, experiments, and laboratory study following a sample re- tions between 0.7 and 3-micron studies of asteroids have proven turn mission. to 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 of carbonaceous 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 (1.7–2.49μm)forB,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, and NIR2 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 contains most 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 carbonaceous chondrite 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 for B, 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 is wavelength 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- and G-class asteroids, and the green circle contains most of the K-complex objects. Note that the black diamonds, representing the carbonaceous chondrite meteorites, fall mainly near 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- Fig. 8. (top) Scaled reflectance of 742 Edisonia (solid line) compared to CO3-type teroid classes examined here. Note that K (including Sk- and K-candidates) and Xk meteorite c1ph42 (dashed line). (bottom) Scaled reflectance of 973 Aralia (solid line) asteroids span the range of 0.9 to 1.2 microns, and that most meteorite classes (ex- compared to CM2-type meteorite c1ph44 (dashed line). 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 not field of fragments indicate that the Eos family is approximately 1.3 enough CV, CH, or CI meteorites to suggest any correlations or trends. billion old (Vokrouhlicky et al., 2006). This can be taken as the approximate age of the surfaces of Eos family members. of asteroids (Rivkin et al., 2002). The 0.7 μm feature was not seen in the K-complex asteroids studied to date. None of the other targets in this study are part of any of the Asteroids 221 Eos, 661 Cloelia, and 742 Edisonia are dynamical known families. According to Bottke et al. (2005), the collisional members of the Eos family (Mothé-Diniz et al., 2008). In fact, these lifetimes of most of our targets can be very long so that they three objects have very similar orbits located in the central part of most likely date back to the pre-Late-Heavy-Bombardment , the Eos family. Dynamical models that include the initial ejection 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 5 References Asteroid orbital parameters.

Number Avg a Min a Max a Bell, J.F., 1988. A probable asteroidal parent body for the CV or CO chondrites. Me- objects (AU) (AU) (AU) teoritics 23, 256–257. Bell, J.F., 1989. Mineralogical clues to the origin of asteroid dynamical families. Bus Icarus 78, 426–440. Xk 39 2.736 1.933 3.414 Bell, J.F., Hawke, B.R., Owensby, P.D., Gaffey, M.J., 1988. The 52-Color Asteroid Sur- K332.779 1.392 3.030 vey: Final results and interpretations. Lunar . Sci. XIX, 57–58. Bell, J.F., Davis, D., Hartmann, W.K., Gaffey, M.J., 1989. Asteroids: The big picture. In: Tholen Binzel, R.P., Gehrels, T., Matthews, M.S. (Eds.), Asteroids II. Univ. of Arizona Press, B82.982 2.331 3.150 Tucson, pp. 921–948. C1392.866 2.197 3.885 Binzel, R.P., 1988. Collisional evolution in the Eos and Koronis families: Observa- F282.664 2.554 3.429 tional and numerical results. Icarus 73, 303–313. G92.883 2.362 3.187 Binzel, R.P., Lupishko, D., Di Martino, M., Whiteley, R.J., Hahn, G.J., 2002. Physi- Note: Values are from the JPL Horizons database (ssd.jpl.nasa.gov). Average semi- cal properties of near-Earth objects. In: Bottke Jr., W.F., Cellino, A., Paolicchi, major axis is calculated for “pure” types (e.g. “C” only, not “Cgh” etc.). Average a for P., Binzel, R.P. (Eds.), Asteroids III. Univ. of Arizona Press, Tucson, pp. 273– B asteroids does not include Chiron at 13.7 AU. 288. Bottke, W.F., Durda, D.D., Nesvorny, D., Jedicke, R., Morbidelli, A., Vokrouhlicky, D., thus substantially younger than our other targets. This is interest- Levison, H.F., 2005. Linking the collisional history of the main to its dynamical excitation and depletion. Icarus 179, 63–94. ing given the observation (by visual inspection of Fig. 2) that the Britt, D., Tholen, D.J., Bell, J.F., Pieters, C.M., 1991. Comparison of asteroid and me- Eos family has deeper band depths near 1 micron than our other teorite spectra: Classification by principle component analysis. Icarus 99, 153– targets. Other workers have suggested a trend between band depth 166. and age for the S-type asteroids (e.g. Binzel et al., 2002). Burbine, T.H., Binzel, R.P., 2002. Small Main-Belt Asteroid Spectroscopic Survey in In Table 5 we summarize some information regarding the or- the infrared. Icarus 159, 468–499. Burbine, T., Binzel, R.P., Bus, S.J., Clark, B.E., 2001. K asteroids and CO3/CV3 chon- bital parameters of our target asteroids. We include information drites. Meteorit. Planet. Sci. 36, 245–253. about our comparison asteroid data sets. It is interesting to note Burbine, T., McCoy, T., Meibom, A., Gladman, B., Keil, K., 2002. Meteorite parent bod- that K and Xk asteroids have an average heliocentric distance of ies: Their number and identification. In: Bottke Jr., W.F., Cellino, A., Paolicchi, P., about 2.7–2.8 AU, while the C, B, and G-class asteroids range from Binzel, R.P. (Eds.), Asteroids III. Univ. of Arizona Press, Tucson, pp. 653–667. Bus, S.J., Binzel, R.P., 2002a. Phase II of the Small Main-Belt Asteroid Spectroscopic about 2.9–3.0 AU, and the S-class asteroids average distance is 2.5– Survey: The observations. Icarus 158, 106–145. 2.7 AU. Bus, S.J., Binzel, R.P., 2002b. Phase II of the Small Main-Belt Asteroid Spectroscopic Survey: A feature-based taxonomy. Icarus 158, 146–177. 6. Conclusion Bus, S.J., Binzel, R.P., Volquardsen, E., 2003. Characterizing the visible and near-IR spectra of asteroids using principal component analysis. Bull. Am. Astron. Soc. 35, 976 (abstract). We have observed a total of 30 K-complex objects (12 K, 2 Sk, Chapman, C.R., Gaffey, M., 1979. Reflectance spectra for 277 asteroids. In: Gehrels, and 13 Xk-type asteroids (from the Bus and Binzel taxonomy), plus T., Matthews, M.S. (Eds.), Asteroids. Univ. of Arizona Press, Tucson, pp. 655– 3 K-candidates from previous work) and we have presented an 687. Clark, R., Roush, T., 1984. Reflectance spectroscopy: Quantitative analysis techniques analysis of their spectral properties from 0.4 to 2.5 μm. Our anal- for remote sensing applications. J. Geophys. Res. 89, 6329–6340. ysis suggests that carbonaceous chondrite meteorites are better Clark, B.E., Bell, J.F., Fanale, F.P., O’Connor, D.J., 1995. Results of the Seven-Color As- spectral analogs for the K-complex asteroids than for the previ- teroid Survey: Infrared spectral observations of 50-km sized S, K, and M-type ously suggested C-, B-, G- and F-type asteroids. Of the 13 Xk aster- asteroids. Icarus 113, 387–402. Clark, B.E., Lucey, P., Helfenstein, P., Bell III, J.F., Peterson, C., Veverka, J., Mc- oids we targeted, three objects were previously classified C-types Connochie, T., Robinson, M., Bussey, B., Murchie, S., Izenberg, N., Chapman, in the Tholen system, and seven objects were previously classified C., 2001. 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