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1917 The Meteoritical Society, 2009. Printed in USA.

Meteoritics & Planetary Science 44, Nr 12, 19171927 (2009)

Abstract available online at http://meteoritics.org

Composition of 298 Baptistina: Implications for the K/T impactor link

Vishnu REDDY1*, Joshua P. EMERY2, Michael J. GAFFEY1, William F. BOTTKE3, Abigail CRAMER4, 5,and Michael S. KELLEY4, 6

1Department of Space Studies, University of North Dakota, Grand Forks, North Dakota 58202, USA2Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee 37996, USA

3Southwest Research Institute, 1050, Walnut St., Suite 300, Boulder, Colorado 80302, USA4Department of Geology and Geography, Box 8149, Georgia Southern University, Statesboro, Georgia 30460, USA

5Present address: S&ME, Inc., 7604 Waters Ave., Savannah, Georgia 31406, USA6Present address: Planetary Science Division, Science Mission Directorate, NASA Headquarters, Washington, D.C. 20546, USA

*Corresponding author. E-mail: [email protected]

(Received 08 December 2008; revision accepted 14 June 2009)

AbstractBottke et al. (2007) suggested that the breakup of the Baptistina asteroid family (BAF)

160+30/20 Myr ago produced an asteroid shower that increased by a factor of 23 the impact flux ofkilometer-sized and larger asteroids striking the Earth over the last ~120 Myr. This result led them topropose that the impactor that produced the Cretaceous/Tertiary (K/T) mass extinction event 65 Myrago also may have come from the BAF. This putative link was based both on collisional/dynamicalmodeling work and on physical evidence. For the latter, the available broadband color andspectroscopic data on BAF members indicate many are likely to be dark, low albedo asteroids. Thisis consistent with the carbonaceous chondrite-like nature of a 65 Myr old fossil meteorite (Kyte 1998)and with chromium from K/T boundary sediments with an isotopic signature similar to that fromCM2 carbonaceous chondrites. To test elements of this scenario, we obtained near-IR and thermal IRspectroscopic data of asteroid 298 Baptistina using the NASA IRTF in order to determine surfacemineralogy and estimate its albedo. We found that the asteroid has moderately strong absorptionfeatures due to the presence of olivine and pyroxene, and a moderately high albedo (~20%). Thesecombined properties strongly suggest that the asteroid is more like an S-type rather than Xc-type

(Moth-Diniz et al. 2005). This weakens the case for 298 Baptistina being a CM2 carbonaceouschondrite and its link to the K/T impactor. We also observed several bright (V Mag. 16.8) BAFmembers to determine their composition.

INTRODUCTION

Catastrophic events have strongly affected the course ofevolution of life on planet Earth. Bottke et al. (2007) arguedthat a factor of 2 increase in the terrestrial and lunar impactflux over the last ~120 Myr (potentially including the K/Timpactor) was likely triggered by the catastrophic disruptionof a ~170 km body in the inner region of the main asteroidbelt. They identified the Baptistina asteroid family (BAF) asthe probable source of this putative increase using a range ofobservational data and numerical modeling work (e.g., theorbital and absolute magnitude distribution of the observedfamily, asteroid color data from the Sloan Digital Sky Survey,and the existing spectroscopy of two large Baptistina familymembers, collisional and dynamical modeling work).According to Bottke et al. (2007), the BAF formed 160+30/20million years ago and since then the family members have

been drifting both toward and away from the Sun via non-gravitational Yarkovsky forces. This has allowed 1020% ofthe kilometer-sized and larger BAF members, which oncemay have exceeded 140,000 members, to reach two nearbymean motion resonances (the 7:2 and 5:9 mean motionresonances with Jupiter and Mars, respectively) that in turndelivered them to orbits that cross those of the terrestrial planets. Numerical modeling work shows that asteroidsexiting these resonances have a nearly 2% chance of strikingthe Earth. Taken together, Bottke et al. (2007) used thesevalues to show that the BAF could have potentially dominatedthe background impact flux on the Earth, Moon, and otherterrestrial planets for several tens of Myr, with the peak of theflux occurring about 100 Myr ago.

Based on this premise, Bottke et al. (2007) speculatedthat there could be a connection between the putative asteroidshower produced by the BAF and the K/T impact, presumably


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1918 V. Reddy et al.

the largest impact on Earth over the last 160 Myr. Usingdynamical modeling work as well as information on thenature of the BAF and the K/T impactor, they estimated therewas a ~90% probability that the K/T impactor was derivedfrom the BAF asteroid shower.

In order to perform the K/T calculation, Bottke et al.(2007) checked to see whether the BAFs taxonomic

properties were consistent with what was known about the K/Timpactor. The deduced composition of the K/T impactor,while somewhat murky, is consistent with it coming from adark, low albedo asteroid. An investigation of a 65 Myr oldhighly-altered fossil meteorite, presumably a fragment fromthe K/T impactor, was found to be most similar tocarbonaceous chondrite meteorites, themselves a darkmeteorite class (Kyte 1998). Similarly, chromium isotopestudies of sediments from several different K/T sites(Trinquier et al. 2006; see also Shukolyukov and Lugmair1998) suggested the impactor was most similar to CM2-typecarbonaceous chondrites. Assuming the latter is true and thatthe BAF indeed produced the K/T impactor, it would suggest

that the BAF was likely similar in composition to CM2-typecarbonaceous chondrites.

Here we attempt to further test the BAF-K/T impactorlink by observing and spectrally characterizing some of thebrightest objects in the BAF. Our goals in this particular paperare to determine whether 298 Baptistina is spectrallyconsistent with CM2-type carbonaceous chondrites, andthereby test the K/T impactor link. The compositionalinvestigation of smaller BAF members, such as thoseidentified by Parker et al. (2008) using SDSS colors, is on-going and will be described in a follow-up paper. It is alsoimportant to recall what these taxonomic classifications meanand their relationship (or the lack of) to meteorites.Taxonomic classification is a useful technique to broadlyclassify large number of objects for applications such asidentifying broad trends throughout the main belt orconstraining membership of dynamical families, but it doesnot equate with compositional characterization of asteroids.

The largest member of the BAF, 298 Baptistina, has beenclassified as Xc-type based on its visible spectrum (Lazzaroet al. 2004). In this paper, we present new near-infrared (0.8 to2.5 m) spectra that we obtained in order to assess the surfacecomposition of Baptistina. This spectral range is morediagnostic of actual silicate mineralogy than the visible region,and will therefore provide a test of the hypothesized CM2-like

composition for Baptistina. We also present a measurement ofthermal emission from Baptistina and the resulting albedoestimate. Albedo provides a significant constraint oncompositional interpretation, with a CM2 assemblage havingan albedo in the 36% range (Gaffey 1976) while an S-typeobject would normally have an albedo in the ~1035% range(Gaffey et al. 1993a). We also observed several bright (V Mag.16.8) BAF members to determine their composition and ifthey make geologic sense as members of the family.

OBSERVATION AND DATA REDUCTION

In an effort to constrain the composition of 298Baptistina, its albedo and diameter, and the composition ofsome of its family members, an observational campaign waslaunched in Spring 2008 using the SpeX instrument (Rayneret al. 2003) at the NASA IRTF. Baptistina was observed in

low-resolution prism mode (0.752.50 m) on Feb. 28, 2008(13:3014:30 UT), March 21, 2008 (13:0013:30 UT), andMarch 22, 2008 (13:0014:00 UT) and in long cross-dispersion mode (LXD) (1.904.20 m) on March 21, 2008(11:2011:45 UT) (Table 1). All observations were made onsite during which local standard stars and solar analog starswere also observed for extinction and solar continuumcorrection of the asteroid spectrum.

The low resolution prism data (0.752.50 m) wereprocessed using methods described in Abell (2003) and theLXD data (1.904.20 m) were processed using methodsdescribed in Emery and Brown (2003).

ALBEDO AND DIAMETER ESTIMATION

Albedo measurement of an asteroid is a valuable datumthat can constrain its compositional characterization. In thecase of 298 Baptistina it could help confirm the compositionallink to a specific meteorite type. Albedo measurements ofBaptistina were not available prior to this study. In an effort toconstrain the albedo, cross dispersion data (1.904.20 m)(Fig. 1) were obtained using the SpeX instrument on NASAIRTF.

Albedo Estimation

The spectrum of Baptistina measured at the IRTF on 21March 2008 rises steeply longward of about 3.6 m (Fig. 1).This is interpreted as a contribution to the spectrum bythermal radiation from the surface, and we use this thermaltail to estimate the albedo of Baptistina. This is part of the testto see whether Baptistina is consistent with a carbonaceouschondrite-like composition, which would imply a low (fewpercent) albedo.

The approach used for estimating the asteroids albedo issimilar to that used by several other authors in theinterpretation of relative spectral data of near-Earth asteroids(e.g., Abell 2003; Rivkin et al. 2005; Reddy et al. 2007;

DeMeo and Binzel 2008). The data are not photometric (i.e.,no reliable absolute flux calibration), prohibiting the normalapplication of the radiometric method to determine both sizeand albedo. Nevertheless, the relative spectral accuracy isquite good, and the location and magnitude of the thermalcontribution provides an indication of the surface temperatureand, since the location of the asteroid is well known, thealbedo.

The thermal excess at each wavelength, (Rivkin


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298 Baptistina and K-T impactor link 1919

et al. 2005) is calculated as

. (1)

In this relation, Rrepresents the reflected component at agiven wavelength, and Tthe thermal component. The quantityR+ Tis the measured relative spectrum. Since we do not havean independent measure of R in the thermal region ( >3.5 m), the model is fit by a straight line to the K-band portionof the spectrum and extrapolating this line through the L-band.This straight line extrapolation is supported by the compositioninferred from the prism (0.8 to 2.5 m) spectrum (Fig. 2). Notethat since both R and T are proportional to the square of the

radius (i.e., the cross-sectional area), this factor cancels out andthe thermal excess is independent of size. In other words, thesolution for the albedo determined from is not degraded byour ignorance of the size of Baptistina.

The task is now to fit the thermal excess using modeledreflected and thermal fluxes. The modeled reflected flux isgiven by

(2)

where rAU is the heliocentric distance in AU, km is thegeocentric distance in km, and is the radius in km. Fsun, is

the solar flux at 1 AU, for which the parameterization given inSmith and Gottlieb (1974) is used. The phase factor, , wascalculated as described in Bowell et al. (1989) assuming aslope parameter, G, of 0.15, giving = 0.48. Finally, pis thegeometric albedo at wavelength . This is related to thevisible geometric albedo, pv, by p=pvfkvR, where fkv is theratio between the visible and K-band geometric albedos andR is the relative reflectance modeled as a straight line fit tothe K-band data as described above. The term fkv is estimatedfrom the measured prism spectrum to be 1.12.

The modeled thermal flux, , is calculated using aslight modification of the standard thermal model (STM;Lebofsky et al. 1986) that is commonly referred to as the near-

Earth asteroid thermal model (NEATM; Harris 1998). Themodifications are that the beaming parameter (), which wasintroduced to adjust the surface temperature to account forsurface roughness but can be used to account for thermalinertia as well, is allowed to float rather than being set at apredetermined value and the mid-infrared phase function ishandled slightly differently. Since adjusts the temperature,the choice of values for this parameter also affects theresulting albedo estimate. In a reanalysis of IRAS asteroid

Table 1.

AsteroidRotation period(h)

Taxonomicclassification

Observing dates(UT)

Visualmagnitude Airmass

Phaseangle

Heliocentric distance(AU)

298 Baptistina 16.22 Xc 28-Feb-08 14.4 1.14 22.5 2.10921-Mar-08 13.85 1.246 13.9 2.12822-Mar-08 13.85 1.28

1126 Otero A 21-Mar-08 14.68 1.17 18.3 1.96

1365 Henyey S 23-Mar-08 14.15 1.18 8.9 2.095

1619 Ueta 2.94 S 22-Mar-08 16.5 1.06 27.1 2.18223-Mar-08 16.5 1.041 27.1 2.184

2093 Genichesk 11.02 C 23-Mar-08 16.39 1.04 16.9 2.392

4375 Kiyomori A 22-Mar-08 14.92 1.13 5.4 2.08323-Mar-08 14.92 1.08 5.9 2.082

4859 Fraknoi S 23-Mar-08 16.82 1.08 26.1 2.106

Fig. 1. Thermal IR data of 298 Baptistina obtained in the cross-dispersion mode (LXD) using the SpeX instrument on NASA IRTF.The thermal models were calculated using NEATM. The black opendiamonds are the data binned by 6 channels (which is the width of theslit), and the grey open circles are data binned by 36 channels.

R T+

R------------------- 1=

Rm

Fsun ,

r2AU

-------------

2

2km

-----------p ,=

Tm


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photometry, Russ Walker (personal communication) foundthat the distribution of in the Main Belt was a Gaussian witha mean near 1 and a standard deviation of ~0.09. In thethermal fits performed here, a default of 1.0 was adopted,but calculations were also performed with values of 0.9 and1.1 to assess uncertainties in the albedo estimate.

The modeled thermal excess is compared to themeasured thermal excess for a given choice of pv and . Thetwo are compared for goodness-of-fit by calculating areduced 2. A minimization algorithm (Press et al. 1992) wasused to quickly and efficiently zero in on the best-fit pv for agiven . For = 1, the best-fit pv is 0.22. The data aresomewhat noisy and are reasonably well fit by a range of pv

from 0.13 to 0.37 (Fig. 1). Taking account of the 1 range of mentioned above, the best-fit pv for = 0.9 is 0.29 and for = 1.1 is 0.17. Adding together in quadrature the uncertaintydue to the noise in the data and that from the 1 spread in ,the final albedo estimate is (i.e., range from0.12 to 0.39). This derived albedo is much higher than the ~36% albedo expected for a carbonaceous chondrite-like body,but is in line with those of S-type asteroids, in agreement withthe silicate absorption features apparent in the prismspectrum.

This albedo value is higher than the preliminary reportedvalue of 14+2/3%by Reddy et al. (2008), which was based ondata set that was not corrected for spectral slope. The error

bars reported here are more realistic based on the scatter in theraw data.

Diameter Estimation

An asteroids diameter can be calculated if its absolutemagnitude (H) and geometric visual albedo (pv) are known.His related topv and the asteroids effective diameter(Deff) inkilometers by the equation (Fowler and Chillemi 1992):

Using an absolute magnitudeH= 11.0 and an albedo of20+15/10%, 298 Baptistina has a diameter of ~19+7/5 km. Thisdiameter is much smaller than the 40 km reported by Bottke etal. (2007) which was estimated based on an assumed loweralbedo.

COMPOSITIONAL CHARACTERIZATION

Near-Infrared Spectra

Asteroid composition is defined in terms of itsmineralogy, mineral chemistry, and the relative abundances ofmajor minerals. Key minerals (i.e., olivine, pyroxene) presenton asteroid surfaces have diagnostic absorption features atvisible to near-IR wavelengths (0.302.50 m) that can beused to determine surface compositions. These absorption bands arise from electronic transitions within a specificmineral structure. Several spectral band parameters (bandcenter, band area ratios, band intensity) can be used toconstrain an asteroids mineralogy. The general position andpattern of the absorption band(s) are a function of the crystalstructure, while the exact positions of the bands are a functionof the mineral composition. The intensity of the feature is

related to the abundance of the absorbing species (e.g., Fe2+)in the mineral and in a mineral mixture to the relativeabundance of different minerals, the distortion of the mineralcrystal structure, particle size, opaques, etc. Since a mineral isdefined by its structure and composition, the positions andintensities of the absorption bands are uniquely related tospecific minerals.

Figure 2 shows the average near-IR spectra of 298Baptistina from March 21 and March 22, 2008 UT. Both

o

Fig. 2. Near-IR spectrum of 298 Baptistina obtained on March 21 (top) and 22 (bottom), 2008. The visible data from Lazzaro et al. (2004) andthe near-IR data from NASA IRTF were combined to show the complete VNIR spectrum. Spectra from both nights are consistent with oneanother and are offset for clarity.

pv 0.22 0.10+0.17=

Def f1329

pv------------*10

H 5

=


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spectra show a well resolved Band I absorption feature at1.0 0.01 m with a band depth of 7 1% and a weaker BandII (band depth 2 1%) feature at 1.96 0.02 m. The Band IIabsorption feature is more apparent on March 22, 2008 data.The average Band Area Ratio (Band II Area/Band I Area) forboth nights is 0.3 0.1. Visible wavelength data (~0.500.75m)

from Lazzaro et al. (2004) agree well with the near-IR dataand confirm the earlier supposition that a weak feature mayexist beyond 0.90 m. The scatter at 1.4 and 1.9 m is due toincomplete correction of atmospheric water vapor features.Majaess et al. (2009) present a rotational lightcurve ofBaptistina that was measured throughout March and April2008, finding a period of 16.23 0.02 h. Since their observingperiod overlapped with some of our dates, we were able tomatch the near-IR spectral observations with rotational phase

of the asteroid. Spectral observations on March 21, 2008,were made at ~0.32 (115) phase and 0.83 (298) phase onMarch 22, 2008. As seen in Fig. 2 no significant rotationalvariation was observed in the measured spectral band parameters (band centers, band depth, Band Area Ratios[BAR]).

The presence of Band I and II strongly indicates the presence of a mafic silicate mineralmost likely pyroxeneon the asteroids surface. Pyroxenes typicallyhave Band I centers between 0.900.935 m for low-Capyroxenes, 0.911.07 m for Type B high-Ca pyroxenes andBand II centers between 1.781.97 m for low-Ca

pyroxenes and ~1.972.36 m for Type B high-Capyroxenes (Cloutis and Gaffey 1991). For 298 Baptistina theBand I center of 1.0 0.01 m suggests that the surfaceassemblage has either a low-Ca and high-Ca pyroxenemixture or low-Ca and olivine mixture. Figure 3 shows aplot of Band I and II centers for ortho and clinopyroxenes.Baptistina plots above this trend line strongly suggesting thepresence of olivine in the mixture.

Based on the Cloutis and Gaffey (1991) calibration theratio of pyroxene in a mixture of olivine + pyroxene is~20:80 10. Correcting the Band I center for olivine usingmethods described in Gaffey et al. (2002), Baptistina plots below the trend line suggesting the presence of a possible

third phase (high-Ca pyroxene) apart from olivine and low-Capyroxene. The Band I center and BAR of 298 Baptistina plotexactly on the olivine + orthopyroxene mixing trend line inthe Gaffey S-asteroid subtype plots (Fig. 4) strengthening thecase for an olivine and low-Ca pyroxene mixture. Based onthe mineralogy, the ratio of olivine/pyroxene, and the locationof the asteroid band parameters on S-asteroid subtype plot,LL chondrites are plausible meteorite analog for 298Baptistina.

Fig. 3. Band-band plot showing the orthopyroxene to clinopyroxene trend line (Adams 1974). 298 Baptistina plots above the trend lineindicating the presence of olivine.

Fig. 4. S-asteroid subtypes plot from Gaffey et al. (1993b) showingthe olivine-pyroxene mixing line. 298 Baptistina plots on this trendsuggesting the presence of olivine and pyroxene in the surfaceassemblage.


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Absorption features on 298 Baptistina are 7 1%(Band I) and 2 1% (Band II) deep, respectively. This isweaker than the average S-asteroid Band I depth (12.3 4.6%) from Gaffey et al. (1993b), but within the range ofS-asteroid Band I depth (2.724%). The presence of weak/suppressed features could be due to a variety of reasons.The presence of opaques (e.g., metal) suppresses silicate

absorption features (Cloutis et al. 1990) and reddens thespectral slope (increasing reflectance with increasingwavelength). Cloutis (personal communication) notedreddening of spectral slope in olivine + metal mixtures withmore than 50% metal. Metal also tends to increase the albedo(moderate albedo ~1020%). Shock-darkened ordinarychondrites also have lower albedo and subdued absorptionfeatures (Britt and Pieters 1994; Gaffey 1976). Other opaques(e.g., carbon) suppress spectral absorption features anddecrease albedo (Clark 1981, 1983). A low abundance of theabsorbing species (e.g., Fe2+) in a moderate albedo mineralcan also cause weak features. Adams (1975) observed theincrease in band depth and shifting of band center as Fe

content of olivine increased in the forsterite-fayalite solidsolution series. Presence of weakly featured minerals, forexample low-Fe orthopyroxene can produce weak featureswith high albedos. Particle size and porosity can also affectthe albedo and intensity of the absorption feature (Gaffeyet al. 1993c). For transparent or weakly absorbing species, adecrease in particle size causes weakening of absorptionfeatures, but increases the albedo. For opaques such ascarbon, decreasing particle size decreases albedo.

CM2 Carbonaceous Chondrite Link

Identifying a meteorite analog of an asteroid helps notonly to link the meteorite to a source region in the asteroid belt but also to understand the petrologic history andformation conditions of the asteroid. Barring direct samplingand analysis of an asteroid, the most robust way to identify ameteorite analog of an asteroid to use spectral band parameters to determine the asteroids surface mineralogyand mineral abundances.

Visible Spectra

The visible wavelength spectrum (~0.500.90 m) of298 Baptistina was published by Lazzaro et al. (2004) as part

of the Small Solar System Object Spectroscopic Survey(S3OS2) and is shown in Fig. 5. The spectrum shows a rise inreflectance from ~0.500.75 m and a drop in reflectance beyond that toward near-IR wavelengths suggesting apossible weak feature at 0.90 m. The sharp narrow feature at~0.77 m is an uncorrected absorption due to atmosphericoxygen (O2). Comparing the visible spectrum of 298Baptistina with those of CM2 meteorites and meteoritemixtures (Fig. 5) shows a significant mismatch.

Near-IR Spectra

Apart from olivine and pyroxene there is also a possiblethird phase (probably an opaque phase such as metal orcarbon) that is suppressing the absorption features. Thealbedo of the asteroid is very helpful in narrowing downpossible meteorite analogs. Based on the preliminary albedoestimate using cross-dispersion data 298 Baptistina has amoderate albedo ~20%.

Using this information the first step is to investigate thelink between 298 Baptistina and CM2 carbonaceouschondrites. The CM2 carbonaceous chondrites all belong to petrologic type 2; meaning they have undergone aqueousalteration but no thermal metamorphism (Brearley and Jones,1998). Near-IR laboratory spectra of Cold Bokkeveld, Migheiand Nogoya show a weak absorption feature (


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(1976) shows most have albedos from 3.3%

(ColdBokkeveld) to 4.7% (Nogoya). Albedo measurements(absolute reflectance at 0.565 m) of Antarctic CM2 samplesshow that all but two samples (PCA 02012 and QUE 99038)have low albedos between 2 and 5%, similar to Gaffey (1976)measurements. PCA 02012 and QUE 99038 are minor tomoderately weathered samples that have albedos of ~13%,which is considerably higher than all other known CM2meteorite albedos. There is a strong possibility that terrestrialweathering has had some effect on the albedos of thesemeteorites. The spectral effects of terrestrial weathering onmeteorites are not well characterized. Vilas and Gaffey (1989)suggested that such high albedos (715%) are possible in

highly aqueously altered CM2 assemblages, but theabsorption features are still very weak (


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(S3OS2). Figure 7 shows the near-IR spectra of 1365 Henyeywith deep absorption bands at 1- and 2-m, and its bandparameters are Band I center 1.01 0.01 m, Band II center2.01 0.01 m and BAR of 0.29. The data plot above thelow- and high-Ca mixing line on the band-band plot (Adams1974) and above the S (IV) region of the Gaffey S-asteroidssubtype plot. The estimated abundance of pyroxene in a

mixture of olivine and pyroxene is 20 5% (Cloutis et al.1986).

1619 Ueta

A member of BAF that shows silicate absorptionbands due to olivine and pyroxene. Due to low SNR of thespectra further analysis was not possible for this asteroidand more observations are planned during the nextopposition.

2093 Genichesk

An ~18 km object (based onH= 12.6, pv = 0.05) with anS3OS2 taxonomic classification of C-type. The near-IRspectrum of 2093 Genichesk (Fig. 7) shows a shallow, broadabsorption feature at ~1.0 m and a relatively flat spectrum beyond that. The shallow absorption feature is similar to1.0 m absorption feature due to phyllosilicates on certaincarbonaceous chondrites like Cold Bokkeveld (CM2) andCO3 meteorite Kainsaz from Gaffey (1976). Genichesk is theonly object in the BAF we have observed and analyzed so farthat has CM2 carbonaceous chondrites as possible meteoriteanalogs. Despite the possibility of Genichesk being a CM2assemblage, its link to the BAF and ultimately the K/Timpactor is tenuous at best.

Note that while observing BAF members isstraightforward, the identification of BAF members is not.The BAF is located in a dynamically complex region of themain belt where it is mixed with members of the Flora andVesta families (Bendjoya and Zappala 2002; Bottke et al.2007). This makes it difficult to sort out true BAF membersfrom interlopers, particularly at the bright end of theabsolute magnitude distribution where low albedo BAFmembers may be mixed with high albedo S- and V-classasteroids. The SDSS color data mainly exists for objectswith H ~1516 and has limited coverage among brighter BAFmembers with H < 14. Moreover, until recently, the BAF

was identified using cluster algorithms set to grab largenumbers of nearby (and potentially unrelated) objects(Bottke et al. 2007). This may explain why the BAF waslong thought to merely be a sub-cluster within the Florafamily (Williams 1992), and why previous attempts tocharacterize bright BAF members have produced a potpourri of taxonomic types (Moth-Diniz et al. 2005).These factors make it impossible to link Genichesk to theBAF or the K/T impactor.

Fig. 7. Normalized near-IR reflectance spectrum of BAF members1126 Otero, 1365 Henyey, 2093 Genichesk, and 4375 Kiyomoriobtained using the SpeX instrument on NASA IRTF.


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4375 Kiyomori

This asteroid has been classified as A-type by Lazzaro etal. (2004) as part of the Small Solar System ObjectSpectroscopic Survey (S3OS2). Figure 7 shows the near-IRspectrum of (4375) Kiyomori with moderately deep Band Iand Band II features (13% and 11% deep, respectively). The

spectral band parameters of this asteroid are Band I center0.93 0.01 m, Band II center 2.0 0.01 m and BAR of1.63. The data plot exactly on the low- and high-Ca mixingline on the band-band plot (Adams 1974) suggesting thepresence of a nearly pure pyroxene assemblage. The asteroidfalls in the basaltic achondrite region of the Gaffey S-asteroids subtype plot. The estimated abundance of olivine ina mixture of olivine and pyroxene is


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