Constraining albedo, diameter and composition of near-Earth asteroids via near-infrared spectroscopy

Icarus 219 (2012) 382–392

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Constraining albedo, diameter and composition of near-Earth asteroids via near-infrared spectroscopy Vishnu Reddy a,b,⇑,1, Michael J. Gaffey c,1, Paul A. Abell d,1, Paul S. Hardersen e,1 a

Department of Space Studies, Room 520, Box 9008, University of North Dakota, Grand Forks, ND 58202, USA Max-Planck Institute for Solar System Research, 37191 Katlenburg-Lindau, Germany c Department of Space Studies, Room 518, Box 9008, University of North Dakota, Grand Forks, ND 58202, USA d Astromaterials Research & Exploration Science Directorate, NASA Johnson Space Center, Mail Code KR, 2101 NASA Parkway, Houston, TX 77058-3696, USA e Department of Space Studies, Room 530, Box 9008, University of North Dakota, Grand Forks, ND 58202, USA b

a r t i c l e

i n f o

Article history: Received 30 August 2011 Revised 5 March 2012 Accepted 8 March 2012 Available online 22 March 2012 Keywords: Asteroids, Composition Near-Earth objects Spectroscopy Meteorites

a b s t r a c t We present a method to constrain the albedo and diameters of near-Earth asteroids (NEAs) based on thermal flux in their near-infrared spectra (0.7–2.5 lm) using the Standard Thermal Model. Near-infrared spectra obtained with the SpeX instrument on NASA Infrared Telescope Facility are used to estimate the albedo and diameters of 12 NEAs (1992 JE, 1992 UY4, 1999 JD6, 2004 XP14, 2005 YY93, 2007 DS84, 2005 AD13, 2005 WJ56, 1999 JM8, 2005 RC34, 2003 YE45, and 2008 QS11). Albedo estimates were compared with average albedo for various taxonomic classes outlined by Thomas et al. (Thomas, C.A. et al. [2011]. Astron. J. 142(3)) and are consistent with their results. Spectral band parameters, like band centers, are derived and compared to spectra of laboratory mineral mixtures and meteorites to constrain their composition and possible meteorite analogs. Based on our study we estimate the albedos and diameters of these NEAs and compare them with those obtained by other techniques such as ground-based mid-infrared, Spitzer thermal infrared and Arecibo radar. Our results are broadly consistent with the results from other direct methods like radar. Determining the compositions of low albedo asteroids is a challenge due to the lack of deep silicate absorption features. However, based on weak absorption features and albedo, we suggest possible meteorite analogs for these NEAs, which include black chondrites, CM2 carbonaceous chondrites and enstatite achondrites. We did not find any specific trends in albedo and composition among the asteroids we observed. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction The size of a near-Earth asteroid has important implications for impact hazard assessment. Currently, accurate diameter measurements are not available for most of the >8526 known near-Earth asteroids. A common method employed to estimate the diameter is to use the absolute magnitude, which is calculated from visual magnitude and an assumed slope parameter, and assume a range of albedos (typically 5–25%) (Fowler and Chillemi, 1992). The uncertainty of this method corresponds to a factor of 2.2 in diameter and 11 in mass. While more direct techniques like radar have been very successful in estimating accurate diameters, they are limited to NEAs that make a close flyby of the Earth. ⇑ Corresponding author at: Department of Space Studies, Room 520, Box 9008, University of North Dakota, Grand Forks, ND 58202, USA. Fax: +1 701 777 3711. E-mail address: [email protected] (V. Reddy). 1 Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under Cooperative Agreement No. NNX-08AE38A with the National Aeronautics and Space Administration, Science Mission Directorate, Planetary Astronomy Program. 0019-1035/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.icarus.2012.03.005

While thermal models based on 10- and 20-lm radiometry have been used to calculate albedo and diameter of NEAs (Lebofsky et al., 1981; Matson, 1986; Veeder et al., 1989; Tedesco, 1989; Tedesco et al., 2002; Harris, 1998; Lim et al., 2005), very few albedo and diameter estimates have been made based on thermal flux between 2.2 and 3.0 lm (Abell, 2003; Rivkin et al., 2005; Kumar et al., 2006; Reddy et al., 2006a, 2006b). Unlike 10- and 20-lm radiometry, where simultaneous observations are needed in the thermal and visual wavelengths, thermal flux in near-infrared wavelengths (2.0–2.4 lm) can be measured from the near-infrared spectrum of an object. So for low albedo NEAs, the composition, albedo and diameter can all be estimated from a single near-infrared spectrum without the need for additional multi-wavelength observations. This method complements 10- and 20-lm radiometry and greatly increases the number of objects for which albedos can be constrained (Rivkin et al., 2005). Here we present albedo and diameter estimates for 12 near-Earth asteroids based on their near-infrared spectrum. We also attempt to constrain their surface composition and propose probable meteorite analogs. Where applicable we have compared

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the diameters and albedos from our method with other techniques like mid-IR, and thermal IR observations from ground-based telescopes (Howell et al., 2008), and radar observations from Arecibo and space missions like Spitzer (Mueller et al., 2011). 2. Observations and data reduction Near-infrared observations of the NEAs were obtained using the SpeX instrument (Rayner et al., 2003), a medium resolution nearinfrared spectrograph, at the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawai’i. The data were obtained using a combination of onsite observations from the summit of Mauna Kea and remote observations. In low-resolution mode, SpeX covers the wavelength range between 0.7 and 2.5 lm with a spectral resolution of 150. A detailed description of the observing protocol is presented in Reddy (2009). Table 1 lists observational information for each of the 12 NEAs studied here. SpeX prism data were processed using the IDL-based Spextool provided by the NASA IRTF (Cushing et al., 2004). Analysis of the data to determine spectral band parameters like band centers, band depths and Band Area Ratio (BAR) was done using SpecPR based on the protocols discussed by Cloutis et al. (1986), Gaffey et al. (2002), and Gaffey (2003, 2005). Band I is the absorption band at 1 lm (typical of olivine and/or pyroxene) and Band II is the band at 2 lm (pyroxene). 3. Thermal modeling The size and albedo of an asteroid can be determined if its reflected and thermally emitted flux are measured. At a given heliocentric distance (for example: 1.0 AU), the transition region between the reflected and emitted components will depend on the asteroid’s albedo (Lebofsky et al., 1986). For NEAs with an albedo of less than 10%, the transition region typically lies between 2.2 and 3.0 lm at 1.0 AU. Various radiometric models—including the Standard Thermal Model (STM) (Lebofsky and Spencer, 1989), Fast Rotating Model (FRM) (Lebofsky et al., 1978) and Near-Earth Asteroid Thermal Model (NEATM) (Harris, 1998)—have been proposed to estimate the albedo and size of asteroids. The underlying principle behind all these models is the fact that any airless body needs to be in thermal equilibrium where a fraction of the incident solar radiation is reflected into space and the remaining fraction is absorbed (depending on the object’s albedo). The absorbed energy is reemitted into space as thermal radiation and the total energy (reflected and emitted) is equal to the incident solar radiation reaching the surface. The method we have employed to constrain the albedo of NEAs is based on STM and is similar to that used by several other authors (e.g., Abell, 2003; Rivkin et al., 2005; DeMeo and Binzel, 2008;

Reddy et al., 2009) in interpretation of relative spectra. Due to the fact that there is no absolute flux calibration for our near-infrared spectra, the standard radiometric method to determine size and albedo cannot be applied. However, the location and magnitude of the thermal contribution in the near-infrared is an indicator of the surface temperature. Using the asteroid’s heliocentric distance, the albedo can be constrained using a simple thermal model such as the STM. A detailed description of the thermal model and methodology employed has been published in Reddy et al. (2009) and Reddy (2009). 3.1. Model parameters Taxonomic classifications are a useful first-order technique to separate asteroids into broadly different groups. However, taxonomies were not designed as compositional determinants and mask geological and mineralogical differences within a given class (Hardersen et al., 2011). Albedos have been used to differentiate spectrally degenerate asteroids, as in the case of the E-, M-, and P-type asteroids (Chapman et al., 1975; Tholen, 1984). Lower-albedo objects absorb more incident solar radiation than those with higher albedos. This absorbed energy is then re-emitted as thermal radiation and produces higher surface temperatures for the loweralbedo objects. The amount of thermal flux emitted by an asteroid is directly related to its albedo. Fig. 1A shows thermal excess of four asteroid taxonomic classes (E, M, S, and C) at 1.0 AU, 30° phase angle, emissivity and IR beaming factor of 0.90. The transition point between reflected and emitted components shifts to shorter wavelength going from high to low albedo objects (30–5%). This would imply that lower-albedo objects show thermal excess at further heliocentric distances (lower temperature) than high-albedo objects. Higher-albedo objects have more reflected flux (0.4–2.5 lm) and lower emitted flux. Asteroids with albedos <20% have a non-zero thermal excess value at near-infrared wavelengths (0.7–2.5 lm), which enables one to estimate or constrain the albedo. A change in phase angle has a minimum effect on the thermal excess at near-infrared wavelengths for fixed albedo and heliocentric distance. Fig. 1B shows relative flux from a C-type NEA (Tedesco, 1989) (5% albedo at 1.0 AU, emissivity and infrared beaming of 0.90) with changing phase angle (0–90°). The thermal excess decreases with increasing phase angle, which is logical as the illuminated surface area is decreasing and the subsolar point (highest temperature) shifts closer to the terminator. One of the primary uncertainties in the STM is the infrared beaming factor (g), which is the inverse of thermal flux enhancement at 0° phase angle over a uniformly radiating sphere. The beaming factor is affected by surface roughness, which decreases its value by increasing the sunward thermal emission and increases its value if a rotating body has non-zero thermal inertia

Table 1 Observing circumstances for SpeX near-infrared data. Object

Type

Rotation period (h)

Observing date (UT)

V. mag.

Ph. angle

Heliocentric distance (AU)

1992 1992 1999 1999 2003 2004 2005 2005 2005 2005 2007 2008

Amor Apollo Aten Apollo Apollo Apollo Apollo Apollo Aten Apollo Amor Apollo

38 12.906 7.68 136.0 168–336 – – 5.4 >30 – – –

30-July-05 31-July-05 31-July-05 20-May-08 13-August-08 5-July-06 11-June-07 26-July-08 6-January-08 31-January-06 5-April-07 1-October-08

15.00 12.40 16.80 15.50 17.1 14.00 15.48 15.13 11.34 15.90 16.3 14.28

44.00 6.00 65.30 66.70 54.50 78.10 47.00 68.00 99.00 36.50 48.30 55.80

1.18 1.08 1.10 1.10 1.14 1.02 1.1 1.03 0.95 1.18 1.04 1.02

JE (13553) UY4 (100085) JD6 (85989) JM8 YE45 (90403) XP14 AD13 RC34 WJ56 (199003) YY93 DS84 QS11

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Fig. 1. (A) The effect on changing albedo (for various taxonomic classes) on the relative flux in visible-near-infrared wavelengths modeled using STM. The plot (A) shows relative flux of four asteroid taxonomic classes (E, M, S, and C) at 1 AU, phase angle 30°, emissivity 0.90, IR beaming factor 0.90. Optically bright taxonomic classes (E, M and S) have high flux in visible wavelengths (reflected component) and lower flux at near-infrared wavelengths (emitted component). (B) The effect on changing phase angle on the relative flux in visible-near-infrared wavelengths of a low-albedo C-type NEA at 1 AU, phase angle 30°, emissivity 0.90, infrared beaming factor 0.90 modeled using STM. As the phase angle increases from 0° to 90° there is a minimal drop in thermal flux. (C) The effect on changing infrared beaming factor on the relative flux in visible-near-infrared wavelengths of a low-albedo C-type NEA at 1 AU, phase angle 30°, emissivity 0.90 modeled using STM. As the beaming factor goes from 0.90 to 0.75, there is a decrease in overall thermal flux in near-infrared wavelengths.

(Lebofsky et al., 1986). The canonical value that has been used for the beaming factor in the IRAS Minor Planet Survey (Tedesco et al., 1992) is 0.756 based on observations of main-belt objects at lowphase angles (Lebofsky et al., 1986). For silicate surfaces, a value

of 0.90 has been typically suggested (Lebofsky and Spencer, 1989). The effect of changing the beaming factor between 0.756 and 0.90 for an asteroid has a higher impact on its thermal excess than its phase angle. Fig. 1C depicts this effect on an asteroid at

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1.0 AU, phase angle of 30°, albedo of 5% and emissivity of 0.90. Since this is a correction factor for infrared beaming, a value of 1.0 would mean zero beaming. 3.2. Difference between STM and NEATM The primary differences between the STM and NEATM are the beaming parameter (g) and the way in which the phase angle is utilized. Lebofsky et al. (1986) calibrated the STM against thermal infrared observations of large asteroids like Ceres and Pallas. Observing stellar occultations of a few calibrated asteroids, they derived the beaming parameter of 0.756. It was however noted that this value might not be appropriate for small asteroids as g is affected by surface roughness. Large asteroids typically have thick layers of regolith compared to smaller asteroids that are thought to be mostly rubble piles. This key difference (macroscopic roughness, thermal inertia) affects the g value used for large asteroids and smaller NEAs. Harris (1998) modified the STM to better-fit thermal infrared observations of NEAs by setting g as a free parameter, but with a different meaning. STM and NEATM also differ in the way the phase angle is used in the model. In STM, the phase angle is accounted for by scaling the infrared flux by 0.01 mag/deg. This phase coefficient was derived and tested for phase angles 30°. In contrast, the phase angle effect in NEATM is accounted for by estimating the actual thermal flux given off a Lambertian sphere visible to the observer at a given solar phase angle. This assumes that there are no emissions from the night side of the asteroid. Unlike STM, which can use the near-infrared tail of the Planck Curve to derive albedos, NEATM needs high quality thermal infrared data with good wavelength coverage (5–20 lm) for accurate modeling. This makes it difficult to use NEATM in its original form for constraining NEA albedos with just near-infrared data (0.7–2.5 lm). 3.3. Identifying thermal excess On a featureless, low-albedo asteroid, thermal excess is identified as a sharp rise in flux beyond 2.0 lm (Reddy, 2009). However, due to the inherent faintness of NEAs coupled with observational difficulties in obtaining high signal-to-noise ratio spectra, the task of identifying and quantifying thermal excess can be challenging. There are two potential anomalous sources of thermal excess that could be misidentified in an NEA spectrum. The first is scatter in the data at long wavelengths due to reduced sensitivity of the detector. Visual inspection of the spectrum should help differentiate between noise and thermal excess in a spectrum. The second is misidentifying a spectral absorption feature, typically the 1.9–2.0 lm orthopyroxene or Type B clinopyroxene feature, as thermal excess. In this case, the albedo can be calculated assuming that the excess is real and one can postulate, based on the depth of the 1.0 lm feature, if the albedo calculated is realistic or not. For example, Fig. 2 shows the near-infrared spectrum of near-Earth Asteroid 2000 OY21 with two absorption bands at 1and 2-lm, typical of the mineral pyroxene. The spectrum could also be interpreted as an olivine + spinel mixture with the rise in flux beyond 2.3 lm as thermal excess. Assuming the extremities in continua, the estimated albedo range is 2.5–7.5% (Fig. 2). Given the depth of the 1-lm feature and the inferred mineralogy it is unlikely that the rise in flux beyond 2.3 lm is thermal excess because an S-type asteroid typically has an albedo much higher than 2.5–7.5%. 3.4. Errors in thermal flux estimation One of the primary sources of uncertainty (apart from model parameters) in the estimation of albedo using the thermal excess

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method arises from the uncertainty in estimating the continuum over which the thermal excess is superimposed. For asteroids with featureless spectra beyond 1.5 lm, usually a linear continuum is assumed and can be computed. In such cases, the choice of continuum has a relatively minimal effect on the thermal excess estimate. The linear continuum is estimated by fitting a linear function to the data (typically in the 1.4–1.8 lm range) that does not show an absorption band or thermal excess. However, residual water vapor features due to insufficient atmospheric correction on a featureless asteroid’s spectrum can distort the spectrum making the task of identifying the continuum a challenge. Some low-albedo asteroids have shallow features that, when sufficiently broad and weak, could be misidentified as the continuum (Reddy, 2009). Uncertainty in the estimated albedo could also arise from the quality of data in the wavelength region encompassing the thermal excess. Although point-to-point scatter at 2.4 lm is relatively lower compared to the noise at longer wavelengths, averaging the flux values that encompass the channels around 2.4 lm helps in reducing this uncertainty. In the SpeX data used here, three channels (2.403-, 2.406-, 2.408-lm) were averaged to obtain the flux value at 2.4 lm. We also fit the observed thermal flux with the modeled thermal flux for different albedos (Fig. 4A–C) for most asteroids that showed a visible thermal excess (1992 JE, 2007 DS84, 1999 JM8, and 2005 YY93) and found that the resulting albedos are essentially the same (±1.5%) for both methods. The uncertainty in the albedo is based on the scatter at 2.4 lm along with the range of modeled albedo curves that can be fit to encompass the observed thermal flux. Typically this value is between 1% and 2% for NEAs with good signal beyond 2.0 lm. 4. Diameter estimation The diameter of an asteroid can be calculated if its absolute magnitude (H) and geometric visual albedo (Pv) are known. Absolute magnitude is the asteroid’s magnitude in V-band measured or extrapolated at 0° phase angle when the object is at unit (1 AU) heliocentric and geocentric distance. H is related to Pv and the asteroid’s effective diameter (Deff) in kilometers is calculated by the following equation (Flower and Chillemi, 1992; Pravec and Harris, 2007):

1329 Deff ¼ p  10H=5 Pv 4.1. Uncertainty in absolute magnitude and slope parameter One of primary sources of uncertainty in calculating diameter is the absolute magnitude (H) of the asteroid. The HG magnitude system was adopted at the 1985 IAU General Assembly and was developed to predict the visual magnitude of an asteroid as a function of changing phase angle (Bowell et al., 1989). H and G are related by:

V obs ¼ HðaÞ þ 5 logðrDÞ HðaÞ ¼ H  2:5 logðð1  GÞU1 ðaÞ þ GU2 ðaÞÞ where H(a) is the reduced magnitude, a is the phase angle, and U1 and U2 are phase functions (Bowell et al., 1989). The above equations are based on empirical and theoretical analysis in which the slope parameter G has no physical interpretation, but is related to the geometric albedo and amount of light scattered (Piironen et al., 1996). G also serves as a weighting parameter for the influence of the phase functions U1 and U2 (Bowell et al., 1989). The HG system was revised at the 1991 IAU General Assembly where a G value of 0.15 was adopted for all asteroids, which did

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Fig. 2. The near-infrared spectrum of 2000 OY21 with two absorption bands at 1- and 2-lm, the extremities in possible continua are shown for estimation of thermal excess.

not have explicitly derived values. This ‘‘replaced a more complicated, ambiguous and confusing procedure used in the 1985 system’’ (Matson and Tedesco, 1992). At present the Minor Planet Center (MPC) uses a default G value of 0.15 for all objects (including NEAs) for which G values have not been derived from phase curves. Due to this assumption there is an uncertainty in the H magnitudes published by the MPC. Diameters calculated using MPC H magnitude would similarly be affected. Harris (1989) and Lagerkvist and Magnusson (1990) constrained the G range for low and high albedo asteroids. According to these authors, G  0.04 ± 0.06 for low albedo asteroids (e.g. C-types) and G  0.45 ± 0.04 for high albedo asteroids (e.g. E-types). Table 2 lists the calculated diameters of the asteroids based on the H magnitude derived by the MPC (G = 0.15). Given the albedo and the H magnitude the diameter is calculated based on the methods described by Flower and Chillemi (1992). 5. Constraining albedo and composition 5.1. (13553) 1992 JE 1992 JE was discovered from Geisei, Japan, in 1992 and is classified as a member of the Amor class of NEAs. This asteroid has a rotation period of 38.0 h with an amplitude range of 1.1 (Pravec, Personal Communication). Radar observations suggest a diameter of 2.38 km (Benner, Personnel Communication). Observational circumstances for all objects are listed in Table 1. Fig. 3A shows the average spectrum of 1992 JE with a weak Band I feature and constant reflectance between 1.6 and 2.1 lm. The sharp rise in flux beyond 2.1 lm is interpreted as strong thermal emission based on its heliocentric distance of 1.180 AU at the time of observation. Assuming a linear continuum, the estimated thermal excess at 2.4 lm is 12 ± 1%. Using STM, we calculated the thermal flux calibration using a value of 0.90 for the emissivity and infrared beaming factor. Based on the thermal flux calibration curve, the albedo of 1992 JE is estimated to be 3 ± 1% (Table 2). The weak absorption band has a continuum-removed band center at 0.99 lm with a weaker inflection at 1.30 lm (Fig. 3A). The constant reflectance between 1.6 and 2.1 lm rules out a 2 lm pyroxene feature. The estimated center of 0.99 lm would be inconsistent with a single phase like orthopyroxene, Type A clinopyroxene or olivine. Band I centers for orthopyroxenes typically ranges from 0.90 to 0.95 lm (Cloutis and Gaffey, 1991),

1.007–1.025 lm for Type A clinopyroxenes (Schade et al., 2004) and 1.04–1.08 lm for olivines (King and Ridley, 1987). The Band I center is more consistent with a mixture of olivine and low-Fe orthopyroxene, which lack a 2-lm feature (Cloutis and Gaffey, 1991). Using the Band I position and the calibration for olivine/ orthopyroxene mixtures by Cloutis et al. (1986), the estimated relative upper limit for pyroxene in a pyroxene–olivine mixture is 20%. An independent estimate of the olivine content in an olivine–orthopyroxene mixture using the calibration by Singer (1981) gives an olivine content of 80%, which is consistent with the earlier estimate. Due to its low albedo, possible meteorite analogs for 1992 JE include black chondrites that have an albedo range 4–9% and CM2 carbonaceous chondrites with an albedo range of 3–5% (Gaffey, 1976). Black chondrites are ordinary chondrites with compositions similar to L type that have been shock blackened by impact (Heymann, 1967). The shock produces metal inclusions that darken the meteorites. The most prominent feature in the spectra of these meteorites is the 0.9 lm pyroxene band. However, the pyroxene relative abundance (of total olivine/orthopyroxene abundance) in L chondrites is between 36% and 52%, which is much higher than the 20% we have for 1992 JE. Dunn et al. (2010) suggested that this discrepancy is due to the fact that the Cloutis et al. (1986) method does not account for the third silicate component, clinopyroxene, which could be causing the lower estimate. Chapman and Salisbury (1973), and Gaffey (1976), noted that in ordinary chondrites (including L type), the 0.9-lm pyroxene feature shifts to longer wavelength with increasing Fe2+ and olivine content. The presence of a high olivine content and a Band I center, consistent with an olivine/orthopyroxene mixture, suggests that 1992 JE has a surface assemblage similar to black chondrites. 5.2. (100085) 1992 UY4 1992 UY4 is an Apollo-type Potentially Hazardous Asteroid (PHAs) with a rotation period is 12.9060 h and an amplitude of 0.26 magnitudes (Warner et al., 2006). Fig. 3B shows the average of 40 spectra of 1992 UY4 from July 31, 2005. The spectrum has a red slope with no detectable absorption features. Residual water vapor features from poor extinction corrections are visible at 1.4- and 1.9-lm. A sharp rise in flux beyond 2.1-lm is due to thermal emission given its heliocentric distance of 1.08 AU at the time of observation. The estimated thermal excess at 2.4 lm is 13 ± 1% assuming a linear continuum. The thermal flux calibration curve of 1992 UY4

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Table 2 Albedo and diameter of NEAs observed as part of this work. Note that the diameters were calculated based on the H magnitude derived by the MPC. Albedo and diameters from other published sources are also listed. Taxonomic class based on albedo and albedo ranges for each class from Thomas et al. (2011) are also listed. Object

Albedo estimate (%)

Albedo range of tax. classes (Thomas et al., 2011)

Taxonomic class based on MPC H albedo mag

Diameter (km)

Other diameter estimates (km)

Source

1992 JE

3±1

2þ2 =1

D

16.0

4.80 ± 0.8

1.9–4.3

MPC

1992 UY4

6±1 5.2 ± 0.5

<10%

P

17.80

1.5 ± 0.3

2 1.68 ± 0.08

þ2:3 2=0:09

2þ2 =1

D

9±1 17 ± 4

<10% 10–30%

17.10

4:7þ5:1 =2:4

13þ6 =5

P M C

1999 JM8

1.5 ± 1

2þ2 =1

D

16.5

5.4 ± 1.2

3.5

Radar Volquardsen et al. (2007) Mueller et al. (2011) Near-IR Howell et al. (2008) Mueller et al. (2011) Radar

2003 YE45

2.5% ± 1 >5 ± 1

<10%

P C

17.8

<1.64 ± 0.2

0.67–0.7

Howell et al. (2008) Radar

2004 XP14

>20 ± 2

19.40

<0.39 ± 0.02 0.30 ± 0.09

Radar

2005 AD13 2005 RC34

17.9 19.7

<0.97 ± 0.03 0.8–1.8 <0.39 ± 0.14 0.09–0.10

2005 WJ56 2005 YY93

>13 ± 1 >15 ± 1 60 ± 15 >20 ± 2 3.5 ± 1

2þ2 =1

M E M M E M D

18.1 17.1

<0.71 ± 0.02 0.50 0.39 ± 0.4 1.1–2.6

MPC Radar Howell et al. (2008) Radar MPC

2007 DS84 2008 QS11

5±1 8±1

<10% <10%

P P

20.8 19.9

0.41 ± 0.04 0.49 + 0.03

MPC Radar

1999 JD6

13þ6 =5 10–30% >30% 10–30% 10–30% >30% 10–30%

was calculated for a heliocentric distance of 1.08 AU, phase angle of 6°, and an emissivity and infrared-beaming of 0.9. Based on the calibration curve, the estimated albedo is 6 ± 1%. This value is insensitive to change in infrared beaming factor (0.75–0.90). Apart from estimating the albedo from flux measurements at 2.4 lm, the STM can be applied to all data points in the thermal excess part of the spectrum simultaneously. The slope-removed spectrum of the asteroid is plotted along with the predicted model emission fluxes in Fig. 4A. If the above estimated albedo is correct, then the observed and calculated thermal emission curves should match. Fig. 4A shows the observed and calculated thermal emission for 5%, 6% and 7% albedo for 1992 UY4. The observed thermal emission closely matches the calculated thermal curve for 6% albedo confirming that the 6 ± 1% albedo estimated for the object above is reasonable. This method could be employed for high signal-tonoise ratio spectra of NEAs where the scatter at longer wavelengths is minimal. Independent near-infrared spectral measurements using the NASA IRTF made by Volquardsen et al. (2007) confirms the albedo and diameter estimated here. Using data from 0.8 to 4.2 lm, they estimated the geometric albedo to be 5.2 ± 0.5% and a diameter of 1.68 ± 0.08 km. A good agreement with our estimate and those by Volquardsen et al. (2007) suggests that thermal excess in the near-infrared wavelengths (0.7–2.5 lm) can be effectively used to determine albedo and diameters of low-albedo PHAs. However, radiometric observations on this asteroid with the Spitzer Space Telescope (SST) by Mueller et al. (2011) suggest an albedo of 2þ2:3 =0:09 %. While this value (1.91–4.3%) is lower than our value (5– 7%), the albedo measured by Volquardsen et al. (2007) (4.7–5.7%) is within the uncertainties quoted above. Compositional analysis of 1992 UY4 is challenging due to its featureless near-infrared spectrum (Fig. 3B). Given the low albedo (6%), probable meteorite analogs would include carbonaceous chondrites that have albedos ranging from 3% to 9% (Gaffey, 1976). Due to the lack of any diagnostic features the spectral slope can be used as a last resort to identify the asteroid’s possible taxonomic class. It is important to note that with rare exceptions, determining the taxonomic class of an asteroid provides virtually

2:549þ0:7 =0:61 1.6 + 0.2

1.2 2:393þ0:66 =0:60

0.20–0.46 0.45 + 0.1

no information on the composition or physical properties. The infrared slope value for 1992 UY4 is 0.13. Using the 52-color survey data (Bell et al., 1988), we calculated the mean infrared slope value for all taxonomic classes with the G taxonomic class having the closest value of 0.13. Volquardsen et al. (2007), using curve-matching techniques, concluded that the spectrum of 1992 UY4 closely matches a P-type asteroid similar to 46 Hestia. Benner et al. (2008) found a distinct correlation between an asteroid’s taxonomic type and its surface roughness, which in radar terms translates to circular polarization ratio (ratio of same sense to opposite sense polarization). Radar observations of this asteroid from Arecibo observatory yield a circular polarization ratio of 0.20 ± 0.02, which is close to 0.19, the mean SC/OC value for P and D-type asteroids. 5.3. (85989) 1999 JD6 1999 JD6 is an Aten-type PHA with a rotation period of 7.68 h and a large lightcurve amplitude of 0.7–1.2 magnitude (Szabo et al., 2001; Pravec et al., 2006; Polishook et al., 2005). This object has been classified as a K-type by Bus and Binzel (2002). Fig. 3C shows an average of 10 spectra of 1999 JD6. Visible wavelength data from the Small Main-Belt Asteroid Spectroscopic Survey (Bus and Binzel, 2002) was combined with the SpeX data. The spectrum is almost featureless with a neutral slope and point-to-point scatter in the data increasing beyond 1.9 lm due to decreased detector sensitivity coupled with the inherent faintness of the object. Assuming a linear continuum, the estimated mean value of thermal excess at 2.4 lm is 3 ± 2% given the scatter in the data. The estimated lower limit albedo for the object using the thermal flux calibration curve given the uncertainties is 9 ± 1%. Howell et al. (2008) observed this object using the NASA IRTF and their spectrum is consistent with the one obtained by us. However, the albedo they estimated is higher (13–21%) than our value (8–10%). We attribute this difference to higher scatter in our data beyond 1.9 lm that prevented us from precisely quantifying the thermal excess. Mueller et al. (2011) estimated the albedo of 1999 JD6 to be 4:7þ5:1 =2:4 % using the Spitzer

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Fig. 3. Near-infrared spectra of 12 NEAs observed as part of this study. The spectra have been normalized to unity at 1.4 lm.

Space Telescope. The higher end of this estimate is closer to the value obtained by us (9 ± 1%) than Howell et al. (2008) (17 ± 4%).

Compositional analysis of the spectrum shows a shallow feature at 0.6 lm in the visible part of the spectrum (Fig. 3C). The

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JD6 has a composition similar to a CV3 chondrite that is consistent with the earlier interpretation by Binzel et al. (2001). 5.4. (53319) 1999 JM8 (53319) 1999 JM8 is an Apollo-type NEA with an extremely slow rotation period of more than 136 h (Pravec et al., 2005) and has been classified as a tumbler/non-principle axis rotator. It has a radar diameter of 3.3 km (Benner et al., 2001) and is classified as X-type (Binzel et al., 2002). The near-infrared spectrum of 1999 JM8 is shown in Fig. 3D. The scatter in the data is due to less than ideal weather conditions on Mauna Kea at the time of observations. The spectrum is featureless with no absorption features (10% level) and a sharp rise in flux beyond 2.0 lm due to thermal emission. Based on the thermal excess of 33%, the estimated albedo of 1999 JM8 is 1.5 ± 1%. Based on the albedo and lack of any spectral features the most probable taxonomic class for this NEA is D (Thomas et al., 2011) (Table 2). Arecibo and Goldstone radar observations of 1999 JM8 provide us with a wealth of physical information about the asteroid. Benner et al. (2001) observed the asteroid for 18 days and achieved very high-resolution images (15 m/pixel), which reveal an asymmetric, irregularly shaped object with a typical overall dimension within 20% of 7 km. Using the radar observations they report optical and radar albedos of 2% and 6% respectively. These values are consistent with our albedo estimate of 1.5% using near-infrared data for this dark object. The object also showed ‘‘regions of pronounced topographic relief, prominent facets several kilometers in extent, numerous crater-like features between about 100 m and 1.5 km in diameter’’ (Benner et al., 2001). The asteroid also showed very high circular polarization ratio. Based on this, Campbell et al. (2005) suggested the presence of a thick regolith layer on the surface of 1999 JM8. 5.5. (90403) 2003 YE45

Fig. 4. (A) Continuum removed spectra of (100085) 1992 UY4 (A), 2007 DS84 (B) and 2008 QS11 (C) showing the observed thermal flux plotted along with modeled thermal flux curves.

near-infrared spectrum shows a very weak inflection at 0.90 lm suggesting a possible Band I feature (Fig. 3C). The presence of a feature at 2.0 lm cannot be verified given the scatter at longer wavelengths. Binzel et al. (2001) suggested that 1999 JD6 belongs to the K taxonomic class, which is similar to the CV chondrites. The estimated albedo of 9 ± 1% would be consistent with such an interpretation. CV3 chondrites typically have albedos ranging from 5% to 10% (Gaffey, 1976). Some CV3 chondrites like Allende show a weak inflection shortward of 0.6 lm, a complex broad feature at 0.9 lm and a very weak shallow feature at 2.0 lm. Given the albedo and weak features at 0.6 and 0.90 lm, we suggest that 1999

Spectrum of (90403) 2003 YE45 (Fig. 3E) exhibits a shallow absorption feature at 1.0 lm and a relatively flat spectrum beyond 1.4 lm. No deep absorption features could be detected at 2.0 lm although weak feature (<10%) cannot be ruled out due to the scatter in the data at these wavelengths. No thermal excess can be detected beyond 2.0 lm but the scatter in the data at 2.4 lm prevents the detection of thermal excess below 5%. Based on this, the lower limit albedo is P5 ± 1%. Howell et al. (2008) also observed this object in long cross dispersion mode (LXD) between 1.9 and 4.3 lm using the NASA IRTF and obtained an albedo of 17 ± 4%. While this albedo value is much higher than the one obtained with our method, it is still consistent (>5%) with our albedo measurement. The lower than expected value we obtained might be due to the high scatter in data beyond 2.0 lm and also the higher albedo which would result in lower thermal emission in the near-infrared wavelengths. Fig. 3E shows the average spectrum with a moderately deep absorption feature at 1.05 lm and a band depth of 17%. The lack of thermal excess helps constrain the lower limit albedo between 5% and 15% (due to the scatter in the data). Hicks (Personal Communication) reported an S-type spectral classification for this object. The band center of 1.05 lm strongly indicates the presence of olivine, which has Band I center range between 1.05 and 1.09 lm (King and Ridley, 1987). Mg-rich forsterite (>Fo90) has a band center around 1.05 lm whereas Fe-rich fayalite has band centers close to 1.09 lm. However, the typical monominerallic olivine feature has much more pointed minimum whereas 2003 YE45 has a shallower concave feature. Using the improved forsterite calibration in Reddy et al. (2011), the Fo% can be estimated using the following equation:

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Fo% ¼ 1946:6  Band I center þ 2139:4 This yields a Fo% of 95 ± 15% for 2003 YE45. The 15% error is due to the uncertainties in the laboratory calibration of olivine. The lack of Band II near 2.0 lm also strengthens the case for a nearpure olivine assemblage. A few olivine-rich meteorites have similar Fo% as 2003 YE45. Ureilites and pallasites have Fo content similar to 2003 YE45, and ureilites have an average albedo of 13%, which is within the range of the lower limit albedo for 2003 YE45. 5.6. 2004 XP14 2004 XP14 is an Apollo-type PHA with a slow rotation period (Benner et al., 2008). The spectrum (Fig. 3F) is featureless with a red slope shortward of 0.9 lm and a neutral slope from 0.9 to 2.5 lm. No thermal excess is seen within the detectable limits. Weak features at 0.94 and 1.14 lm are incompletely corrected atmospheric water vapor features. The estimated lower limit albedo for the object assuming zero thermal excess is >20 ± 2% given the scatter at 2.4 lm. Based on the lower limit albedo and a featureless spectrum, possible taxonomic classes are M and E. Spectra of some M asteroids have a weak 0.90-lm feature suggesting the presence of low-Fe pyroxene in the surface assemblage (Hardersen et al., 2005, 2011). E asteroids are featureless or weakly featured with very high albedo and slightly reddish slope. These asteroids have been identified as possible parent bodies of enstatite achondrites (Zellner, 1975; Zellner et al., 1977; Gaffey et al., 1989, 1992). A prominent E asteroid in the NEA population is the Apollo-type Asteroid 3103 Eger, considered the probable parent body of aubrite meteorites (Gaffey et al., 1989, 1992). Radar observations of 2004 XP14 suggest a high circular polarization ratio (SC/OC) (0.90 ± 0.15). E-type asteroids have the highest SC/OC (mean: 0.892), which would suggest an extremely rough surface (Benner et al., 2008). 2004 XP14’s circular polarization ratio (SC/OC) is very close to the mean SC/OC values for E-type asteroids. Using the Gaffey and Kelley (2010) classification scheme, E-type asteroids are divided into the classes E[I], E[II] and E[III]. The E[I] objects are featureless with a slightly reddish slope in the visible spectral range, characteristic of enstatite. The E[II] group has relatively strong absorption features at 0.49 lm and sometimes between 0.90 and 0.96 lm due to the calcium sulfide mineral, oldhamite [(Ca,Mg,Fe)S]. The last type is the E[III], which have a flat or reddish spectra but with a well defined feature centered at 0.89– 0.90 lm due to low-Fe pyroxene containing Fe2+. Using this classification scheme, 2004 XP14 can be classified as an E[I] type and having a surface dominated by enstatite. 5.7. 2005 AD13 2005 AD13 is an Apollo-type NEA with a diameter between 0.80 and 1.8 km. The near-infrared spectrum (Fig. 3G) shows no absorption features (down to 10% level) and the overall slope is blue (decreasing reflectance with increasing wavelength). No thermal excess is seen within the detectable limits. One can constrain the lower limit albedo for the object by assuming 3 ± 2% thermal emission at 2.4 lm. Based on this the estimated lower limit albedo for this asteroid is 13 ± 1%. The lack of diagnostic features could be due to the presence of an opaque phase (carbon), which results in low albedo. Some carbonaceous chondrites like Grenada (CV3) have a blue slope with an albedo range of 5–10% (Gaffey, 1976). Using the moderate albedo and lack of spectral features it is reasonable to suggest Type 3 carbonaceous material as a possible analog for 2005 AD13.

5.8. 2005 RC34 The spectrum of 2005 RC34 (Fig. 3H) shows a weak inflection at 0.90 lm (<10%) that could be an absorption feature. No apparent thermal excess is detected beyond 2.0 lm. Using STM the estimated lower limit albedo for the object using the thermal flux calibration curve given the uncertainties is >15 ± 1%. 5.9. (199003) 2005 WJ56 2005 WJ56 is an Aten-type NEA with a rotational period of 4.3786 h and an amplitude of 0.15 (Betzler and Novaes, 2008). Fig. 3I shows an average spectrum of (199003) 2005 WJ56 with no visible absorption features and a reddish slope. No thermal emission was detected beyond 2.0 lm. We can constrain the lower limit albedo for the object by assuming 2 ± 1% thermal emission at 2.4 lm. Based on this, the estimated lower limit albedo for this asteroid is >20 ± 2%. The lack of spectral features makes it difficult to constrain the composition of the object, but the minimum albedo of 20% helps narrow down the possibilities. Taxonomically, featureless/ weakly-featured dark asteroids (B, C, D, F, G, P, and T) have albedos lower than 10%; M- and E-type asteroids are the only moderate to high albedo (10–60%) asteroids that are weakly featured or featureless. So we can restrict the taxonomic type of 2005 WJ56 to M- or E-type. Like 2004 XP14, the object was also observed with radar from Arecibo observatory during its close flyby in 2008. 2005 WJ56 has an SC/OC ratio of 0.92 ± 0.15, which is very close to the mean SC/OC values for E-type asteroids (0.892). Using the Gaffey and Kelley (2010) classification scheme, 2005 WJ56 can be classified as an E[I] type with a pure enstatite spectrum. If the aubrite interpretation is correct, then the parent body of 2005 WJ56 experienced temperatures of at least 1400 °C during the formation epoch of the Solar System. 5.10. 2005 YY93 The near-infrared spectrum (Fig. 3J) of 2005 YY93 shows a weak Band I feature and no detectable Band II feature. The sharp rise in flux beyond 2.3 lm appears to be thermal emission. Assuming a straight-line continuum, the estimated thermal excess at 2.4-lm is 4 ± 1%, which corresponds to an albedo of 3.5 ± 1%. The weak Band I has a center at 0.91 ± 0.01 lm and a band depth of 6%. Given the low albedo (3%) estimated from the thermal excess, the most probable meteorite analogs are carbonaceous chondrites. Vilas and Gaffey (1989) observed several low albedo main belt asteroids that displayed a weak absorption feature at 0.9 lm and attributed them to iron oxides in phyllosilicates. These clay minerals are thought to have formed by aqueous alteration of the original parent material on the asteroid surface. Spectra of Murray and Cold Bokkeveld, both CM2 chondrites, display a distinct 0.90 lm feature attributed to phyllosilicates. Fig. 5 shows the spectrum of the CM2 chondrite, Cold Bokkeveld, and the continuum-removed spectrum of 2005 YY93. The low albedo coupled with a weak feature at 0.90 lm suggests that CM2 chondrites are possible analogs for 2005 YY93. 5.11. 2007 DS84 The near-infrared spectrum (Fig. 3K) of 2007 DS84 shows a weak absorption feature (band depth 8 ± 1%) with a band center of 0.96 ± 0.01 lm. The rise in flux beyond 2.2 lm appears to be thermal emission. Assuming a straight-line continuum, the

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Fig. 5. The near-infrared spectrum of 2005 YY93 obtained using the SpecX instrument on NASA IRTF and laboratory spectrum of CM2 carbonaceous chondrite meteorite Cold Bokkeveld.

estimated thermal excess at 2.4 lm is 9 ± 1%, which corresponds to an albedo of 5 ± 1% (Fig. 4B). The presence of a 0.9 lm feature along with a low albedo suggests possible meteorite analogs such as the CO3 carbonaceous chondrites Warrenton and Ornans (Gaffey, 1976) and CM2 carbonaceous chondrites like Cold Bokkeveld. While spectrally 2007 DS84 looks similar to Warrenton and Ornans, the estimated asteroid albedo (5 ± 1%) is lower than CO3 meteorites (typically 7–12%). The asteroid albedo is similar to CM2 carbonaceous chondrites, which have 3–5% albedos. 5.12. 2008 QS11 The spectrum of 2008 QS11 (Fig. 3L) shows a weak feature at 1.0 lm and a steep reddish slope similar to 1992 JE. A sharp rise in flux beyond 2.0 lm is due to thermal emission. The measured thermal excess at 2.4 lm is 11þ1:5 =0:5 %, which results in an estimated albedo of 8 ± 1% (Fig. 4C). The weak absorption band has a band center of 0.98 ± 0.02-lm and band depth of 7 ± 1%. The Band I center would be inconsistent with a single phase like orthopyroxene, Type A clinopyroxene or olivine. The Band I center is more consistent with a mixture of olivine and low-Fe orthopyroxene, which lack a 2-lm feature. Using the Band I position and calibration for olivine/orthopyroxene mixtures by Cloutis et al. (1986), the estimated upper limit relative abundance for pyroxene in an orthopyroxene–olivine mixture is 25%. An independent estimate of the olivine content in the olivine–orthopyroxene mixture made using the calibration by Singer (1981) gives an olivine content of 75%, which is consistent with the earlier estimate. Due to its low albedo of 5%, possible meteorite analogs for 2008 QS11 would include black chondrites that have an albedo range 4–9% and C2 carbonaceous chondrites with an albedo range from 3% to 5% (Gaffey, 1976).

6. Limitations and conclusion Depending on their heliocentric distance, thermal emission in near-infrared spectra of low-albedo NEAs can be used to determine their albedo. Accuracy of the estimated albedo is limited by the assumptions made in the thermal model and the quality of the near-infrared spectrum. The diameter of the asteroid can be calculated using the albedo and absolute magnitude. A major limitation of the diameter estimation is the accuracy of the absolute magnitude, which is calculated using an assumed slope parameter of 0.15. Despite this, the work presented here provides ample evidence for the robustness of employing near-infrared spectra to

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determine albedo and diameter of low-albedo asteroids and constraining the lower limit albedo of high albedo asteroids. Based on this limited survey we have observed no specific trends that would help classify these NEAs in different categories based on albedo or composition. Comparison of the results obtained by the method used here (Table 2) with other techniques like thermal radiometry (Spitzer) and radar (Arecibo) suggests that they are consistent. The lack of specific trends suggests that each object we have observed is unique and needs to be interpreted using all available constraints rather than binning them into specific categories. We also compared albedos of asteroids with average albedos of various taxonomic classes calculated by Thomas et al. (2011) in Table 2. The albedos we have obtained are broadly consistent with the ranges they have defined for each class suggesting a general correlation between albedo and taxonomic classes. It is not surprising to see a majority of the NEAs from this study belonging to C, D, and P taxonomic classes. Most of these objects have very low to moderate albedo and hence show thermal excess in the near-infrared wavelengths. Apart from C, D, and P type asteroids, E and M types are also present in our sample, which have moderate to high albedos. Acknowledgments This research was supported by NASA NEOO Program Grant NNX07AL29G, and NASA Planetary Geology and Geophysics Grant NNX07AP73G. We thank the IRTF TAC for awarding time to this project, and to the IRTF TOs and MKSS staff for their support. References Abell, P.A., 2003, Near-IR Reflectance Spectroscopy of Main Belt and Near-Earth Objects: A Study of Their Composition, Meteorite Affinities and Source Regions. PhD Dissertation, Rensselaer Polytechnic Institute, Troy, NY. Bell, J.F., Owensby, P.D., Hawke, B.R., Gaffey, M.J., 1988. The 52-color asteroid survey: Final results and interpretation. Lunar Planet. Sci. Conf. XIX, 57–58. Benner, L.A.M. et al., 2001. Recent radar observations of four near-Earth Asteroids 2000 EH 26, 2000 YA, 4183, 1999 JM8. Bull. Am. Astron. Soc. 33, 918. Benner, L.A.M., Ostro, S.J., Magri, C., Nolan, M.C., Howell, E.S., Giorgini, J.D., Jurgens, R.F., Margot, J.-L., Taylor, P.A., Busch, M.W., Shepard, M.K., 2008. Near-Earth asteroid surface roughness depends on compositional class. Icarus 198 (2), 294– 304. Betzler, S.A., Novaes, A.B., 2008. Colors of potentially hazardous Asteroids 2005 WJ56 and 2007 TU24. Minor Planet Bull. 35, 108–109. Binzel, R.P., Harris, A.W., Bus, S.J., Burbine, T.H., 2001. Spectral properties of nearEarth objects: Palomar and IRTF results of 48 objects including spacecraft targets (9969) Braille and (10302) 1989 ML. Icarus 151, 139–149. Binzel, R.P., Lupishko, D.F., Di Martino, M., Whiteley, R.J., Hahn, G.J., 2002. Physical properties of near-Earth objects. In: Bottke, W.F., Cellino, A., Paolicchi, P., Binzel, R.P. (Eds.), Asteroids III. The University of Arizona Press, pp. 255–271. Bowell, E., Hapke, B., Domingue, D., Lumme, K., Peltoniemi, J., Harris, A.W., 1989. Application of photometric models to asteroids. In: Binzel, R.P., Gehrels, T., Matthews, M.S. (Eds.), Asteroids II. The University of Arizona Press, pp. 524– 556. Bus, S.J., Binzel, R.P., 2002. Phase II of the small main-belt asteroid spectrographic survey: A feature-based taxonomy. Icarus 158, 106–145. Campbell, D.B., Carter, L.M., Campbell, B.A., Stacy, N.J., 2005. Planetary surface properties from radar polarimetric observations. In: Workshop on Radar Investigations of Planetary and Terrestrial Environments, Houston, Texas, February 7–10, 2005. Abstract No. 6026. Chapman, C.R., Morrison, D., Zellner, B., 1975. Surface properties of asteroids: A synthesis of polarimetry, radiometry, and spectrophotometry. Icarus 25, 104– 130. Chapman, C.R., Salisbury, J.W., 1973. Comparisons of meteorite and asteroid spectral reflectivities. Icarus 19, 507–522. Cloutis, E.A., Gaffey, M.J., 1991. Pyroxene spectroscopy revisited: Spectralcompositional correlations and relationship to geothermometry. J. Geophys. Res. (Planets) 96, 22809–22826. Cloutis, E.A., Gaffey, M.J., Jackowski, T.L., Reed, K.L., 1986. Calibrations of phase abundance, composition and particle size distribution for olivine– orthopyroxene mixtures from reflectance spectra. J. Geophys. Res. 91 (B11), 11641–11653. Cushing, M.C., Vacca, W.D., Rayner, J.T., 2004. Spextool: A spectral extraction package for SpeX, a 0.8–5.5 lm cross-dispersed spectrograph. Publ. Astron. Soc. Pacific 116 (818), 362–376.

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