SPECTRAL CHARACTERISTICS OF HAYABUSA 2 NEAR-EARTH ASTEROID TARGETS 162173 1999 JU3 AND 2001 QC34

Many space probes with surface sampling included as part of their objectives focus on the near-Earth asteroids (NEAs). These efforts are driven by the easy accessibility of NEAs to spacecraft from the Earth, and the possibility of acquiring pristine samples of building-block materials at various stages of the development of the solar system. Following the visit of the spacecraft Hayabusa to NEA 25143 Itokawa in 2005, the Japanese Space Agency (JAXA) now plans to send a second space probe (Hayabusa 2) to Apollo asteroid 162173 1999 JU3 (Abe et al. 2007). The objective of JAXA is to sample an asteroid having an unaltered or barely altered composition (C or D class). Asteroid 162173 1999 JU3 was chosen both for its easy accessibility from the Earth (a low ΔV of 3.238 km s⁻¹, Abe et al. 2007) and its spectral class. In addition, Apollo asteroid 2001 QC34 was identified as a possible target for Hayabusa 2. As such, 2001 QC34 was observed spectrally for the first time on UT 2007 September 11.

To date, the only published asteroid spectrum or multicolor photometry of 162173 1999 JU3 is the narrowband visible–near-infrared (VNIR) reflectance spectrum taken by Binzel and coworkers during the asteroid's 1999 discovery apparition (Binzel et al. 2002). This spectrum was classed Cg as part of the SMASSII VNIR asteroid classification program (Binzel et al. 2002). As defined by Bus & Binzel (2002a), asteroids of class Cg are part of the C complex, and show a pronounced UV/blue intervalence charge-transfer (IVCT) transition absorption starting near an upper wavelength of 0.55 μm, a relatively flat spectrum across the 0.55 to 0.9 μm wavelength range, and occasionally a small amount of absorption beginning near 0.9 μm. Figure 1 contains examples of sample Cg asteroid spectra from the SMASSII collection (Bus & Binzel 2002b), including the 1999 spectrum of 162173 1999 JU3.

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Very little observational data on 162173 1999 JU3 have yet been obtained beyond the discovery observations of its orbital properties and the spectrum from 1999. We have no currently published information about the rotational period or albedo. Applying the H, G formulae to determine a size from absolute magnitude, and assuming a mean albedo of 0.06 for a C-class asteroid, Binzel et al. (2002) calculate a diameter of 0.7 km.

In 2007, NEA 162173 1999 JU3 again approached the Earth, reaching opposition during early September at a relative magnitude V ∼ 18. The asteroid was observed in order to characterize it further in support of the Japanese Hayabusa 2 mission. The NEA 2001 QC34, a potential alternative target for Hayabusa 2, was also observed as a target of opportunity during September.

The MMT 6.5 m telescope was used with the facility Red Channel spectrograph, coupled with a new deep-depletion charge-coupled device (CCD), to obtain VNIR reflectance spectra of NEA 162173 1999 JU3 on the three nights of UT 2007 July 11 and September 10 and 11. Reflectance spectra of NEA 2001 QC34 were also obtained on 2007 September 11. Table 1 contains information about the observations. For all three nights, the sky was clear but probably not photometric, with atmospheric water content varying within each night. The Landolt photometric standard stars SA 114-654 and, on September 11 only, SA 113-276 (Landolt 1973; Skiff 1998) were observed across the same (or close) airmass range as the asteroids in order to provide extinction correction and solar spectral correction for the same area of sky. The spectra were obtained with a 1 arcsec wide, 150 arcsec long slit, providing a spectral resolution of 20 Å. The spectra obtained in July cover the 0.5–0.9 μm wavelength range, limited by the poorer signal-to-noise ratio (S/N) beyond those wavelengths. The spectra obtained in September cover a wavelength range of 0.42 to 0.93 μm. Data reduction followed the description of Vilas & Smith (1985) and Massey et al. (1992).

Figure 2 shows the 2007 July spectrum and the composite 2007 September spectrum, as well as the SMASSII reflectance spectrum, from 1999 (Binzel et al. 2002), of 162173 1999 JU3. A median filter of three spectra of 162173 1999 JU3 obtained on the nights of September 10 and 11, all covering the same wavelength range, produced the composite September spectrum. The peak-to-peak scatter among points in an individual spectrum is here considered the most conservative estimate of the error in that spectrum (see Vilas & Smith 1985). Thus, the September spectrum should be considered as the highest-quality spectrum presented here. The fainter magnitude of 162173 1999 JU3 during the July observations represents a source of uncertainty that should be considered when interpreting this spectrum. The significant differences among the three spectra suggest, however, that surface mineralogical differences are exposed by the individual spectra. The spectra are therefore discussed in this context.

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The 2007 July spectrum shows an absorption feature centered near 0.7 μm similar to those absorption features commonly seen in the spectra of C-type asteroids (Ch and Cgh, Bus & Binzel 2002a; about 50% C-class and C-subclasses B, F, G, Tholen 1989). This absorption feature has been attributed to an Fe²⁺ → Fe³⁺ charge-transfer transition in oxidized iron in phyllosilicates (Vilas & Gaffey 1989). In earlier studies, Vilas et al. (1998a) demonstrated that the 0.7 μm feature can be identified in spectra with a minimal S/N of 10. Thus, the presence of the 0.7 μm feature in the July spectrum is likely real. The presence of the 0.7 μm feature in the reflectance spectra of NEAs is rare (Vilas 2005; Binzel et al. 2002). Likewise, spectral data of NEAs pointing to aqueous alteration based on different spectral characteristics in the NIR are also rare (Abell et al. 2002; Volquardsen et al. 2007).

In Figure 3, the July spectrum of 162173 1999 JU3 is compared to C-class asteroid 407 Arachne, CM2 carbonaceous chondrite ALH 82001, and terrestrial phyllosilicate antigorite spectra, all showing this absorption feature. A linear continuum has been removed in order to intercompare the feature near 0.7 μm more clearly among all of these spectra. Among the main-belt C-type asteroid spectra that show the 0.7 μm absorption feature, the lower-wavelength edge of the feature occurs near 0.57 μm and the upper-wavelength edge occurs near 0.83 μm. The depth of the absorption feature in the 1999 JU3 spectrum is greater than that observed in the spectra of the main-belt asteroids and carbonaceous chondrite meteorites, but comparable to the VNIR spectra of the terrestrial samples.

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Compared to the July spectrum, the composite September spectrum has a much improved S/N. Figure 4 shows the September spectrum compared to the July spectrum and the C-class asteroid 407 Arachne. A linear continuum has been removed in order to delineate any absorption features in the spectra. A shallow absorption feature ranging in width from 0.48 to 0.74 μm, centered near ∼0.6 μm, is present. Shallow absorption features have been observed in visible asteroid spectra (e.g., Vilas et al. 1994; Thibault et al. 1995; Bus & Binzel 2002b). Some have been associated with asteroids that have spectra showing the 0.7 μm and the 3.0 μm absorption features, sometimes varying temporally with the rotation period of the asteroid. No mineralogical explanation is offered here for this absorption feature.

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The action of aqueous alteration (the alteration of material by the interaction of that material with liquid formed by melting of incorporated ice) of near-subsurface material has been inferred for many asteroids based on the spectrophotometric evidence of phyllosilicates and iron alteration minerals (see Rivkin et al. 2002 for a review). The definitive indication of aqueous alteration in asteroid spectra is the 3.0 μm absorption feature attributed to structural hydroxyl (OH) and interlayer and adsorbed water (H₂O) in phyllosilicates (clays) (see Lebofsky 1978, 1980). This absorption feature has been observed in NIR photometry of many C-class asteroids.

The presence of the absorption feature centered near 0.7 μm has been observed in reflectance spectra of 50–60% of the main-belt C-class asteroids (see Sawyer 1991; Vilas 1994; Bus & Binzel 2002a; Carvano et al. 2003). The relationship between this feature and the 3.0 μm absorption feature has been explored. Vilas (1994) showed an 85% correlation between both features. Using concurrent observations in the visible and NIR spectral regions, Howell and Rivkin have shown that the presence of the 0.7 μm absorption feature always indicates the presence of the 3.0 μm absorption feature, although the converse is not always true (Howell et al. 1999; Rivkin et al. 2002).

Spatial distribution of the mineralogy governing the 0.7 μm feature across an asteroid's surface, as indicated by the varying presence and absence of the feature in the VNIR spectra of the same asteroid, has also been observed among main-belt C-class asteroids (Thibault et al. 1995; Howell et al. 2001; Rivkin et al. 2002). Where a rotational period is available for an asteroid, the presence/absence of the 0.7 μm absorption feature has been tied to different sides of the asteroid (see Rivkin et al. 2002). Surface variegation is observed among main-belt asteroids having the SMASS Cgh classification: main-belt asteroid 776 Berbericia is cited as an example of the Cgh class, exhibiting a strong UV/blue IVCT and a strong 0.7 μm absorption feature (Figure 4) (Bus & Binzel 2002a). An earlier spectrum of 776 Berbericia shows a sharp UV/blue IVCT, however, no 0.7 μm absorption feature (Figure 5) (Vilas et al. 1998b). These two spectra demonstrate that 776 Berbericia shows a variegated surface composition.

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Taken collectively, the spectra of 162173 1999 JU3 show that this NEA has a variegated surface composition. One difference here is in the depth of the 0.7 μm absorption feature in the July spectrum compared to the spectra of main-belt C-class asteroids. Coupled with the significantly smaller size of the asteroid, this suggests that 162173 1999 JU3 represents an intermediate object between the main-belt C-class objects (having spectra showing the shallower 0.7 μm absorption feature) and the CM2 carbonaceous chondrites (having reflectance spectra with a deeper 0.7 μm absorption feature).

What do these differences imply for the composition of 162173 1999 JU3?

First, any interpretation must take into account the fact that the asteroid observations are predominantly being made of a point source; any spatial resolution of the asteroid's surface is due to different sub-Earth points on the asteroid being exposed through the rotational period of the asteroid. During an observation, the contribution of light from the surface area forming a cone described by a ≤45° angular separation around the sub-Earth point dominates the spectral signature. The asteroid spectrum will represent the collective spectral attributes of coexisting materials in that surface area. Thus, differences due to materials of different compositions could be observed in the single spectrum of a much larger asteroid. In the larger main-belt asteroid population, variations in spectral properties across the surfaces are observed (see Thibault et al. 1995; Howell et al. 2001).

In the NEA population, the asteroids are of a much smaller size. Thus, an asteroid could be a coherent fragment of a parent body created as part of a catastrophic event. Alternatively, it could be smaller pieces of a parent body that were created in a catastrophic event and then gravitationally re-accreted to form a new asteroid. In the case of 162173 1999 JU3, the structure of the asteroid must allow for spatial variations in surface mineralogy. A coherent fragment of a parent body could have sampled a contact between two different geological units of the original parent body. Likewise, two smaller bodies of re-accreted material having different mineralogical compositions could have joined together to form one NEA. Under this scenario, however, the re-accreted material is likely to have mixed due to shifting caused by gravitational settling, removing spectral evidence of variation in mineralogical composition.

I propose that 162173 1999 JU3 will likely be the coherent fragment of a larger parent body, and that we are sampling different geological units of the parent body. Two effects could cause the difference in strength of the spectral feature near 0.7 μm compared to other main-belt C-class asteroids showing this feature. Spectra of the larger main-belt C-type asteroids include a larger amount of surface area of the asteroid. Thus, they probably indicate the presence of aqueously altered material having spectra that show this feature coupled with an unknown and diverse variety of other surface materials. Since the C-type asteroids generally have lower albedos, darkening agents in the surface materials combined with materials that do not have the 0.7 μm feature as part of their spectral properties could reduce the spectral depth of the 0.7 μm absorption feature. In contrast, 162173 1999 JU3 is a much smaller asteroid. In July, its spectrum sampled primarily the aqueously altered material showing the 0.7 μm feature in its spectrum. The spectral properties of the material are therefore closer to those of isolated phyllosilicates found in the terrestrial samples of phyllosilicates, and phyllosilicate combinations found in the CM2 carbonaceous chondrites. In September, its spectrum sampled material indicating another mineral that was plausibly created by the action of aqueous alteration. In 1999, its spectrum sampled material not showing the effects of aqueous alteration. Consequently, JAXA could sample a very interesting, primitive asteroid.

Figure 6 shows the relative reflectance spectrum of NEA 2001 QC34 obtained on 2007 September 11. The reflectance spectrum shows a clear, deep mafic silicate absorption beginning near 0.73 μm. A moderately steep absorption due to the ferric iron UV/blue IVCT is observed from 0.42 to 0.55 μm, continuing more shallowly from 0.55 to 0.73 μm. The peak reflectance value at ∼0.73 μm is estimated to be 1.07 (taken here as the mid-range of the reflectance values around 0.73 μm). Based on the context of different classes provided by Bus & Binzel (2002a), this asteroid is closest in properties to the Q-class and the O-class asteroids. Both classes are found only in the NEA population (Bus & Binzel, 2002a). This asteroid does not represent the type of object sought by JAXA for their Hayabusa 2 mission.

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New reflectance spectra of NEA 162173 1999 JU3 show both an absorption feature centered near 0.7 μm that has been previously observed in main-belt and outer-belt reflectance spectra of the C-type asteroids, and possibly a shallow absorption feature centered near 0.6 μm of undetermined origin. The 0.7 μm absorption feature is attributed to an Fe²⁺ → Fe³⁺ charge-transfer transition in oxidized iron in phyllosilicates. Combined with a reflectance spectrum obtained of the asteroid during its discovery apparition (Binzel et al. 2002), 162173 1999 JU3 shows a variation in spectral properties (Figure 2). This suggests a variation of surface properties across the asteroid's surface, similar to variations observed in main-belt C-type asteroid spectra (see Rivkin et al. 2002). The strength of the absorption feature suggests that the material contributing to the spectrum is largely concentrated iron-bearing phyllosilicates. Reflectance spectra of NEA 2001 QC34 show a mafic silicate absorption feature, and are similar to the Q-class asteroids observed only among the NEA population. Near-Earth asteroid 162173 1999 JU3 would be a very interesting primitive target for Hayabusa 2.

I thank Bobby Bus for his review and discussions that have made this a stronger paper. These observations were made at the MMT Observatory, a joint facility operated by Smithsonian Astrophysical Observatory and the University of Arizona.