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Near-Infrared Spectral Results of Asteroid Itokawa from the Hayabusa Spacecraft

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Science  02 Jun 2006:
Vol. 312, Issue 5778, pp. 1334-1338
DOI: 10.1126/science.1125718

Abstract

The near-infrared spectrometer on board the Japanese Hayabusa spacecraft found a variation of more than 10% in albedo and absorption band depth in the surface reflectance of asteroid 25143 Itokawa. Spectral shape over the 1-micrometer absorption band indicates that the surface of this body has an olivine-rich mineral assemblage potentially similar to that of LL5 or LL6 chondrites. Diversity in the physical condition of Itokawa's surface appears to be larger than for other S-type asteroids previously explored by spacecraft, such as 433 Eros.

Visible and near-infrared spectroscopic observations (from 0.3 to 3.3 μm) have been used extensively to study the mineralogy and physical properties of asteroid surfaces. These data are compared with similar laboratory measurements of meteorites from asteroids to determine the geologic history of the asteroid regions. Because most of the asteroids have not experienced major mineralogical alteration since the formation of the solar system, study of their chemical and physical properties tells us about the earliest epochs of planet formation. Asteroid 25143 Itokawa (previously 1998 SF36) has been observed unresolved from ground-based telescopes at mineralogically diagnostic wavelengths (1, 2) and has been found to be similar to ordinary chondrite and/or primitive achondrite meteorites. Here, we report spectral observations of Itokawa at high spatial resolution and place them in the context of their geologic interpretation.

The near-infrared spectrometer (NIRS) onboard the Japanese Hayabusa spacecraft (also known as MUSES-C) obtained more than 80,000 spectra of asteroid Itokawa during mapping operations at the asteroid from September to November 2005. NIRS has a 64-channel InGaAs photodiode array detector and a grism (a diffraction grating combined with a prism). The dispersion per pixel is 23.6 nm. Spectra were collected from 0.76 to 2.1 μm (3). The NIRS field of view (0.1° × 0.1°) was aligned with the LIDAR (Light Detection and Ranging) and the AMICA (Asteroid Multiband Imaging Camera) fields of view before launch. During the cruising and mapping phases, this coalignment was verified multiple times.

The first spectrum from Itokawa was obtained by NIRS on 10 September 2005 at a distance of 50 km from the asteroid. NIRS spectra were obtained at solar phase angles ranging from near 0° to 38° and at footprint sizes ranging from 6 to 90 m2 (not including the touchdown phase of the mission). Most spectra were obtained between 7° and 11° solar phase angle during the first mapping phase of the mission in September.

During this mapping phase, while the spacecraft was 7 to 20 km from the asteroid, an equatorial scan was performed using only the rotation of the asteroid to shift the pointing of NIRS. During the second mapping phase in October 2005, a two-dimensional scan was obtained by slewing the attitude of the spacecraft in the direction of the rotational axis of the asteroid. The spacecraft hovered at a distance of 3.5 to 7 km above the asteroid during this time. The position of the spacecraft was limited to near 0° Earth phase angle during the first mapping phase, but after arriving at a distance of 7 km from the asteroid′s surface, the spacecraft was moved off the Earth-asteroid line to vary the solar phase angle and aspect angle while observations were performed.

Itokawa′s phase curve (brightness as a function of varying solar phase angle) at NIRS wavelengths can be compared to phase curves of similar asteroids at similar wavelengths. There are differences in reflectance at the same phase angle (Fig. 1A), because the incidence and emission angles within the NIRS′ footprint vary. To compare Itokawa to other asteroids, we binned the data with respect to phase angle and selected spectra with maximum reflectance within a 0.2° phase angle bin and fitted a Hapke five-parameter shadow-hiding model photometric function (4). Overplotted on the data is the fitted phase curve for asteroid 433 Eros at 950 nm (5). Itokawa appears to have an opposition surge (nonlinear brightening near zero phase) with a lower amplitude and narrower angular width than Eros. In theory, this would indicate that the average particle on Eros is more opaque than the average particle on Itokawa, and/or that Eros has a lower surface porosity than Itokawa. This is consistent with the data that so far indicate a brighter geometric albedo (at 0.55 μm) for Itokawa (∼30%) (69) than for Eros (∼25%) (5, 10). However, we hesitate to use phase curves to predict other surface characteristics (such as grain sizes) of Itokawa and Eros on the basis of the information available because of the following factors: (i) The Itokawa data have a limited phase angle range (0° to 38°), (ii) the five-parameter photometric model is not accurate to the level required, and (iii) the viewing angles of the Itokawa spectra are preliminary.

Fig. 1.

(A) Reflectance at 952 nm as a function of solar phase angle. Data shown are those observed at a distance of 4 to 11 km from the surface, at an average resolution of 12 m × 12 m. Data points in boxes denote the top of the envelope for each 0.2° phase angle bin. The top envelope is used in generating a photometric model for two reasons: It restricts the size of the data set and allows more tractable computation time in fitting five free parameters, and it selects the data with the highest signal-to-noise ratio. Use of a selection of the data is defensible if no biases are introduced. For example, if the top envelope consisted only of the lowest emission angle (or lowest incident angle) data at a given phase angle, then it would not be a representative sample of the data. We carefully checked the range in incidence angles (0° to 80°) and emission angles (0° to 80°) of the top envelope and found no evidence for restriction in these angles. The fitted phase curve of Itokawa (solid line) is plotted with that of Eros (dashed line) at similar wavelengths (5). (B) Ratio of the reflectance at 1565 nm to that at 952 nm, plotted as a function of phase angle. The fitted model curve of Itokawa (solid line) is plotted with that of Eros (dashed line) at similar wavelengths.

Itokawa is observed to spectrally redden with increasing phase angle (Fig. 1B). Phase reddening has been observed in laboratory studies of particulate media (11) and was predicted to occur on asteroids; the Near Earth Asteroid Rendezvous (NEAR) spacecraft made the first such actual observation at Eros (5, 12). The physical mechanism for the effect is still unclear [e.g., (13)]. However, it has been suggested that the single-scattering albedo, and hence multiple scattering, are more important controls of phase reddening than is the single-particle phase function (13). We find the long-wavelength reflectance to be up to 9% higher at 38° phase angle than at 0° phase angle. This is consistent with the phase reddening detected by NEAR at Eros (5).

During the mapping phase, we performed several equatorial scans. Throughout each equatorial scan, the footprint of NIRS was fixed at the image center. A comparison between the spectrum obtained near 8° phase angle and that obtained near 30° phase angle reveals a phase reddening of ∼6% across the wavelength range of 1.0 to 1.6 μm (Fig. 2). The 30° spectrum was redder than the 8° spectrum.

Fig. 2.

Equatorial averaged NIRS spectra of Itokawa observed on 21 September 2005 (red circles) and 8 October 2005 (green triangles). The distance from the asteroid is 18 km and 11 km, respectively, for these dates; the solar phase angles are 8° and 30°, respectively. The difference in error bars for each spectrum is mainly due to the difference in the number of individual spectra stacked together to produce an average spectrum for that particular date. Also plotted are two independent ground-based disk-integrated spectra [blue diamonds (1) and purple squares (2)]. All reflectance spectra are normalized at 1565 nm.

An average NIRS spectrum of Itokawa over a large range of incidence and emission angles compares well with the ground-based disk-integrated spectra (1, 2). The largest discrepancy occurs at wavelengths less than 0.9 μm. At these wavelengths, the apparent reflectance of Itokawa is lower in the NIRS spectra than in the ground-based data. In part this discrepancy is due to the difference of the viewing geometry and the footprint size. Although the discrepancy may also be partly due to calibration error, we cannot confirm this until we can compare our results to the calibrated AMICA data at 0.86 and 0.96 μm; AMICA data are not yet available for analysis.

The NIRS spectra shown in Fig. 2 are replotted in Fig. 3 with the background continuum removed in the natural logarithmic scale. The wavelength range of NIRS does not completely span the 1-μm absorption band, so we used the shortest wavelength of NIRS as one of the contact points to define the 1-μm band continuum background. Because NIRS data do not cover the entire 2-μm band, the 2-μm band continuum background is assumed to be a constant (horizontal line). After continuum removal and further adjustment to the vertical scale by a factor of 0.5, both NIRS spectra are consistent with the spectrum of an Alta′ameem (14) LL5 chondrite sample with particle sizes of <125 μm measured at 30° incidence and 0° emergence angles by the RELAB spectrometer (15). The 2-μm band center of the NIRS spectra may not match that of Alta′ameem, which suggests mineralogical differences that are not yet understood.

Fig. 3.

Natural log average reflectance spectra of Itokawa observed by NIRS on 21 September 2005 (solid circles) and 8 October 2005 (solid triangles). Background continua (broken curves) are subtracted from them and they are replotted (open circles and triangles) together with the scaled, continuum-removed spectrum of an Alta'ameem LL5 chondrite sample (solid line).

NIRS data points were calculated by deriving the relative band strength values from each of the spectra measured and obtaining the daily average and standard deviation (Fig. 4). Key wavelength positions for characterizing the 1-μm absorption bands of pyroxene and olivine are located at 0.95, 1.05, and 1.25 μm (16, 17). We compared Itokawa data points with those for Eros (18), ordinary chondrites, and some primitive achondrites on the basis of their reflectance spectra measured at RELAB (Fig. 4). In addition, we plotted data points for tricomponent mixtures of olivine, pyroxene, and plagioclase (19). Note that the continua of NIRS data may be different from those for the meteorite samples used in Fig. 4 because of their incomplete coverage of the 1-μm band. The 0.95-μm pyroxene band strength may be underestimated for this same reason. Therefore, the actual NIRS data points would probably shift to the lower left, toward the LL5 (Alta′ameem) points (Fig. 3).

Fig. 4.

Natural log absorption strengths at 1.05 and 1.25 μm relative to that at 0.95 μm of average Itokawa spectra acquired by NIRS on 21 September and 8 October 2005 in comparison with Eros (18), powder samples of ordinary chondrites, primitive achondrites [acapulcoites (Acap) and lodranites (Lod)], and orthopyroxene-olivine-plagioclase mixtures (15). This figure indicates that Itokawa has an olivine-rich mineral assemblage similar to LL5 and LL6 chondrites.

Itokawa NIRS data points appear to have an olivine-rich mineral assemblage similar to LL5 or LL6 chondrites. More specifically, this plot suggests that the olivine/(olivine + pyroxene) ratio of the average surface material of Itokawa is about 70 to 80%. This range represents the uncertainty of the estimate of the continuum based on the shortest wavelength of NIRS. In contrast, Eros is plotted closer to the L chondrite region. This result suggests that Itokawa may be more olivine-rich than Eros, which is estimated to have a surface composed of 50 to 80% olivine (16, 20, 21). Elemental investigation by the x-ray fluorescence spectrometer also supports this result (22).

Spectra of several distinct areas on Itokawa were obtained (Fig. 5, A and B): boulder-rich areas, high-albedo areas (so-called brighter areas), and the Muses Sea. The spectrum footprint within each area was verified with AMICA imaging (Fig. 5, C to E). The spectra were obtained at phase angles near 22°, incidence angles near 25°, and emission angles near 30°. Comparison of these three areas shows that the albedo and absorption band depth of the reflectance spectrum varies by more than 10% (Fig. 5, A and B). The spectral properties of these distinct areas indicate that the high-albedo areas have deeper 1-μmfeaturesthan the other two terrains. These spectral trends are inconsistent with differences resulting from photometric viewing geometry, but they may be consistent with differences in optical freshness and/or particle size. These spectral trends were confirmed in imaging and spectral comparisons obtained at 4°, 6°, and 15° phase angle.

Fig. 5.

(A) Reflectance spectra of three typical areas on Itokawa. A boulder-rich area (red), a high-albedo area (green), and Muses Sea (blue) are plotted. (B) Replotted spectra are normalized at 1565 nm. (C) The footprint for observing the boulder-rich area is denoted by a small red square at the center of the simultaneously obtained AMICA image. (D) Same as (C) for the observed high-albedo area. (E) Same as (C) for the Muses Sea area.

The boulder-rich areas have lower albedos than the other two areas and shallower 1-μm bands than the high-albedo areas. Surface roughness and/or space weathering may explain why the boulder-rich areas have a low albedo and shallow absorption features (23).

Muses Sea is one of the smooth terrains discussed in the AMICA report (23). This area shows a shallower 1-μm absorption band than the other two terrains, but is not as reddened as the high-albedo area. These spectral trends suggest that the smooth terrain may consist of a different range of particle sizes than that of the high-albedo areas (24), because powders with very large particle sizes may have bluer colors and shallower absorption band depths (25). Close-up images by AMICA show that Muses Sea is densely filled with size-sorted gravels from millimeter to centimeter scales (26). This finding supports the above interpretation, although fine particles may also exist in Muses Sea.

In conclusion, NIRS confirms that Itokawa has a spectrum characteristic of S-class asteroids with absorption bands of pyroxene and olivine. NIRS data suggest that Itokawa has an olivine-rich mineral assemblage similar to LL5 and LL6 chondrites. Given the general decrease in olivine abundance with heliocentric distance (27), the most probable source region of Itokawa is the inner part of the main belt (28). On the surface of Itokawa there are differences in the absorption band depth, color, and albedo. It is likely that such diversity is a result of combinations of different degrees of space weathering and different grain sizes (physical properties).

References and Notes

  1. Observation wavelength and pixel number of the detector are related by the following expression: Embedded Image The observational limit at the long-wavelength end was determined by the cutoff wavelength of the InGaAs photodiode array to be ∼2100 nm. The short-wavelength cutoff is 763.60 nm.
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