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Itokawa Dust Particles: A Direct Link Between S-Type Asteroids and Ordinary Chondrites

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Science  26 Aug 2011:
Vol. 333, Issue 6046, pp. 1113-1116
DOI: 10.1126/science.1207758

Abstract

The Hayabusa spacecraft successfully recovered dust particles from the surface of near-Earth asteroid 25143 Itokawa. Synchrotron-radiation x-ray diffraction and transmission and scanning electron microscope analyses indicate that the mineralogy and mineral chemistry of the Itokawa dust particles are identical to those of thermally metamorphosed LL chondrites, consistent with spectroscopic observations made from Earth and by the Hayabusa spacecraft. Our results directly demonstrate that ordinary chondrites, the most abundant meteorites found on Earth, come from S-type asteroids. Mineral chemistry indicates that the majority of regolith surface particles suffered long-term thermal annealing and subsequent impact shock, suggesting that Itokawa is an asteroid made of reassembled pieces of the interior portions of a once larger asteroid.

The Hayabusa spacecraft arrived at S(IV)-type asteroid 25143 Itokawa (formerly 1998 SF36) in September 2005 (1). Remote-sensing measurements from the spacecraft suggest that Itokawa consists of rocks similar to LL5 and LL6 ordinary chondrites (2, 3), confirming ground-based spectral characterization (4). On 20 and 26 November 2005, the spacecraft descended to touchdown and capture dust particles from MUSES-C Regio. This area consists of dust and gravel deposits dominated by grains up to 1 cm in diameter (5). Although the sampler did not operate as planned, an elastic sampling horn impacted onto the asteroid surface, directing dust particles into the spacecraft’s sample catcher device (5). The Hayabusa sample capsule successfully landed in the Woomera Prohibited Area in South Australia on 13 June 2010. Dust particles collected at the second touchdown were recovered by two methods. In one method, we used a Teflon spatula to sweep particles from about 10% of the surface of a sample catcher. In the other method, we gently tapped on the exterior of the sample catcher, causing particles to drop onto a pure silica glass slide (6).

On the Teflon spatula, we identified 1534 rocky particles by means of a field-emission scanning electron microscope. The particles have diameters ranging from 3 to 40 μm but are mostly smaller than 10 μm (7). Most Itokawa particles are angular and are probably broken pieces of larger rocks. Among the 1534 harvested rocky particles, 1087 are monomineralic, including 580 olivine particles, 126 low-Ca pyroxenes, 56 high-Ca pyroxenes, 186 feldspars (172 plagioclase and 14 K-feldspar), 113 troilites, 13 chromites, 10 Ca phosphates, and 3 Fe-Ni metal grains. The remaining 447 particles are polymineralic mixtures, mainly silicates. Several other particles are silica minerals and K-bearing halite, all of uncertain origin.

Of the 40 particles removed by tapping (diameters ranging from 30 to 180 μm) that were analyzed by x-ray computed microtomography (7) and x-ray diffraction, 38 were subjected to more detailed mineralogic analysis. Backscattered electron images of selected particles are shown in Fig. 1, A to D. RA-QD02-0030 (Fig. 1A) and RA-QD02-0024 (Fig. 1B) have a platy morphology, are polymineralic, and have many mineral grains 1 to 10 μm in diameter adhering to their surfaces. Their appearance is typical of most Itokawa particles. Two particles show different morphologies. RA-QD02-0013 (Fig. 1C) has a smoother soccer-ball shape, whereas RA-QD02-0027 (Fig. 1D) consists of a large troilite crystal and smaller silicates. Particles that contain troilite or taenite as major components like RA-QD02-0027 are rare.

Fig. 1

(A to D) Backscattered electron (BSE) images of RA-QD02-0030 (A), RA-QD02-0024 (B), RA-QD02-0013 (C), and RA-QD02-0027 (D).

Mineralogical analysis of individual “tapped” particles indicates that they consist mainly of coarse [typically 10 to 50 μm in diameter (7)] crystalline silicates, the most abundant being olivine. The next most abundant minerals are low- and high-Ca pyroxene and plagioclase (fig. S6A). Low-Ca pyroxene is exclusively composed of orthopyroxene, except for RA-QD02-0060, which is dominated by low-Ca clinopyroxene (monoclinic structure was confirmed by x-ray diffraction). The degree of crystallinity of silicates differs between and within particles, particularly for plagioclase. Some particles contain chromite, chlorapatite, merrillite, and troilite up to 25 μm in size. Small inclusions (up to 10 μm) of taenite, kamacite, troilite, and chromite occur within coarse silicates in many particles. The occurrence of triple junctions at the boundary between coarse silicates suggests that most particles have been thermally annealed. Internal, frequently elongate, pores in silicates defining curved planes are interpreted as partially healed, impact-generated fractures (7).

Silicates in 38 of the “tapped” Itokawa particles show a limited compositional range. Of these, olivine was found in 29, with an average compositional range of Fa28.6±1.1 (Fig. 2, A and B). Low-Ca pyroxene (present in 15 particles) is Fs23.1±2.2Wo1.8±1.7 (Fig. 2, A and C); high-Ca pyroxene (present in 14 particles) is Fs8.9±1.6Wo43.5±4.5 (Fig. 2C); and plagioclase (present in 23 particles) is Ab83.9±1.3Or5.5±1.2. FeNi metallic inclusions within silicates show a narrow compositional range: Ni and Co concentrations for kamacite are 3.8 to 4.2 weight percent (wt%) and 9.4 to 9.9 wt%, respectively, and those for taenite are 42 to 52 wt% and 2.0 to 2.5 wt%, respectively. Compositional ranges of olivine, low-Ca pyroxene, and Co and Ni concentrations in kamacite for equilibrated members (petrologic types 4 to 6) of the main ordinary chondrite groups are as follows: H (Fa16–20, Fs14.5–18, 0.44 to 0.51 wt% Co, ~6.9 wt% Ni), L (Fa22–26, Fs19–22, 0.70 to 0.95 wt% Co, ~6.54 wt% Ni), and LL (Fa26–32, Fs22–26, 1.42 to 37.0 wt% Co, ~4.98 wt% Ni) (8, 9). Olivine, low-Ca pyroxene, and kamacite compositions of most Itokawa particles fall within the range of LL chondrites, indicating that MUSES-C Regio is covered with LL-chondrite particles.

Fig. 2

(A) Mean Fa and Fs contents of all tapped Itokawa particles in comparison with those of H, L, and LL chondrites. (B) Fa distribution of olivine crystals from 29 Itokawa particles. Approximately 10 analyses were made for each olivine crystal. (C) Pyroxene compositions from 5 poorly equilibrated particles (left) and from 18 highly equilibrated particles (right); 5 to 10 analyses were made for each pyroxene crystal. (D) Chromite compositions of three highly equilibrated particles: RA-QD02-0030, -0031, and -0047. Chromite data of meteorites are from (16).

Close examination of the mineral chemistries of these 38 Itokawa particles revealed that they belong to two different populations: six poorly equilibrated particles and 32 highly equilibrated particles. The poorly equilibrated particles contain olivine and low-Ca pyroxene with compositional ranges Fa24.4–28.9 and Fs11.2–23.8, respectively (Fig. 2, B and C); pyroxene is more heterogeneous because of slower cation diffusion (10). RA-QD02-0060 contains a large low-Ca clinopyroxene crystal with lamellar-like Fe-Mg zoning (Fig. 3A) and Mg-rich host (Fs11.3Wo0.5; table S1) with many Mg-poor lamella (~Fs20.4Wo0.6; table S1). Three particles contain mesostasis consisting of tiny diopside and troilite crystals embedded in albitic glass (Fig. 3B). The texture and composition of the mesostasis is similar to that of chondrule mesostasis, which suggests that these particles are pieces of chondrules.

Fig. 3

(A to F) Poorly equilibrated particles RA-QD02-0060 (A) and RA-QD02-0011-1 (B). Highly equilibrated particles RA-QD02-0030 (C), RA-QD02-0013 (D), RA-QD02-0024 (E), and RA-QD02-0032 (F). (A) Thin, FeO-rich light lamellae repeatedly occur in Low-Ca clinopyroxene. (B) Mesostasis glass-like portions occur between coarse silicate crystals. Inset: An EBSD map (red box) indicates that mesostasis shows no diffraction and thus it is glass. (C) A variety of minerals are included in this particle. (D) Three parallel exsolution lamella of K-feldspar sanidine occur in albite. (E) Low-Ca pyroxene consists of orthopyroxene with many stacking disorders parallel to (100). Inset: Diffraction spots show streaks (indicated by arrows) due to disturbance of the 1.8-nm repetition (indicated by white lines). (F) The dislocation density in this olivine is 1.6 × 108 cm−2, suggestive of moderate shock. (A) to (D), BSE images; (E) and (F), bright-field transmission electron microscope images. Ol, olivine; LPx, low-Ca pyroxene; LCPx, low-Ca clinopyroxene; Di, diopside; Chr, chromite; Tr, troilite; K, kamacite; Tae, taenite; Pl, plagioclase; Kfs, K feldspar; Mes, mesostasis.

The highly equilibrated Itokawa particles (Fig. 1, A to D) have narrow olivine and low-Ca pyroxene compositional ranges of Fa27.1–30.7 and Fs22.5–25.7, respectively (Fig. 2, B and C). Mesostasis is absent, and coarse-grained diopside and plagioclase (Fig. 3C)—some of which include exsolved K-feldspar (Fig. 3D)—are abundant. Major and minor element concentrations in silicates in these particles are nearly constant within and between particles. The mean compositions of olivine and low- and high-Ca pyroxene are Fa29.0±0.7, Fs24.0±0.6Wo1.4±0.3, and Fs8.6±0.8Wo44.8±0.9, respectively (table S2). Virtually constant Fs and Wo contents in both low- and high-Ca pyroxenes indicate that the highly equilibrated particles experienced intense thermal metamorphism (11); Mg, Fe, and Ca partitioning between pyroxenes has almost progressed to completion.

Ordinary chondrite meteorites exhibit a complete thermal metamorphic sequence from unequilibrated type 3, through types 4 and 5, to totally equilibrated type 6 (11). Mg-rich clinopyroxene in type-3 ordinary chondrites is progressively transformed to orthopyroxene with increasing metamorphism. Devitrified chondrule mesostasis glass (table S1), clinopyroxene, and the characteristic lamellar-like Fe-Mg zoning (like that in the poorly equilibrated Itokawa particles) is present in type 4 ordinary chondrites but absent in type 5 (12, 13). Thus, the poorly equilibrated Itokawa particles are weakly metamorphosed material, classified as petrologic type 4.

The narrow compositional variations of the highly equilibrated Itokawa pyroxenes (Fig. 2C) are similar to LL5 and LL6 chondrites (10, 14). The characteristics of high-temperature metamorphism of grades 5 and 6 (14) that we observe in highly equilibrated Itokawa particles include (i) very low CaO and Cr2O3 contents (<0.02 wt%) in olivine, (ii) predominance of low-Ca orthopyroxene over clinopyroxene (Fig. 3E), (iii) preferential occurrence of diopside and absence of augite (Fig. 2C), and (iv) presence of coarsened plagioclase (typically 20 to 50 μm in diameter; Fig. 3C) and diopside (typically 10 to 30 μm). Similarly, chromite in the highly equilibrated particles has compositions (Fig. 2D and table S3) similar to those of LL5 and LL6 chondrites (15, 16). Thus, the highly equilibrated particles formed during intense thermal metamorphism. Itokawa is a breccia of poorly equilibrated LL4 and highly equilibrated LL5 and LL6 materials. The conclusion is confirmed by independent results of a companion paper (7).

With our mineralogic data of the highly equilibrated particles, it is possible to follow three critical phases of Itokawa’s thermal evolution. The prograde peak crystallization temperature is provided by plagioclase triclinicity (6, 17, 18) from five particles: 570°C for RA-QD02-0010, 560°C for RA-QD02-0025-01, 570°C for RA-QD02-0055, 575°C for RA-QD02-0067, and 820°C for RA-QD02-0013. These plagioclases crystallized at different points as temperatures rose to a peak of 820°C, with each crystal locking in a different triclinicity (18, 19). The peak metamorphic temperature (i.e., the temperature of last equilibration) is revealed by two-pyroxene geothermometry (20). Low- and high-Ca pyroxenes with homogeneous compositions coexist in three Itokawa particles, and peak metamorphic temperatures at 0.1 MPa were calculated using QUILF 95 software: 783° ± 12°C for RA-QD02-0013, 814° ± 21°C for RA-QD02-0024, and 837° ± 10°C for RA-QD02-0010. These temperatures are slightly lower than the range reported for LL6 chondrite meteorites [875° to 945°C (19)]. To characterize the subsequent cooling period, we applied the olivine-spinel geothermometer (21) to three particles containing olivine-chromite pairs: RA-QD02-0030 (Figs. 1A and 3C), -0031, and -0047, in which yielded equilibration temperatures of 636°C, 625°C, and 595°C, respectively. These temperatures are lower than those obtained by two-pyroxene thermometry, reflecting continued Fe-Mg exchange between chromite and olivine during cooling, which occurred more quickly than Ca diffusion in pyroxene (10). The olivine-spinel geospeedometry (22) applied to mean chromite size (~20 μm) and the equilibrated temperatures and the results indicate slow cooling at a rate of ~0.5 K per 1000 years near 600°C. The combined results of these temperature estimates suggest that the highly equilibrated particles experienced a peak metamorphic temperature of ~800°C and cooled slowly to 600°C.

During thermal metamorphism in asteroids, temperatures increase with depth and thus higher petrologic-type materials form deeper than the lower types, assuming internal heating due to decay of short-lived radioisotopes such as 26Al (2325). The slow cooling of the highly equilibrated Itokawa particles suggests that they formed at considerable depth. The temperature of 800°C experienced by many Itokawa particles requires a diameter of the original asteroid larger than 20 km (25). The current size of Itokawa (0.5 × 0.3 × 0.2 km) (1) is much smaller than this required size. Therefore, the Itokawa parent S-class asteroid was originally much larger, experienced intense thermal metamorphism, and was then catastrophically disaggregated by one or more impacts into many small pieces, some of which re-accreted into the present greatly diminished, rubble-pile asteroid, consistent with previous suggestions (1). Impact processes are responsible for cracks in crystals (7), dislocations in some olivine crystals (Fig. 3F), and local variations in the degree of silicate crystallinity. These impact effects vary greatly between particles (fig. S7), which is typical of moderately shocked astromaterial corresponding shock stages up to S4 (6, 26).

MUSES-C Regio probably formed by segregation and accumulation of fine gravel into areas close to the gravitational center of Itokawa due to global-scale electrostatic grain levitation, vibration-induced granular migration, and deposition of slow moving ejecta launched from surface impacts (2729). Therefore, particles in MUSES-C Regio originally derived from diverse regions of Itokawa. Fortunately, despite the small mass of the recovered Itokawa samples, they record the critical steps in the history of this asteroid. Itokawa was classified as an S-type asteroid from terrestrial remote sensing, and it has been commonly suggested that S-type asteroids, the most abundant asteroids in the inner asteroid belt, are the parent bodies of ordinary chondrites. Our petrologic data from MUSES-C Regio confirm that Itokawa is indeed an ordinary chondrite (LL4 to LL6), thereby finally linking these asteroids and meteorites.

Supporting Online Material

www.sciencemag.org/cgi/content/full/333/6046/1113/DC1

Figs. S1 to S8

Tables S1 to S5

References (3040)

References and Notes

  1. See supporting material on Science Online.
  2. Acknowledgments: We thank the Hayabusa project team for sample return; KEK for synchrotron experiments; H. Nakano, Y. Yamazaki, K. Shimada, Y. Kakazu, T. Hashimoto, M. Konno, Y. Katsuya, and Y. Matsushita, for technical support; and J. Grossman, T. Ikeda, T. Hokada, K. Ozawa, Y. Nakamuta, and S. Wakita for helpful discussions. Supported by NASA grant 769583.07.03 (M.E.Z. and S.A.S.).
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