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Incipient Space Weathering Observed on the Surface of Itokawa Dust Particles

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Science  26 Aug 2011:
Vol. 333, Issue 6046, pp. 1121-1125
DOI: 10.1126/science.1207794

Abstract

The reflectance spectra of the most abundant meteorites, ordinary chondrites, are different from those of the abundant S-type (mnemonic for siliceous) asteroids. This discrepancy has been thought to be due to space weathering, which is an alteration of the surfaces of airless bodies exposed to the space environment. Here we report evidence of space weathering on particles returned from the S-type asteroid 25143 Itokawa by the Hayabusa spacecraft. Surface modification was found in 5 out of 10 particles, which varies depending on mineral species. Sulfur-bearing Fe-rich nanoparticles exist in a thin (5 to 15 nanometers) surface layer on olivine, low-Ca pyroxene, and plagioclase, which is suggestive of vapor deposition. Sulfur-free Fe-rich nanoparticles exist deeper inside (<60 nanometers) ferromagnesian silicates. Their texture suggests formation by metamictization and in situ reduction of Fe2+.

The surfaces of airless bodies exposed to interplanetary space gradually have their structures, optical properties, chemical compositions, and mineralogy changed by solar wind implantation and sputtering, irradiation by galactic and solar cosmic rays, and micrometeorite bombardment. These alteration processes and the resultant optical changes are known as space weathering (13). Our knowledge of it depends almost entirely on studies of the surface materials returned from the Moon (1, 4), with the addition of recent analyses of regolith breccia meteorites (5). Space weathering reduces (darkens) the albedo of lunar soil and regolith (6), steepens the slopes of their reflectance spectra (reddening), and attenuates the characteristic absorption bands of their reflectance spectra (1, 7). It is caused by vapor deposition of small (<40 nm) metallic Fe nanoparticles within the grain rims of lunar soils (5, 711) and agglutinates (12).

Spectra of near-Earth asteroids suggest that ordinary chondrite-like spectra change to S-type (mnemonic for siliceous) ones by space weathering (13, 14). The Galileo spacecraft observed ongoing space weathering on the S-type asteroids Ida and its satellite Dactyl (15). Laboratory studies apparently succeeded in simulating space weathering observed on asteroids (2). Laboratory and astronomical studies indicate that asteroid spectra are modified in their early history by solar wind irradiation operating on unexpectedly short time scales of 104 to 106 years, and later by micrometeoroid bombardment operating on longer time scales of 108 to 109 years (1619).

The Hayabusa spacecraft touched down at and lifted off from the smooth terrain of the MUSES-C Regio on asteroid 25143 Itokawa on 19 and 25 November 2005 UTC (20). Although pebbles (debris ranging from 4 mm to 6.4 cm in diameter) that cover the regio uniformly (20, 21) appeared to lack a distinct powdery covering, Hayabusa successfully collected submillimeter particles from the regio (22). Because no impactors were fired by the sampling mechanism (21), the collected samples are unlikely to be fragments formed during sampling and are thus likely to retain their original surfaces. In addition, Itokawa shows space-weathered S-type spectra (23, 24), and the Itokawa particles have LL chondrite-like mineralogy (22). Therefore, the particles are ideal for investigation of asteroidal space weathering.

Ten Itokawa particles (average diameter 52 μm) were embedded in epoxy resin and ultramicrotomed into ~100-nm-thick sections. We used high-angle annular dark-field scanning transmission electron microscope (HAADF–STEM) images, in which Fe-rich nanoparticles appear as bright spots (25). Figure 1, A and B, show backscattered electron images of an Itokawa particle and a lunar soil grain, both of which have Fe-rich nanoparticle–bearing (npFe-bearing) rims. The npFe-bearing rims are found on five of the Itokawa particles (Fig. 1C). The thickness of the rims ranges from 30 to 60 nm, which overlaps with the typical thickness of those on lunar soil grains (9). The average grain size of the npFe’s in the Itokawa particles (~2 nm; fig. S3) is similar to that of lunar soils (~3 nm) (7, 9, 12), although the lunar soil contains larger npFe’s (5 to 10 nm across) (Figs. 1D; 3S).

Fig. 1

(A) Backscattered electron images of a space-weathered particle (RA-QD02-0035) and (B) a space-weathered lunar soil particle (15004, 194) for comparison. Abbreviations: olivine, Ol; plagioclase, Pl; high-Ca pyroxene, HPx; vesicle, V. (C) HAADF-STEM images of nanoparticle-bearing rims on the surface of olivine in RA-QD02-0041 and (D) an inclusion-rich rim on the surface of glass in 15004, 194. Because the surface of the ultrathin section in (C) is not edge-on, a slanted surface is shown. Sigmoidal cracks in (C) and parallel cracks in the inset of (D) are artifacts formed during ultramicrotomy.

Cross sections of olivine, low-Ca pyroxene, and plagioclase show that they have a 5- to 15-nm-thick amorphous surface layer containing a densely arranged layer of npFe’s (1 to 2 nm) within or at the bottom of the layer (zone I in Fig. 2). Below zone I, the texture of the rim depends on whether host minerals contain abundant Fe2+ or not. Ferromagnesian silicates have a npFe-rich zone with variable thickness from 20 to 50 nm (zone II in Fig. 2, A to D). In contrast to lunar npFe-bearing rims, in which npFe’s are embedded in amorphous material (5, 7, 9, 12), in Itokawa grains the host minerals in zone II show various degrees of amorphization (Fig. 3, A and B). Zone II gives way to the well-crystalline minerals in zone III. Although lattice fringes of the substrate minerals hamper the identification of npFe’s in bright-field (BF)–STEM images, HAADF-STEM images suggest that the bright npFe’s are enclosed in darker material (composed of lighter elements), which can be recognized in Fig. 2D. On the contrary, in zone II of plagioclase, no npFe’s were observed (Fig. 2, E and F). Therefore, when plagioclase is adjacent to olivine, only the olivine rim has the npFe-rich zone II (Fig. 3, C and D). Because plagioclase was rapidly damaged during analysis by the intensely focused electron beam, we could not observe the texture of amorphization of plagioclase in zone II and the boundary between zones II and III (Fig. 2, E and F).

Fig. 2

Edge-on BF-STEM and HAADF-STEM images of (A and B) olivine in RA-QD02-0041, (C and D) low-Ca pyroxene in RA-QD02-0042, and (E and F) plagioclase in RA-QD02-0042. The rims of these minerals are divided into three zones based on their texture: zone I, amorphous surface layer containing npFe; zone II, partially amorphized area; zone III, crystalline substrate minerals. Zone II of olivine and pyroxene contains abundant npFe. The boundaries between zones are indicated by dotted curves.

Fig. 3

High resolution BF-STEM images of three zones in (A) olivine and (B) low-Ca pyroxene shown in Fig. 2. They were obtained at the boundary between zones I and II, within II, and at the transitional area from zone II to III. (C and D) BF-STEM and HAADF-STEM images showing the boundary between olivine and plagioclase in RA-QD02-0042. The boundary is indicated by a dotted curve. High-resolution BF-STEM images of (E) an S-bearing npFe in zone I of RA-QD02-0042 and (F) an S-free npFe in zone II of RA-QD02-054. The former shows 0.22- to 0.23-nm lattice fringes and the latter 0.20-nm lattice fringes.

Elemental distribution maps clearly show that the zone I areas of olivine and low-Ca pyroxene are enriched in Fe, S, and Mg, and depleted in Si (Fig. 4). As seen by comparing Figs. 2 and 4, the npFe’s in zone I are enriched in S, Fe, and probably Mg. In this zone, small amounts of elements that are not included in the substrate minerals were ubiquitously detected. Sulfur, Al, Na, and K were detected on ferromagnesian silicates; Mg, Fe, and S on plagioclase; and Na, Mg, Si, K on troilite. On the other hand, abundant npFe’s (1 to 3 nm) in zone II do not contain sulfur. With the exception of zone I, the bulk chemical compositions of the rims are similar to those of their substrate minerals (Figs. 2 and 4, and fig. S4).

Fig. 4

Elemental distribution maps of nanoparticle-bearing rims on (A) olivine and (B) low-Ca pyroxene shown in Fig. 2. In both minerals, the very surface (<10 to 15 nm thick) of these minerals is enriched in S.

Sulfur-bearing npFe’s in zone I show 0.22- to 0.23-nm lattice fringes (Fig. 3E), which is not consistent with those of troilite and pyrrhotite. This discrepancy may be related to the presence of Mg accompanied by the npFe’s in zone I. On the other hand, S-free npFe’s in zone II have 0.20-nm lattice fringes, which is consistent with the spacing of (110) of α-Fe (d110 = 0.203 nm) (Fig. 2D). The surface of the rim contains npFe sulfide (npFeS), and the deep (>10 to 15 nm) interior of the rim contains npFe metal (npFe0).

Studies of lunar space weathering have suggested that abundant soil particles in the 10- to 100-μm range are required to form sufficiently large numbers of npFe0’s to affect the spectra (7). Because abundant fine-grained regolith did not exist on the MUSES-C Regio, the major lunar process operating to form abundant npFe’s on the Moon is ineffective on Itokawa. The high albedo of Itokawa (0.23), within the range of that of S-type asteroids (0.11 to 0.22) (26), suggests poor development of npFe’s. However, Itokawa is as red as large main-belt S-type asteroids (26). Because small npFe’s (<10 nm) redden the spectra, and only large ones (especially >50 nm) darken the spectra (10, 11, 27, 28), it is likely that the small npFe’s (~2 nm) in the Itokawa particles efficiently redden the spectra.

Asteroid studies indicate that their surfaces do not necessarily experience lunar-style space weathering (17, 29, 30). This is attributable to variations in surface composition, differences in micrometeoroid flux and velocities, solar wind particle flux, and the porosity and grain size of the regolith (12). Elements found in the outermost surface (zone I) can be derived from major minerals of Itokawa particles (ferromagnesian silicates, plagioclase, and troilite) (22). Zone I is likely to represent vapor recondensation products attributed to neighboring minerals. Laboratory laser irradiation of ordinary chondrites, which simulate micrometeorite bombardment, forms an S-rich surface layer (31). A thin npFeS-bearing layer could have been formed by both vapor deposition attributable to micrometeorite bombardment (31) and solar wind irradiation and sputter deposition (1). However, it cannot presently be determined which process dominates.

Below zone I (15 to 60 nm from the surface), the structure of the rims clearly depends on whether the substrate minerals contain abundant Fe2+. The texture of partially amorphized ferromagnesian silicates in zone II is similar to that of radiation-damaged zircon (32). Although radiation-induced amorphization has been proposed as the mechanism of asteroidal space weathering (30), our observations suggest that not only amorphization but also contemporaneous reduction of Fe2+ to Fe0 by deeply implanted solar wind ions are required to form npFe0’s in zone II.

Multilayered rims with structure similar to that of Itokawa ferromagnesian silicates have been identified on ilmenite in lunar soils (33). Because abundant solar wind protons that have ~1 keV/atomic mass unit kinetic energy (34) penetrate only the very thin surface to cause sputtering and deposition (1), heavy ions and/or solar flare protons may contribute to the formation of npFe0’s deeper in the rims of the lunar ilmenite (~50 nm) (33) and of zone II of the Itokawa ferromagnesian silicates (~60 nm). Noble gas study of the Itokawa particles shows that the degree of noble gas implantation is variable between particles (35), possibly related to the presence or absence of the npFe-bearing rims. In conclusion, (i) the optical properties of asteroids are altered by space weathering that produces npFe0’s and npFeS’s, and (ii) the structure of the rims on ferromagnesian silicates suggests that npFeS’s near the surfaces are vapor deposits, and npFe0’s existing deep inside the rims were formed by radiation-induced amorphization and in situ reduction of Fe attribute to solar wind irradiation.

Supporting Online Material

www.sciencemag.org/cgi/content/full/333/6046/1121/DC1

Materials and Methods

Figs. S1 to S4

References (3639)

References and Notes

  1. The surface of the Moon is covered by unconsolidated debris referred to as the lunar regolith. The fine-grained fraction of the regolith is called lunar soil. Irregularly shaped clusters composed of a mixture of lithic fragments and interstitial glass in lunar soil are called agglutinates.
  2. S-type (or S-class) asteroids are characterized by moderate albedos and reddish-sloped spectra with moderate absorption features at 1 and 2 mm, which correspond to olivine and pyroxene absorption bands. They are most abundant among both the inner asteroid belt and the near-Earth asteroids.
  3. Materials and methods are available as supporting material on Science Online.
  4. Acknowledgments: Special thanks to the Hayabusa project team for sample return. We are grateful to Y. Suzuki, S. Tsujimoto, R. Sagae, R. Hinoki, and M. Kawamoto for supporting N2 purge sample preparation at the Institute of Space and Astronautical Sciences of the Japan Aerospace Exploration Agency and Ibaraki University, and for STEM observation at Hitachi High-Technologies. Thanks to H. Hidaka, we could compare the rims of the Itokawa particles and those of space-weathered lunar soil. M. Zolensky was supported by NASA’s Muses-C/Hayabusa Program.
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