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Irradiation History of Itokawa Regolith Material Deduced from Noble Gases in the Hayabusa Samples

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Science  26 Aug 2011:
Vol. 333, Issue 6046, pp. 1128-1131
DOI: 10.1126/science.1207785

Abstract

Noble gas isotopes were measured in three rocky grains from asteroid Itokawa to elucidate a history of irradiation from cosmic rays and solar wind on its surface. Large amounts of solar helium (He), neon (Ne), and argon (Ar) trapped in various depths in the grains were observed, which can be explained by multiple implantations of solar wind particles into the grains, combined with preferential He loss caused by frictional wear of space-weathered rims on the grains. Short residence time of less than 8 million years was implied for the grains by an estimate on cosmic-ray–produced 21Ne. Our results suggest that Itokawa is continuously losing its surface materials into space at a rate of tens of centimeters per million years. The lifetime of Itokawa should be much shorter than the age of our solar system.

The Hayabusa spacecraft arrived at asteroid 25143 Itokawa in November 2005. Itokawa is a small (535 by 294 by 209 m) (1) S-type asteroid with the appearance of a rubble pile. Global remote-sensing observations revealed that there are two geological settings, boulder-rich rough terrains and smooth terrains (24). Hayabusa carried out two touchdowns on a smooth terrain, MUSES-C Regio, and collected regolith particles disturbed by the touching down of the sampler horn (5). After the sampling, Hayabusa returned to Earth, and the sample capsule was successfully recovered on 13 June 2010. The sample container was opened at the curation facility of the Japan Aerospace Exploration Agency (JAXA), and a large number of small particles were found. Scanning electron microscope energy-dispersive x-ray analyses revealed that at least 1500 grains were of extraterrestrial origin and were definitely from asteroid Itokawa (6).

The Hayabusa samples are pristine undamaged grains collected from the surface of Itokawa and are essentially different from other extraterrestrial materials: Micrometeorites recovered on Earth have experienced frictional heating and ablation of the surface layer during passage through the atmosphere and have then suffered from exposure to terrestrial atmosphere. Even the Stardust samples from comet 81P/Wild 2 have suffered from frictional heating and decomposition in the aerogel collector (7). Although interplanetary dust particles (IDPs) recovered in the stratosphere contain high concentrations of solar noble gases and are not observed to have been heated much upon entry into Earth’s atmosphere, they have been contaminated by atmosphere. Samples returned by Hayabusa were simply recovered from the sample return capsule and handled in the clean chambers of the JAXA curation facility under vacuum or clean nitrogen gas with low concentrations of noble gases, which minimized contamination of atmospheric noble gases, oxygen, and water vapor (8).

Regolith on the surface of small asteroids is exposed to various energetic particles, including solar wind (SW), solar cosmic rays (SCRs), and galactic cosmic rays (GCRs). SW (with energy of ~keV/nucleon), which includes noble gases, is implanted within 1 μm of the grain surfaces. SCRs are composed of more energetic solar particles (1 to 100 MeV/nucleon), and GCRs have even higher energies of >0.1 GeV. The high-energy protons from SCRs penetrate several centimeters, whereas GCRs penetrate up to 1 m or more beneath the surface. Nuclear reactions caused by these cosmic rays can produce noble gases with characteristic isotopic compositions that are different from those associated with SW. Unlike the regolith breccia represented by meteorites and on the Moon, which formed on relatively large parent bodies, the regolith on Itokawa, which is small and thus has little gravity, is unconsolidated. Here, we use noble gas concentrations and isotope ratios in the Hayabusa samples to assess the exposure histories and regolith processes on this microgravity asteroid.

We measured three grains: RA-QD02-0015 (#0015), RA-QD02-0053 (#0053), and RA-QD02-0065 (#0065). These grains are blocky and translucent olivine with dimensions of ~40, 40, and 60 μm (6), and their estimated masses are 0.06, 0.06, and 0.2 μg for #0015, #0053, and #0065, respectively [see supporting online material (SOM)]. Noble gases were extracted from each grain by means of stepwise heating at 200° and 300°C and finally melted for complete extraction (CE) using a yttrium-aluminum-garnet–Nd laser. The extracted gases were measured with a modified-VG5400(MS-III) at the Geochemical Research Center, University of Tokyo.

Signals from the extracted heavy noble gas isotopes, such as 40Ar, 84Kr, and 132Xe, are indistinguishable from the blank levels because of the small amounts of gases released. This is reasonable in that the concentrations of these trapped and radiogenic gases should be low in chondritic olivine. In contrast, released amounts of 4He, 20Ne, and 36Ar from the samples are clearly distinguished from the respective blank levels. Isotopic ratios of Ne obtained (table S1) are close to the SW composition (9) as depicted in Fig. 1. Argon isotopic ratios, 38Ar/36Ar, also are close to that of SW (table S1). 3He/4He ratios (0.00035 to 0.00038) are similar to those observed for IDPs and Antarctic micrometeorites (1012) and lunar soils (1315) but lower than the Genesis-SW value of 0.000464 (9) and also below the lowest value of ~0.00041 in a bulk metallic glass (BMG) target measured by closed system stepwise etching (CSSE) (16). The concentrations of 4He of the Hayabusa samples are (1.8 to 3.5) × 10−2 cm3STP/g (table S1), which are as high as those for IDPs and olivines in lunar soils (Fig. 2). These results indicate that the three grains had been exposed directly to corpuscular SW particles on the surface of Itokawa. Gas release profiles of 4He, 20Ne, and 36Ar, and respective 3He/4He, 20Ne/22Ne, and 40Ar/36Ar ratios (Fig. 3) are different among the samples: Most of He, Ne, and Ar in #0015 was released at the last CE step; in #0053, most of the He was released at 200° and 300°C, but 36Ar was released at the CE step; in #0065, 4He, 20Ne, and 36Ar were released at 200°C. All the isotopic ratios at the peak releases are similar to those of SW except for 40Ar/36Ar ratios, for which exact correction for contaminating atmospheric 40Ar (40Ar/36Ar = 296) was difficult. The resulting values as bulk 40Ar/36Ar ratios for each sample (3 to 22) (table S1) are much higher than those for the SW (~10−4), albeit with large experimental uncertainties.

Fig. 1

Isotopic ratios of Ne released from the three Hayabusa samples by stepped laser heating. Bulk compositions of the three samples plot close to the SW values. The inset shows data of each temperature step of the samples varying between SW and fractionated SW (FS), but not being shifted toward cosmogenic Ne produced via nuclear reaction by cosmic rays. CE, complete noble gas extraction.

Fig. 2

Elemental ratios of 4He/20Ne and 4He concentrations for the Hayabusa samples, along with those for olivine grains in lunar soil with dimensions of 300 to 700 μm (13) and 150 to 200 μm (14, 15), IDPs of 8 to 40 μm (10), unmelted Antarctic micrometeorites (AMMs) of ~100 μm (11, 12), and cosmic spherules of 50 to 250 μm (26). The 4He concentrations in the Hayabusa samples are as high as those for lunar soils and IDPs.

Fig. 3

Release profiles of 4He, 20Ne, and 36Ar through stepped laser heating extractions, together with respective 3He/4He, 20Ne/22Ne, and 40Ar/36Ar. The release profiles are different among the samples, although isotopic ratios of He and Ne in the three samples are similar to those of SW.

The 4He/20Ne ratios depicted in Fig. 4 (110−170) (table S1) for the Hayabusa samples are distinctly lower than those of Genesis SW value (656) (9), whereas the 36Ar/20Ne ratios are close to the SW value (0.024) (9). Most measurements from gas-rich meteorites (17) plot between the Hayabusa and SW data, suggesting that there is a common mechanism for He depletion in regolith materials on the asteroidal surface. A similar noble gas composition was found in the Genesis BMG target, where 4He/20Ne ratios decreased to 100 to 400 after ~30 nm was removed from the target surface (16). At deeper levels, the He and Ne isotopic ratios, as well as the elemental ratio 20Ne/36Ar, were almost constant. Most He was found at shallow levels in the BMG target (<30 nm), whereas Ne was retained at depths of 30 to 70 nm, with only small isotopic fractionation. The deeper layer (>70 nm) contains noble gases fractionated in respect of both elemental and isotopic ratios, which is commonly observed in lunar soils and IDPs (18, 19), but not in the case of Hayabusa samples. Thus, we infer that the noble gas compositions observed for the Hayabusa samples result from repeated SW implantation in addition to a preferential loss of SW-He, which is implanted at shallower depths. One process that may remove the rims of grains is mechanical erosion caused by friction among the grains or sputtering by other SW particles. Some He is probably also lost by diffusion; this should reasonably lower the 3He/4He ratios below the SW value (9, 16). Noguchi et al. (20) describe space-weathered rims containing metallic nanoparticles in the Itokawa grain surfaces, which would be fragile and easily removed mechanically from the grain surface.

Fig. 4

Elemental ratios of 36Ar/20Ne and 4He/20Ne for Hayabusa samples and gas-rich meteorites. The 4He/20Ne ratios for the Hayabusa samples are distinctly lower than those of SW (9), whereas 36Ar/20Ne ratios keep the SW value. 4He/20Ne released from Genesis BMG by CSSE analysis with estimated depths from the surface is shown (16).

SW implantation occurs only when a grain is exposed directly to SW on Itokawa’s surface, because the low velocity of SW particles (mostly 300 to 800 km/s) results in a penetration depth of 20 to 100 nm in olivine (21, 22). We estimated the time needed for 20Ne to accumulate to the observed concentration by assuming that the irradiation occurred in a single stage and ignoring erosion of grain surface and backscatter effects of Ne ions (SOM). The resulting time spans are 410, 150, and 550 years for #0015, #0053, and #0065, respectively. The estimated time spans, however, should be lower limits of total periods for SW implantation, and the actual SW irradiation ages might be several times longer because the release profiles of noble gases among the samples (Fig. 4) indicate rather complex SW implantation and loss histories for these grains.

The more energetic particles (i.e., SCRs and GCRs) can reach much deeper levels in the asteroid and can produce cosmogenic nuclides through nuclear reactions (table S2). Neon data for the Hayabusa samples, however, do not show a noticeable contribution of the cosmic-ray–produced 21Ne in excess of the SW-Ne isotopic ratios beyond experimental errors (Fig. 1). We estimated the upper limit of cosmic-ray exposure ages for the Hayabusa samples based on the Ne isotopic compositions (SOM). The best constraint obtained from #0065 is ≈3 million years (My) if the grain remained in the uppermost regolith layer, and at most 8 My if the grain remained several tens of centimeters below the surface. These are much shorter than the nominal exposure ages of over ~400 My for mature lunar regolith samples (21).

The cosmic-ray exposure and SW-implantation ages for the Hayabusa samples suggest that the regolith materials have been continuously escaping from the Itokawa surface at a relatively high rate, several tens of cm/My. This estimate supports observations of Itokawa surface: Only a small portion (~20%) of the Itokawa surface is covered with regolith, and where it is present the regolith is estimated to be several meters thick (23). The weak gravity on Itokawa (escape velocity ~0.2 m/s) (24) may allow its grains to easily escape.

The release profiles of the Hayabusa samples (Fig. 3) and the difference in diffusion rates of noble gases between the space-weathered rim and crystalline part (25) allow the history of each Hayabusa grain in regolith on Itokawa to be explained: Grain #0015 contained the highest amounts of 4He, 20Ne, and 36Ar among the three samples, and these gases were released at temperature (CE) higher than 300°C, which suggests that noble gases were trapped deep in the crystals. Therefore, this grain must have been exposed to SW particles for a time long enough to accumulate large amounts of solar particles. After implantation, the grain was buried in the regolith layer and shielded from SW. Solar particles that remained in the uppermost surface layer of the grain were completely lost by removal of the fragile rim during mechanical contact and by diffusion. After that, the grain was picked up by Hayabusa. For sample #0053, 4He was released at low temperatures of 200° and 300°C, mainly from the rim (25) but scarcely at melting. The profile suggests that grain #0053 appeared recently on the uppermost regolith layer and experienced SW implantation for ~150 years, thereby accumulating only a low amount of SW particles. The release pattern of #0065 reflects a combination of two histories for #0015 and #0053. At first, #0065 was exposed to SW and the uppermost rim (<30 nm) was weathered, after which the rim was eroded. The grain again experienced SW irradiation and space weathering. Consequently, the gases in the present rim consists of at least two different SW gases, one implanted during the latest irradiation and another implanted deeper (>30 nm) during the earlier irradiation.

Itokawa regolith materials have formed from surface rocks by impacts of meteoroids after its formation and then migrated to low gravitational potential areas forming smooth terrains (23). The short period for cosmic-ray exposure and SW implantation obtained for the regolith grains indicates that Itokawa is continuously shrinking by losing its surface materials into space at a loss rate of several tens of cm/My. The lifetime of Itokawa should be 100 to 1000 My.

Supporting Online Material

www.sciencemag.org/cgi/content/full/333/6046/1128/DC1

Fig. S1

Tables S1 and S2

References (2735)

References and Notes

  1. Acknowledgments: We thank the Hayabusa project team for their major effort in the success in the mission returning the samples to Earth. The National Institute for Materials Science is acknowledged for the special thin thermocouple used for the calibration of laser heating. We acknowledge constructive comments from reviewers.
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