Irradiation History of Itokawa Regolith Material Deduced from Noble Gases in the Hayabusa Samples
Geochemical Research Center, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan. Science
(Impact Factor: 33.61).
08/2011; 333(6046):1128-31. DOI: 10.1126/science.1207785
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.
Available from: Tatsuaki Okada
- "Category 1 particles are composed only of transparent minerals, whereas category 2 particles consist of not only transparent particles but also opaque minerals (Yada et al. 2014). Particles belonging to categories 1 and 2 have been confirmed as Itokawa regolith (e.g., Ebihara et al. 2011; Nagao et al. 2011; Nakamura et al. 2011; Noguchi et al. 2011; Tsuchiyama et al. 2011; Yurimoto et al. 2011). "
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ABSTRACT: Three category 3 (organic) particles (RB-QD04-0001, RB-QD04-0047-02, and RA-QD02-0120) and so-called `white object' found in the sample container have been examined by micro-Raman and infrared (IR) spectroscopy. In addition, several artificial substances that could occur as possible contaminants and chondritic insoluble organic matter (IOM) prepared from the Murchison CM2 chondrite were analyzed. The Raman spectra of the particles show broad G-band and weak D-band. The G-band parameters plot in the disordered region and close to the artifact produced from a Viton glove after laser exposure rather than chondritic IOM. The particles were therefore originally at low maturity level, suggesting that they have not experienced strong heating and are therefore not related to the LL4-6 parent body. The IR spectra are not similar to that of chondritic IOM. Furthermore, the particles cannot be identified as some artificial carbonaceous substances, including the white object, which are the possible contaminants, examined in this investigation. Although it cannot be determined exactly whether the three category 3 particles are extraterrestrial, the limited IR and Raman results in this investigation strongly suggest their terrestrial origin. Although they could not be directly related to the artificial contaminants examined in this investigation, they may yet be reaction products from similar substances that flew on the mission. In particular, RB-QD04-0047-02 shows several infrared spectral absorption bands in common with the `white object.' This may relate to the degradation of a polyimide/polyamide resin.
- "In any case, the relative timescale difference remains the same, i.e., the particle RA-QD02-0174 has been exposed to the solar ion flux for a duration ~4 times longer than particles RA-QD02-0163 and RA-QD02-0213. The relative short residence times estimated for Itokawa's particles (Nagao et al. 2011; Keller and Berger 2014; Noguchi et al. 2014), the relatively red VIS-NIR spectra presented in this work, and the weathering timescales previously estimated through distinct laboratory experiments (e.g., Sasaki et al. 2001; Brunetto et al. 2006b) confirm that solar ion irradiation (and not micrometeorite bombardment) is the main cause for rapid asteroidal space weathering, as demonstrated by Vernazza et al. (2009). "
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ABSTRACT: Hayabusa-returned samples offer a unique perspective for understanding the link between asteroids and cosmomaterials available in the laboratory, and provide insights on the early stages of surface space weathering. This study characterizes the mineralogy and the extent of space weathering of the three Itokawa particles RA-QD02-0163, RA-QD02-0174, and RA-QD02-0213 provided by JAXA to our consortium. We report here a series of results based on nondestructive analyses through visible-near-infrared reflectance and Raman spectroscopy. Results were obtained on the raw particles, both in their original containers and deposited on diamond windows. Identification of the minerals, characterization of their elemental compositions, and measurements of their relative abundances were led through Raman spectroscopy in punctual and automatic mode. Reflectance spectra in the visible and near-IR wavelengths constrain the mineralogy of the grains and allow direct comparison with the surface of Itokawa. The spectra reflect the extent of space weathering experienced by the three particles. Particle RA-QD02-0163 consists of a heterogeneous mixture of minerals: olivine (Fo76) dominates an assemblage with both Ca-rich (En50, Wo50) and Ca-poor (En85) pyroxenes. The elemental compositions of the silicates are consistent with those previously reported for distinct Hayabusa particles. Particles RA-QD-0174 and RA-QD02-0213 are solely composed of olivine, whose chemical composition is similar to that observed in RA-QD02-0163. It has been previously shown that the S-type asteroid 25143 Itokawa is a breccia of poorly equilibrated LL4 and highly equilibrated LL5 and LL6 materials. The three particles studied here can be related to the least metamorphosed lithology (LL4) based on the high forsterite content of the olivine. Neither carbonaceous matter nor hydrated minerals were detected through Raman on the three allocated particles. The NIR-VIS reflectance (incidence = 45°, light collection at e = 0°) spectra of the three particles, in particular the 1 μm band, are consistent with the presence of both olivine and pyroxene detected via Raman. The spectra of particles RA-QD02-0163 and RA-QD02-0213 are also fully compatible with the ground-based observations of asteroid (25143) Itokawa in terms of both spectral features and slope. By contrast, particle RA-QD02-0174 has a similar 1 μm band depth but higher (redder) spectral slope than the surface of Itokawa. This probably reveals a variable extent of space weathering among the regolith particles. RA-QD02-0174 may contain a higher amount of nanophase metallic iron and nanophase FeS. Such phases are products by space weathering induced by solar wind, previously detected on other Itokawa particles.
Available from: Tatsuaki Okada
- "The preliminary examination of Hayabusa-returned samples has successfully unveiled the mineralogical, petrographic, chemical, and isotopic relationships between an S(IV)-type asteroid and ordinary LL chondrite meteorites as predicted by ground-based observation (e.g.,Binzel et al. 2001Binzel et al. , 2010) and provided the first direct evidence that meteorites originate from asteroids (Ebihara et al. 2011;Nakamura et al. 2011;Noguchi et al. 2011;Tsuchiyama et al. 2011;Yurimoto et al. 2011). The particle-size distribution and existence of rounded grains (Tsuchiyama et al. 2011) as well as the noble gas isotopic compositions (Nagao et al. 2011) have also recorded asteroid surface processes such as meteoroid impacts, possible granular flow, and solar wind irradiation on the asteroid surface, which were not observed in meteorites. Organic analyses have been also performed (Kitajima et al. 2011;Naraoka et al. 2012), but indigenous organic compounds have not been identified from the samples to date. "
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