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Kitazato, Kohei*; Milliken, R. E.*; Iwata, Takahiro*; Abe, Masanao*; Otake, Makiko*; Matsuura, Shuji*; Takagi, Yasuhiko*; Nakamura, Tomoki*; Hiroi, Takahiro*; Matsuoka, Moe*; et al.
Nature Astronomy (Internet), 5(3), p.246 - 250, 2021/03
Times Cited Count：43 Percentile：96.93(Astronomy & Astrophysics)Here we report observations of Ryugu's subsurface material by the Near-Infrared Spectrometer (NIRS3) on the Hayabusa2 spacecraft. Reflectance spectra of excavated material exhibit a hydroxyl (OH) absorption feature that is slightly stronger and peak-shifted compared with that observed for the surface, indicating that space weathering and/or radiative heating have caused subtle spectral changes in the uppermost surface. However, the strength and shape of the OH feature still suggests that the subsurface material experienced heating above 300 
C, similar to the surface. In contrast, thermophysical modeling indicates that radiative heating does not increase the temperature above 200 C at the estimated excavation depth of 1 m, even if the semimajor axis is reduced to 0.344 au. This supports the hypothesis that primary thermal alteration occurred due to radiogenic and/or impact heating on Ryugu's parent body.
Kitazato, Kohei*; Milliken, R. E.*; Iwata, Takahiro*; Abe, Masanao*; Otake, Makiko*; Matsuura, Shuji*; Arai, Takehiko*; Nakauchi, Yusuke*; Nakamura, Tomoki*; Matsuoka, Moe*; et al.
Science, 364(6437), p.272 - 275, 2019/04
Times Cited Count：259 Percentile：99.73(Multidisciplinary Sciences)The near-Earth asteroid 162173 Ryugu, the target of Hayabusa2 sample return mission, is believed to be a primitive carbonaceous object. The Near Infrared Spectrometer (NIRS3) on Hayabusa2 acquired reflectance spectra of Ryugu's surface to provide direct measurements of the surface composition and geological context for the returned samples. A weak, narrow absorption feature centered at 2.72 micron was detected across the entire observed surface, indicating that hydroxyl (OH)-bearing minerals are ubiquitous there. The intensity of the OH feature and low albedo are similar to thermally- and/or shock-metamorphosed carbonaceous chondrite meteorites. There are few variations in the OH-band position, consistent with Ryugu being a compositionally homogeneous rubble-pile object generated from impact fragments of an undifferentiated aqueously altered parent body.
Fukuda, Akari*; Hagiwara, Hiroki; Ishimura, Toyoho*; Kozuka, Mariko*; Ioka, Seiichiro*; Amano, Yuki; Tsunogai, Urumu*; Suzuki, Yohei*; Mizuno, Takashi
Microbial Ecology, 60(1), p.214 - 225, 2010/05
Times Cited Count：29 Percentile：65.76(Ecology)To better understand the geochemical and microbiological relationships, we characterized granitic groundwater collected from a 1,148 to 1,169 -m deep borehole interval at the Mizunami Underground Research Laboratory site, Japan, in 2005 and 2008. Geochemical analyses of the groundwater samples indicated that major electron acceptors, such as NO
and SO
, were not abundant, while dissolved organic carbon (not including organic acids), CH and H
were moderately rich. The most common phylotypes were both related to 
spp., the cultivated members of which can utilize minor electron donors, such as aromatic and aliphatic hydrocarbons. Geomicrobiological results suggest that deep granitic groundwater has been stably colonized by spp. probably owing to the limitation of O, NO and organic acids.
Amano, Yuki; Iwatsuki, Teruki; Ioka, Seiichiro*; Sasao, Eiji
Genshiryoku Bakkuendo Kenkyu, 14(1), p.61 - 67, 2007/12
How would microbial production of reduced compounds contribute to keep the repository host rock reduced? Japan Atomic Energy Agency (JAEA) has studied redox buffer capacity of the water-rock (organics)-microbes interaction. In this paper, the results of water-rock (organics)-microbes interaction in the subsurface environment around Tono area to assess microbial effect on redox condition were presented.
Iwatsuki, Teruki; Amano, Yuki; Ioka, Seiichiro; Saegusa, Hiromitsu; Takeuchi, Ryuji
Nihon Genshiryoku Gakkai Wabun Rombunshi, 6(1), p.73 - 84, 2007/03
Hydrochemical changes of the groundwater in sedimentary rocks were observed during construction of the Mizunami Underground Laboratory. The observation of hydraulic head and groundwater chemistry indicates that rock formations with low hydraulic conductivity act as barriers to hydraulic disturbances, while higher conductivity zones provide a preferential flow path. Groundwater flow to the drifts creates chemical changes by mixing among chemically different groundwaters. It is therefore meaningful to monitor the water pressure and chemistry at highly conductive rock formations during construction and operation of underground facilities. Pre-construction investigations to determine the geology, hydraulic conductivity and baseline hydrochemistry are essential for understanding the disturbance around the underground facility caused by construction. An approximate four month interruption in pumping allowed the underground workings to temporarily fill with groundwater.
Ioka, Seiichiro; Iwatsuki, Teruki; Amano, Yuki; Furue, Ryoji*
Nihon Suimon Kagakkai-Shi, 37(1), p.3 - 8, 2007/02
For safety assessment of geological isolation, it is important to evaluate in situ redox buffer capacity in high-permeability zone as groundwater flow path. This study evaluated in situ sulfate reduction rate as redox buffer capacity in the conglomerate bedding in Toki Lignite-bearing Formation, which occurs at the lowest part of sedimentary rocks overlying basement granite. The bedding plays an important role as the main groundwater flow path. The result showed that in situ redox buffer capacity in the conglomerate bedding has been identified on first nine months, whereas in the following period the redox buffer capacity has not been identified for about fifteen months. This will be caused by the bedding became inappropriate for microbial survival as the organic matter which is needful for microbial activity was consumed. Thus, there will be limited redox buffer capacity in groundwater flow path even in formation including organic matter-bearing layer.
Ioka, Seiichiro; Furue, Ryoji*; Iwatsuki, Teruki
Nihon Suimon Kagakkai-Shi, 36(4), p.181 - 190, 2006/11
Hydrogeological and geochemical investigations are required when the deep underground environment is explored or when the water from these depths are to be used as a resource. This study demonstrates the methodology on geochemical sampling of groundwater to evaluate the redox condition in a deep borehole with large downward hydraulic gradient. In the case of losses of drilling fluids during drilling, groundwater was sampled using single and double packer equipment at the time of the fluid loss. When there was no loss of drilling fluids groundwater samples where successfully obtained using a long term multi-packer system.
Ioka, Seiichiro; Iwatsuki, Teruki; Kato, Osamu*; Imakita, Tsuyoshi*
Bunseki Kagaku, 55(10), p.793 - 797, 2006/10
Times Cited Count：1 Percentile：4.65(Chemistry, Analytical)Performance test for redox potential measurement of solution was conducted using continuous polished and untreated Platinum working electrodes in under inert condition. The redox potential measured by polished electrode immediately shows steady value, and approximately coincides with the equilibrium value of redox reaction between HS and S. Then the redox potentials would be controlled by the equilibrium of the redox reaction. On the other hand, redox potential measured by untreated electrode was +0.2V higher than that measured by continuous polished electrode. The redox potential measured implies that the SO is dominant species in solution. Disagreement of redox potentials measured by polished and untreated electrodes may be caused by deactivation of platinum electrode of adsorption of sulfuric acid anions.
Iwatsuki, Teruki; Furue, Ryoji; Mie, Hideki; Ioka, Seiichiro; Mizuno, Takashi
Applied Geochemistry, 20(12), p.2283 - 2302, 2005/12
Times Cited Count：54 Percentile：71.59(Geochemistry & Geophysics)Hydrochemical conditions up to 1000m below ground level around the Mizunami Underground Research Laboratory were investigated to construct a baseline condition model descriving the undisturbed condition.
Mie, Hideki; Iwatsuki, Teruki; Furue, Ryoji; Mizuno, Takashi; Ioka, Seiichiro
JNC TN7400 2004-006, 64 Pages, 2004/10
JNC-TN7400-2004-006.pdf:9.26MBJNC-TN7400-2004-006(errata).pdf:0.08MB
This report describes the results of the investigation on the hydrochemical characteristic of groundwater in DH-15 borehole.
Iwatsuki, Teruki; Mie, Hideki; Mizuno, Takashi; Amano, Yuki; Ioka, Seiichiro
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no abstracts in English
Ioka, Seiichiro; Iwatsuki, Teruki; Kato, Osamu*; Imakita, Tsuyoshi*
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Ioka, Seiichiro; Iwatsuki, Teruki; Mizuno, Takashi; Kato, Osamu*; Imakita, Tsuyoshi*
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Ioka, Seiichiro; Iwatsuki, Teruki; Amano, Yuki
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Ioka, Seiichiro
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Ioka, Seiichiro
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Takir, D.*; Kitazato, Kohei*; Milliken, R. E.*; Iwata, Takahiro*; Abe, Masanao*; Otake, Makiko*; Matsuura, Shuji*; Arai, Takehiko*; Nakauchi, Yusuke*; Nakamura, Tomoki*; et al.
no journal, ,
JAXA spacecraft and sample return mission Hayabusa2 has arrived at the near-Earth asteroid 162173 Ryugu, which is classified a primitive carbonaceous object. Here we report recent results of near-infrared spectrometer (NIRS3) on the Hayabusa2 spacecraft. The observations provide direct measurements of the surface composition of Ryugu and context for the returned samples. NIRS3 has detected a weak and narrow absorption feature centered at 2.72 micrometer across entire observed surface. This absorption feature is attributed to the presence of OH-bearing minerals. The NIRS3 observations also revealed that Ryugu is the darkest object to be observed up-close by a visiting spacecraft. The intensity of the OH feature and low albedo are consistent with thermally-and/or shock-metamorphosed, and/or carbon-rich space-weathered primitive and hydrated carbonaceous chondrites.
