Takahito Osawa

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

Laboratory analysis of samples returned from an asteroid establishes a direct link between asteroids and meteorites and provides clues to the complex history of the asteroid and its surface. 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.

Deep-sea record of impact apparently unrelated to mass extinction in the Late Triassic

The 34-million-year (My) interval of the Late Triassic is marked by the formation of several large impact structures on Earth. Late Triassic impact events have been considered a factor in biotic extinction events in the Late Triassic (e.g., end-Triassic extinction event), but this scenario remains controversial because of a lack of stratigraphic records of ejecta deposits. Here, we report evidence for an impact event (platinum group elements anomaly with nickel-rich magnetite and microspherules) from the middle Norian (Upper Triassic) deep-sea sediment in Japan. This includes anomalously high abundances of iridium, up to 41.5 parts per billion (ppb), in the ejecta deposit, which suggests that the iridium-enriched ejecta layers of the Late Triassic may be found on a global scale. The ejecta deposit is constrained by microfossils that suggest correlation with the 215.5-Mya, 100-km-wide Manicouagan impact crater in Canada. Our analysis of radiolarians shows no evidence of a mass extinction event across the impact event horizon, and no contemporaneous faunal turnover is seen in other marine planktons. However, such an event has been reported among marine faunas and terrestrial tetrapods and floras in North America. We, therefore, suggest that the Manicouagan impact triggered the extinction of terrestrial and marine organisms near the impact site but not within the pelagic marine realm.

Sequential variation of atmospheric mercury in Tokai‐mura, seaside area of eastern central Japan

Concentrations of gaseous elemental mercury (GEM) were continuously measured from October 2005 to August 2006 in Tokai-mura (36.27{sup o}N, 140.36{sup o}E), a small city on the Pacific side of central Japan. The concentration of atmospheric mercury in summer and autumn is higher than in winter and spring, and an 11-month average of 1-hour measurements is 3.78 {+-} 1.62 ng/m{sup 3} (n = 7551). Our results showed that changes in the GEM level in Tokai-mura are largely determined by temperature and relative humidity, but are independent of wind speed and direction. Two contrary patterns were observed in the daily cycle of GEM concentrations: the most frequent pattern of lower concentrations at noon and a pattern of higher concentrations at noon, which was only observed in November. Our results showed that two nearby potential anthropogenic sources of mercury, a coal-fired power plant and incineration facility, have only little effect on the gaseous elemental mercury (GEM) in Tokai-mura based on only GEM measurements.

Mass-dependent isotopic fractionation of a solid tin under a strong gravitational field

Pure tin metals were centrifuged at 1 × 106g and at 220°C for 100 hours, at 0.40 × 106g at 220–230°C for 24 hours, and at 0.25 × 106g at 220°C for 24 hours. Their isotopic compositions were measured by a secondary ion mass spectrometer (SIMS). 116Sn/120Sn and 124Sn/120Sn ratios of the 1.02 × 106g sample were considerably different than the initial compositions, and the magnitude of isotopic fractionation reached 2.6 ± 0.1%. A three-isotope diagram of 116Sn/120Sn vs. 124Sn/120Sn shows conclusively that isotopic fractionation caused by a gravitational field depended only on the isotopic mass.

Solar Wind Noble Gases in Micrometeorites

Most extraterrestrial materials discovered on the Earth have no solar wind noble gases. In fact, only four types of extraterrestrial materials contain noble gases attributed to the solar wind or its fractionated component: gas-rich meteorites, lunar materials collected by the Apollo missions, asteroid samples returned from Itokawa by the Hayabusa mission, and micrometeorites. Except for micrometeorites, all of these have a specific history of solar wind irradiation on the surface of their parent bodies. On the other hand, solar wind noble gases in micrometeorites are implanted during orbital evolution in interplanetary space. Micrometeorites have a different origin and irradiation history from the other three materials and from typical meteorites, meaning that these tiny particles that fell on the Earth can provide us valuable information about the activity of the solar system. Of all the analytical methods in planetary science, noble gas analysis of extraterrestrial materials is one of the most useful, because the analysis can reveal not only their origin and age but also their history of irradiation by galactic and solar cosmic rays and solar wind. In particular, the most reliable positive proof of an extraterrestrial origin for micrometeorites is the solar wind noble gases. In this chapter, solar wind noble gases trapped in micrometeorites are reviewed.

Argon retentivity of carbonaceous materials: feasibility of kerogen as a carrier phase of Q-noble gases in primitive meteorites

Extremely large amounts of heavy noble gases are concentrated in phase Q, which seems to be a carbonaceous phase analogous to terrestrial Type III kerogen. Phase Q must have very high noble gas retentivity based on the presence of such extremely large amounts of heavy noble gases in a very minor fraction of the meteorite. To verify that kerogen is a carrier phase of Q-noble gases, X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) using synchrotron radiation were carried on for kerogens (coals) and carbon allotropes that had been bombarded by 3-keV Ar ions, and the Ar retentivities of the two materials were compared. This comparison of the estimated Ar concentrations in the target materials revealed that carbon allotropes (graphite, fullerene, carbon nanotube, and diamond) have a much higher Ar retentivity than kerogens. This unexpected result clearly shows that the terrestrial kerogens tested in our study are not suitable as a carrier phase of Ar and, consequently, that phase Q may not be similar to the terrestrial kerogen tested. If heavy noble gases are really concentrated in carbonaceous components of primitive meteorites, phase Q may have a more ordered structure than terrestrial kerogen based on the fact that the greatest difference between terrestrial kerogen and carbon allotropes is the degree of order of the molecular structure.

An evaluation method of reflectance spectra to be obtained by Hayabusa2 Near-Infrared Spectrometer (NIRS3) based on laboratory measurements of carbonaceous chondrites

We conducted ground-based performance evaluation tests of the Near-Infrared Spectrometer (NIRS3) onboard Hayabusa2 spacecraft in November 2013 and from April to May 2014 and established a method for evaluating its measured reflectance spectra. Reflectance spectra of nine powdered carbonaceous chondrite samples were measured by both NIRS3 and a Fourier transform infrared (FT-IR) spectrometer. We have established two methods for correcting the NIRS3 data by comparing them with the corresponding FT-IR data because raw data obtained by NIRS3 underwent spectral distortion caused by systematic offsets in sensitivity of individual pixels. The corrected NIRS3 spectra of carbonaceous chondrite samples are comparable with their FT-IR spectra. The depth of each band component Dλ is defined for each wavelength λ (μm) to characterize the absorption bands in NIRS3 spectra. It is suggested that the relationship between the D2.72/D2.79 ratio and the D2.76/D2.90 ratio would be useful for estimating the degree of heating of the asteroid surface, if contributions of terrestrial adsorbed water on D2.79 and D2.90 are properly corrected. The degrees of heating and space weathering are also comprehensively evaluated by the relationship between D2.90 and the D2.76/D2.90 ratio. Reflectance spectra of asteroid Ryugu, the target asteroid of Hayabusa2, to be recorded by the NIRS3 instrument are expected to reveal the characteristics of the surface materials by using the evaluation technique proposed in this paper. Such information will be used for choosing the touchdown points for sampling and also for investigating the distribution of the materials similar to the returned samples on Ryugu.Graphical abstract.

Development of an automatic prompt gamma-ray activation analysis system

An automatic prompt gamma-ray activation analysis system was developed and installed at JRR-3M. This system is mainly composed of two personal computers, four programs, a vertical revolute joint robot, and data acquisition devices. The main control software, referred to as AutoPGA, was developed using LabVIEW 2011 and the program can control all functions of the analytical system. Up to 14 samples can be automatically measured by the system. Therefore, this system can not only dramatically increase the efficiency of routine measurements but also decrease the background level of gamma-rays for the analysis.

Development of special rotor for centrifugal separation of isotopes in solid pure metals

A prototype rotor with two grooves for the multistage centrifugal isotope separation in solid state was developed to test a new idea. This idea is based on the sedimentation of constitutional atoms in solid. In the performance test using indium specimen, it is verified that the developed rotor can receive all injected molten-indium droplets from an automatic raw-material feeding system even at the high rotational speed of 97,000 rpm without the loss of rotational stability, and the received indium specimens can be transferred in/between two grooves through the plastic flow under the influence of strong centrifugal force even in the solid state. The isotope ratio of centrifuged indium specimens was analyzed employing the secondary ion mass spectrometry, and it is confirmed that intended isotope separation by the centrifugation is realized in the solid state. The developed rotor can be used to perform the isotope separation on at least solid metals under the conditions of up to 400 degrees C in specimen temperature and 0.4x10(6)g in centrifugal force field.

Isotope Fractionation due to Sedimentation of Atoms in Centrifuged Indium-Lead Alloy

The atomic-scale graded structure of In-Pb alloy was formed by an ultracentrifuge under a gravitational field of 0.81 x 106 g for 100 hours at 150 °C in solid state. The isotope ratio measurements were performed on the centrifuged sample with secondary ion mass spectrometer (SIMS, CAMECA IMS-6f). 206Pb/208Pb and 207Pb/208Pb isotope ratio changed with negative gradient in the direction of centrifugal force approximately 1.5% and 0.8%, respectively. There was a tendency that the heavy 208Pb isotope abundance increased and the light 206Pb isotope abundance decreased in the direction of centrifugal force. Three-isotope diagram of 206Pb/208Pb versus 207Pb/208Pb proved that the isotope fractionation depends on the isotopic mass difference. These results showed that a strong gravitational field not only affected the inter-diffusion but also the self-diffusion in this alloy by causing isotope fractionation effect, which was dependent on the mass-difference.