Solar system exploration is a key driver for the progress in cosmochemistry. This special issue “Future of solar system exploration and cosmochemistry: Hayabusa, Kaguya, Hayabusa2 and beyond” focuses on the past, current, and future contributions of cosmochemistry to the solar system exploration. It consists of five articles covering a wide array of topics; exploration of Moon and Mars, in situ dating of surface rocks with a lander, a curation facility for extraterrestrial returned samples, and an asteroidal sample return mission “Hayabusa2.” We hope that cosmochemistry continues to contribute to, or even lead, the solar system exploration.

抄録全体を表示

The Extraterrestrial Sample Curation Center of JAXA curates the Hayabusa-returned samples in conditions of minimum terrestrial contaminants, because these samples are very tiny. We evaluated the cleanliness of the handling instruments, the cleanroom environments and the sample storage chamber to improve the cleanliness of particles, organic molecules and metallic elements to a level not to affect the analyses of the Hayabusa-returned samples. In the environment of the clean chamber No. 2 where the samples have been stored, the organic molecule abundance was lower than the detection limit, furthermore, metallic elemental concentrations were the lowest among other evaluated place. A multi-stage ultrasonic cleaning by organic solvents and the ultrapure water have been applied to instruments made of stainless steel and/or aluminum alloy, and additionally, acid-alkali liquids cleaning have been performed for those made of quartz glasses. For the cleanliness of quartz glasses after the cleaning, the organic molecules abundances were blank level, and the metallic element concentrations were 1～100×10⁹atom/cm²/24 h. It was confirmed by optical microscope that no particle of size more than 10 μm was observed on quartz glasses after the cleaning.

抄録全体を表示

Since Mars has attracted much interest as a potentially accessible habitable planet, the greatest number of spacecraft has been sent to this planet among any of the other extraterrestrial bodies. This paper summarizes our activities of robotic explorations and the heritage of our knowledge on Mars in decades past.

抄録全体を表示

Age is one of the most important observables in planetary science. The ages of geologic events on planets including Mars, however, are estimated with the crater chronology approach, which is largely based on radiometric age data derived from very limited places on the Moon. Obtaining age data for the inner planets, asteroids, or the satellites of the outer planets is essential for accurate understanding of the history of our solar system. Although sample-return missions can achieve this goal, they are technically very challenging and extremely expensive. Thus, in-situ geochronology measurements with one-way lander/rover missions would be very important. This paper reviews (1) primary scientific issues on planetary chronology, such as uncertainties in the crater chronology models on Mars, (2) recent developments for in-situ dating instruments based on K–Ar, Rb–Sr, or Pb–Pb dating methods, (3) in-situ K–Ar age measurements conducted by NASA's Curiosity rover, and (4) the development status of our in-situ K–Ar isochron dating method based on a combination of laser-induced breakdown spectroscopy and quadrupole mass spectrometry for measurements of potassium and argon, respectively. We also discuss perspectives for a future Japanese in-situ geochronology mission to Mars.

抄録全体を表示

In order to decipher complex geological history of a planetary body, it is essential to integrate the global remote sensing data and laboratory analysis of the returned sample. Earth's Moon is the best example to demonstrate this synergy because we have both a large collection of lunar samples and tremendous amount of remote sensing data obtained by recent lunar exploration missions. Reanalysis of archived Apollo samples revealed that the Moon has preserved water in the interior, while the traditional scenario of the Earth-Moon system formation, known as “Giant Impact,” leads to complete loss of volatile components. The global data sets provided by SELENE/Kaguya suggest that a leftover planetesimal of several hundred kilometers in size could have delivered water to the Moon just after solidification of the highland crust from magma ocean. Studies on the volatile inventory of the Moon will help us to understand the formation of Earth's ocean, atmosphere and life.

抄録全体を表示

Hayabusa2 is the sample return mission to the near-Earth C-type asteroid (162173) 1999 JU₃ (2014–2020). Samples from C-type asteroids, which are abundantly present in the asteroid belt and of which reflectance spectra resemble those of carbonaceous chondrites, may well preserve the information covering the entire history of the Solar System; the epoch prior to the birth of the Sun, planetesimals and planet formation including a behavior of volatiles to terrestrial planets. Moreover, asteroidal surface samples record current surface geological processes and the dynamical evolution of small bodies in the Solar System, which are not recorded in meteoritic samples. The Hayabusa2 spacecraft will launch off in 2014, and arrive at the asteroid 1999 JU₃ in mid-2018. Investigations by several remote sensing techniques and sample collections at three different locations will be carried out during its 18-month stay. The spacecraft will return to Earth with asteroidal samples in December 2020. In this article, we describe the outline and cosmochemical rationales of the Hayabusa2 mission with its significance in primitive small body exploration missions.

抄録全体を表示

Along with the progresses in analytical procedure and mass spectrometry, the number of papers using a Re–Os isotope system has rapidly increased, although Re and Os are trace elements whose typical concentrations in geological materials are ppt～ppb levels. Rhenium has two isotopes and ¹⁸⁷Re is a radio isotope which generates ¹⁸⁷Os through β- decay in the half-life of 41.6 Gyr. Since both Re and Os are highly siderophile and chalcophile elements, they are concentrated into sulfide minerals. Besides the ¹⁸⁷Re–¹⁸⁷Os decay system, Re is more incompatible than Os during a magma differentiation process, producing a large variety of the ¹⁸⁷Re/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os ratios among various reservoirs on the Earth. The seawater ¹⁸⁷Os/¹⁸⁸Os ratio is mainly controlled by riverine flux having a high ¹⁸⁷Os/¹⁸⁸Os ratio (～1.4) and hydrothermal fluid/cosmic dust fluxes with low values (0.12～0.13). Thus, the Re–Os isotope system is a powerful tool for (1) geochronology of sulfide deposit, black shale and petroleum deposit using an isochron method, (2) geochronology of ferromanganese crust whose sedimentary age can be determined by fitting their ¹⁸⁷Os/¹⁸⁸Os ratios with the secular variation curve of the marine Os-isotope ratio, (3) decoding the trigger and processes of global climate change and impact event, and (4) unraveling the magma source and formation processes of volcanic rocks. Here, we review the recent geochemical study using the Re–Os isotope system and especially focus on a geochronology of the sulfide deposit.

抄録全体を表示

A variety of volatile organic compounds (VOCs) play an important role in the global environment, such as depletion of stratospheric ozone, and aerosol formation. I have studied their sources, distributions and trends in attempt to understand their fate and impact in the environment. Major results include, (1) the discovery of strong emission of methyl chloride from tropical forests, accounting for most of its missing source, (2) the first detection of pinonaldehyde in the forest aerosol, providing an evidence of secondary biogenic aerosol, (3) the discovery of long-term variation of atmospheric methyl iodide which is likely linked to global environmental change, (4) development and the first application of high frequency measurements of VOCs including most volatile fluorocarbons in East Asia. I am highly honored to have been awarded the 2011 Geochemical Society of Japan Award.

抄録全体を表示

The Antarctic ice core records revealed atmospheric CO₂ concentrations (pCO₂) during glacial periods were ～80 ppm lower than those of interglacial periods. The mechanism of understanding the low atmospheric pCO₂ of glacial periods is one of the most important problems in the global carbon cycle study. Most of paleoclimatologists consider that the deep-sea must have been an active pool of global carbon cycle on this timescale. However, there is no broadly accepted evidence that the deep sea played a role as the effective carbon pool during glacial periods. The reconstruction of deep-sea carbonate chemistry that is provided by oceanic CaCO₃ cycle (CaCO₃ preservation and dissolution) with glacial-interglacial cycles is a key to solve the mechanism of ocean carbon cycle. Number of previous studies reconstructed the pattern of CaCO₃ dissolution on the seafloor during glacial-interglacial periods. These previous studies employed dissolution intensity proxies based on carbonate microfossils preserved in sediments. However, existing proxies are insufficient in order to obtain quantitative data for ocean carbon budget. This is due to the low accuracy of existing dissolution proxies and the lack of knowledge about carbonate dissolution mechanism. This review paper summarizes the principles, characteristics, application to paleoceanography and problems of carbonate dissolution proxies, and then discusses future possible development.

抄録全体を表示