This special issue of "Chikyu Kagaku" contains five papers from three sessions for cosmochemistry, "Cosmochemistry of extraterrestrial materials: From presolar history to early solar history", "Origin and evolution of Moon, Mars and small bodies in the solar system", and "Analytical technique, analytical organization and the scientific results of a new age of sample return mission", in the annual meeting of the Geochemical Society of Japan 2008. The five papers cover several topics on formation of water molecules in circum stellar environments, noble gas compositions of individual chondrules, discovery of chondrules from a comet sample, new analytical technique of extraterrestrial organics, and advances of lunar science under the cosmochemical view. These papers should be helpful to understand the trends of current research subjects of cosmochemistry. We hope this special issue could act as useful media for the cosmochemical community and the new comers to this research field.
Formation of water by reaction of oxygen molecules (O2) with hydrogen atoms (H) were investigated at low temperatures (10-40 K) in order to reveal the possible reaction pathways to yield water on grain surfaces in molecular clouds. Two types of experiments for surface reaction between O2 and H were performed: one of which was that solid O2 was deposited onto the surface, followed by H irradiations to the O2, another of which was that gaseous O2 was continuously codeposited with H onto the surface and was reacted with each other. H2O (D2O) and H2O2 (D2O2) were quickly produced through successive reactions of O2 with H (D). Reaction kinetics and isotope effect are discussed for the former experiment. For the latter experiment, the dependence of compositions of the products on the temperature and O2/H ratio, and also the structure of the H2O ice produced are examined.
More than 300 small chondrules (50-200 μm) were found in the Sayh al Uhaymir (SaU) 290 CH3 chondrite which is a solar wind noble gas bearing meteorite and shows a brecciated texture. We analyzed noble gases in the individual chondrules using a laser ablation noble gas extraction system which enables to reduce blank noble gases. Isotopic ratios of He and Ne show no clear evidence of solar wind exposure, suggesting that the chondrules have not been exposed to solar winds. A chondrule shows an excess of cosmogenic 3He relative to other chondrules, which is explained by cosmic-ray exposure on the parent body. The derived parent exposure age is longer than 2.0 Ma, while the space exposure age is estimated as 1.4 Ma.
Many small rock particles have been successfully recovered from a short-period comet 81P/Wild 2 by the Stardust mission. They are believed to be very primitive dust at outer regions of the protoplanetary disk, because short-period comets originally formed as Kuiper-belt objects that currently locate at 30-50 AU from the Sun. Chondrules are sub-millimeter to millimeter size objects formed in the inner protoplanetary disk by total or partial melting of silicate-rich dust particles. They are a major constituent of chondrites from asteroids. Among the particles from the comet Wild 2, we found pieces of chondrules that show igneous texture and have mineral compositions and major and most minor element concentrations very similar to chondrules in primitive meteorites derived from asteroids. Oxygen isotope ratios are highly heterogeneous from - 50 to + 5‰ in δ18OSMOW and plotted nearly along the slope = 1 mass independent fractionation line in the oxygen three isotope diagram, which characterizes chondrules in carbonaceous chondrites that comprise middle to outer asteroid belt. These observations suggest that the particles are pieces of chondrules formed through the least degree of melting, crystallization, and elemental and isotopic equilibration at high temperatures. The presence of chondrules in a short-period comet from the Kuiper belt indicates that chondrules migrated from hot inner regions to cold outer regions of the disk and spread widely over the early solar nebula.
Synchrotron-based soft X-ray micro analysis is a powerful technique for the quantitative molecular characterization of submicron-sized organic samples without damage. Recent development of X-ray Absorption Near Edge Structure (XANES) spectroscopy using Scanning Transmission X-ray Microscope (STXM) has enabled the comprehensive study on the chemical history of the early solar system as recorded in organic molecules ranging from the most primitive to altered extraterrestrial materials. In this review paper we describe our two achievements: one is the results of STXM μ-XANES analyses performed on organic-containing particles extracted from 81P/Wild 2 cometary dust tracks collected by the Stardust comet sample return mission. The XANES spectra have revealed highly complex organic structures with a large heterogeneity in heteroatom content and different functional groups, suggesting that the comet organics may have multiple precursors. Another result is the molecular spectroscopic data on meteoritic organic matter spanning various chondrite classes, groups, and petrologic types, using carbon XANES spectroscopy. The sample analyses and the kinetics through heating experiment have shown that the intensity of 1s→σ* exciton derived from highly conjugated sp2 carbon in the type 3+ chondritic organic matter appears to quantitatively determine the parent body metamorphism.
The Earth's moon has made significant contributions both in advances of space exploration and planetary science. Rock samples returned from the Moon by Apollo and Luna missions provided us with basic understandings of the origin and evolution of Moon, Earth, and other rocky planets in the solar system. Yet, the global compositional data from subsequent orbital satellites (Galileo, Clementine, and Lunar Prospector) and analyses of lunar meteorites have revealed the diversity of the surface composition and the nearside-farside asymmetry of the Moon. These data suggest that the crustal evolution and thermal history of the Moon should be by far more complex than that inferred from the Apollo samples which only represent the central nearside. KAGUYA (SELENE) mission now provides high-precision, high-spacial-resolution and high-energy-resolution remote sensing data on physical and chemical properties of the global Moon. The state-of-art data set, coupled with the latest results of lunar sample analyses, enables us to demonstrate the currently proposed model for a bimodal crustal evolution, and provides clues to answer the primary issues on the lunar origin and evolution.
Volatile organic compounds (VOC) are ubiquitously trace species in the atmosphere and play a variety of roles in the chemistry of atmosphere, including as precursors of photochemical smog formation, carriers of ozone-destroying halogens into the stratosphere, and radiatively active gases that affect on the Earth's climate system. This review summarizes our geochemical studies regarding emission sources, transport processes, and past trend of VOC in the atmosphere. The major results are (1) vertical profiles of non-methane hydrocarbons over east Asia are caused mainly by the mixing of fresh emissions with aged air masses containing reactivity-determined amount of photochemically aged non-methane hydrocarbons, (2) atmospheric concentration of methyl chloride, a naturally occurring ozone-depleting substance, was relatively constant, being similar to the present levels, during the pre-industrial Holocene, and (3) tropical plants produce methyl chloride with a biogenic mechanism and global methyl chloride emission by tropical plants represents 30-50% of the global emissions.
The presence of several short-lived, now extinct, radionuclides in the early solar system has been confirmed through measurements of excesses of daughter isotopes that correlate with the abundances of their parent elements in cogenetic minerals in meteorites. Some of such short-lived radionuclides should have formed just prior to or soon after the solar system formation either by stellar nucleosynthesis or by energetic-particle irradiation. A short-lived rdionuclide 60Fe ins produced only in stars and thus provides a constraint on the stellar contribution to solar system radionuclides. Clear evidence of the presence of 60Fe in the early solar system has been found in various components in meteorites. The estimated initial abundance of 60Fe in the solar system cannot be explained byheritage from the interstellar medium, but requires the injection of 60Fe into the proto solar materials from a nearby star. Although no previous model succeeded to explain the abundances of the short-lived radionuclides (26Al, 41Ca, 53Mn, and 60Fe) by injection from asingle stellar source, I propose here that a faint super nova with mixing and fallback can match the solar-system abundances of 26Al, 41Ca, 53Mn, and 60Fe suggesting that the solar system formed nearby a massive star.