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: 31.2). 08/2011; 333(6046):1128-31. DOI: 10.1126/science.1207785
Source: PubMed

ABSTRACT 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 (21)Ne. 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.

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A wide variety of organic compounds have been found in space, and their relevance to the origin of life is discussed. Interplanetary dust particles (IDPs) are most promising carriers of extraterrestrial organic compounds, but presence of bioorganic compounds are controversial since they are so small and were collected in the terrestrial biosphere. In addition, IDPs are directly exposed to cosmic and solar radiation. Thus, it is important to evaluate the stability of organics in IDPs in space environment. We are planning a novel astrobiology mission named Tanpopo by utilizing the Exposed Facility of Japan Experimental Module (JEM/EF) of the International Space Station (ISS). Two types of experiments will be done: Capture experiments and exposure experiments. In the exposure experiments, organics and microbes will be exposed to the space environments to examine possible alteration of organic compounds and survivability of microbes. Selected targets for the exposure experiments of organic compounds are as follows: Amino acids (glycine and isovaline), their possible precursors (hydantoin and 5-ethyl-5-methyl hydantoin) and complex precursors "CAW" synthesized from a mixture of carbon monoxide, ammonia and water by proton irradiation. In addition to them, powder of the Murchison meteorite will be exposed to examine possible alteration of meteoritic organics in space. We will show the results of preparatory experiments on ground by using a UV lamp, synchrotron facilities, and a heavy ion irradiation facility.
    International Symposium on Space Technology and Science; 06/2013
  • [Show abstract] [Hide abstract]
    ABSTRACT: Kr and Xe ion bombardment experiments were conducted on Mo, Hf, Ta, W, Re, Au, and five platinum group elements. The implanted Kr and Xe ions were detected, and their concentrations were determined by X-ray photoelectron spectroscopy (XPS) using synchrotron radiation. In the case of 5d transition metals, the binding energies of Kr2p3/2, Xe3d3/2, and Xe3d5/2 in the target metals correlate with the number of d electrons of metals. Furthermore, the Xe concentration in the 5d transition metals correlates with atomic number, but there is no such correlation for Kr. The observed trend for the Xe concentration can approximately be reproduced by a theoretical calculation that takes sputtering yield into consideration. The retentivities of Kr and Xe are defined as the ratio of the experimental and theoretical surface concentrations. Although there is no correlation between retentivity and the atomic number of the target metal, the elastic energies in the cases of Kr and Xe atoms implanted in metals positively correlate with the corresponding retentivities. If the activation energy of noble-gas diffusion in the target metals can be represented in terms of the elastic energy, the noble-gas retentivity can be qualitatively explained.
    Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 03/2012; 274:93–99. · 1.27 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report a summary of the initial analysis of space weathering of the Itokawa particles. In addition to the two-layered nanoparticle-bearing rims, we found vesicular rims with nanoparticles and quite thin vapor deposition layers on intact minerals.