SOLAR NOBLE GASES IN EIGHT HAYABUSASAMPLES FROM ITOKAWA’S SURFACE WITH SHORTDURATION OF COSMIC RAY EXPOSURE.
K. Nagao1, R. Okazaki2, Y.N. Miura3, T. Osawa4, M.K. Haba1,Y. Tobimatsu2, J. Gilmour5, and Y. Nishimura5
1
Geochemical Research Center, The University of Tokyo (nagao@eqchem.s.u-tokyo.ac.jp), 2Department of Earth and Planetary Sciences, Kyushu University, 3Earthquake Research Institute, The University of Tokyo, 4Japan Atomic Energy Agency,
5
School of Earth, Atmospheric and Environmental Sciences,
University of Manchester.
Introduction: Solid materials on the surface of small asteroids without atmosphere are exposed to particles emitted fromthe Sun, and the particles are implanted into surface layer (≤ m)of the materials. The asteroidal surface is also bombarded to thedepth of several meters by energetic galactic cosmic-rays, producing cosmogenic noble gases with characteristic isotopic compositions through nuclear reactions. The cosmogenic and solar
noble gases show histories of cosmic ray irradiation, surface gardening, and surface erosion rate of small asteroids. We havemeasured noble gases of eight Hayabusa grains: RA-QD02-0015,
RA-QD02-0053, and RA-QD02-0065 (Initial analysis, [1]), RAQD02-0144 and RA-QD02-0160 (1st A/O, [2]), RB-CV-0001,
RB-CV-0009, and RB-CV-0039 (2nd A/O).
Experimental Method: The samples were heated stepwise
using a Nd-YAG CW laser at 200, 300°C, and melt (initialanalysys), 200, 300, 800°C and melt (1st A/O), and 100, 200,400°C and melt (2nd A/O). In the 1st and 2nd A/O analyses, thin
W-Re thermo-couples (25 μm in diameter) were set at each position of the samples to measure temperature during the heating ofsamples for noble gas extraction. Extracted noble gases at eachtemperature were measured with a modified-VG5400 (MS-III) at
the University of Tokyo. About 50% of Kr and Xe of the 1st and2nd A/O samples were sealed in gold tubes and sent to the University of Manchester for ultra-sensitive mass spectrometry.
Results and Discussion: He and Ne of solar wind (SW)origin were detected for all the Hayabusa samples. SW-gases
were mostly released at the lowest heating temperatures (100 or200°C), except for RA-QD02-0015 which released at melt. Concentrations of SW He and Ne for the samples are as high as thosefor interplanetary dust particles or micrometeorites. The observation confirms that the samples were collected on Itokawa’s surface, where the grains were irradiated by solar wind. 3He/4He ratios are similar to that of SW-He [3] or slightly lower. Ne isotopic compositions were close to SW-Ne when enhanced release ofNe from the samples was observed. He/Ne ratios are lower thanthe SW-ratio, which might be caused by selective loss of He fromthe samples during the SW irradiation.
Concentrations of galactic cosmic-ray produced noble gases,e.g., 3He and 21Ne, were almost negligible within experimentalerrors. Although we proposed that the cosmic-ray exposure age
for the surface materials of Itokawa is no more than 8 My [1],much shorter exposure age, ≤0.5 My, is inferred from the large
sample RB-CV-0009 allocated as the 2nd A/O.
References: [1] Nagao et al. 2011. Science 333:1128–1131.
[2] Nagao et al. 2013. Abstract #1976. 44th Lunar and PlanetaryScience Conference. [3] Heber et al. 2009. Geochimica et
Cosmochimica Acta 73:7414–7432.