LITHIUM, BORON AND LIGHT NOBLE GAS ANALYSESON THE SURFACE OF THE ITOKAWA ASTEROIDALREGOLITH RETURNED BY THE HAYABUSA MISSION.
W. Fujiya1, P. Hoppe1, U. Ott2 and M.M.M. Meier3. 1Max Planck
InstituteforChemistry,Germany.
E-mail:wataru.fujiya@mpic.de. 2University of West Hungary, Hungary.
3
Lund University, Sweden.
Introduction: Asteroidal regolith from the surface of Itokawa was brought to the Earth by a Japanese spacecraft, Hayabusa.
Because the sample was taken from the uppermost layer of theasteroid, it had been exposed to solar wind (SW) [1]. In thisstudy, we investigated the SW Li, B and light noble gas isotopic
compositions on the surface of Hayabusa samples.
Experimental: The allocated samples, RA-QD02-0167 andRA-QD02-0209 (hereafter 0167 and 0209), are ~50 m sized
olivine grains. We pressed them onto ultra-pure Au foils. First weanalyzed them with a NanoSIMS 50 at MPI. We obtained depth
profiles of secondary 6,7Li+, 10,11B+ and 30Si+ ions produced by anO− primary ion beam (100 pA) by acquiring a series of 200 ion
images [2]. The total analyzed depth was ~120 nm. Subsequently, we measured He and Ne isotopic ratios at ETH Zurich.
Results and Discussion: No Li isotopic anomalies were detected throughout the depth profiles, as opposed to the results by[3] for grains from lunar soil. As for B, grain 0167 is clearly enriched in 10B by ~16 %, with 10B/11B = 0.288 ± 0.023 (2; solar =0.248) in the region deeper than ~30 nm. The B concentration ofthis grain is ~250 ppb. Grain 0209 shows solar B isotopic composition within error (0.240 ± 0.015). Because [1] found no detectable cosmogenic 21Ne excesses for three Hayabusa grains, weinvoke SW implantation rather than cosmogenic B to explain theobserved high 10B/11B of grain 0167. There are several ways toincrease the 10B/11B ratio in SW and/or the solar photosphere. 1)
Spallation reactions in the Sun’s atmosphere [3]. However, spallogenic B accounts for only <1 % of total B in the Sun’s atmosphere, so its contribution is minimal. 2) Gravitational settlingwithin the Sun, which also plays a minor role [4]. 3) Nuclear
burning of B at the base of the Sun’s convective zone [5]. Since
11
B is more easily destroyed than 10B, this results in higher
10 11B/ B. But if a substantial amount of 11B is destroyed, 9Be
should be completely destroyed and depleted in the Sun, which is
not observed. 4) Isotope fractionation between SW and the Sun’souter convective zone, which is expected to occur during SW
acceleration (e.g., [6]). The inefficient Coulomb drag model, e.g.,predicts enrichment in 10B relative to 11B by ~4 % (assuming B+5in SW) to ~40 % (B+3). In any case, if we assume the SW 10B/11B
to be in the range of 0.265 to 0.347 (the latter corresponds to a40 % enrichment in 10B), then the observed 10B/11B of >0.265 canbe explained by a SW B contribution of 46 to 250 ppb in the analyzed zone. For a SW B implantation rate at 1.3 AU of 0.14
cm−2s−1 (based on the SW He flux and the solar B/He ratio;[7,8]), the corresponding irradiation age is between 5.5 and 30 ×104 years. The data reduction of the noble gas analysis is in progress, and the results will be presented at the workshop.
References: [1] Nagao K. et al. 2011. Science 333:1128. [2]Fujiya W. et al. 2013. MetSoc 2013: abst. #5061. [3] Chaussidon
M. and Robert F. 1999. Nature 402:270. [4] Turcotte S. andWimmer-Schweingruber R. F. 2002. JGR 107:1442. [5] Bochsler
P. and Geiss J. 1973 Solar Phys. 32:3. [6] Heber V. S. et al.
2012. ApJ 759:121. [7] Heber V. S. et al. 2009. GCA 73:7414.
[8] Asplund M. et al. 2009. Rev. Astron. Astrophys. 47:481.