THE COSMIC-RAY EXPOSURE AGE PROBABILITYDENSITY DISTRIBUTION OF HAYABUSA GRAINS
M. M. M. Meier1,2, C. Alwmark2, S. Bajt3, U. Böttger4, H. Busemann5,6, W. Fujiya7, J. Gilmour5, U. Heitmann8, P. Hoppe7, H.W. Hübers4, F. Marone9, U. Ott7, S. Pavlov4, U. Schade10, N.
Spring5, M. Stampanoni9, I. Weber8. 1CRPG CNRS-Nancy,France (mmeier@crpg.cnrs-nancy.fr), 2Dept. of Geology, Lund
Univ., Sweden, 3Photon Sci., DESY, Hamburg, Germany, 4DLR,Planet. Res. Inst., Berlin, Germany, 5SEAES, Univ. of Manchester, UK, 6Earth Sciences, ETH Zurich, Switzerland, 7MPICMainz, Germany, 8Inst. f. Planetologie, Univ. of Münster, Germany, 9Swiss Light Source, PSI, Villigen, Switzerland,10Helmholtz-Zentr. Berlin, Germany.
Background: The analysis of He and Ne in individual mineral grains brought back by the JAXA space probe Hayabusa
from near-Earth asteroid Itokawa has revealed that these grains
all have exposure times to galactic cosmic rays (cosmic-ray exposure = CRE ages) of <8 Ma [1-5]. This is surprisingly shortgiven the expected survival time of Itokawa in the inner solarsystem (on the order of a few 10 Ma) [6], and the observed crater
distribution on Itokawa (at least 75 Ma) [7], suggesting higher
exposure ages for grains from that asteroid. Possible explanations
include: 1) a high surface erosion rate on the order of ~0.1-1 m
per Ma [1]; 2) a relatively recent (<8 Ma) mass-shedding/resurfacing event, either through break-up and re-assembly, partial
tidal disruption [3,4] or YORP-induced effects [8]; 3) a localeffect, i.e., most of the grains sampled by Hayabusa derive from anearby boulder or “ridge” on the surrounding surface in “MusesSea” (where Hayabusa touched down on Itokawa) which wasexposed <8 Ma ago by, e.g., a small impact; 4) a coincidence
favored by small-number statistics; 5) a combination of different
scenarios. Scenarios 1-4 should result in characteristic (althoughperhaps not unique) shapes of the CRE age probability density
distribution. For example, a single event leading to the simultaneous exposure of all grains would result in a single large peak in
the probability distribution, while a continuous, constant CRE ina regolith would result in a flat distribution.
Methods & Results: We use published (as well as our own,yet unpublished) data for He, Ne amounts. For grain masses, weeither use Synchrotron X-Ray Tomographic Microscopy(SRXTM) at the Swiss Light Source/TOMCAT beam-line of thePaul-Scherrer-Institute in Switzerland for volume determination(and subsequent mass determination via density from #Fo [2]), orvolumes determined from available SEM grain images from theHayabusa grain database, combined with a statistical model linking cross-sectional areas with volumes (calibrated by SRXTM[2,9,10]). We then construct the cosmic-ray exposure age probability density distribution for all Hayabusa grains analyzed fortheir He, Ne content up to the time of the meeting.
References: [1] Nagao K. et al. 2011. Science 333, 11281131. [2] Nagao K. et al., 2013. Abstract #1976. 44th LPSC. [3]Meier M. M. M. et al. 2013, First Hayabusa Symposium. [4]Meier M. M. M. et al. 2014. Abstract #1247, 45th LPSC. [5] Fujiya W. et al. 2014. Abstract #1802, 45th LPSC. [6] Michel P. andYoshikawa M. 2005. Icarus 179, 291-296. [7] O'Brien M. P. etal. 2009. Icarus 200, 503-513. [8] Connolly H. C. et al. 2014. Abstract #5075. 77th Annual Meteoritical Society Meeting. [9] MeierM. M. M. et al. 2014, Geochimica et Cosmochimica Acta 125,338-350. [10] Tsuchiyama A. et al. 2014. Meteoritics & Planetary Science 49, 172-187.