STUDIES OF ITOKAWA’S SURFACE EXPOSURE BY
MEASUREMENTS OF COSMIC-RAY PRODUCEDNUCLIDES.
M. W. Caffee1, K. Nishiizumi2, A. Tsuchiyama3, M. Uesugi4, andM. E. Zolensky5. 1Dept of Physics, Purdue Univ., West Lafayette,
IN 47907. 2Space Sci. Lab., Univ. of Calif. Berkeley, CA 94720.
3
Div. of Earth and Planet. Sci., Kyoto Univ., Kyoto.
4
JAXA/ISAS, Sagamihara, Kanagawa. 5NASA JSC, Houston,TX 77058.
Introduction: We plan to investigate the evolutionary
history of surface materials from 25143 Itokawa, the Hayabusasamples. Our studies are based on the measurement of nuclides
produced in asteroidal surface materials by cosmic rays.
Cosmogenic radionuclides are used to determine the duration andnature of the exposure of materials to energetic particles. Ourgoals are to understand both the fundamental processes on theasteroidal surface and the evolutionary history of its surface
materials. They are also key to understanding the history of
Itokawa’s surface and asteroid-meteoroid evolutionary dynamics.
To achieve our key goals, in particular reconstructing theevolutionary histories of the asteroidal surface, we proposed: (1)characterizing Itokawa particles using SXCT, SXRD, and FESEM without modification of the sample; (2) embedding eachparticle in acrylic resin, then slicing a small corner with an ultramicrotome and examining it using super-STEM and SIMS for
characterizing surface morphology, space weathering, andoxygen three-isotope analysis; and finally (3) measuring small
amounts of cosmogenic radionuclides (104-105 atoms) inHayabusa samples by AMS. However, we have to modify ourplan due to unexpected situation.
Experiemetals: Two Hayabusa samples, RA-QD02-0188
and RB-CV-0022 were allocated our studies.
RA-QD02-0188. Since RB-CV-0022 is not avilable forfurther studies (see bleow), we determined that RA-QD02-0188(175 µm, olivine, FeNi) is to be used exclusively for cosmogenic
radionuclide measurement, excepting of course opticalobservations that can be done without touching the sample. We
determined the shock state of the samples in the conventionalmanner under a standard petrographic microscope throughHayabusa container’s window before any surface morphology
change [1]. We anticipate that the 10Be and 26Al measurements
will be perfomed before the symposium.
RB-CV-0022. To avoid polymalized glue by synchrotron Xray radiation, the RB-CV-0022 was inserted into a JAXA’s newly
developed Si3N4 holder [2]. The synchrotron radiation X-ray
microtomography (SXCT) was performed at SPring-8 to makeobservations of textures, mineralogy, and three-dimensional
structures of RB-CV-0022 before cosmogenic nuclide
measurements [3]. The preliminary results indicate the size ofRB-CV-0022 is 5.1x106 µm3 and 146x98x74 µm instead of 164µn in Hayabusa catalogue. The particle is mainly consisted with
low-Ca pyroxene with minor olivine and troilite. When the RBCV-0022 was returned to JAXA curatorial facility after SXCT,
they found that the Si3N4 container was broken and the particle
escaped from the container to clean chamber and never recoveredyet.
This work was supported by NASA LARS program.
References: [1] Zolensky M. E. et al. 2014. This Symposium.
[2] Uesugi M. et al. 2014. This Symposium. [3] Tsuchiyama A. etal. 2011. Science 333:1125-1128. and This Symposium.