OUTLINE OF HAYABUSA SAMPLE PRELIMINARY
EXAMINATION RESULTS.
A. Tsuchiyama1, T. Nakamura2, H. Yurimoto3, M. Ebihara4, T.
Noguchi5, K. Nagao6, H. Naraoka7, F. Kitajima7, T. R. Ireland8, S.
A. Sandford9, M. E. Zolensky10, M. Abe11, T. Okada11, T. Yada11,
Y. Ishibashi11, Y. Karouji11, M. Uesugi11, and HAYABUS sample preliminary examination team members. 1Kyoto Univ. atsuchi@kueps.kyoto-u.ac.jp, 2Tohoku Univ., 3Hokkaido Univ.,
4Tokyo Metropolitan Univ., 5Ibaraki Univ., 6 Tokyo Univ.,78910Kyushu
Univ.,
ANU,NASA/ARC,NASA/JSC,
11JAXA/ISAS.
Introduction: Fine particles on S-type asteroid Itokawa
25143 were successfully recovered from the smooth terrain by
the Hayabusa mission. They are not only the first samples recovered from an asteroid, but also the second extraterrestrial regolithto have been sampled, after the Moon. Itokawa samples permit a
direct validation of the relation between asteroids and meteorites.
In addition, the properties of Itokawa particles permit studies ofregolith formation and evolution. About fifty particles (30-180µm in size) were allocated and examined systematically in preliminary examination (PE) throughout 2011. This paper presents
its comprehensive results [1-8].
Materials on Itokawa: The samples are mainly composed ofolivine, low- and high-Ca pyroxene, plagioclase, and troilite [1].
Modal abundances [5], elemental [1,3] and oxygen isotopic compositions [2] of the minerals indicate that the samples correspondto LL chondrites as estimated from the reflectance spectra of theS-type asteroid observed by ground-based telescopes and theHayabusa spacecraft.
Preexisting parent body: Sample textures mainly indicateclassification as LL5 and/or LL6, while some are LL4 [1,5]. The
maximum temperature (~800oC) estimated from the twopyroxene thermometer and a heating model using 26Al indicatethat thermal metamorphism occurred in the preexisting parental
body of Itokawa, which was larger than 20 km in radius [1].
Surface processes on Itokawa: Fe-rich nanoparticles embedded in thin surface amorphous layers (<60 nm) of the particles
show evidence of space weathering [4], which caused a change(reddening and darkening) in the reflectance spectrum of the airless body. Solar wind noble gasses, which were detected in theparticles [6], seem to be responsible for the space weathering.
The 3D shape distribution of the particles is consistent withthe results of mechanical disaggregation, primarily as a response
to impacts [5]. No particles show large-scale melting, indicating
relatively low-impact velocities (<~5 km/s) [5]. Rounded edges
observed on some particle surfaces were probably formed from
more angular edges by abrasion as grains migrated during impacts [5]. The noble gas release pattern is also consistent withgrain migration. [6] The noble gas contents indicate a short residence time of the uppermost regolith layer (~150 yr.) and an upper limit of the residence time at Itokawa’s surface (~8 Myr.) [6].
Organic materials have not yet been observed [7,8].
References: [1] Nakamura, T. et al. (2011) Science,333:1113-1116. [2] Yurimoto H. et al. (2011) Science, 333:11161119. [3] Ebihara M. et al. (2011) Science, 333:1119-1121. [4]Noguchi T. et al. (2011) Science, 333:1121-1125. [5] Tsuchiyama
A. et al. 2011. Science, 333:1125-1128. [6] Nagao K. et al.
(2011) Science, 333:1128-1131. [7] Naraoka H. et al. (2012) Geochem. J. 46:61-72. [8] Kitajima F. et al. (2011) LPSC XLII, abstract #1855.