Micro-nano impact features on the surface of an Itokawa
particle, RA-QD02-0265.
S. Yakame1, M. Uesugi1, T. Yada1, Y. Karouji1, Y. Ishibashi1, T.
Okada1 and M. Abe1. 1JAXA/ISAS.
E-mail: uesugi@planeta.sci.isas.jaxa.jp.
Introduction: We studied a particle Extra-0013, which was
not picked up by a normal procedure from the Hayabusa sample
catcher. The particle was found unexpectedly on an Au SEM
holder. After detailed investigations, the particle was confirmed
as one of Itokawa particles and named as RA-QD02-0265.
Observation of the surface of RA-QD02-0265 by FE-SEM shows
anomalous number of micro-impact features.
In this paper, we show the result of chemical composition
analysis of RA-QD02-0265 by FE-EPMA, and also show theresult of observation of the impact features on the surface of theparticle by FE-SEM.
RA-QD02-0265: Samples unexpectedly found during themanipulations and do not have records of picking up from aQuartz Disk or a sample catcher were named EXTRA particles
and excluded from the normal list of Hayabusa-returned samples.
The particle, named Extra-0013, is around 20 µm along the longaxis, and is mainly composed of high-Ca pyroxene, olivine andplagioclase. There are two distinct parts in the particle, a large
high-Ca pyroxene crystal part and the aggregate part made ofsmall particles of high-Ca pyroxenes, olivines and plagioclases.
The particle was divided into two pieces by FIB in order to obtainthe smooth surface, and analyzed by FE-EPMA at JEOL. The
result showed that the distribution of chemical composition ofhigh-Ca pyroxene crystals in the particle agrees well to thediopsides found in LL6 chondrites and in Itokawa particles
analyzed by the preliminary examination [1]. This particle wasnamed as RA-QD02-0265 after the analysis, and listed in thenormal sample list of the Hayabusa-returned samples.
Surface observation: Before cutting by FIB, the surface ofthe particle was precisely observed by FE-SEM. The observation
appears that anomalous number of the impact features, micronano impact craters and impact-melted iron-sulfide. Six micronano impact craters were reported in the preliminary
examinations [2]. However, such craters have not been found inother Itokawa particles so far. On the surface of RA-QD02-0265,
15 craters were found on the surface of the largest pyroxene
crystal. The diameter of these craters ranges from 250 nm to 500
nm, and most of them are surrounded by a droplet-array. Thecraters are heterogeneously distributed on the surface of theparticle, and 12 craters were found within 5 µm2 area of thesurface of the pyroxene crystal. In addition to the craters, there
are amount of iron-sulfides on the surface of the pyroxene crystal.
The size of the iron-sulfides ranges from less than 100 nm tolarger than 1 µm. Some iron-sulfides show melt-splashed shape,indicating high velocity impact after melting of the sulfide. It isinteresting that the surface of the large pyroxene crystal whichcontains micro-nano craters and iron-sulfide droplets shows sharp
steps. It indicates that the surface is relatively fresh, and theduration of exposure to the space would be short. The highnumber density of the impact-feature on such a fresh surface andsmall area suggest a bombardment of a cluster of tiny particles ina short period on the surface of the particle. It indicates highly
heterogeneous spatial distribution of such small dust particles in
the interplanetary space.
References: [1] Nakamura et al. 2011. Science 333:11131116. [2] Nakamura et al. 2012 PNAS, 109, E624。