TRANSMISSIONELECTRONMICROSCOPYOFITOKAWA REGOLITH GRAINS. L. P. Keller1 and E. L.
Berger2,1. 1Robert M. Walker Laboratory for Space Science,Code KR, ARES, NASA/JSC, Houston, TX 77058. E-mail:
Lindsay.P.Keller@nasa.gov. 2GeoControl Systems, Inc./JETS,Houston, TX 77058.
Introduction: In a remarkable engineering achievement, theJAXA space agency successfully recovered the Hayabusa spacecraft in June 2010, following a non-optimal encounter and surface sampling mission to asteroid 25143 Itokawa. These are thefirst direct samples ever obtained and returned from the surfaceof an asteroid. The Hayabusa samples thus present a special opportunity to directly investigate the evolution of asteroidal surfaces, from the development of the regolith to the study of theeffects of space weathering. Here we report on our preliminary
TEM measurements on two Itokawa samples.
Methods: We were allocated particles RA-QD02-0125 andRA-QD02-0211. Both particles were embedded in low viscosity
epoxy and thin sections were prepared using ultramicrotomy.
High resolution images and electron diffraction data were obtained using a JEOL 2500SE 200 kV field-emission scanningtransmission electron microscope. Quantitative maps and analyses were obtained using a Thermo thin-window energydispersive x-ray (EDX) spectrometer.
Results: Both particles are olivine-rich (Fo70) with µm-sized
inclusions of FeS and have microstructurally complex rims. Particle RA-QD02-0125 is rounded and has numerous sub-µm
grains attached to its surface including FeS, albite, olivine, andrare melt droplets. Solar flare tracks have not been observed, but
the particle is surrounded by a continuous ~50 nm thick, stucturally disordered rim that is compositionally similar to the core ofthe grain. One of the surface adhering grains is pyrrhotite showing a S-depleted rim (~8-10 nm thick) with nanophase Fe metalgrains (<5 nm) decorating the outermost surface. The pyrrhotite
displays a complex superstructure in its core that is absent in theS-depleted rim.
Particle RA-QD02-0211 contains solar flare particle tracks(~2x109 cm-2) and shows a structurally disordered rim ~100 nmthick. The track density corresponds to a surface exposure of~103-104 years based on the track production rate of [1]. The disordered rim is nanocrystalline with minor amorphous material
between crystalline domains. Quantitative element maps show
the outermost ~10 nm of the disordered rim is Si-rich.
Discussion and Conclusions: Both particles record the effects of space weathering processes on Itokawa. Noguchi et al.
[2] proposed that the disordered rims they observed on Itokawa
particles largely result from solar wind radiation damage and wearrive at a similar conclusion for the two particles we analyzed.
The microstructure of the S-depleted layer on the pyrrhotite grain
in RA-QD02-0125 is similar to that observed in troilite irradiatedwith ~1018 4 kV He+ [3, 4]. Prolonged irradiation has also beenshown to disorder pyrrhotite such that the superstructure reflections are lost [5].
References: [1] Sandford S. A. 1986. Icarus 68:377-386. [2]Noguchi T. et al. 2013. MAPS 48: in press. [3] Keller L. P. et al.
2010. Abstract #1172. 41st Lunar and Planetary Science Conference. [4] Loeffler M. J. et al. 2008. Icarus 195:622-629. [5]Christoffersen R. and Keller L. P. 2011. MAPS 46:950-969.