MICROCHEMICAL AND STRUCTURAL EVIDENCEFOR SPACE WEATHERING IN SOILS FROM ASTEROID
ITOKAWA.
M. S. Thompson,1 R. Christoffersen2,3, and T. J. Zega1. 1Lunarand Planetary Laboratory, Department of Planetary Sciences,
University of Arizona, 1629 E. University Blvd, Tucson, AZ
85721, mst@lpl.arizona.edu. 2Jacobs Technology, Houston, TX
77258-8847, 3NASA JSC Mail Code KR, Houston, TX 77058.
Introduction: The chemistry, microstructure and opticalproperties of grains on the surfaces of airless bodies are continuously modified due to their interactions predominantly with solar
energetic ions and micrometeorite impacts. Collectively known
as space weathering, this phenomenon results in a discrepancy
between remotely sensed spectra from asteroids and those acquired directly from meteorites. The return of pristine samples
from the asteroid Itokawa provides insight into surface processes
on airless bodies and will help in correlating remote sensing datawith laboratory analysis of meteorites.
Samples and Methods: We examined Itokawa samples RAQD02-0042-01 and RA-QD02-0042-02, ultramicrotomed sections of a singular grain prepared by the Hayabusa sample curation team. We analyzed these slices using a 200 keV JEOL 2010F
transmission electron microscope (TEM) at Arizona State University and a 200 keV JEOL 2500SE TEM at NASA JSC. Bothfield emission TEMs are equipped with energy-dispersive X-ray
spectrometers (EDS) and scanning TEM (STEM) detectors.
Results and Discussion: TEM observations reveal that thesectioned grain predominantly consists of a single crystal of lowCa orthopyroxene, with subsidiary smaller regions of olivine, FeNi sulfide, and Fe-Ni metal. EDS-spectrum imaging and highresolution TEM (HRTEM) show local, nanocrystalline regions ofthe outermost 2 to 5 nm of the pyroxene are composed of an FeMg-S-rich and Si- and O-depleted layer that is underlain by a 2to 5-nm thick amorphous zone enriched in Si. These layers occur
in multiple microtome slices and have uniform thicknesses. We
also observe localized ‘islands’ of material on the surface of thepyroxene which HRTEM imaging indicates are amorphous andEDS measurements show are compositionally heterogeneous. A
10- to 60-nm thick partially amorphous zone occurs below thecompositionally distinct rim. While this zone is associated withthe compositionally heterogeneous outer layer, it also occurs as alocal stand-alone feature on the exterior rim of the grain. Areas ofthe pyroxene grain rim also exhibit a vesicular texture.
The TEM data indicate a complex history of space weathering for samples RA-QD02-0042-01 and -02. The outermost layerof nanocrystalline material with varied composition is consistent
with previously suggested [3-4] chemical and structural processing by solar wind ions, with a possible additional role for impact vapor deposition [3-4]. The amorphous and compositionallydistinct islands on the surface of this grain, similar to lunar glasses, suggest formation through vapor deposition via micrometeorite impact events. In comparison, the amorphization and vesiculation textures are likely a product of radiation damage from thesolar wind. The depth and degree of amorphization, in conjunction with model calculations, will help provide an upper limit on
exposure time for these particles.
References: [1] Hapke B. 2001. Journal of Geophysical Research- Planets 106:10039-10073. [2] Pieters C. M. et al. 2000.
Meteoritics and Planetary Science 35:1101-1107. [3] Noguchi T.
et al. 2011. Science 333:1121-1125. [4] Noguchi T. et al. 2013.
Meteoritics and Planetary Science.