SPACE-WEATHERING FEATURES ON TWO HAYABUSAPARTICLES IMAGED BY HELIUM ION MICROSCOPY
R. C. Ogliore1, K. Nagashima1, and V. Shutthanandan 2 1Univ. Of
Hawai`i at Manoa. (ogliore@higp.hawaii.edu). 2Environmental
and Molecular Sciences Laboratory, Pacific Northwest NationalLaboratory.
Introduction: Particles returned from the S-type asteroid
Itokawa by JAXA's Hayabusa mission show evidence of space
weathering [e.g. 1]. Quenched melt splashes, shock lamellae, andmicrocraters decorate the outer surfaces of Hayabusa particles.
These features can be very small (<1 μm in size) and very shallow (within a 100 nm of the surface). The smallest space-weathering surface features and textures are difficult to resolve by secondary electron (SE) imaging in a field-emission SEM (FEGSEM): SEs created from backscattered electrons from the incident beam, electron-electron scattering in the sample surface, anda relatively high mean SE energy degrades spatial resolution andsurface sensitivity. TEM has excellent spatial resolution, but it isdifficult to reconstruct the three-dimensional structure of surfacefeatures from electron-transparent thin sections. By using He ionsinstead of electrons, He ion microscopy (HIM) can produce asmaller probe size (< 0.1 nm) than FEG-SEM as well as a smaller
surface interaction volume for SE imaging [2]. Additionally, HIM
has 5–10 the depth of field of FEG-SEM. SE images producedby HIM can resolve the smallest and shallowest surface spaceweathering features that cannot be seen in a FEG-SEM.
Methods: We used the Zeiss Helium Ion Microscope at theEnvironmental and Molecular Sciences Laboratory at the PacificNorthwest National Laboratory to image two Hayabusa particles:
RB-QD04-0062 (“#62”: 40 μm, olivine and plagioclase) and RBQD04-0091 (“#91”: 43 μm, olivine and plagioclase). We imaged
with a 35 keV, ~0.6 pA He ion beam. The particles were removed
from their glass slides with a Sutter micromanipulator and tungsten needle and then stuck on a SEM stub with a thin layer of
Post-It note glue. The stub was coated with ~6 nm of carbon forelectrical conductivity. We collected SE images of the surfaces of
the two Hayabusa particles with magnifications up to 190,000
Results: Both Hayabusa particles show variable surface textures, a variety of splash melt features, adhering grains, and smallholes. Two porous particles, with structures reminiscent of chondritic-porous interplanetary dust particles, were found adhering to
the surface of the Hayabusa grains (a 1.2 μm object on #91, a 350
nm object on #62). Much of the surface of #62 is covered withsmall bumps 25–100 nm in size, whereas other regions (usuallyobvious cleavage faces) are free of small bumps. There is a large,
6 μm quenched melt splash feature on the surface of #62. We also
imaged a 30 μm grain of lunar soil which had a much higher density of sub-μm splash melt features than the Hayabusa grains. We
saw no obvious impact craters (holes with raised rims). A face of#91 shows multiple concoidal fractures and splash melt droplets
and pancakes, but fewer and smaller surface bumps compared to#62.
Conclusions: Helium ion microscopy images of the surfacesof two Hayabusa grains revealed diverse space-weathering features on scales from several nm to several μm. The variations insurface textures indicates that these grains experienced a complex
history of fracturing and exposure to space-weathering processeson the surface of asteroid Itokawa.
References: [1] E. Nakamura et al. 2012. Proceedings of theNational Academy of Sciences 109:11 E624–E629. [2] B. W.
Ward et al. 2006. Journal of Vacuum Science & Technology 24:6
2871–2874.