Abstract
Here we report microchemical and microstructural features indicative of space weathering in a particle returned from the surface of asteroid Itokawa by the Hayabusa mission. Space weathering features include partially and completely amorphous rims, chemically and structurally heterogeneous multilayer rims, amorphous surface islands, vesiculated rim textures, and nanophase iron particles. Solar-wind irradiation is likely responsible for the amorphization as well as the associated vesiculation of grain rims. The multilayer rims contain a nanocrystalline outer layer that is underlain by an amorphous inner layer, and both have compositions that are distinct from the underlying, crystalline orthopyroxene grain. The multilayer rim features could be derived from either radiation-induced sputter deposition or vapor deposition from micrometeorite impact events. The amorphous islands on grain surfaces have a distinctive morphology and composition suggesting that they represent surface deposits of melt derived from micrometeorite impact events. These observations indicate that both irradiation damage and micrometeorite impacts play a role in surface modification and space weathering on asteroid Itokawa.
Highlights
We report microchemical and microstructural features indicative of space weathering in a particle returned from the surface of asteroid Itokawa by the Hayabusa mission
A further detailed analysis of 12 particles was performed, and these results have shown that several grains exhibit space weathering features including radiationdamaged rims, Fe nanoparticles, vesiculated textures, and chemically distinct, surface-deposited rims (Noguchi et al 2014)
We investigated the samples using several analytical scanning transmission electron microscopes (STEM instruments) including (1) a 200-keV FEI Osiris ChemiSTEM equipped with a high brightness field-emission gun (X-FEG), quad-annular silicon drift detector (SDD) for energy-dispersive X-ray spectroscopy (EDS), and both dark-field (DF) and bright-field (BF) STEM detectors previously located at FEI headquarters (Portland, OR, USA); (2) a 200-keV JEOL 2500SE analytical S/transmission electron microscopy (TEM) (JEOL Ltd., Tokyo, Japan) at NASA Johnson Space Center equipped with a Thermo thin-window Li-drifted Si [Si(Li)] EDS detector and BF and DF STEM detectors (Thermo Fisher Scientific, Waltham, MA, USA); and (3) a 200-keV JEOL 2010F S/TEM at Arizona State University equipped with a DF STEM detector and EDAX thin-window Si(Li) EDS system
Summary
We report microchemical and microstructural features indicative of space weathering in a particle returned from the surface of asteroid Itokawa by the Hayabusa mission. Space weathering features include partially and completely amorphous rims, chemically and structurally heterogeneous multilayer rims, amorphous surface islands, vesiculated rim textures, and nanophase iron particles. The multilayer rim features could be derived from either radiation-induced sputter deposition or vapor deposition from micrometeorite impact events. The amorphous islands on grain surfaces have a distinctive morphology and composition suggesting that they represent surface deposits of melt derived from micrometeorite impact events.
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