Radiolytic solar wind water in rims on an Itokawa regolith grain
John P. Bradley1 and Hope A. Ishii11Hawaii Institute of Geophysics and Planetology, University of Hawai‘i, Honolulu, HI 96922, USAIntroduction: The detection using electron energy-loss spectroscopy (EELS) of OH/H2O in-situ in vesicles within sputteredrims on interplanetary exposed to the solar wind for ~104-105 years provided the first direct evidence that the interaction of thesolar wind with oxygen-rich silicates produces water; additional confirmation was provided by detection of water using EELS
in the sputtered rims on silicate mineral standards following proton irradiation at solar wind fluences [1]. The results are
consistent with OH/H2O signatures on the surface of airless bodies observed by remote spectroscopy, e.g. [2]. Regolith fromthe S-type asteroid Itokawa returned by the Hayabusa mission enables us to further explore the effects of solar wind irradiation
on airless bodies. Enrichments in water and hydroxyl in the solar wind-irradiated rim of an Itokawa olivine grain and a labirradiated olivine standard have been detected using atom probe tomography [3]. Here we describe detection of OH/H2O inthe surface of another Itokawa olivine grain using electron energy loss spectroscopy (EELS).
Experimental procedure: Hayabusa regolith particle RA-QD02-0332 was carbon-coated for imaging and elementmapping in a focused ion beam-scanning electron microscope (FIB-SEM: FEI Helios 660 with Oxford Instruments EDS), and
FIB sections of selected regions were extracted at Univ. Hawai‘i using Pt protective straps. These were further analyzed on(scanning) transmission electron microscopes at the Molecular Foundry (S/TEM: FEI Titan ChemiSTEM with EDS, TEAM
1.0 with Gatan Continuum GIF for EELS).
Results: RA-QD02-0332 is a ~48µm grain that consists of olivine, pyroxene, plagioclase and, likely, K-feldspar. In SEM
imaging, glassy melt splashes with degassing vesicles are visible in multiple locations on the surface. Fractures are observed aswell as significant fine-grained adhering material, including angular fragments and spheroidal particles with melt droplet
appearance. Two FIB sections crossing fine-grained material contain small mineral crystals of albitic plagioclase and fayalitic
olivine with lesser amounts of high-Ca pyroxene and iron-sulfide. Fine grain sizes range from ~100 nm to several microns
across, and grain shapes range from angular to euhedral to rounded, consistent with some brecciation on the parent body.
Another FIB section contains a ~4 µm plagioclase grain with fracture and defects in the interior consistent with shock, anamorphized rim on one face that has lost most of its Na and some Ca relative to the underlying crystal, and surficial lacy Ferich material that is likely remnants of the impactor.
A FIB section through an olivine crystal shows melt splash glass and a ~40 nm in thick vesiculated amorphous rim on thesurface (Figure 1a). Energy dispersive spectral mapping shows that the amorphous rim has a composition similar to theunderlying olivine but with minor additional Al and Ca present. An EELS spectrum from one of the vesicles shows features at~8 and ~13.5 eV features. These features correspond to the energy gap and H-K core scattering edge from hydroxyl and/or
molecular water, like those we previously observed in rims on IDPs, water in a liquid cell, proton irradiated silicate standards
and electron beam-damaged talc and brucite [1,4]. Weaker features are observed in the amorphous rim off the vesicle.
Figure 1: (a) Darkfield image and highermagnification inset show vesicles in amorphous
rim on Itokawa olivine,.(b)EELS low-loss
spectrum shows ~8 eV and ~13 eV featuresfrom one of the vesicles superimposed on theamorphous rim volume plasmon.
Discussion: The vesiculated amorphous rimon the olivine crystal in Itokawa grain RAQD02-0332 establishes that it was exposed tothe solar wind on the surface Itokawa.
Detection of water by EELS confirms the prioratom probe tomography detection of water inanother solar wind-irradiated rim on an Itokawa
olivine [3]. The relatively low intensity of the features observed here from the vesicle in the amorphous rim on Itokawa olivine
compared to those observed in some vesicles in IDPs (Fig 1c), may be a consequence of a shorter (solar wind) exposure agesof Itokawa regolith grains relative to IDP orbital exposure ages.
References: [1] Bradley J.P. et al. (2014) Proceedings of the National Academy of Sciences 111:1732. [2] Li S. andMilliken R.E. (2017) Science Advances 3:e1701471. [3] Daly L. et al. (2021) Nature Astronomy 5:1275. [4] Zhu C. et al.
(2019) Proceedings of the National Academy of Sciences 116:11165-11170.