Space weathering of sulfides and silicate minerals from asteroid Itokawa
Laura Chaves1, Michelle Thompson1
1
Purdue University
Space weathering refers to the progressive spectral, microstructural, and chemical alterations of mineral grains on the surfacesof airless planetary bodies [1]. These changes are produced by high-velocity micrometeoroid impacts and solar wind irradiation,
causing vesiculation, melt and vapor deposits, sputtering, ion implantation, and amorphization on regolith grains. Spectrally,
space weathering produces multiples alterations in the visible near-infrared (VNIR) wavelengths, including reddening,
darkening, and attenuated absorption bands [1]. The spectral anomalies have been attributed to the presence of Fe nanoparticles(npFe) that are mainly composed of metallic iron (npFe0). Analyses on individual lunar grains show that the spectral alterationsin the VNIR depend on the particle size, where npFe with diameters < 5 nm produce reddening, whereas particles >10nm produce
darkening [2]. In 2010, the Japan Aerospace Exploration Agency (JAXA) Hayabusa mission successfully collected 1534 regolithparticles from the surface of S-type asteroid Itokawa [3]. Prior to sample return, S-type asteroids were hypothesized to be theparent bodies of ordinary chondrites; however, the spectral characteristics of the meteorites in our collections differed from theseasteroids. Geochemical analyses in Itokawa particles indicated a composition similar to LL4-LL6 chondrites [3], corroborating
the link between ordinary chondrites and S-type asteroids and demonstrating the importance of space weathering studies for
accurate characterization of asteroidal surfaces.
Previous studies have identified space weathering features in Itokawa regolith particles, including disordered rims, chemicallydistinct layers, whisker-like structures, sulfur depleted rims, and Fe0 and FeS nanoparticles [4,5,6,7,8]. Most space weatheringstudies have concentrated on understanding the response of silicate minerals to interplanetary space [4,9,10] as these phases are
the most common in lunar samples and Itokawa particles. However, our understanding of the behavior of other minerals underspace weathering conditions is still in a very early stage. Among these understudied minerals are sulfides which are present inthe sample collection of asteroid Itokawa [11] and are relevant minerals in carbonaceous chondrites [12], thought to be meteoritic
counterparts of asteroids Bennu, Ryugu, and Psyche [13,14,15].
To compare the responses of silicates and sulfides under space weathering conditions, we performed coordinated analyses of theRC-MD01-0025 Itokawa grain previously identified to contain olivine, low-Ca pyroxene, and Fe-Ni- sulfides. We embedded
the particle in low-viscosity epoxy and prepared electron transparent thin sections with an approximate thickness of 50 nm usinga Leica EM UC7 ultramicrotome. To identify microstructural and chemical properties associated to space weathering in silicate
and Fe-Ni- sulfide grains in the Itokawa particle, we used a FEI Talos 200 KeV transmission electron microscope (TEM) coupledwith a Super –X silicon drift detector (SDD) energy-dispersive X-ray spectrometer (EDS). The sample preparation of the Itokawa
particle and the electron microscopy analyses of the ultramicrotomed samples were performed at Purdue University.
Bright-field (BF) TEM imaging shows the presence of a ~50 nm mottled rim in an olivine grain (Fig. 1a). Chemically, EDSmaps show the rim presents three layers (Fig. 1b,c). Layer 1 (L1) thickness is ~30 nm and shows similar O, Mg, Fe, and Si
concentrations as the bulk grain. Layer 2 (L2) has a thickness of 5-10 nm and presents depletion of Mg, Fe, and enrichment of
Si compared to L1. Layer 3 (L3) has a thickness of 5-10 nm and is Si depleted compared to L1 and L2; it is enriched in Fe andMg compared to L2 but depleted in these elements compared to L1. High resolution (HR) TEM images show an amorphous
region in the rim's outer 5-10 nm with some nanocrystalline regions. The nanocrystalline domains present d-spacing values of
0.20 nm that correspond to npFe0. Metallic iron nanoparticles were previously identified in silicates and Fe-sulfides in Itokawa
regolith particles [4,5]. The identification of chemically distinct layers in returned samples [4,6,7] and in H+ irradiation
experiments on olivine [16] suggests this multilayer rim might correspond to a combination of sputtering, redeposition, and ion
irradiation damage processes.
High resolution (HR) TEM imaging of the Fe-Ni- sulfide grain shows d-spacing values of 0.28 nm similar to (222) pentlandite
and the presence of a ~5-10 nm rim that presents nanocrystallinity (Fig. 2a). EDS mapping shows that the rim is depleted in Ni
and S but enriched in Fe compared to the bulk mineral (Fig. 2b,c). Previous studies in Itokawa samples have identified sulfurdepleted rims Fe-sulfides [5,17], and the origin of these rims is attributed to solar wind damage. The depletion in S and Ni andthe enrichment of Fe of the rim suggest it might have formed in a complex process of sputtering and redeposition. The
microstructural and chemical characteristics in the olivine and pentlandite grains further indicate that solar wind irradiation isthe main contributor to the space weathering of both mineral phases. Future work will include the TEM and EDS analyses of thelow-Ca pyroxene grains present in the RC-MD01-0025 regolith particle. In addition to TEM and EDS, we will perform electronenergy-loss spectroscopy (EELS) to compare the oxidation state of Fe between the mineral grains and the space weathered rims.
Understanding how different mineral phases respond to space weathering conditions using returned samples is crucial toaccurately interpret remote sensing observations of the surfaces of airless bodies and adequately characterize the regolith samples
collected from asteroids Ryugu and Bennu.

Figure 1. Space weathered rim on olivine grain. a) Bright field (BF) TEM image shown the ~50 nm mottled rim on olivine. b)
High-angle annular dark field image overlain with EDS maps (O, Mg, Si, Fe). c) EDS line scan showing the chemical layeringof the rim.

Figure 2. Space weathered rim on Fe-Ni- sulfide grain. a) High-resolution (HR) TEM showing the presence of a 5-10
nanocrystalline rim. b) High-angle annular dark field (HAADF) image overlain with EDS maps of S, Fe, and Ni. c) EDS line
scan showing the Ni and S depletion on rim compared to the bulk mineral.

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