Fe‐redox changes in Itokawa space‐weathered rims

Abstract

Synchrotron Fe-K X-ray absorption spectroscopy and transmission electron microscopy have been used to investigate the mineralogy and Fe-redox variations in the space-weathered (SW) rims of asteroidal samples. This study focuses on the FIB lift-out sections from five Itokawa grains, returned by the Hayabusa spacecraft, including samples RB-QD04-0063, RB-QD04-0080, RB-CV-0011, RB-CV-0089, and RB-CV-0148. Each of the samples featured partially amorphized SW rims, caused by irradiation damage from implanted low mass solar wind ions, and the impacting of micrometeorites. Using brightfield and HAADF-STEM imaging, vesicular blistering and nanophase Fe metal (npFe 0) particles were observed within grain rims, and solar flare tracks were observed in the substrate host grain, confirming the presence of SW zones. We use Fe-K XANES mapping to investigate Fe-redox changes between the host mineral and the SW zones. All SW zones measured show some increases in the ferric-ferrous ratio (Fe 3+ /ΣFe) relative to their respective host grains, likely the result of the implanted solar wind H + ions reacting with the segregated ferrous Fe in the surface material.

Comparison of space weathering features in three particles from Itokawa

“…The samples were stored in a nitrogen dry box but exposed to ambient air several times during transfer from the sample container to the epoxy stub. Overall, the material in the Itokawa particles is not oxidized, with EELS measurements of interior regions of grains showing only Fe 2+ present in olivine and pyroxene, consistent with previous XANES analyses of Itokawa HPx and LPx and with other LL OCs (Noguchi et al 2014a), although Hicks et al (2020) reported minor Fe 3+ in several samples. The formation of an oxidized rim due to laboratory exposure is consistent with XPS measurements of experimentally irradiated olivine, which have shown that initial metallic Fe formed during the experiment is quickly oxidized once removed from vacuum as oxygen from the atmosphere is able to interact with damaged and broken bonds in the material (Loeffler et al 2009).…”

“…The oxidation on most of the rims occurs to depths of 10-20 nm, similar to the depth of sulfur depletion in the troilite rim and consistent with the depth of greatest damage by hydrogen from the solar wind (Noguchi et al 2014b;Matsumoto et al 2020). Hicks et al (2020) recently reported oxidation of the space weathered rim in several Itokawa particles, with up to 14% more Fe 3+ in one olivine rim compared to the host grain, and confirmed metallic npFe 0 within the oxidized rim matrix. Their XANES measurements average over the full rim depth (~50 nm), and thus are consistent with our higher oxidation values at very shallow depths.…”

“…Hicks et al. (2020) recently reported oxidation of the space weathered rim in several Itokawa particles, with up to 14% more Fe 3+ in one olivine rim compared to the host grain, and confirmed metallic npFe 0 within the oxidized rim matrix. Their XANES measurements average over the full rim depth (˜50 nm), and thus are consistent with our higher oxidation values at very shallow depths.…”

“…2014a), although Hicks et al. (2020) reported minor Fe 3+ in several samples. The formation of an oxidized rim due to laboratory exposure is consistent with XPS measurements of experimentally irradiated olivine, which have shown that initial metallic Fe formed during the experiment is quickly oxidized once removed from vacuum as oxygen from the atmosphere is able to interact with damaged and broken bonds in the material (Loeffler et al.…”

“…The fact that lattice spacings in the oxidized rim on the Itokawa troilite, while present, could not be clearly matched to any specific phase could indicate that it is poorly crystalline and only partially oxidized, which would be possible if a highly reactive metal layer was present before terrestrial exposure. Oxidation that did take place while on the asteroid surface could be due to the iron disproportionation reaction, as hypothesized by Hicks et al (2020), which would then link the degree of matrix oxidation to the amount of npFe 0 present in the rim. However, in the grain where we see oxidized material beneath a less oxidized rim, npFe 0 remains concentrated mostly in the rim.…”