Fe-K XANES and HR-TEM analyses of Apollo lunar grain space weathered surfaces

Introduction

Space weathering due to the bombardment of electrons and solar wind upon the exposed lunar surface shows as an apparent spectral darkening and reddening in ground-based and lunar-orbital observations [1].

Space weathered rims have been observed on soil surface samples, returned by the Apollo landings [1,2], featuring amorphized material and nanophase Fe metal (npFe⁰) particles formed due to the implantation of solar wind H⁺ ions reducing the host grain mineral oxides to form metal [2,3]. Oxidation of these Fe particles has also been shown, and a suggested correlation between oxidation and lunar soil maturity [3].

In this study, we investigate Fe-redox changes in the space weathered rims of Apollo 17 lunar surface soil samples, using TEM and X-ray nanoprobe Fe-K XANES.

Samples and Methods

The lunar sample number is 78481,29, collected from the top 1 cm of trench soils during the Apollo 17 lunar landing [4]. Observing an abundance of blisters (a feature of space weathering [1,5]) on the grain surfaces indicated space weathering to be investigated, and FIB-SEM lift-out sections were extracted from at least three sample grains.

The three grains consisted of two augite pyroxenes, En₈₁Fs₁₆ (#A17-3) and En₈₅Fs₁₂ (#A17-5), and one olivine Fa₃₉ (#A17-6), with partially amorphised space weathered rims measuring up to ~100 nm thick featured in all three (e.g. Figure 1a), observed in HAADF-STEM imaging using a JEOL JEM-ARM300CF at ePSIC in the Diamond Light Source synchrotron facility. A deposition of tungsten (W-dep) on the grain surface was used during FIB lift-out preparation which can be seen in Figure 1a.

Fe-redox was analysed using the I-14 X-ray Nanoprobe Beamline at Diamond, similarly to previously investigated Itokawa asteroid samples [6,7,8]. A series of XRF maps are obtained over the Fe-K XAS energy range 7000 and 7300 eV, with a higher resolution of energy increments over the XANES features (~7100-7150 eV). Using Mantis 2.3.02 [9] to process the XANES mapping, spectra can be isolated for the space weathered zone (SWzone), the grain ‘Host’ mineralogy, and the W-dep. All measured spectra are normalized in Athena 0.8.056 [10].

Results

Nano-grains measuring ~2-3 nm in diameter within the partially amorphised space weathered zones (see Figure 1b), confirmed to be Fe metal when measuring lattice fringe spacings. Figure 1c shows a nano-grain with lattice spacings measuring ~2.06 Å, observed in the #A17-3 augite sample. Other lattice spacing measurements of up to ~2.10 Å in each of the three lunar grains confirms the presence of Fe metal, similar to previous studies of Itokawa samples [5].

In the Fe-K XANES analysis, the 1s→3d pre-edge centroid positions are defined by the intensity-weighted average across baseline subtracted peaks (see Figure 2). A shift in the 1s→3d centroid position from the host mineral to the SWzone suggests Fe redox variation, where an positive shift in energy position is an increase in ferric (Fe³⁺) content, based on a ferric-ferrous ratio (Fe³⁺/ΣFe) defined by Fe-K XANES measurements of standard reference minerals. The largest variation observed between SWzone and the respective substrate host mineral is a positive shift of 0.23 ±0.06 eV, equivalent to a ferric increase of Fe³⁺/ΣFe = 0.14 ±0.03 from host to SWzone. There is a consistent positive shifting in the 1s→3d centroid energy positions observed in all three lunar samples, with average ferric increases of: Fe³⁺/ΣFe = 0.08 ±0.03 (#A17-3); Fe³⁺/ΣFe = 0.11 ±0.03 (#A17-5); and Fe³⁺/ΣFe = 0.10 ±0.03 (#A17-6).

Discussion

A minor Fe-redox variation of up to Fe³⁺/ΣFe ~0.14 ±0.03 has been observed in these lunar grains, similar to the space weathered rims of asteroid Itokawa grains with increased ferric contents ranging Fe³⁺/ΣFe ~0.02-0.14 ±0.03 [6,7,8]. The ferric increase is in the dominant silicate phase of the amorphized rims, as Fe metal appears to have had no influence on the Fe-K XAS spectra. Additionally, the npFe⁰ particles show no oxidation present, confirmed by the Fe metal lattice spacings, unlike the oxidised Fe particles observed in previous space weathered lunar regolith samples [3].

The minor increase in ferric material is likely the result of solar wind H⁺ implantation on the ferrous silicate grain surfaces, segregating the Fe to form Fe metal. The resulting water vapour then oxidises some of the remaining Fe to Fe³⁺ in the silicate phase.

It has been proposed that the spectral reddening observed on surfaces of airless bodies is due to the npFe⁰ particles formed in space weathering [1], and spectral reflectance models have also shown that Fe³⁺ can increase reddening too [3]. Continued investigations into the mineralogical complexities associated with space weathering could reveal more spectral effects. Further samples to be investigated will include new Apollo samples, as well as potential space weathered material collected by asteroid missions Hayabusa2 and OSIRIS-REx.

Conclusions

The three Apollo lunar surface grains feature space weathered rims up to ~100 nm thick. The npFe⁰ particles measured lattice spacings of ~2.06-2.10 Å, suggesting they had not been oxidised. However, Fe-K XANES analyses suggest a minor oxidation of up to Fe³⁺/ΣFe ~0.14 ±0.03 occurring in the dominant silicate phase of the space weathered rims. These results are consistent with previously analysed space weathered asteroid Itokawa samples, suggesting Fe-redox variations in silicate material exposed on the surfaces of airless planetary bodies may be a key part of space weathering effects.

References

[1] Pieters C. A. and Noble S. K. (2016) JGR: Planets, 121, 1865-1884. [2] Hapke B. (2001) JGR, 106, 10039-10073. [3] Thompson M. S. et al. (2016) Meteorit. Planet. Sci., 51, 1082-1095. [4] Butler P. (1973) MSC 03211 Curator’s Catalog. pp 447. [5] Noguchi T. et al. (2014) Meteorit. Planet. Sci., 49, 188–214. [6] Hicks L. J. et al. (2019) LPSC L, Abstract #1805. [7] Hicks L. J. et al. (2019) 82nd Annual Meeting of The Meteoritical Society, Abstract #6330. [8] Hicks L. J. et al. (2019) 2nd British Planetary Science Conference, Abstract. [9] Lerotic M. et al. (2014) J. Synchrotron Radiat., 21, 1206–1212. [10] Ravel B. and Newville M. (2005) J. Synchrotron Radiat., 12, 537–541.