Coordinated Nanoscale Compositional and Oxidation State Measurements of Lunar Space‐Weathered Material

Abstract

Space weathering on airless bodies includes a number of processes, such as micrometeorite impacts and solar wind bombardment, which leads to a variety of microscale to nanoscale alteration features, including vapor deposited layers on grain and rock surfaces and creation of nanophase opaque inclusions. The nanophase inclusions cause reddening and darkening of the visible to near-infrared spectra of space weathered material, features associated with increasing space exposure of many airless body regoliths. On the Moon, most nanophase inclusions are metallic iron (npFe 0 ), but recent work using aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy has provided evidence of oxidized nanoparticles in space weathered lunar soil grains. We examined three different lunar soils in order to confirm the finding of oxidized nanophase inclusions and to provide detailed elemental and mineralogical information about the surrounding material. Our data show that substrate and rim composition are key factors in determining whether highly localized oxidation occurs; for example, nanophase inclusions in rims on low Fe substrates are more prone to oxidation. Detailed understanding of the phases and features present in these samples is necessary for the correct interpretation of remotely sensed data as well as extrapolation from processes on the lunar surface to those on other airless bodies.Plain Language Summary The Moon and asteroids are being constantly altered by interactions with the space environment, a process known as space weathering. These interactions, such as with solar wind ions and micrometeoroids, can be studied by examining samples from the Moon and analyzing the nanoscale features present in surface coatings or rims on small lunar dust grains. Using techniques such as energy dispersive X-ray spectroscopy and electron energy loss spectroscopy in the scanning transmission electron microscope, we are able to image and measure the composition and oxidation state of the nanoscale inclusions and layers in these rims. These data provide evidence of complex material interactions, such as formation of metallic iron inclusions at a variety of sizes contained within specific sublayers, and oxidation to magnetite of some of those inclusions. From this information, we can better interpret spacecraft and telescopic data of the Moon and asteroids that can be significantly influenced by space weathering and the presence of nanoscale inclusions. We can also compare these data to samples that have been experimentally space weathered in the laboratory using pulsed laser or ion irradiation in order to gain a better understanding of the conditions necessary to create observed features.

Comparison of space weathering features in three particles from Itokawa

“…3). The surfaces of the olivine and LPx in A0114‐4 are oxidized to a depth of 30–50 nm with a calculated Fe 3+ /ΣFe ˜0.4 at the surface (although with very large error bars due to noise; see supplementary material and methods in Van Aken and Liebscher 2002; Burgess and Stroud 2018a), and very small (<5 nm) npFe 0 are observed to a maximum depth of ˜60 nm. In several locations, we were able to confirm that the nanoparticles are indeed metallic.…”

“…The monolayer could show the initial stages of plagioclase rim formation. In lunar plagioclase, complex npFe 0 ‐bearing rims composed of layers only a few nanometers thick have been noted (Burgess and Stroud 2018a).…”

“…The EELS measurements have a typical energy resolution, determined from the full-width half-maximum of the zero loss peak, of 0.5 eV. The degree of oxidation of Fe-bearing phases was determined according to the Fe-L edge and whiteline ratio methods described by Burgess and Stroud (2018a). Briefly, Fe 2+ mostly contributes to peaks at 708 and~722 eV for the L 3 and L 2 edges, respectively, while the Fe 3+ contribution is centered at~710 and (Potapov and Lubk 2019).…”

Nanophase iron carbides in fine‐grained rims in CM2 carbonaceous chondrites: Formation of organic material by Fischer–Tropsch catalysis in the solar nebula

“…There are two known possible mechanisms by which nanoparticles of Fe,Ni metal can form in a parent body environment, specifically space weathering of Fe‐bearing silicates (Thompson et al. 2016; Burgess and Stroud 2018) and by shock melting of Fe‐bearing silicates (e.g., Guo et al. 2020).…”

Space Weathering of FeS Induced via Pulsed Laser Irradiation

“…This spectral mismatch, which particularly refers to the region between ~0.5 and 2.5 μm, is manifested through a reddened (positive) slope, attenuated absorption features, and darker overall reflectance compared with the fresh unirradiated surface (Chapman, 2004; Clark et al, 2002; Gaffey, 2010). While recent electron microscope analysis has shown that lunar soil contains a variety of Fe (Fe 0 , Fe 2+ , and Fe 3+ ) nanoparticles (Thompson et al, 2016; Burgess & Stroud, 2018), it is the chemically reduced iron nanoparticles (Fe 0 or npFe) created by either solar wind irradiation or micrometeorite impacts that are likely causing this spectral mismatch (e.g., Brunetto et al, 2006; Hapke, 2001; Loeffler et al, 2009; Loeffler et al, 2016; Sasaki et al, 2001). Depending on the size of the npFe, the spectra of the material will appear more sloped (reddened) and, in some cases, darker (Noble et al, 2007).…”