Kate Burgess

Space weathering of lunar minerals, due to bombardment from solar wind (SW) particles and micrometeoroid impacts, modifies the mineralogy within tens of nanometers of the surface, i.e., the rim. Spectroscopic signatures of these modifications, observed via remote sensing, have long been used to gauge surface exposure times on the Moon. However, the relative contributions of SW and micrometeoroids in the creation of rim features are still debated, particularly for the nanometer-scale clusters known as nanophase iron (npFe ⁰ ), which commonly form in ferrous minerals. We address this issue in the laboratory, using deuterium ions and low-energy electrons as a synthetic solar wind plasma to irradiate ilmenite (FeTiO ₃ ), a common lunar mineral. Characterization by high-resolution scanning transmission electron microscopy and electron energy-loss spectroscopy shows that the SW alone creates rims with all the main characteristics of lunar samples. We conclusively identify npFe ⁰ and quantify its distribution as a function of depth and fluence, allowing us to estimate the SW exposure of Apollo soil 71501. Our results confirm that small npFe ⁰ particles (<10 nm in diameter) form from SW irradiation. Such experiments provide microscopic details of space weathering, improving the link between surface modification processes and macroscopic remote-sensing data.

Lunar mare basalts are often rich in titanium, hosted predominantly within the mineral ilmenite (Fe²⁺Ti⁴⁺O₃). Here, we examine ilmenite in a ~ 3.8 billion-year-old mare basalt (Apollo rock 75035) using high-resolution electron microscopy and electron energy loss spectroscopy. A key finding is that 75035 ilmenite is itself enriched in Ti, beyond the end member of the conventional solid solution series. Using energy loss near-edge spectroscopy, we determine that the excess Ti is trivalent, with Ti³⁺ accounting for 13% of the total Ti content. This discovery confirms the presence of trivalent Ti in lunar ilmenite, long hypothesized based on the Moon’s reducing environment. Accounting for the change in implied stoichiometry, a review of literature data suggests that Ti³⁺ may be present in ilmenite across a wide range of lunar samples. We extrapolate known relationships from the literature to connect Ti³⁺ to redox conditions, estimating the oxygen fugacity during crystallization of 75035 to be below the iron-wüstite buffer, ΔIW≤ − 1.6. Further quantifying the relationship between Ti valence state and oxygen fugacity would allow Ti³⁺-bearing ilmenite to serve as an oxybarometer able to access the reducing conditions found on many planetary bodies.

The interactions between micrometeoroid impacts and solar wind on the surface of the Moon remain a major area of investigation in the space weathering field. In this study, we first irradiated Fe-bearing enstatite [(Mg,Fe) ₂ Si ₂ O ₆ ] (1.50 at% Fe) with a nanosecond-pulse UV excimer laser ( λ = 248 nm) to simulate micrometeoroid impacts, using fluences above and below the ablation threshold to produce four different impact-processed regions. Next, we irradiated the surfaces with 4 keV He ⁺ to explore how prior impact processing changes the way solar wind ions interact with mineral surfaces. We performed comprehensive micro-scale to nano-scale characterization of the experimental products using scanning electron microscopy, scanning transmission electron microscopy, electron energy loss spectroscopy, and energy dispersive X-ray spectroscopy. In the samples irradiated above the ablation threshold, we observed the coexistence of nanophase Fe ( np Fe ⁰ ) and Fe ³⁺ -bearing glass, indicating that np Fe ⁰ formation was driven by charge disproportionation. In the sample irradiated at the highest laser fluence, we produced helium-bearing vesicular np Fe ⁰ , an enigmatic space weathering product known to occur widely on the Moon. Finally, we show that the temperature of impact melting and the phases produced by impact processing have a direct influence on the target’s sputtering behavior and overall helium capture capacity. Our findings demonstrate that micrometeoroid impacts play a critical role in modifying the crystal structure and chemistry of regolith grain surfaces and that variation in impact processing can fundamentally shift the subsequent interactions that solar wind ions have with affected surfaces.

Lunar mare basalts are often rich in titanium, hosted predominantly within the mineral ilmenite (Fe ²⁺ Ti ⁴⁺ O ₃ ). Of all Apollo samples returned from the Moon, only two are olivine-free basalts, making them of interest for their potential to constrain models of lunar evolution. Here, we use electron microscopy and electron energy loss spectroscopy (EELS) to investigate ilmenite in one of these samples: Apollo rock 75035. A key finding is that 75035 ilmenite is itself enriched in Ti, beyond the end member of the conventional solid solution series. Using energy loss near-edge spectroscopy (ELNES), we determine that the excess Ti is reduced to trivalent Ti, with Ti ³⁺ mole fraction ~ 0.07. This discovery confirms a longstanding hypothesis based on the Moon's reducing environment. Its impact for lunar models comes through the implied oxygen fugacity during crystallization of 75035, which we estimate to be below the iron-wüstite buffer (IW): ∆IW ≤ −1.5. Our research also suggests that Ti ³⁺ -bearing ilmenite could serve as a single-phase oxybarometer for the reducing conditions found on many planetary bodies.

The surface morphology of regolith grains from the C‐type asteroid Ryugu was studied in search of evidence of impact events on the asteroid. Scanning electron microscopy revealed that ~8% of C0105‐042 Ryugu grains have a smooth surface on one side of the grains. One of these grains has striated linear grooves (striations) on its smooth surface. Transmission electron microscopy of the grain showed that a porous fine‐grained Mg‐Fe phyllosilicate assemblage, which is the main component of Ryugu grains, is compacted near the smooth surface. The smooth surface with striations closely resembles a slickenside, a characteristic texture found in terrestrial fault rocks formed by shear deformation. There is no evidence of melting/decomposition in the Mg‐Fe phyllosilicates near the smooth surface, indicating that the shear heating temperature is less than ~1100 K. Assuming that the average length of the striations corresponds to the minimum displacement of the micro‐fault, the shock pressure recorded in the C0105‐042 Ryugu grain is estimated to be <~4.5 GPa by a fault mechanics calculation. The shock pressures of C0105‐042, together with those of C0014 (~2 GPa) and C0055 (>~3.9 GPa) in previous studies suggest that the impact velocities recorded in these grains are < ~0.89–1.63 km s⁻¹. Based on the impact velocities, these grains may record an impact event that occurred when asteroid Ryugu was in the orbit in Main Belt.

Regolith samples returned from asteroid 162173 Ryugu by the Hayabusa2 mission provide direct means to study how space weathering operates on the surfaces of hydrous asteroids. The mechanisms of space weathering, its effects on mineral surfaces, and the characteristic time scales on which alteration occurs are central to understanding the spectroscopic properties and the taxonomy of asteroids in the solar system. Here, we investigate the behavior of the iron monosulfides mineral pyrrhotite (Fe_1−xS) at the earliest stages of space weathering. Using electron microscopy methods, we identified a partially exposed pyrrhotite crystal that morphologically shows evidence for mass loss due to exposure to solar wind ion irradiation. We find that crystallographic changes to the pyrrhotite can be related to sulfur loss from its space‐exposed surface and the diffusive redistribution of resulting excess iron into the interior of the crystal. Diffusion profiles allow us to estimate an order of magnitude of the exposure time of a few thousand years consistent with previous estimates of space exposure. During this interval, the adjacent phyllosilicates did not acquire discernable damage, suggesting that they are less susceptible to alteration by ion irradiation than pyrrhotite.

The Moon’s surface is directly exposed to the space environment and subject to alteration by space weathering. One agent of space weathering, the solar wind, enriches the lunar surface with helium. Although we understand how helium is delivered to the Moon, certain aspects of helium concentration processes on the surface remain unknown, such as why impact-generated glass aggregates contain more helium than equally sized soil grains of other types. Here we have analyzed the contents of vesicular iron nanoparticles in lunar impact glasses using aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy and show that the nanoparticles contain high concentrations of helium (10-24 atoms/nm³). The widespread occurrence of vesicular iron nanoparticles among lunar samples suggests that they may be an important helium reservoir. These results also suggest that space weathering of iron-rich minerals plays a role in helium sequestration on the Moon and potentially on other airless bodies.

Transmission electron microscopy analyses of Hayabusa2 samples show that Ryugu organic matter exhibits a range of morphologies, elemental compositions, and carbon functional chemistries consistent with those of carbonaceous chondrites that have experienced low‐temperature aqueous alteration. Both nanoglobules and diffuse organic matter are abundant. Non‐globular organic particles are also present, and including some that contain nanodiamond clusters. Diffuse organic matter is finely distributed in and around phyllosilicates, forms coatings on other minerals, and is also preserved in vesicles in secondary minerals such as carbonate and pyrrhotite. The average elemental compositions determined by energy‐dispersive spectroscopy of extracted, demineralized insoluble organic matter samples A0107 and C0106 are C₁₀₀N₃O₉S₁ and C₁₀₀N₃O₇S₁, respectively, with the difference in O/C slightly outside the difference in the standard error of the mean. The functional chemistry of the nanoglobules varies from mostly aromatic C=C to mixtures of aromatic C=C, ketone C=O, aliphatic (CH_n), and carboxyl (COOH) groups. Diffuse organic matter associated with phyllosilicates has variable aromatic C, ketone and carboxyl groups, and some localized aliphatics, but is dominated by molecular carbonate (CO₃) absorption, comparable to prior observations of clay‐bound organic matter in CI meteorites.

The Hayabusa2 mission from the Japan Aerospace Exploration Agency (JAXA) returned to the Earth samples of carbonaceous asteroid (162173) Ryugu. This mission offers a unique opportunity to investigate in the laboratory samples from a C‐type asteroid, without physical or chemical alteration by the terrestrial atmosphere. Here, we report on an investigation of the mineralogy and the organo‐chemistry of Hayabusa2 samples using a combination of micro‐ and nano‐infrared spectroscopy. Particles investigated with conventional FTIR spectroscopy have spectra dominated by phyllosilicate‐related absorption, as observed for samples of CI‐chondrites, selected ungrouped carbonaceous chondrites, and selected hydrated micrometeorites. Ryugu samples show smaller sulfate‐related absorption than CI‐chondrites. Our samples that were only briefly exposed to the Earth atmosphere show absorptions related to molecular water, revealing fast terrestrial contamination of the spectral signature at 3 μm. Overall, our FTIR data are in agreement with other work done on Ryugu samples, revealing a low degree of mineralogical variability across Ryugu samples. AFM‐IR mapping of the grains shows the presence of a micrometer‐sized organic globule in one of our analyzed grains. The AFM‐IR spectra obtained on this globule are similar to IR spectra obtained on IOM suggesting that it is constituted of refractory organic matter. This globule may host silicate in its interior, with a different mineralogy than bulk Ryugu phyllosilicate. The shape, presence of peculiar silicate, and the nature of organic constituting the globule point toward a pre‐accretionary origin of this globule and that at least part of Ryugu organics were inherited from the protosolar nebulae or the interstellar media. Altogether, our results show the similarities between Ryugu samples and CI chondrites.