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All sourcesNASA NTRSDOE OSTI
NASA NTRS2015

Fe and O EELS Studies of Ion Irradiated Murchison CM2 Carbonaceous Chondrite Matrix

Introduction: The physical and chemical response of hydrated carbonaceous chondrite materials to space weathering processes is poorly understood. Improving this understanding is a key part of establishing how regoliths on primitive carbonaceous asteroids respond to space weathering processes, knowledge that supports future sample return missions (Hayabusa 2 and OSIRISREx) that are targeting objects of this type. We previously reported on He+ irradiation of Murchison matrix and showed that the irradiation resulted in amorphization of the matrix phyllosilicates, loss of OH, and surface vesiculation. Here, we report electron energy-loss spectroscopy (EELS) measurements of the irradiated material with emphasis on the Fe and O speciation. Sample and Methods: A polished thin section of the Murchison CM2 carbonaceous chondrite was irradiated with 4 kilovolts He(+) (normal incidence) to a total dose of 1 x 10(exp 18) He(+) per square centimeter. We extracted thin sections from both irradiated and unirradiated regions in matrix using focused ion beam (FIB) techniques with electron beam deposition for the protective carbon strap to minimize surface damage artifacts from the FIB milling. The FIB sections were analyzed using a JEOL 2500SE scanning and transmission electron microscope (STEM) equipped with a Gatan Tridiem imaging filter. EELS spectra were collected from 50 nanometer diameter regions with an energy resolution of 0.7 electronvolts FWHM at the zero loss. EELS spectra were collected at low electron doses to minimize possible artifacts from electron-beam irradiation damage. Results and Discussion: Fe L (sub 2,3) EELS spectra from matrix phyllosilicates in CM chondrites show mixed Fe(2+)/Fe(3+) oxidation states with Fe(3+)/Sigma Fe approximately 0.5. Fe L(sub 2,3) spectra from the irradiated/ amorphized matrix phyllosilicates show higher Fe(2+)/Fe(3+) ratios compared to spectra obtained from pristine material at depths beyond the implantation/amorphization layer. We also obtained O Ka spectra from phyllosilicates in both regions of the sample. The O Ka spectra show a pre-edge feature at approximately 530.5 electronvolts that is related to O 2p states hybridized with Fe 3d states. The intensity ratio of the O Ka pre-edge peak relative to the main part of the O Ka edge (that results from transitions of O 1s to 2p states) is lower in the irradiated layer compared to the pristine material and may reflect the loss of O (as OH) as was observed by IR spectroscopy. Conclusions: In addition to amorphization and OH loss, EELS spectra of He(+) irradiated matrix phyllosilicates in Murchison show that some of the Fe(3+) is reduced to Fe(2+). Spectral deconvolution is underway to extract quantitative ratios from the EELS spectra.

Keller, L. P.↗
NASA NTRS2023

Investigating the Role of Incident Ion Flux in Solar Wind Space Weathering of Carbon-Rich Asteroidal Regolith Via H+ and He+ Irradiation of the Murchison Meteorite

We present results from a set of low and high flux 1 keV/amu H + and He + irradiation experiments performed on slabs of the Murchison CM2 carbonaceous chondrite. The low flux conditions for H + and He + irradiation were ~1-1.5 orders of magnitude lower than the high flux conditions, and each experiment was irradiated to a total fluence between ~3 × 10 16 to ~6 × 10 16 ions/cm 2 . Irradiation-induced changes in the surface chemistry and optical properties of the Murchison samples were evaluated using in situ X-ray photoelectron spectroscopy (XPS) and visible and near-infrared spectroscopy (VNIR). We characterized the microstructure and composition of ion damaged rims in focused ion beam (FIB) cross-sections extracted from olivine and matrix material in each irradiated Murchison slab using transmission electron microscopy (TEM). XPS results suggest that both low flux and high flux H + and He + irradiation cause minor sputtering of surface carbon as well as a reduction in the valence state of iron, from Fe 3+ to Fe 2+ . Slope bluing is observed in VNIR spectra of the irradiated samples which may reflect carbonization and dehydrogenation of organic species and contrasts with reddening trends associated with npFe 0 formation. Although we do not observe a strong flux-dependence on the crystallinity of ion-damaged olivine, TEM analyses reveal a variety of microstructures in all olivine FIB-sections, suggesting that crystallographic orientation affects amorphization efficiency. Analyses of matrix FIB-sections indicate that phyllosilicate alteration is mainly driven by He+ irradiation, where the higher incident flux leads to greater amorphization and the formation of more distinct ion-damaged layers, similar to smooth layers in returned Ryugu particles. TEM results also provide some evidence that higher ion flux leads to greater vesiculation, with He + irradiation being more efficient at vesiculation than H + irradiation, and that higher ion flux may promote the segregation of Mg and Si into laterally extensive lenses and layers in olivine samples. We discuss the implications of these findings for constraining the role that ion flux plays in the development of space weathering characteristics in silicate phases present in carbonaceous asteroidal regoliths. These results will be important for understanding the complexity of this process and how it operates on carbon-rich airless bodies like asteroids Bennu and Ryugu.

meteorites↗
NASA NTRS2018

Novel Applications of Focused Ion Beam Technique for Planetary Sample Analyses

We are using innovative FIB techniques to prepare samples of planetary materials for different types of coordinated analyses using ion microprobes, synchrotron beamlines, and specialized transmission electron microscopy (TEM) techniques. In these cases, the FIB sample preparation is the critical step in enabling these specialized analyses. We discuss several examples below utilizing the FEI Quanta3D instrument at the NASA Johnson Space Center. Trace element analyses utilizing synchrotron x-ray fluorescence. The trace element content of mineral grains in comet dust provides important clues on their formation and processing in the early solar system. We preformed coordinated analyses of a comet dust particle that had been prepared using ultramicrotomy for TEM analysis. Following the TEM analyses, we extracted a 70 nm thick section from a region of the carbon (C) film of the TEM grid, for additional analyses. A carbon ring ~2-3 μm thick was deposited on top of the C film using the FIB. The C film on the outer rim of the ring was milled away using various patterns to uniformly release the stresses on the film, preventing rupture and collapse, and was attached to the micromanipulator needle. We then isolated the ring completely and transferred the section to a silicon sample holder for analysis using the HXN (hard X-ray nanoprobe) beamline at NSLSII at Brookhaven National Lab. Coordinated Analyses of Presolar Grains. Rare sub-m presolar grains that originate in evolved stars and supernovae, occur in primitive astromaterials and are identified by their exotic isotopic compositions. Coordinated analyses of these grains using NanoSIMS, TEM, and other techniques on the same grain is enabled by innovative FIB sample preparation. In order to obtain subsequent isotopic analyses of Mg and Fe, contributions from surrounding grains were minimized. We precisely deposited a protective cap of Pt on top of the grain to preserve the grain of interest and then milled away about 5 μm diameter of the surrounding material. Following the isotopic analyses, the spindle was extracted and thinned to electron transparency for TEM microstructural analyses. In situ heating TEM experiments on lunar samples. We extracted a FIB thin section from Apollo 17 lunar rock 76015. To avoid ion-beam damage, e-beam deposition was used to deposit the first 500 nm of the C strap, followed by ion beam-assisted deposition of ~3 μm carbon. We performed an ex situ lift-out of the section and placed the section on one of the elements of a microelectromechanical systems (MEMS) - specialized heating substrate and attached the section to the substrate by depositing small C straps with the FIB. The heating chips utilize silicon nitride windows to support the samples and provide uniform heating while enabling TEM imaging. The heating chip was loaded into a Hitachi “Blaze” heating holder and analyzed using a Hitachi HF5000 at the University of Arizona.

Rahman, Z.↗
NASA NTRS2014

Insights into Regolith Dynamics from the Irradiation Record Preserved in Hayabusa Samples

The rates of space weathering processes are poorly constrained for asteroid surfaces, with recent estimates ranging over 5 orders of magnitude. The return of the first surface samples from a space-weathered asteroid by the Hayabusa mission and their laboratory analysis provides "ground truth" to anchor the timescales for space weathering. We determine the rates of space weathering on Itokawa by measuring solar flare track densities and the widths of solar wind damaged rims on grains. These measurements are made possible through novel focused ion beam (FIB) sample preparation methods.

Keller, Lindsay P.↗
NASA NTRS2014

A Novel Hybrid Ultramicrotomy/FIB-SEM Technique: Preparation of Serial Electron-Transparent Thin Sections of a Hayabusa Grain

The Japanese space agency's (JAXA) Hayabusa mission returned the first particulate samples (typically <100micron) from the surface of an asteroid (25143 Itokawa). These precious samples provide important insights into early Solar System processes, but their sizes pose tremendous challenges to coordinated analysis using a variety of nano- and micro-beam techniques. The ability to glean maximal information from individual particles has become increasingly important and depends critically on sample preparation. We developed a hybrid technique combining traditional ultramicrotomy with focused ion beam (FIB) techniques, allowing for more thorough in situ investigations of grain surfaces and interiors. Using this method, we increase the number of FIB-prepared sections that can be recovered from a particle with dimensions on the order of tens of microns. These sections can be subsequently analyzed using a variety of analytical techniques. Particle RA-QD02-0211 is a approx. 40×40×20 micron particle from Itokawa containing olivine and Fe sulfides. It was embedded in low viscosity epoxy and partly sectioned to a depth of approx 10 micron; sections are placed on Cu grids with thin amorphous films for transmission electron microscope (TEM) analyses. With the sample surface partly exposed, the epoxy bullet is trimmed to a height of approx. 5mm to accommodate the allowable dimensions for FIB work (FEI Quanta 600 3D dual beam FIB-SEM). Using a diamond trim knife, the epoxy surrounding the grain is removed on 3 sides (to within a few microns of the grain); the depth of material removed extends well below the bottom of the particle. The sample is attached to an SEM pin mount, the epoxy coated with conductive paint, and the entire assembly coated with approx. 40nm of carbon to eliminate sample charging during FIB work. A protective carbon cap is placed according to the plan for the 15 FIB sections. The central 'spine' of the cap runs perpendicular to the front of the sample, and the 'ribs' protruding from either side run parallel. Each rib indicates the location of a planned FIB section, and the spine contains the final two planned sections. We use a cap with a 4 micron-wide spine and 2micron-wide ribs that have ≳3.5 micron of space between them (narrower cuts result in too much re-deposition of material inside the trenches). Using a 30kV, 3nA ion-beam we expose the front surface of the grain and commence milling trenches between sections. Rather than using the typical C-cut to prepare the sample for lift-out, an L-cut is used instead, leaving the sample connected by an interior tab. tab. Sections are lifted out, attached to TEM grids and thinned to electron transparency. TEM analyses show that our hybrid technique preserves both interior and edge features, including surface modifications from exposure to the space environment, such as damaged rims that form in response to solar wind implantation effects and adhering grains. In addition, the FIB sections provide larger areas that are free of fractures and chatter effects in comparison to the microtome thin sections, thus enabling more accurate measurements of solar flare particle track densities that are used to determine the surface exposure age of the particles.

Berger, Eve L.↗
NASA NTRS2015

Characterization of Meteorites by Focused Ion Beam Sectioning: Recent Applications to CAIs and Primitive Meteorite Matrices

Focused ion beam (FIB) sectioning has revolutionized preparation of meteorite samples for characterization by analytical transmission electron microscopy (TEM) and other techniques. Although FIB is not "non-destructive" in the purest sense, each extracted section amounts to no more than nanograms (approximately 500 cubic microns) removed intact from locations precisely controlled by SEM imaging and analysis. Physical alteration of surrounding material by ion damage, fracture or sputter contamination effects is localized to within a few micrometers around the lift-out point. This leaves adjacent material intact for coordinate geochemical analysis by SIMS, microdrill extraction/TIMS and other techniques. After lift out, FIB sections can be quantitatively analyzed by electron microprobe prior to final thinning, synchrotron x-ray techniques, and by the full range of state-of-the-art analytical field-emission scanning transmission electron microscope (FE-STEM) techniques once thinning is complete. Multiple meteorite studies supported by FIB/FE-STEM are currently underway at NASA-JSC, including coordinated analysis of refractory phase assemblages in CAIs and fine-grained matrices in carbonaceous chondrites. FIB sectioning of CAIs has uncovered epitaxial and other overgrowth relations between corundum-hibonite-spinel consistent with hibonite preceding corundum and/or spinel in non-equilibrium condensation sequences at combinations of higher gas pressures, dust-gas enrichments or significant nebular transport. For all of these cases, the ability of FIB to allow for coordination with spatially-associated isotopic data by SIMS provides immense value for constraining the formation scenarios of the particular CAI assemblage. For carbonaceous chondrites matrix material, FIB has allowed us to obtain intact continuous sections of the immediate outer surface of Murchison (CM2) after it has been experimentally ion processed to simulate solar wind space weathering. The surface amorphization and loss of OH produced by the irradiation provides important clues regarding space weathering on primitive asteroids such as the OSIRIS-Rex target 101955 Bennu.

Christoffersen, Roy↗
NASA NTRS2016

TEM Analyses of Itokawa Regolith Grains and Lunar Soil Grains to Directly Determine Space Weathering Rates on Airless Bodies

Samples returned from the moon and Asteroid Itokawa by NASA's Apollo Missions and JAXA's Hayabusa Mission, respectively, provide a unique record of their interaction with the space environment. Space weathering effects result from micrometeorite impact activity and interactions with the solar wind. While the effects of solar wind interactions, ion implantation and solar flare particle track accumulation, have been studied extensively, the rate at which these effects accumulate in samples on airless bodies has not been conclusively determined. Results of numerical modeling and experimental simulations do not converge with observations from natural samples. We measured track densities and rim thicknesses of three olivine grains from Itokawa and multiple olivine and anorthite grains from lunar soils of varying exposure ages. Samples were prepared for analysis using a Leica EM UC6 ultramicrotome and an FEI Quanta 3D dual beam focused ion beam scanning electron microscope (FIB-SEM). Transmission electron microscope (TEM) analyses were performed on the JEOL 2500SE 200kV field emission STEM. The solar wind damaged rims on lunar anorthite grains are amorphous, lack inclusions, and are compositionally similar to the host grain. The rim width increases as a smooth function of exposure age until it levels off at approximately 180 nm after approximately 20 My (Fig. 1). While solar wind ion damage can only accumulate while the grain is in a direct line of sight to the Sun, solar flare particles can penetrate to mm-depths. To assess whether the track density accurately predicts surface exposure, we measured the rim width and track density in olivine and anorthite from the surface of rock 64455, which was never buried and has a surface exposure age of 2 My based on isotopic measurements. The rim width from 64455 (60-70nm) plots within error of the well-defined trend for solar wind amorphized rims in Fig. 1. Measured solar flare track densities are accurately reflecting the surface exposure of the grains. Track densities correlate with the amorphous rim thicknesses. While the space-weathered rims of anorthite grains are amorphous, the space-weathered rims on both Itokawa and lunar olivine grains show solar wind damaged rims that are not amorphous. Instead, the rims are nanocrystalline with high dislocation densities and sparse inclusions of nanophase Fe metal. The rim thicknesses on the olivine grains also correlate with track density. The Itokawa olivine grains have track densities that indicate surface exposures of approximately 10(exp 5) years. Longer exposures (up to approximately 10(exp 7) years) do not amorphize the rims, as evidenced by lunar soil olivines with high track densities (approximately 10(exp 11) cm(exp -2)). From the combined data, shown in Fig. 1, it is clear that olivine is damaged (but not amorphized) more rapidly by the solar wind compared to anorthite. The olivine damaged rim forms quickly (in approximately 10(exp 6) y) and saturates at approximately 120nm with longer exposure time. The anorthite damaged rims form more slowly, amorphize, and grow thicker than the olivine rims. This is in agreement with numerical modeling data which predicts that solar wind damaged rims on anorthite will be thicker than olivine. However, the models predict that both olivine and anorthite rims will amorphize and reach equilibrium widths in less than 10(exp 3) y, in contrast to what is observed for natural samples. Laboratory irradiation experiments, which show rapid formation of fully amorphous and blistered surfaces from simulated solar wind exposures are also in contrast to observations of natural samples. These results suggest that there is a flux dependence on the type and extent of irradiation damage that develops in olivine. This flux dependence suggests that great caution be used in extrapolating between high-flux laboratory experiments and the natural case, as demonstrated by. We constrain the space weathering rate through analysis of returned samples. Provided that the track densities and the solar wind damaged rim widths exhibited by the Itokawa grains are typical of the fine-grained regions of Itokawa, then the space weathering rate is on the order of 10(exp 5) y. Space weathering effects in lunar soils saturate within a few My of exposure while those in Itokawa regolith grains formed in approximately 10(exp 5) y. Olivine and anorthite respond differently to solar wind irradiation. The space weathering effects in olivine are particularly difficult to reconcile with laboratory irradiation studies and numerical models. Additional measurements, experiments, and modeling are required to resolve the discrepancies among the observations and calculations involving solar wind amorphization of different minerals on airless bodies.

Berger, Eve L.↗
NASA NTRS2017

Experimental Hypervelocity Dust Impact in Olivine: FIB/TEM Characterization of Micron-Scale Craters with Comparison to Natural and Laser-Simulated Small-Scale Impact Effects

The space weathering of regoliths on airless bodies and the formation of their exospheres is driven to a large extent by hypervelocity impacts from the high relative flux of micron to sub-micron meteoroids that comprise approximately 90 percent of the solar system meteoroid population. Laboratory hypervelocity impact experiments are crucial for quantifying how these small impact events drive space weathering through target shock, melting and vaporization. Simulating these small scale impacts experimentally is challenging because the natural impactors are both very small and many have velocities above the approximately 8 kilometers-per-second limit attainable by conventional chemical/light gas accelerator technology. Electrostatic "dust" accelerators, such as the one recently developed at the Colorado Center for Lunar Dust and Atmospheric Studies (CCLDAS), allow the experimental velocity regime to be extended up to tens of kilometers-per-second. Even at these velocities the region of latent target damage created by each impact, in the form of microcraters or pits, is still only about 0.1 to 10 micrometers in size. Both field-emission analytical scanning electron microscopy (FE-SEM) and advanced field-emission scanning transmission electron microscopy (FE-STEM) are uniquely suited for characterizing the individual dust impact sites in these experiments. In this study, we have used both techniques, along with focused ion beam (FIB) sample preparation, to characterize the micrometer to nanometer scale effects created by accelerated dust impacts into olivine single crystals. To our knowledge this work presents the first TEM-scale characterization of dust impacts into a key solar system silicate mineral using the CCLDAS facility. Our overarching goal for this work is to establish a basis to compare with our previous results on natural dust-impacted lunar olivine and laser-irradiated olivine.

Christoffersen, R.↗
NASA NTRS2012

Coordinated STEM/FIB/NanoSIMS Analyses of Presolar Silicates in Comet Dust and Primitive Meteorites

Silicate grains were among the most abundant mineralogical building blocks of our Solar System. These grains were the detritus from earlier generations of stars that have been recycled in the early solar nebula. Rare sub-micrometer survivors of this processing have been identified in meteorites, micrometeorites and interplanetary dust particles (IDPs). These silicate grains are recognized as presolar in origin because of their extremely anomalous isotopic compositions that reflect nucleosynthetic processes in their stellar sources (evolved stars, novae and supernovae). We perform coordinated chemical, mineralogical and isotopic studies of these grains to determine their origins and histories. We examine the complex mineralogy and petrography of presolar silicates using imaging, diffraction and chemical data obtained from thin sections with the JSC JEOL 2500 field-emission STEM equipped with a Noran thin window energy dispersive x-ray (EDX) spectrometer and a Gatan Tridiem GIF. Quantitative element x-ray maps (spectrum images) are acquired by rastering a 4 nm incident probe whose dwell time is minimized to avoid beam damage and element diffusion during mapping. Successive image layers are acquired and combined in order to achieve approx 1% counting statistics for major elements. The IDP samples are prepared by ultramicrotomy of particles embedded in epoxy or elemental sulfur. After EDX mapping, the sections are subjected to C, N, and O isotopic imaging with the JSC NanoSIMS 50L ion microprobe. We prepare sections of some meteorite grains using the JSC FEI Quanta 3D focused ion beam (FIB) instrument. The specimen surface is protected from the FIB milling process by layers of electron beam-deposited C and Pt followed by an ion-deposited Pt layer. We also use the FIB to preferentially remove surrounding grains to reduce the background in subsequent NanoSIMS measurements. For mineralogical studies, we again employ the FIB instrument to deposit a protective cap over the grain of interest and then extract the grain and thin it to electron transparency for TEM analysis.

Keller, Lindsay↗
NASA NTRS2015

Solar Flare Track Exposure Ages in Regolith Particles: A Calibration for Transmission Electron Microscope Measurements

Mineral grains in lunar and asteroidal regolith samples provide a unique record of their interaction with the space environment. Space weathering effects result from multiple processes including: exposure to the solar wind, which results in ion damage and implantation effects that are preserved in the rims of grains (typically the outermost 100 nm); cosmic ray and solar flare activity, which result in track formation; and impact processes that result in the accumulation of vapor-deposited elements, impact melts and adhering grains on particle surfaces. Determining the rate at which these effects accumulate in the grains during their space exposure is critical to studies of the surface evolution of airless bodies. Solar flare energetic particles (mainly Fe-group nuclei) have a penetration depth of a few millimeters and leave a trail of ionization damage in insulating materials that is readily observable by transmission electron microscope (TEM) imaging. The density of solar flare particle tracks is used to infer the length of time an object was at or near the regolith surface (i.e., its exposure age). Track measurements by TEM methods are routine, yet track production rate calibrations have only been determined using chemical etching techniques [e.g., 1, and references therein]. We used focused ion beam-scanning electron microscope (FIB-SEM) sample preparation techniques combined with TEM imaging to determine the track density/exposure age relations for lunar rock 64455. The 64455 sample was used earlier by [2] to determine a track production rate by chemical etching of tracks in anorthite. Here, we show that combined FIB/TEM techniques provide a more accurate determination of a track production rate and also allow us to extend the calibration to solar flare tracks in olivine.

Berger, Eve L.↗
NASA NTRS2005

Ultrasonic Micro-Blades for the Rapid Extraction of Impact Tracks from Aerogel

The science return of NASA's Stardust Mission with its valuable cargo of cometary debris hinges on the ability to efficiently extract particles from silica aerogel collectors. The current method for extracting cosmic dust impact tracks is a mature procedure involving sequential perforation of the aerogel with glass needles on computer controlled micromanipulators. This method is highly successful at removing well-defined aerogel fragments of reasonable optical clarity while causing minimal damage to the surrounding aerogel collector tile. Such a system will be adopted by the JSC Astromaterials Curation Facility in anticipation of Stardust s arrival in early 2006. In addition to Stardust, aerogel is a possible collector for future sample return missions and is used for capture of hypervelocity ejecta in high power laser experiments of interest to LLNL. Researchers will be eager to obtain Stardust samples for study as quickly as possible, and rapid extraction tools requiring little construction, training, or investment would be an attractive asset. To this end, we have experimented with micro-blades for the Stardust impact track extraction process. Our ultimate goal is a rapid extraction system in a clean electron beam environment, such as an SEM or dual-beam FIB, for in situ sample preparation, mounting and analysis.

Ishii, H. A.↗
NASA NTRS2013

Miniature Trace Gas Detector Based on Microfabricated Optical Resonators

While a variety of techniques exist to monitor trace gases, methods relying on absorption of laser light are the most commonly used in terrestrial applications. Cavity-enhanced absorption techniques typically use high-reflectivity mirrors to form a resonant cavity, inside of which a sample gas can be analyzed. The effective absorption length is augmented by the cavity's high quality factor, or Q, because the light reflects many times between the mirrors. The sensitivity of such mirror-based sensors scales with size, generally making them somewhat bulky in volume. Also, specialized coatings for the high-reflectivity mirrors have limited bandwidth (typically just a few nanometers), and the delicate mirror surfaces can easily be degraded by dust or chemical films. As a highly sensitive and compact alternative, JPL is developing a novel trace gas sensor based on a monolithic optical resonator structure that has been modified such that a gas sample can be directly injected into the cavity. This device concept combines ultra-high Q optical whispering gallery mode resonators (WGMR) with microfabrication technology used in the semiconductor industry. For direct access to the optical mode inside a resonator, material can be precisely milled from its perimeter, creating an open gap within the WGMR. Within this open notch, the full optical mode of the resonator can be accessed. While this modification may limit the obtainable Q, calculations show that the reduction is not significant enough to outweigh its utility for trace gas detection. The notch can be milled from the high- Q crystalline WGMR with a focused ion beam (FIB) instrument with resolution much finer than an optical wavelength, thereby minimizing scattering losses and preserving the optical quality. Initial experimental demonstrations have shown that these opened cavities still support high-Q whispering gallery modes. This technology could provide ultrasensitive detection of a variety of molecular species in an extremely compact and robust package. With this type of modified WGMR, one can inject a gas sample into the open gap, allowing highly sensitive trace molecule detection within a roughly 1-cm volume. Other critical components of the instrument, such as the detector and a semiconductor laser, could be directly packaged with the resonator so as to not significantly increase the size of the device. Besides its low mass, volume, and power consumption, the monolithic design makes these resonators intrinsically robust devices, capable of handling significant temperature excursions, without moving parts to wear out or delicate coatings that can be easily damaged. A sensor could integrate with microfluidics technology for a chip-scale device. It could be mounted to the end of a deployable arm, or inserted into a borehole. Also, a network of individual sensors could be dispersed to monitor conditions over a wide region

Aveline, David C.↗
NASA NTRSDate not supplied

Unraveling Solar Wind Space Weathering of Carbon-Rich Asterods: Low-Flux VS. High-Flux Ion Irradiation of Murchison

Introduction: Space weathering processesalter the microstructural, compositional, and optical properties of airless planetary surfaces.Solar wind space weatheringforms ion-damaged rims within the outermost ~100 nm of regolith grains through atomic-displacements and implantation of energetic H+and He+ions[1]. Transmission electron microscope(TEM)studies of space weathered Itokawa and lunar olivine particles reveal predominantly nanocrystalline solar wind-damaged rims with only minor regions of amorphization[2-5]. This finding differs from laboratoryion irradiation experiments in which samples become amorphous at fluences 2-3 orders of magnitude lower than the independentlyconstrained fluences for still-crystalline natural samples [6-7].Ion flux is generally considered to have a secondaryeffectcompared to fluence in these irradiation experiments. However, solar wind irradiation of natural samples occurs at a flux that is ~4-5 orders of magnitude lower than whatisused inlaboratory simulations (e.g., [6,8-9]). Constraining how the very low ion flux of solar wind influences the microstructural, chemical, and spectral alteration of silicate minerals will improve our understanding of solar wind space weathering and aid in the analysis of returned samples from the Hayabusa2 and OSIRIS-REx missions. Here we report results from low-and high-flux ion irradiation experiments on the Murchison (CM2)meteoritewithcomparable total fluences. Methods: We performed low-flux and high-flux He+(4 keV) and H+(1 keV) irradiation experiments on separate, dry-cut Murchison slabs. Low-flux He+irradiation reached a total fluence of 2.1 x 1016ions/cm2(~400 years of exposure at Bennu) using a flux of 3.6 x 1011ions/cm2/s; low-flux H+irradiation reached a total fluence of 3.9 x 1016ions/cm2(~30 years) using a flux of 6.6 x 1011ions/cm2/s. High-flux He+irradiation reached a total fluence of 2.0 x 1016ions/cm2(~400 years)using a flux of 9.1 x 1012ions/cm2/s; high-flux H+irradiation reached a total fluence of 5.8 x 1016ions/cm2(~50 years)using flux of 8.4 x 1012ions/cm2/s. By irradiating to similar total fluences in both the low-flux and high-flux experiments, we can better isolate and evaluatethe effects of ion flux. Differences between unirradiatedand irradiated surfaces are determined using threeanalytical techniques:(1) in situX-ray photoelectron spectroscopy(XPS) reveals changes in surface chemistry,(2) visible to near-infrared spectroscopy (VNIR; 0.35-2.5 μm) highlights changes in spectraltrends, such as albedo and slope, and (3) TEM and energy-dispersive X-ray spectroscopy (EDS) identifieschanges in nanoscale structure and elemental composition within olivine and matrix electron-transparent thin sections prepared using focused ion beam scanning electron microscopy (FIB-SEM). Results and Discussion: XPS analyses of the low-fluxand high-fluxHe+-and H+-irradiated samples showa minorreduction in surface carbon contentand chemicalreduction ofFe3+to Fe2+. The latter result is consistent with the higher Fe2+/Fe3+ratios observed in phyllosilicate-rich space weathered rims of Ryugu grains [10].VNIR analyses suggest that the low-flux H+-irradiated spectrum has a slightly bluer slope (over 0.65-2.5 μm) compared to the unirradiated VNIR spectrum from [9] while the low-flux He+-irradiated spectral slope is comparable to the unirradiated surface. Both the high-flux He+-and H+-irradiated VNIR spectra exhibitslightlyhigheralbedos at shorter wavelengths relativeto their unirradiated counterparts, with He+irradiation yielding a greater change. Thus far, the low-flux He+-irradiated matrix and olivine FIBsections as well as the low-flux H+-irradiated olivine FIBsection have been examined with TEM. The surfaces of all three samplesshowless vesiculation than high-flux/high-fluenceirradiated matrix and olivine samples from [9]. The depth of phyllosilicate amorphization in thelow-fluxHe+-irradiated matrix FIBsection varieslaterally, with crystalline domains occurring as shallow as ~25 nm in some regions. Thelow-flux He+-irradiated olivine FIBsection has acontinuous,~40-90 nm thick ion-affected layer with polycrystalline and amorphous microstructures. The low-flux H+-irradiated olivine sampleexhibits a distinct, ~10-40 nm ion-affected layer whose uppermost ~20 nmin thicker regions is predominantly amorphous.Continuing to compare results from this work to those from [9] will provide a comprehensive understanding of how ion flux impacts the microstructural, chemical, and optical modification of various mineral phases and, in turn,help identify space weathering features from parent body alteration in Hayabusa2 and OSIRIS-REx returned samples.

D L Laczniak↗