Linking Cause and Effect: Nanoscale Vibrational Spectroscopy of Space Weathering from Asteroid Ryugu

Airless bodies are subjected to space-weathering effects that modify the first few microns of their surface. Therefore, understanding their impact on the optical properties of asteroids is key to the interpretation of their color variability and infrared reflectance observations. The recent Hayabusa2 sample return mission to asteroid Ryugu offers the first opportunity to study these effects, in the case of the most abundant spectral type among the main-asteroid belt, C-type objects. This study employs vibrational electron energy-loss spectroscopy in the transmission electron microscope to achieve the spatial resolution required to measure the distinct mid-infrared spectral signature of Ryugu's space-weathered surface. The comparison with the spectrum of the pristine underlying matrix reveals the loss of structural -OH and C-rich components in the space-weathered layers, providing direct experimental evidence that exposure to the space environment tends to mask the optical signatures of phyllosilicates and carbonaceous matter. Our findings should contribute to rectifying potential underestimations of water and carbon content of C-type asteroids when studied through remote sensing with new-generation telescopes

Four‐dimensional‐STEM analysis of the phyllosilicate‐rich matrix of Ryugu samples

Ryugu asteroid grains brought back to the Earth by the Hayabusa2 space mission are pristine samples containing hydrated minerals and organic compounds. Here, we investigate the mineralogy of their phyllosilicate-rich matrix with four-dimensional scanning transmission electron microscopy (4D-STEM). We have identified and mapped the mineral phases at the nanometer scale (serpentine, smectite, pyrrhotite), observed the presence of Ni-bearing pyrrhotite, and identified the serpentine polymorph as lizardite, in agreement with the reported aqueous alteration history of Ryugu. Furthermore, we have mapped the d-spacings of smectite and observed a broad distribution of values, ranging from 1 to 2 nm, with an average d-spacing of 1.24 nm, indicating significant heterogeneity within the sample. Such d-spacing variability could be the result of either the presence of organic matter trapped in the interlayers or the influence of various geochemical conditions at the submicrometer scale, suggestive of a range of organic compounds and/or changes in smectite crystal chemistry

Caractérisation 4D-STEM de matériaux sensibles : Etude de l’astéroide Ryugu et des météorites carbonées

International audienceFour-dimensional scanning transmission electron microscopy (4D-STEM) has emerged as a powerful tool for the characterization of beam-sensitive materials, enabling structural analysis at the nanometer scale with minimal radiation damage. In a 4D-STEM experiment, a onverging electron probe is scanned across a thin area of a sample and a 2D diffraction pattern is recorded at each scan position, yielding a four-dimensional dataset (Fig 1). 4D-STEM coupled with the new direct electron detection technology allows for the acquisition of diffraction information with unprecedented sensitivity and speed. The unique advantages of 4D-STEM make it an attractive option for studying a wide range of materials, including those that have previously been challenging for structural characterization using conventional transmission electron microscopy (TEM) techniques. Among these materials, carbonaceous meteorite and asteroid samples represent some of the most primitive specimens in our solar system, containing a wealth of information about its formation and evolution. The study of such samples is rendered particularly difficult due to the very fine scale mixing of beam sensitive phases composing their phyllosilicate matrix.In this work, we report an application of the 4D-STEM technique to investigate the mineralogy of Ryugu asteroid samples from the Japanese mission Hayabusa2 [1] and the composition-close Orgueil carbonaceous meteorite. Experiments have been carried out on a probe corrected 60-300 kV Thermo Fisher Titan Themis (S)TEM coupled with a Medipix 3 direct electron detector (Quantum Detectors) installed behind a post-column high-resolution energy filter (Gatan Quantum ERS/966), enabling energy-filtered 4D-STEM. Data have been analyzed using the open-source Python packages py4DSTEM and HyperSpy software. We introduce an analysis protocol for mapping the different mineralogical phases in the phyllosilicate matrix of the samples (Fig 2). We find that the serpentine in Ryugu is, with a good level of confidence, present in the form of Lizardite. We also map the interplanar spacing in the smectite phase(Fig 2). We will discuss the implications of such findings

A 4D-STEM approach for the study of complex mineralogical samples: The case of Ryugu samples

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Etude structurale par 4D-STEM de phases minéralogiques complexes dans les astromatériaux : Cas de l’astéroïde carboné Ryugu

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Cooperative Synthesis of Raspberry‐Like Covalent Organic Framework‐Polymer Particles with a Radial Single‐Crystal Grain Orientation

International audienceAbstract Despite many efforts devoted toward the design of covalent organic frameworks (COFs) at the framework level by selecting the building blocks, their organization in the nano to meso regimes is often neglected. Moreover, the importance of processability for their applications has recently emerged and the synthesis of COF nanostructures without agglomeration is still a challenge. Herein, the first example of hybrid COF‐polymer particles for which polymers are used to manipulate the 2D COF growth along a specific direction is reported. The study examines how the nature, chain‐end functionality, and molar mass of the polymer influence the shaping of hybrid 2D boronate ester‐linked COF‐polymer particles. Catechol‐poly(N‐butyl acrylate) leads to the self‐assembly of crystallites into quasi‐spherical structures while catechol‐poly(N‐isopropylacrylamide) mediates the synthesis of raspberry‐like COF‐polymer particles with radial grain orientation. Scanning and transmission electron microscopies (SEM and TEM) and 4D‐STEM‐ACOM (automated crystal orientation mapping) highlight the single‐crystal character of these domains with one plane family throughout the particles. Interestingly, the presence of PNIPAm on the particle surface allows their drying without co‐crystallization and enables their resuspension. Kinetic investigations show that catechol‐P n BuA acts as a modulator and catechol‐PNIPAm induces a template effect, introducing supramolecular self‐assembly properties into particles to create new morphologies with higher structural complexity, beyond the framework level

Études à la nano-échelle des échantillons de l’astéroïde Ryugu : Caractérisation des effets d’altération spatiale par microscopie électronique

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Spectroscopie vibrationnelle à la nano-échelle en STEM : étude des effets d’altération spatiale sur la signature IR de l’astéroïde Ryugu

International audienceLe récent développement de microscopes électroniques monochromatés dédiés à la spectroscopie EELS à haute résolution met sur le devant de la scène la possibilité d’atteindre la gamme spectrale de l’infrarouge moyen avec, au mieux, une résolution spectrale de 40 cm-1 [1]. Cette technique ouvre une porte vers des comparaisons avec des études IR à plus large échelle.En parallèle, la mission spatiale Hayabusa2 de la JAXA a ramené, fin 2020, 5,4 g de la surface de l’astéroïde carboné Ryugu. Une partie de ces grains présentent une surface modifiée par des effets d’altération spatiale [2]. Ce phénomène est principalement dû à l’irradiation par vent solaire et aux bombardements micro-météoritiques sur la surface d’objets dépourvus d’atmosphère. L’étude de ces échantillons permet donc de mieux comprendre comment un astéroïde de type-C évolue lorsqu’il est exposé à ces sollicitations externes. Les surfaces modifiées par ces phénomènes d’altération spatiale ont été identifiées au MEB et des lames minces y ont été prélevées (FIB). La minéralogie de Ryugu est principalement constituée de phyllosilicates (silicates hydratés). Les phénomènes d’altération spatiale entraînent leur amorphisation ainsi que la formation de couches vésiculées (jusqu’à ~3 μm d’épaisseur). Nos résultats montrent qu’il est possible d’isoler les signatures spectrales IR de ces couches fondues de celles de la matrice hydratée. En particulier, nous mettons en évidence la perte des modes vibrationnels d’H2O et d’-OH dans ces couches fondues. Cela pourrait expliquer pourquoi la bande à 2.7 μm, correspondant aux groupements hydroxyles, est deux fois moins intense dans les données de NIRS3 que dans celles des échantillons collectés [3]. Nous observons également la perte des modes liés à la matière organique dans les couches fondues, en adéquation avec les donées EDS. Nous démontrons donc que l’altération spatiale peut significativement modifier les signatures IR d’astéroïdes de type-C hydratés, pouvant entraîner une sous-estimation de leur teneur en eau et en carbone lors de leur analyse à distance (ie. James Webb Space Telescope)

Space weathering influence on Ryugu’s IR signature: insights from nanoscale vibrational spectroscopy in the STEM

International audienceJAXA’s Hayabusa2 mission brought back 5.4 g of material from the surface of the carbonaceous asteroid Ryugu. Some grains show evidence of a modified surface attributed to space-weathering effects [1], which result from solar-wind irradiation and/or micrometeoroid bombardment. In collaboration with the Hayabusa2-Initial-Analysis Min-Pet Fine Team and the Hayabusa2-Initial-Analysis core, their study should allow to investigate, for the first time, how a C-type, hydrated asteroid surface evolves when subjected to space weathering. Alongside, the recent development of monochromated electron microscopes dedicated to high resolution EELS makes it possible to reach the Mid-IR spectral range (~40 cm-1 spectral resolution at best) [2]. It thus opens the possibility for comparison with bulk IR studies with the advantage of the TEM spatial resolution. Here, we study the modified surfaces and underlying matrix of several small Ryugu grains, originating from both touchdown A and C [3]. Surfaces modified by space weathering were first identified by SEM and FIB sections were prepared.The main product of space weathering is glassy and vesiculated layer likely resulting from the melting of Ryugu materials due to micrometeorite impacts. Our results show that it is possible to distinguish the spectral signature of both space weathered melt layers and underlying preserved matrix. In particular, we evidence the loss of the H2O and -OH vibrational modes in the melt layers. It could explain why the 2.7 µm band, corresponding to hydroxyl groups in phyllosilicates, is twice less intense in NIRS3 spectra than in the returned samples ones [4]. We also observe that the organic modes are absent in the melt layers, in agreement with the EDS data. We thus demonstrate that space weathering strongly modifies the IR signature of hydrated and C-rich airless bodies, and may lead to the underestimation of their water and carbon content when analyzed by remote sensing

Linking Cause and Effect: Nanoscale Vibrational Spectroscopy of Space Weathering from Asteroid Ryugu

Airless bodies are subjected to space-weathering effects that modify the first few microns of their surface. Therefore, understanding their impact on the optical properties of asteroids is key to the interpretation of their color variability and infrared reflectance observations. The recent Hayabusa2 sample return mission to asteroid Ryugu offers the first opportunity to study these effects, in the case of the most abundant spectral type among the main-asteroid belt, C-type objects. This study employs vibrational electron energy-loss spectroscopy in the transmission electron microscope to achieve the spatial resolution required to measure the distinct mid-infrared spectral signature of Ryugu's space-weathered surface. The comparison with the spectrum of the pristine underlying matrix reveals the loss of structural -OH and C-rich components in the space-weathered layers, providing direct experimental evidence that exposure to the space environment tends to mask the optical signatures of phyllosilicates and carbonaceous matter. Our findings should contribute to rectifying potential underestimations of water and carbon content of C-type asteroids when studied through remote sensing with new-generation telescopes