Measurements inside the sample holder (N2 atmosphere)
The samples arrived at SOLEIL (France) in early July. The sample holders were first inspected using an optical microscope through the KBr window (Fig. 2).
We successfully identified 28 out of 32 of the original Ryugu particles by their morphological correspondence with the grains prepared at Tohoku University and their characteristic spectral feature at 2.7 µm, typical of Ryugu (see Fig. 3). The remaining 4 samples moved during transportation and were not found on the gold mirror.
Once the samples were identified, the analytical pipeline began with a full spectral characterization using IR synchrotron beam, from 1 to 20 µm, while keeping the sample holder sealed to minimize exposure to air. The goal of this first step in our analytical pipeline was threefold:
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Identify spectral features of interest in each Ryugu grain and derive their spectral parameters (for instance, position and depth of the (M)-OH stretching feature around 2.7 µm, position of the Si–O stretching silicate feature around 10 µm, presence/absence of the carbonate feature around 7 µm).
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Select grains to be mounted on needles for 3D IR characterization.
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Have a track record of the spectral signature of the grains prior to the opening of the sample holder, to follow possible grain alteration by terrestrial processes.
Our sample holder design allowed us to easily acquire measurements using all the available IR microscopes available at the SMIS-beamline (SOLEIL Synchrotron) without the risk of compromising the samples. To cover the above-mentioned spectral range, we used three different FTIR microscopes (see Fig. 4):
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1.
a Continuum microscope with a FTIR spectrometer equipped with an MCT/B detector, synchrotron-radiation-fed, allowing us to probe both the near and mid-IR spectral ranges (from 1 to 18 µm);
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2.
a NicPlan microscope with a IS50 FTIR spectrometer (Thermo Fisher), equipped with a bolometer detector (boron doped silicon, 4.2 K cooled, Infrared Laboratories) and a solid-state Si beamsplitter, allowing us to probe the far-IR range (from 15 to 50 µm);
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3.
an Agilent Cary 670/620 micro-spectrometer using the internal Globar source, equipped with a focal plane array (FPA) detector, allowing us to acquire spectral maps and hyperspectral images in the mid-IR range (from 2.5 to 12 µm).
The large spectral coverage obtained by coupling all these instruments allowed us to detect carbonates (around 7 µm), organics (around 3.4 and 6.2 µm), and phyllosilicates (around 2.7 µm for the metal-OH stretching vibration and 10 µm for the SiO stretching vibration). Hyperspectral imaging in the mid-IR allowed us to start probing the composition heterogeneity of individual grains.
Raman spectra and maps were also acquired to investigate the characteristics of the endemic aromatic organics in Ryugu’s grain, as well as to complement mineral identification data. Raman data is acquired using a DXR Raman microspectrometer from Thermo Fisher with a 532 nm exciting laser radiation, using a low power—typically in the 0.5–0.5 mW range—to avoid heating of the samples. Nonetheless, due to the preciousness of the samples, these measurements were done on isolated small fragments detached from the main grains, to avoid alteration from the Raman laser. An example of a measured Raman spectrum is shown in the figure below (Fig. 5).
The D and G bands of the endemic organic matter are clearly visible and their position is compatible with CI-chondrites, supporting the results and discussion lead in Nakamura et al. (2022) (see science publication for a more in-depth scientific discussion).
Raman was also used to detect molecular oxygen inside one of the sample holders (SH2, see Fig. 6), indicating that the holders had lost their air-shut condition at some point, probably in flight from Japan to France.
Upon reception, we realized that the small static-shielding bags holding the sample holders were torn open, possibly due to the pressure difference between the inside and the outside of the bags during the flight. This had probably led to the air from the larger static-shielding bag entering the sample holders. The grains may have been exposed to air for about 72–96 h during transportation, before putting them again in a dry N2 atmosphere. However, we did not observe any modification on the sample holder’s KBr window (a control KBr window we exposed to air for 24 h showed clear modifications, such as opaqueness, creases and wavy patterns across the surface). We inferred that Ryugu grains remained in a relatively dry environment in spite of the presence of O2, probably thanks to the presence of numerous desiccant packs in the traveling case, which prevented an increase of humidity.
To easily acquire FTIR spectroscopic measurements through the KBr window, the grains were arranged onto a gold mirror. This fact had unforeseen consequences on the spectroscopic measurements of all small particles (size smaller than 100 µm), with the collected spectra showing some peculiarities affecting the surface scattering spectral region (from approximately 9 µm and above). In standard conditions, the IR beam would shine onto the particle surface, be reflected by the grain’s surface and be then collected for analysis. This is what we observed for the largest particle in our set of grains, which had a size of approximately 150 µm (Fig. 7, left panel). However, for smaller particles (size < 100 µm), the IR beam is able to go through the grain, similar to what happens in transmission measurements. The transmitted beam would then hit the gold mirror where the particle would rest, shining back inside and through the measured grain, to be collected for spectral analysis (Fig. 7, right panel). This means that the collected beam would be a mix of reflected signal and double-transmitted signal. Their respective contributions may be difficult to gage, but for small grains one would think that this double-transmitted signal would dominate. The consequences of this effect on the spectra measured in these conditions are the following:
This quirk makes the interpretation of the collected signal from small grains more delicate (band intensity and band ratio not straightforward to discuss, the feature position is easier to apprehend), but does not invalidate the usefulness of the measured spectra. It is a well-known phenomenon that the reflectance may contain signals originating from the substrate material in addition to the signal originating from the sample under investigation, and one of the most common effects is the flipping of the IR spectrum. This has been observed in the studies involving thin meteorite sections or particularly small grain size meteorite powders (Skulteti et al. 2020).
Measurements out of the sample holder (ambient air)
Based on the spectral properties (clarity/heterogeneity of the hydration feature, interesting silicate features) obtained with the first step of our analytical pipeline, we selected 9 grains to be mounted on W and Al needle for 3D IR characterization, with sizes ranging approximately from 20 to 100 µm. The sample holders were opened, and the grains were mounted on W or Al needles using Pt-weld at two different FIB-SEM microscopes, a FEI Thermofischer Helios Nanolab 660 at MSSMAT in Saclay and a FEI Strata DB 235 SEM–FIB microscope at IEMN in Lille (Aléon-Toppani et al. 2021). These mounted grains underwent then 3D characterization in both transmission and reflectance, using Infrared Computed Tomography (IR-CT) and Infrared Surface Imaging (IR-SI) respectively (see Fig. 8). IR-CT allows us to assess the compositional heterogeneity of small particles in a 3D space (Dionnet et al. 2020; Yesiltas et al. 2017; Martin et al. 2013), while IR-SI allows us to assess the surface composition for larger particles, treating the grain as a planetary surface by projecting the 2D IR hyper-spectral maps on a 3D shape model (Dionnet et al. 2022). For IR-CT, a 25 × objective is used in combination with a high magnification system placed in front of the 128 × 128 pixel FPA detector: the projected pixel size is approximately 0.66 µm and the field-of-view is 84 µm. For IR-SI, only the 25 × objective is used to maximize signal-to-noise ratio, and the projected pixel size and FOV are 3.3 µm and 105 µm respectively (the detector size is reduced to 32 × 32).
From mounted grains, FIB sections are extracted to perform TEM analysis, following similar procedures to what are described in Aléon-Toppani et al. (2021).
The grains that have not yet been mounted remained in their respective sample holder. Some of these grains underwent complementary measurements, such as Raman micro-spectroscopy (Maupin et al. 2020). All grains are kept in a N2 atmosphere when they are not being measured.