III. a 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 (Figure 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 Figure 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 50 µm, while keeping the sample-holder sealed to minimize exposure to air. The goal of this first step in our analytical pipeline was threefold :
- 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 at 2.7 µm, position of the Si-O stretching silicate feature at 10 µm, presence/absence of the Carbonate feature at 7 µm).
- Select grains to be mounted on needles for 3D IR characterization.
- Have a trace 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 SMIMS-beamline (SOLEIL Synchrotron) without the risk of compromising the samples. In order to cover the above-mentioned spectral range, we used three different FTIR microscopes (see Figure 4):
- 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);
- 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);
- 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 minerals (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. These measurements were done on isolated small fragments detached from the main grains, to avoid alteration from the Raman laser.
Raman was also used to detect molecular oxygen inside one of the sample holders (SH2, see Figure 5), 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 hours 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 24h showed clear modifications). 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 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, 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 (Figure 6, 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 (Figure 6, 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 gauge, 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 :
- Inversion of the spectral bands in the surface scattering region (above 9 µm) ;
- Interpretation of band intensity is not straightforward.
This quirk makes the interpretation of the collected signal from small grains more delicate but does not invalidate the usefulness of the measured spectra.
III. b Measurements out of the sample-holder (ambient air)
Based on the spectral properties 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 in Saclay and 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 Figure 7). IR-CT allows us to assess the compositional heterogeneity of small particles in a 3D space (Zelia Dionnet et al. 2020), 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 (Zélia Dionnet et al. 2022). 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 and visible near IR (Vis-NIR) micro-spectroscopy (Maupin et al. 2020). All grains are kept in a N₂ atmosphere when they are not being measured.