Nano-infrared spectroscopy of Ryugu samples using AFM-IR measurement
Van T. H. Phan1, Pierre Beck1, Rolando Rebois1 and Takaaki Noguchi2, the Hayabusa2-initial-analysis Sand team.
1

Université Grenoble Alpes, CNRS, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), France2
Kyoto University, Division of Earth and Planetary Science, Kyoto, Japan

Introduction: In late 2020, samples of Ryugu (C-type) were collected from its asteroid and returned to Earth, led by theJapan Aerospace Exploration Agency (JAXA), thought to be similar in composition to CI chondrites 1. Understanding how andunder what conditions the organic matter interacted with inorganic phases, is a key step in hypothesizing the mechanism ofaccreted organic matter. This “fresh” carbonaceous material allows us to investigate the best preservation sample without
terrestrial impact 2. In addition, the “bulk” Ryugu samples also show some spectral reflectance bands corresponding to the NHrich compounds 3. Altogether, our study aims to characterize the minerology, organic matter and the possible N-bearing
molecule. To gain insight into fine meteorites matrices containing an interplay of ingredients at the sub-micro scale, it requires
analytical techniques with the sub micrometer spatial resolution to separate their composition and understand theirpetrographic relations. Infrared spectroscopy (IR) is an effective method and a non-destructive technique for molecular-atomic
scale vibrations of organic and inorganic compounds in the extra-terrestrial materials 4,5. However, conventional IR
spectrometers use mirror-based optics to focus the beam on the sample and thus the spatial resolution is limited by thediffraction limit. This IR diffraction limitation can be overcome by using AFM-IR (Atomic Force Microscopy-InfraredSpectroscopy), based on the combination of infrared spectroscopy and atomic force microscopy to efficiently distinguishspectral signatures of the different constituents 6,7.
Our study aims to characterize two different grains of Ryugu that can be affected by the weathering space prepared by“Min-pet fine team” (Sand team)8 as crushed sample between two diamond windows, as well as two different Orgueil samples:
one comes from the Paris Museum (Orgueil Museum) and the another one expected “fresher” kept in the sealed flask (OrgueilFlask) from collection of M. Bizzaro. Here, we report the comparison of AFM-IR results between Ryugu and Orgueil as a CI
chondrite both in minerology and organic structure.
Samples preparation and methods: Two grains of Ryugu (P0032 and P0038), received from the Sand initial analysis
team, were prepared by crushing 10 – 20 µm fragments between two diamond windows. As with Ryugu, the Orgueil Museumand Orgueil Flask samples were also pressed down to the appropriate thickness for AFM-IR measurement. Before AFM-IRanalysis, all samples were performed the micro-FTIR analysis to select the region of interests (ROI).
IR spectroscopy and AFM-IR measurement: The micro-infrared spectra were first analyzed with a Bruker Hyperion
3000 infrared microscope. The nano-IR measurements were obtained with a Bruker nanoIR3s at IPAG (Grenoble, France),
which includes an AFM probe capable of scanning the sample and generating topographic images at nm vertical resolution.
The AFM-IR measurement can be performed in two acquisition modes: AFM and chemical imaging, and local, single point IRspectrum. First, the wavenumber is fixed and the tip moves along the surface to obtain the AFM topography and IR absorption
at the given wavenumber. Second, the position of the tip is fixed and the IR laser is tuned to the possible spectral range (2000 –700 cm-1 with the APE laser at IPAG) which provides the local absorption spectrum at the point of interest 6.
Results and Discussion: We are able to characterize two bulk grains of Ryugu with the region of interest 5×5 µm2 (grainP0032) in the in Figure 1 and the ROI of 3×3 µm2 (grain P0038) in Figure 2.

Figure 1. AFM-IR of a 5×5 µm2 region of interest (ROI)
acquired with the NanoIR3s at IPAG (Grenoble, France)of grain P0032. Left images: AFM image, the AFM-IRimages of 1720 (red), 1450 (blue), 1100 (yellow), 1000
cm-1 (green) corresponding to C=O, carbonate, sulfate andSi-O; and RGB (red, green, blue) composition image fromthe combination of three AFM-IR absorption images: 1720,1450 and 1000 cm-1 respectively. Right panel: comparisonof local AFM-IR spectra on the carbonate rich area (inblue), and the phyllosilicate area (in green) withphyllosilicate signature of Orgueil Flask, µ-FTIR spectraof Ryugu, calcite and dolomite minerals.

In the first grain, P0032, AFM-IR images at different wavenumber were obtained at 1720, 1450, 1100 and 1000 cm-1 inthe left panel corresponding to carbonyl (C=O), carbonate, sulfate and phyllosilicate (Si-O), respectively (Figure 1). The threemaps of 1720, 1450 and 1000 cm-1 wavenumber were overlaid in the RGB composite image, allowed us to identify the twodifferent domain compositions in the mapped area: the green area can be distinguished by a strong absorption at 1000 cm-1
attributed to the presence of phyllosilicate; the blue/purple area shows a strong absorption at 1450 cm-1 related to carbonate.
Numerous spectra were obtained across this ROI shown in the right panel and compared to AFM-IR spectra of Orgueil Flask,
dolomite, calcite and µ-FTIR of the same P0032 grain. Spectrum A, the average of hundreds of spectra in the green region
shows signature at 1020 cm-1 corresponding to phyllosilicate (e.g., saponite, serpentine). Compared to the Orgueilphyllosilicate spectra, Ryugu is lack of a sulfate signature at 1100 cm-1. However, we observed that some purple areas havestronger absorption at 1100 cm-1 and AFM-IR spectra also confirm the two peaks at 1100 and 840 cm-1 (B spectrum). This
Ryugu sample was collected in the fine-grained fraction, affected by space weathering 8, the phyllosilicate-rich region can bemodified. C spectrum in the blue region suggests the presence of carbonate with two strong signatures at 1440 and 860 cm-1,
all indicative of dolomite or calcite.
In the second Ryugu grain, P0038, we first collected the different images at 1720 (red), 1450 (blue) and 1000 cm-1(green) in ROI of 20×20 µm2 and overlaid them in the RGB composite image in the upper left panel. We found a ~ 1µmdiameter globule with strong absorption at 1720 cm-1 corresponding to C=O in the organic material (Figure 2). Therefore, theROI of 3×3 µm2 was selected to obtain the different images at 1720, 1600, 1450 and 1000 cm-1. The RGB composite(combined between the images at 1720, 1450 and 1000 cm-1) show an organic globule in red with strong absorption at 1720
cm-1. Later, the contact AFM-IR spectra also confirm the presence of organic matter with two distinct features at 1720 and1610 cm-1 corresponding to C=O and C=C, respectively. Although the AFM-IR images only show the strong organic
appearance, the AFM-IR spectra of organic globule still mixes with the minerals. Compared to the organic matter obtained at
Orgueil described in 9, the Ryugu organic material is more aromatic. The more diffuse organic may present in this ROI but still
finely mixed with phyllosilicate.

Figure 2. AFM-IR of a 3×3 µm2 region of interest (ROI)
acquired with the NanoIR3s at IPAG (Grenoble, France) ofgrain P0038 from the RGB image of 20×20 µm2 region on theupper left panel. Right images: AFM image, the AFM-IR
images of 1720 (red), 1600 (pink), 1450 (blue), 1000 cm-1(green) corresponding to C=O, C=C, CO3/CH2 bend, and Si-O;
and RGB (red, green, blue) composition image from thecombination of three AFM-IR absorption images: 1720, 1450
and 1000 cm-1, respectively. Lower left panel: comparison oflocal AFM-IR spectra on the organic globule (red), and thephyllosilicate area (green) with organic matter of Orgueil andµ-FTIR spectra of Ryugu.

Conclusion: The complete results and discussion of this study will be presented at the conference. So far, we confirm
that the AFM-IR technique allows us to disentangle the IR signatures of phyllosilicate, carbonate and organic matter in Ryugu
at the sub-micron scale. In minerology, compared to Orgueil, the sulfate signatures almost absent from the phyllosilicate, butsome modifications in Si-O-rich areas were observed. In the organic structure, the weak organic signature is diffuse andsignificantly intermixed with phyllosilicate, similar to the diffuse organic of Orgueil. A 1 µm-organic globule was also
observed with clear C=O and C=C features in its structure. This organic matter globule is more aromatic compared to theorganic matter in Orgueil meteorites.
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