Nanoscale infrared characterization (AFM-IR) of Ryugu samples returned by the Hayabusa 2
space mission
J. Mathurin1, E. Dartois2, A. Dazzi1, A. Deniset-Besseau1, L. Bejach3, C. Engrand3, J. Duprat4, Y. Kebukawa5, H. Yabuta6, H.
Yurimoto7, T. Nakamura8, T. Noguchi9, R. Okazaki10, H. Naraoka10, K. Sakamoto11, S. Tachibana11,12, S. Watanabe13, Y.
Tsuda11, and the initial-analysis IOM and STONE teams
1

Institut Chimie Physique (ICP), Univ. Paris-Saclay/CNRS, 91405 Orsay, France2
ISMO, Univ. Paris-Saclay/CNRS, 91405 Orsay, France
3
IJCLab, Univ. Paris-Saclay/CNRS, 91405 Orsay, France
4
IMPMC/MNHN, SU, Paris, France
5
Faculty of Engineering, Yokohama National University, Japan
6
Hiroshima University, Higashi-Hiroshima 739-8526, Japan
7
Hokkaido University, Sapporo 060-0810, Japan
8
Tohoku University, Sendai 980-8578, Japan
9
Kyoto University, Kyoto 606-8502, Japan
10
Kyushu University, Fukuoka 812-8581, Japan
11
ISAS/JAXA, Sagamihara 252-5210, Japan
12
The University of Tokyo, Tokyo 113-0033
13Nagoya University, Nagoya 464-8601, Japan.

The distribution of chemical bonds in organic matter (OM) of interplanetary samples (meteorites andmicrometeorites) can be efficiently and non-destructively characterized using infrared vibrational spectroscopy (FTIR) [1].
Conventional FTIR microscopy provides a global view of the dust grain physico-chemical composition but remains spatiallylimited by the diffraction. In state-of-the-art synchrotron-based FTIR microscopy, the typical spot sizes in the mid-IR range
can, at best, sample a few µm. This spatial resolution limitation often hampers a direct comparison with complementarytechniques such as isotopic imaging with NanoSIMS, transmission electron or X-ray microscopy. Such IR diffraction
limitation can be circumvented using AFM-IR microscopy [2]. AFM-IR is now a well-established microscopy technique in thevibrational field, combining an atomic force microscope (AFM) and a tunable IR illuminating source to detect photo-thermal
effect and access chemical information. This technique is applied in a wide diversity of scientific fields [3] and we recently
used it to analyze various extraterrestrial samples [4-8]. For this study the tunable IR illuminating source is a QCL laser with aspectral range from 1900 cm-1 – 900 cm-1 with a top-down illumination. This configuration allows two modes of acquisition. In
the first one, the wavenumber is fixed and the tip moves along the surface, allowing to acquire simultaneously AFM
topography and an IR absorption map of the sample at the selected wavenumber. In the second one, the tip position is fixed ona sample position and the IR laser is tuned to explore the whole available spectral range of the QCL laser, giving access tolocal IR spectra at the point of interest.
Ryugu samples from chamber A (surface) and chamber C (sub-surface) were received from the “IOM” and “Stone”
Hayabusa2 initial analysis teams. Several samples from chambers A and C were prepared by crushing small fragments on
diamond windows, down to a thickness of ~1 µm. Because of the high resolution of AFM-IR (tens of nanometers), regions ofinterest were first localized and selected using synchrotron-based FTIR microscopy [8].
Figure 1: Study of chamber C sample C109-04. B1: 3 x 3µm RGB composite image combining the AFM-IR
absorption mapping obtained at different wavenumbers
corresponding to the absorption of Si-O silicates (1020cm-1, green), C=C (1600 cm-1, blue) and C=O (1720 cm1, red). The size of the image here corresponds to the sizeof one pixel in the synchrotron maps - B2: AFM
topography of the 3 x 3 µm area studied in AFM-IR B3: optical image - B4: from top to bottom : average IR
synchrotron spectra obtained on the whole sample, local
spectra obtained by AFM-IR highlighting the presenceof OM with and without a C=O signature at 1720 cm-1,
on the locations highlighted by arrows in B1. The red,
blue and green dashes indicate the wavenumber positionsof the IR mapping with the same color - B5-B7: IR maps
obtained by transmission synchrotron µFTIR at samewavenumber (and corresponding colors) as the AFM-IRmaps. Adapted from [7]A large-scale map (20 µm x 20 µm) was first recorded to characterize the main components' distribution within the sample
with a spatial resolution of ~75 nm [8]. As the OM contribution in the IR spectra is weaker than that of minerals, highlyspatially resolved maps of 3 x 3 µm in size were acquired with a lateral spatial resolution of ~25 nm. It was then possible tolocalize OM inclusions in the samples (Fig.1 – B1) by comparing the signal from the Si-O of the silicates (Fig. 1 – B1 ingreen) and that from the OM contributions of the C=C absorption (Fig. 1 – B1 in blue) and C=O absorption (Fig. 1 – B1 inred). On the larger OM inclusion displayed in Fig. 1 – B1, the OM signal is a mix between C=C and C=O (which appears inpurple), but chemical heterogeneities are observed at small scales: parts of the inclusion seem to exhibit local enrichment in
C=O (redder) or in C=C (bluer). Local AFM-IR spectra were recorded on these spots, showing locally different C=C and C=O
contributions (Fig.1 – B4) [7].
Based on this first measurement, a more complete analysis of various samples was conducted either on whole-rocksamples or on IOM extracted from chambers A and C samples [9]. The results obtained not only confirmed the heterogeneityof the OM distribution already observed but also highlight the fact that OM is mainly distributed in two phases, a diffuseorganic component intimately mixed with the mineral matrix and nanoglobule-like organic particles (Fig. 2). These particles
are texturally resembling nanoglobules present in primitive meteorites and were identified in both whole-rock and IOM
samples from chambers A and C Ryugu samples. The local IR spectra of nanoglobule-like particles also clearly showenhanced carbonyl (C=O) and CH contributions with respect to the diffuse organic matter in Ryugu whole-rock and IOM
residue (Fig. 2 – (c)). Such spectra are compatible with that of organic residues formed from UV or ion irradiated ices at lowtemperatures, simulating the environment of the outer regions of the protoplanetary disk, or in the protosolar cloud.

Figure 2: Whole-rock Ryugu sample A0108-19. (a) RGB image of the region including an organic nanoglobule-like particle (C=O at 1720
cm−1 in red, C=C at 1600 cm−1 in green, CHX at 1460 cm−1 minus 1520 cm−1 in blue). The white line shows the location of individual AFMIR spectra taken across the nanoglobule-like feature. The colored dots (S1 to S3) correspond to average spectra taken on each side (S1 andS3 in blue) and on the nanoglobule-like feature (S2 in red). (b) Intensity ratios corresponding to CHX/C=C (1460 cm−1/1600 cm−1) and
C=O/C=C (1700 cm−1/1600 cm−1) along the white line shown in (a). The location of the S1 to S3 spectra is recalled on the left axis.
(c) Average spectra on each side (blue, S1 and S3) and on the nanoglobule-like feature (red, S2). The red dot-dashed line spectrum labelled‘Nanoglobule-like’ is obtained by scaling the average of S1 and S3 spectra to the same silicate band intensity as in the S2 spectra, and by
subtracting this scaled spectrum from S2. The resulting ‘Nanoglobule-like’ spectrum is thus freed from the matrix contribution underneaththe globule. The nanoglobule-like spectrum shows elevated CHX signal (intense methyl and methylene deformation modes at ∼1460 cm−1and ∼1370 cm−1), as well as high carbonyl contributions around 1700 cm−1. The blue dot-dashed line spectrum labelled ‘Matrix’ corresponds
to the expected contribution from the matrix for equivalent probe depth as for the nanoglobule-like spectrum. Adapted from [9].

AFM-IR measurements demonstrate the presence of organic inclusions intimately mixed with minerals in Ryugu samples
at the sub-micron scale. Focusing on the OM-rich zones of Ryugu samples it is possible to unveil, without any chemicaltreatment, the IR signature in the chemical bondings of chemical OM heterogeneities as well as the sub-micrometric spatial
distribution of the different components. This study shows that OM exists in (at least) two forms in Ryugu samples, as adiffuse phase in the minerals matrix, and as isolated inclusions resembling nanoglobules.
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