Three-dimensional analysis of Ryugu sample particles using X-ray nanotomography.
Akira Tsuchiyama1,2, Megumi Matsumoto3, Junya Matsuno1, Tomoki Nakamura3, Takaaki Noguchi4,5, Kentaro Uesugi6,Akihisa Takeuchi6, Masanori Yasutake6, Miyake Akira4, Shota Okumura4, Yuri Fujioka3, Mingqi Sun2, Aki Takigawa7,Toru Matsumoto4, Satomi Enju8, Itaru Mitsukawa4, Yuma Enokido3, Tomoyo Morita3, Naoto Nakano4 and Stefano Rubino9,Tsukasa Nakano10 Hisayoshi Yurimoto11, Ryuji Okazaki12, Hikaru Yabuta13, Hiroshi Naraoka12, Kanako Sakamoto14,Shogo Tachibana7,14, Seiichiro Watanabe15 and Yuichi Tsuda14, and the Hayabusa2-initial-analysis Stone and Sand teams
1Research Organization of Science and Technology, Ritsumeikan University, 2CAS, Guangzhou Institute of Geochemistry,
3Guraduate School of Science, Tohoku University, 4Guraduate School of Science, Kyoto University, 5Faculty of Arts andScience, Kyushu University, 6SPring8/JASRI, 7Guraduate School of Science, University of Tokyo, 8Guraduate School of Scienceand Engineering, Ehime University, 9Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 10GSA/AIST,
11Guraduate School of Science, Hokkaido University, 12Guraduate School of Science, Kyushu University, 13Guraduate Schoolof Advanced Science and Engineering, Hiroshima University, 14ISAS/JAXA
We have performed initial analysis of samples returned from asteroid 162173 Ryugu by the Hayabusa2 spacecraft usingsynchrotron radiation-based X-ray nanotomography to elucidate the 3D structures and features of the samples with sub-micron
spatial resolution as parts of the Hayabusa2 initial analysis Stone and Sand sub-teams.
We analyzed 29 and 20 particles from the first and second touchdown sites (chambers A and C), respectively. Particles of10 ~ 150 µm in apparent size were picked up and attached to the tips of Ti needles with Pt deposition using FIB or amorphous
carbon fibers with glycol phthalate. They were imaged by two different methods (dual-energy tomography (DET) [1] andscanning-imaging X-ray microscopy (SIXM) [2]) at BL47XU of SPring-8 [3]. In DET, X-ray absorption contrast images wereobtained as the spatial distribution of linear attenuation coefficient (LAC) at two different X-ray energies of 7 and 7.35 keV,which are below and above the K-absorption energy of Fe (7.11 keV), respectively. The LAC images at 7 keV correspondclosely to compositional (Z) contrasts, except for Fe, and those at 7.35 keV strongly depended on Fe content. In SIXM, weobtained X-ray differential phase contrast images as the spatial distribution of refractive index decrement (RID), which is thedifference between the X-ray refractive index (RI) and unity (RI = 1 - RID), at X-ray energy of 8 keV. The RID image isclosely proportional to the density of the object. The voxel sizes of LAC 7 keV, LAC 7.35 keV and RID images are 47.4×
47.4×47.4 nm3, 51.1×51.1×51.1 nm3 and 111.1×111.1×109.2 nm3, respectively.
The image analysis was made by combining ImageJ and codes made with C and Python. The registration among the threetypes of images was made with the reference of LAC 7 keV images. Then, RGB-CT images were made by assigning blue, red,and green to LAC 7 keV, LAC 7.35 keV, and RID images, respectively. Different phases including organic materials show
different colors in the RGB-CT images. Minerals are quantitatively discriminated by comparing the LAC and RID values ofthe objects in CT images and those of minerals with known chemical compositions and densities. The solid portions of thesample particles images were extracted in LAC 7.35 keV using Chan-Base segmentation. The volume, V, surface area, S, 3axial lengths, A≥B≥C, fractal dimension, FD, and closed porosity, p0, were then obtained. Porosity by considering pores andcracks that are open to the outside, named as open porosity, p∞, were also estimated by a wrapping method [4]. The density, r,
was estimated from the averaged RID value of the particle as the lower limit. Density considering open pores, r∞, wascalculated as r×(1- p∞). Sphere-equivalent diameter, D, was calculated from V and sphericity, Y, from V and A.
No distinct difference between the samples from chambers A and C was recognized. The sample particles are mainlycomposed of Mg-rich phyllosilicates (serpentine and/or saponite; Mg#~0.8-0.9) in the matrix (Fig. 1A). The phyllosilicates are
not homogeneous in some particles (Fig. 1B) probably due to the difference of Mg# and/or nanoporosity below the spatialresolution. FeS (mostly pyrrhotite and minor pentlandite) grains are commonly present as hexagonal plates shown as arectangle shape in a slice image (Fig. 1A). We found whiskers of probably FeS (Fig. 1C) in some matrices as well. Magnetite
grains with different morphologies (framboidal, spherulitic, plaquette and equant) are also commonly present (Fig. 1A). Submicron grains of FeS and magnetite are usually present in the matrix. Dolomite is usually present mostly as aggregates ofeuhedral or subhedral grains (Fig. 1A) and contains minor amounts of Fe and some heavy element (possibly Mn). Aggregatesof micron-sized dolomite grains are also present (Fig. 1B). Breunerite is present as a rhombohedron in shape and have thecomposition roughly estimated as (Mg0.6Fe0.4)CO3 (Fig. 1D). CaCO3 (aragonite or calcite) is rarely present. Apatite is usuallypresent mostly as aggregates of subhedral or anhedral grains (Fig. 1E). Small grains of forsterite or enstatite (Mg#~1) arerarely observed. Small objects (mostly <1 µm) of probably organic materials are commonly present in the matrix (Fig. 1B) but
their abundance is small (probably a few % or less). Inclusions, which are empty or filled with low-Z materials, were observedin FeS, dolomite, breunerite and apatite grains (Fig. 1D). Spherical objects of phyllosilicate surrounded by fine grains of FeS

or dolomite (Fig. 1E) and object composed of Fe-rich and mostly anhydrous silicates (Fig. 1E) and fine grains of FeS and/ormagnetite named “dark inclusion” (Fig. 1A) were sometimes observed. In addition to the above phases, several unidentified
phases are observed as well. Some sample particles are unique; a low-density material (~1 g/cm3) with a lot of fine cracks andorganic-like material with embedded mineral grains. Porous objects like the ultra-porous lithology in Acfer 094 [3] were notobserved.

Figure 1. RGB-CT images of Ryugu particles(R: LAC at 7 keV = 0-950 cm-1, G: RID = 020×10-6, B: LAC at 7.35 keV = 0-450 cm-1).
(A) C0103-FC007. (B) A3_MPF_X003
showing dark and bright matrices (upper andlower regions, respectively). (C)A3_MPF_X007 showing a whisker ofprobably FeS. (D) A0067-FC0003: breunerite
grain with inclusions. (E) A3_MPF_X007
having wide cracks. (F) A3_MPF_X004.

D ranges from 9 to 60 µm. The 3D shape distribution indicates that theRyugu particles (B/A =0. 66 and C/B =0.65 in average) are less spherical inshape than the Itokawa particles and fragments of impact experiments (c.f.,silver ratio: B/A=C/B=0.71) [1]. Some particles examined here may not be theoriginal impact fragments on the Ryugu surface but artificial fragments during
the sampling by the space craft and/or in the laboratory. Y is small (<0.4) andFD of some particles are <3 together with a weak correlation between them,indicating that the particle shapes are irregular and complicated. Some surfacesof the particles are convexo-concave indicating fragmentation, while some are
more or less smooth and may be formed by mechanical abrasion on the asteroid
as proposed for Itokawa particles [4]. Cracks less than a few µm in width arecommonly present in all particles except for those composed of single crystals.
Some cracks develop along the boundaries of some objects and mineral grains
Figure 2. Density, r, estimated from the(Fig. 1E), implying that they are cracks formed due to shrinkage by
averaged RID value (lower limit) and density
dehydration. Some cracks are subparallel to flattened particle surfaces
considering open pores, r∞, plotted against
open porosity, p∞.
probably due to impact (Fig. 1F). p0 is usually less than a few%. p∞ mostly
ranges from 5 to 20% (Fig. 2), which is less than the porosity of the CI
chondrite Orgueil (34.9% [5]). This can be explained by the different size of pores; the p∞ counts relatively large pores (>~afew hundred nm) while that of CI counts nanopores. r mostly ranges from 1.3 to 1.8 g/cm3 and and r∞ from 1.1 to 1.6 g/cm3(Fig. 2). They are not inconsistent with the bulk density of Orgueil (1.57 g/cm3 [5]) if we consider r and r∞ as the lower limits.
The examined samples closely resemble CI in mineralogy and textures although we did not surely find any sulfates, such as
gypsum, epsomite and blödite. The porosity and density are also consistent with those of CI. Any signature of dehydration as
suggested from the IR spectrum [6] was not recognized. Highly porous materials as expected from the low thermal inertia [7]were not observed. FIB sections were prepared based on the 3D information and examined by TEM [8, 9]. The present
examination is ongoing and we note that some of the ranges of physical properties, such as D, B/A, C/B, Y, FD, p0, p∞, r andr∞, are still preliminary.
References[1] Tsuchiyama A. et al. 2013. Geochim. Cosmochim. Acta 116;5. [2] Takeuchi A. et al. 2014. J. Synchrotron Radiat. 20:793.
[3] Matsumoto M. et al. 2019. Science Advances 5;eaax5078. [4] Tsuchiyama A. et al. 2011. Science 333;1125. [5] Flynn G. J.
et al. 2018. Chemie der Erde 78: 269. [6] Kitazato K. et al. 2019. Science 268:272. [7] Okada T. et al. 2020. Nature 579:518.
[8] Nakamura T. et al. 2021. Abstract in this volume. [9] Noguchi T. et al. 2021. Abstract in this volume.