Elastic property of Ryugu samples collected at the second touch-down site
Keisuke Onodera1, Yuta Ino2,3, Satoshi Tanaka3, Taichi Kawamura4, Takuya Ishizaki3, Ryota Fukai3, Rei Kanemaru3,Takahiro Iwata3, Tomoki Nakamura5, HAYABUSA2 initial sample analysis team1
Earthquake Research Institute, The University of Tokyo
2
Kwansei Gakuin University
3
Institute of Space Astronautical Science, Japan Aerospace Exploration Agency
4
Institut de Physique du Globe de Paris, Université Paris Cité
5
Department of Earth Sciences, Tohoku University
Introduction
Since the HAYABUSA2’s successful return from the 6 years of space journeyon December 5th 2020, many analyses of the collected samples have been
conducted (e.g., [1]-[4]). Those studies help us better understand the ancienthistory of the solar system as well as the Ryugu’s characteristics and its origin.
According to the initial analysis [1]-[3], it turned out that Ryugu showed highsimilarity to CI chondrites (e.g., Ivuna, Orgueil) from the petrological or
mineralogical aspect. For example, Visible-Near infrared reflectance spectra of
Ryugu samples show a sharp absorption at 2.72 µm – consistent with the remotesensing observation [5] – and weak absorptions at 3.4 and 3.95 µm as seen in CI
chondrites. These observations provide us with chemical features, allowing us tointerpret how these particles were formed under what kind of environment.
In addition to chemical aspects, physical properties such as density, porosity,and rigidity are also of great importance. Especially, rigidity is one of theparamount parameters to constrain the formation process from the catastrophic
impact to re-accumulation to make a rubble pile body [4]. Also, the elasticbehavior like seismic wave propagation is closely related to the asteroid’s surface
evolution (e.g., seismic shaking).
In this study, through the measurement of seismic wave velocity, we are trying
to provide a detailed description on elastic properties such as seismic wave
velocity, Young’s modulus, and attenuation quality factor. Moreover, bycomparing the derived parameters with those for other carbonaceous chondrites,
we discuss which type is the most similar to Ryugu from the viewpoint of elastic
property.
Samples used for measurements
For the measurements, C0002-No3 and No4 were allocated to us. Bothparticles are mm scale with a thickness of 0.5 – 1.0 mm, cut out of one of thelargest samples collected at the second touch-down site. Figure 1 shows themicrographs of the respective particles. Note that the C0002-No4 was broken into
two pieces over the course of the physical property measurements [4], and wemeasured their seismic waves individually.
Seismic wave velocity measurement and estimation of Young’s modulus
We adopted the pulse transmission method [6] to measure P and S wave
velocities. The experimental setting is shown in Figure 2, where a sample issandwiched with two transducers placed on an electronic scale. This kind ofsetting allows us to measure the wave velocity monitoring the loading
simultaneously. The principle is to transmit a pulse from one side transducer toanother side for two settings: (i) transducers without sample, (ii) transducers witha sample interposed. Then, we can obtain the time delay by comparing two signals(Figure 3). Since the thickness and density of each sample are known, the seismic
wave velocity and Young’s modulus can be obtained in the end. As a result, weobtained Vp=1.9 – 2.3 km/s, Vs=1.2 – 1.4 km/s, and 6.2 – 8.6 GPa for Young’s
modulus under 1 MPa loading (Table 1).

Figure 1. Micrographs of Ryugu sample (a:C0002-No3, b: C0002-No4). For themeasurements, we used the divided piecesof C0004-No4 (1&2).

Figure 2. Schematic illustration of theexperimental setting.

Figure 3. Transmitted signal without sample(top) and with sample (bottom). Time delay
between them gives seismic wave velocity.

modulus under 1 MPa loading (Table 1). While Nakamura et al. [4] reported the average values of these samples, thispresentation will show the individual results. Moreover, we discuss whether Ryugu is similar to CI chondrites also fromelastic property through the comparison of our results with those for various carbonaceous chondrites.
Table 1. P- and S-wave velocities and Young’s modulus for each sample
Sample nameVp (km/s)
Vs (km/s)
C0002-No32.25±0.01
1.42±0.15
C0002-No4-1
1.94±0.01
1.20±0.01
C0002-No4-2
1.96±0.01
1.32±0.07

Young’s modulus (GPa)8.58±3.17
6.24±2.80
6.88±2.34

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