Abstract
The elastic property of asteroids is one of the paramount parameters for
understanding their physical nature. For example, the rigidity enables
us to discuss the asteroid’s shape and surface features such as craters
and boulders, leading to a better understanding of geomorphological and
geological features on small celestial bodies. The sound velocity allows
us to construct an equation of state that is the most fundamental step
to simulate the formation of small bodies numerically. Moreover, seismic
wave velocities and attenuation factors are useful to account for
resurfacing caused by impact-induced seismic shaking. The elastic
property of asteroids thus plays an important role in elucidating the
asteroid’s evolution and current geological processes. The Hayabusa2
spacecraft brought back the rock samples from C-type asteroid (162173)
Ryugu in December 2020. As a part of the initial analysis of returned
samples, we measured the seismic wave velocity of the Ryugu samples
using the pulse transmission method. We found that P- and S-wave
velocities of the Ryugu samples were about 2.1 km/s and 1.2 km/s,
respectively. We also estimated Young’s modulus of 6.0 – 8.0 GPa. A
comparison of the derived parameters with those of carbonaceous
chondrites showed that the Ryugu samples have a similar elastic property
to the Tagish Lake meteorite, which may have come from a D-type
asteroid. Both Ryugu and Tagish Lake show a high degree of aqueous
alteration and few high-temperature components such as chondrules,
indicating that they formed in the outer region of the solar system.