Keisuke Onodera

and 32 more

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.

Masanori Kanamaru

and 14 more

Asteroid 162173 (Ryugu) is a carbonaceous asteroid that was visited by Japan’s Hayabusa2 spacecraft in 2018. The formation mechanism of spinning-top shape of Ryugu is an essential clue to the dynamical history of the near-Earth asteroid. In this study, we address the spin-state evolution of Ryugu induced by the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect, i.e., the thermal recoil torque that changes the rotation period and spin-pole direction. Given the current orbit, spin state, and three-dimensional shape observed by Hayabusa2, we computed the YORP torque exerted on Ryugu using a simplified thermal model approximating zero thermal conductivity. Despite differences in meter-scaled topography, all 20 shape models that we examined indicate that the spin velocity of Ryugu is currently decreasing at a rate of (-0.42—6.3)*10-6 deg/day2. Our findings also suggest that the thermal torque on the asteroid is responsible for maintaining the spin pole upright with respect to the orbital plane. Therefore, the YORP effect could explain the significant spin-down from a period of 3.5 h initially to 7.6 h currently. The corresponding time scale of the rotational deceleration is estimated to be 0.58–8.7 million years, depending on the input shape models. This time scale is comparable to e.g., the formation period of the largest crater, Urashima (5–12 Ma) or the western bulge (2–9 Ma) as derived from previous studies on crater statistics in Ryugu. It is considered that the rotation of the asteroid started to decelerate in the wake of the major crater formation or the resurfacing event on the western hemisphere.