9:30 AM - 9:45 AM
[MZZ40-03] Temporal evolution of boulder size distribution on near-Earth asteroid Ryugu
Keywords:Ryugu, Near-Earth asteroid, Boulder, Size distribution evolution
On the surface of Ryugu, fragmentation of boulder due to meteoroid impacts and thermal fatigue is thought to contribute to the mass loss of the asteroid and to the supply of materials to the planets. The fragmentation evolves the size distribution of boulder on the asteroid surface by transferring mass from larger to smaller size, making it important for understanding the surface evolution of asteroids from the boulder size distribution. The size distribution evolution of particles smaller than 1 m on asteroid surface was investigated by numerical simulations of particle fragmentation [1]. However, mass transfer from boulders larger than 1 m influences the size distribution evolution of smaller particles. Therefore, the size distribution evolution of boulders larger than 1 m, which constitute the majority of Ryugu surface and serve as the origin of mass transfer, is important.
The impact strength, the minimum collisional energy required per unit mass to disrupt the boulder, constitutes fundamental information in understanding collisional fragmentation on asteroid surfaces and of parent bodies. Regarding the physical properties of boulders on the asteroid Ryugu, porosity estimation, and mechanical strength assessment through thermal infrared observations [2-3], as well as thermal and mechanical property measurements on returned samples [4], have been conducted. However, due to the complex nature of collisional fragmentation processes, a quantitative assessment of the impact strength of boulders on Ryugu has not been conducted.
In this study, using the impact flux around the near-Earth orbit derived from observations of fireball [5] and a destruction model based on experimental and numerical studies of collisions (e.g., [6]), we constructed a numerical model for the boulder size distribution evolution on the surface of near-Earth rubble-pile asteroids. Using this model, we investigated the relationship between the timescales of boulder fragmentation and boulder impact strengths. Furthermore, using images obtained by the Optical Navigation Camera (ONC) onboard the Hayabusa2 spacecraft, we determined the proportion of boulders larger than 10 m that have experienced catastrophic fragmentation and compared with the simulation results to constrain the impact strength of the boulders.
The results of numerical simulations indicate that as the boulder strength increases, the timescale of boulder fragmentation becomes longer, and the proportion of boulders experienced catastrophic fragmentation is inversely proportional to the boulder strength. Within the analysis area of the ONC images, 11 candidates for boulders larger than 10 m, which may have undergone catastrophic fragmentation, were identified. The estimated proportion of these boulders compared to the total number of boulders larger than 10 m (291 in total) within the same area is below 3.8%. Based on the relationship between the impact strength and the proportion of catastrophically fragmentated boulders derived from the simulation, the impact strength of Ryugu boulders larger than 1 m was estimated to be larger than 200 J/kg. The timescale required to disrupt boulders larger than 1 m within this strength range exceeds 8 × 107 years, surpassing both the typical lifetime of near-Earth asteroids [7] and the cosmic ray exposure age of returned samples [8]. Therefore, it is likely that boulders larger than 1 m on Ryugu have experienced minimal collisional disruption after its migration to a near-Earth orbit, and their size distribution has been preserved.
Reference: [1] Hsu et al., 2022, Nat. Astron., 6, 1043-1050. [2] Grott et al., 2019, Nat. Astron., 3, 971-976. [3] Hamm et al., 2020, MNRAS, 496(3), 2776-2785. [4] Nakamura et al., 2023, Science, 379(6634), eabn8671. [5] Brown et al., 2002, Nature, 420, 294-296. [6] Petit & Farinella, 1993, Celest. Mech. Dyn. Astron., 57, 1-28. [7] Gladman et al., 2000, Icarus, 146(1), 176-189. [8] Okazaki et al., 2023, Science, 379(6634), eabo0431.
The impact strength, the minimum collisional energy required per unit mass to disrupt the boulder, constitutes fundamental information in understanding collisional fragmentation on asteroid surfaces and of parent bodies. Regarding the physical properties of boulders on the asteroid Ryugu, porosity estimation, and mechanical strength assessment through thermal infrared observations [2-3], as well as thermal and mechanical property measurements on returned samples [4], have been conducted. However, due to the complex nature of collisional fragmentation processes, a quantitative assessment of the impact strength of boulders on Ryugu has not been conducted.
In this study, using the impact flux around the near-Earth orbit derived from observations of fireball [5] and a destruction model based on experimental and numerical studies of collisions (e.g., [6]), we constructed a numerical model for the boulder size distribution evolution on the surface of near-Earth rubble-pile asteroids. Using this model, we investigated the relationship between the timescales of boulder fragmentation and boulder impact strengths. Furthermore, using images obtained by the Optical Navigation Camera (ONC) onboard the Hayabusa2 spacecraft, we determined the proportion of boulders larger than 10 m that have experienced catastrophic fragmentation and compared with the simulation results to constrain the impact strength of the boulders.
The results of numerical simulations indicate that as the boulder strength increases, the timescale of boulder fragmentation becomes longer, and the proportion of boulders experienced catastrophic fragmentation is inversely proportional to the boulder strength. Within the analysis area of the ONC images, 11 candidates for boulders larger than 10 m, which may have undergone catastrophic fragmentation, were identified. The estimated proportion of these boulders compared to the total number of boulders larger than 10 m (291 in total) within the same area is below 3.8%. Based on the relationship between the impact strength and the proportion of catastrophically fragmentated boulders derived from the simulation, the impact strength of Ryugu boulders larger than 1 m was estimated to be larger than 200 J/kg. The timescale required to disrupt boulders larger than 1 m within this strength range exceeds 8 × 107 years, surpassing both the typical lifetime of near-Earth asteroids [7] and the cosmic ray exposure age of returned samples [8]. Therefore, it is likely that boulders larger than 1 m on Ryugu have experienced minimal collisional disruption after its migration to a near-Earth orbit, and their size distribution has been preserved.
Reference: [1] Hsu et al., 2022, Nat. Astron., 6, 1043-1050. [2] Grott et al., 2019, Nat. Astron., 3, 971-976. [3] Hamm et al., 2020, MNRAS, 496(3), 2776-2785. [4] Nakamura et al., 2023, Science, 379(6634), eabn8671. [5] Brown et al., 2002, Nature, 420, 294-296. [6] Petit & Farinella, 1993, Celest. Mech. Dyn. Astron., 57, 1-28. [7] Gladman et al., 2000, Icarus, 146(1), 176-189. [8] Okazaki et al., 2023, Science, 379(6634), eabo0431.