2:30 PM - 2:45 PM
[PPS07-10] Numerical simulations of catastrophic collisions of asteroids and fractions of impactor fragments included in remnants
Keywords:Ryugu, S-type boulders, impactor fragments, catastrophic collisions
The Japanese spacecraft Hayabusa2 visited the asteroid Ryugu with the 500 m radius and conducted the detailed proximity observations. The observations show that Ryugu is a rubble-pile asteroid and it was formed through a catastrophic collision of a parent body and subsequent reaccumulation of fragments (Watanabe et al. 2019). A catastrophic collision of a parent body forms remnant asteroids with similar heliocentric orbits, that is, an asteroid family. Ryugu’s spectral features and the studies of orbital evolutions of asteroids suggest that Ryugu was a member of the asteroid family Eulalia or Polana (Sugita et al. 2019). A radius of an original parent body of Ryugu is estimated to be about 50 km from the total mass of the Eulalia or Polana family (Walsh et al. 2013).
Hayabusa2 showed that Ryugu’s surface is almost homogeneously composed of rocks with C-type spectra and low albedos (Kitazato et al. 2019). On the other hand, the high-resolution observation by ONC-T found that several tens of rocks have S-type spectra and high albedos. These S-type boulders are considered to be fragments of an S-type impactor that caused a catastrophic collision of a C-type parent body of Ryugu (Tatsumi et al. 2021). The ratio of the volume of whole Ryugu to that of total S-type boulders is estimated to be about 10-5 (Sugimoto et al. 2021). Constraining impact conditions of a catastrophic collision that forms remnants with such small amounts of impactor fragments may lead to clarify the collisional history of a parent body of Ryugu.
For this purpose, we conducted numerical simulations of catastrophic collisions of asteroids using Smoothed Particle Hydrodynamics (SPH) method. Our SPH code includes the fragmentation model for brittle rocks (Benz and Asphaug 1995) and friction model for granular materials (Jutzi 2015). Our code is parallelized by Framework for Developing Particle Simulator (FDPS: Iwasawa et al. 2015, 2016). In this study, we focused on a catastrophic collision forming the Eulalia or Polana family and investigated the impact condition dependence on amounts of impactor fragments included in produced remnants.
From the total mass of the Eulalia or Polana family and that of the largest member of each family, a catastrophic disruption forming the family is considered to be a collision between a target with about 50 km radius and an impactor, which results in the mass of the largest remnant Mlr to be about 0.1 of the target mass Mt. We firstly conducted low resolution simulations with about 200,000 SPH particles and investigated which impact velocity results in Mlr / Mt ~ 0.1 for each combination of the impact angle of 15, 30, and 45 degrees and the ratio of Mt to the impactor mass of 4, 16, 32, and 64. Then we chose twelve impact conditions with Mlr / Mt ~ 0.1.
Secondly, we conducted the simulations of the same twelve impacts but with high resolutions using about 3,000,000 SPH particles to investigate impactor fractions of produced remnants. As a result, we found that the impact angles of 15 and 30 degrees, i.e., nearly head-on collisions, produce only remnants with impactor fractions of 10-1 - 10-3. Although impacts with higher impact angles produce more remnants without mixing of impactor and target fragments, we also found that impactor fractions drastically decrease with increasing the impact angle and the impact angle of 45 degrees produces remnants with the impactor fractions of ~ 5x10-5. Focusing on the largest remnants, which enables us to investigate smaller impactor fractions, we estimate the impactor fraction dependence on the impact angle and suggest that impact angles of about 60 degrees may produce remnants with impactor fractions of ~ 10-5. Although the investigation of whether impacts with such high impact angles really mix such small amounts of impactor fragments will be our future work, it may be possible that the Eulalia or Polana family formation impact explains amounts of S-type boulers on Ryugu.
Hayabusa2 showed that Ryugu’s surface is almost homogeneously composed of rocks with C-type spectra and low albedos (Kitazato et al. 2019). On the other hand, the high-resolution observation by ONC-T found that several tens of rocks have S-type spectra and high albedos. These S-type boulders are considered to be fragments of an S-type impactor that caused a catastrophic collision of a C-type parent body of Ryugu (Tatsumi et al. 2021). The ratio of the volume of whole Ryugu to that of total S-type boulders is estimated to be about 10-5 (Sugimoto et al. 2021). Constraining impact conditions of a catastrophic collision that forms remnants with such small amounts of impactor fragments may lead to clarify the collisional history of a parent body of Ryugu.
For this purpose, we conducted numerical simulations of catastrophic collisions of asteroids using Smoothed Particle Hydrodynamics (SPH) method. Our SPH code includes the fragmentation model for brittle rocks (Benz and Asphaug 1995) and friction model for granular materials (Jutzi 2015). Our code is parallelized by Framework for Developing Particle Simulator (FDPS: Iwasawa et al. 2015, 2016). In this study, we focused on a catastrophic collision forming the Eulalia or Polana family and investigated the impact condition dependence on amounts of impactor fragments included in produced remnants.
From the total mass of the Eulalia or Polana family and that of the largest member of each family, a catastrophic disruption forming the family is considered to be a collision between a target with about 50 km radius and an impactor, which results in the mass of the largest remnant Mlr to be about 0.1 of the target mass Mt. We firstly conducted low resolution simulations with about 200,000 SPH particles and investigated which impact velocity results in Mlr / Mt ~ 0.1 for each combination of the impact angle of 15, 30, and 45 degrees and the ratio of Mt to the impactor mass of 4, 16, 32, and 64. Then we chose twelve impact conditions with Mlr / Mt ~ 0.1.
Secondly, we conducted the simulations of the same twelve impacts but with high resolutions using about 3,000,000 SPH particles to investigate impactor fractions of produced remnants. As a result, we found that the impact angles of 15 and 30 degrees, i.e., nearly head-on collisions, produce only remnants with impactor fractions of 10-1 - 10-3. Although impacts with higher impact angles produce more remnants without mixing of impactor and target fragments, we also found that impactor fractions drastically decrease with increasing the impact angle and the impact angle of 45 degrees produces remnants with the impactor fractions of ~ 5x10-5. Focusing on the largest remnants, which enables us to investigate smaller impactor fractions, we estimate the impactor fraction dependence on the impact angle and suggest that impact angles of about 60 degrees may produce remnants with impactor fractions of ~ 10-5. Although the investigation of whether impacts with such high impact angles really mix such small amounts of impactor fragments will be our future work, it may be possible that the Eulalia or Polana family formation impact explains amounts of S-type boulers on Ryugu.