5:15 PM - 6:45 PM
[PPS07-P12] Study on Shapes and Internal Structures of Rubble-Pile Asteroids by Rigid-Body Phyisics Simulation of Their Formation Processes
Keywords:asteroid, rubble-pile, Rigid-body physics simulation, size-frequency distribution
The existence of rubble-pile asteroids such as Itokawa, Ryugu, and Bennu is widely known. Despite sharing a common formation process through the accumulation of collisional fragments from their parent bodies, these rubble-pile asteroids exhibit significant diversity in appearance. The diversity in asteroid shapes is also suggested by light curve observations conducted using ground-based or space telescopes. It was thought that maintaining a large macroporosity, as observed in Itokawa, was a characteristic feature of rubble-pile asteroids. However, the large bulk porosity observed in Ryugu and Bennu is now thought to originate from thier large microporosities. Thus, it can be inferred that rubble-pile asteroids exhibit diversity not only in their bulk porosity rates but also in their microstructure.
This study aims to investigate the factors leading to the diversity in overall shape and internal structure of rubble-pile asteroids by reproducing the aggregation process of rubble-pile asteroids using rigid-body physics simulations. We utilized the open-source physics simulation engine, Chrono, to simulate the aggregation and merging processes of collisional fragments constituting rubble-pile asteroids and measured the triaxial ratios and porosity of the formed rubble-pile asteroids. We prepared two types of models: one representing the oligopolistic case where the collisional fragments follow a power-law size frequency distribution (SFD) with an exponent of -2.5 and there are four largest fragments of the same size, and the other representing the monopolistic case where only one largest fragment exists. By repeating simulations of aggregating fragments in random order according to these models, we verified whether the oberved distribution of triaxial ratios and porosity of rubble-pile asteroids could be reproduced, and whether asteroids with extreme shapes like Itokawa could be formed.
It was found that rubble-pile asteroids with triaxial ratios close to 1, similar to Ryugu and Bennu, could be naturally formed in both the oligopolistic and monopolistic cases. However, the range of triaxial ratios obtained across the simulations was biased towards more spherical shapes compared to the distribution range estimated from light curve observations of asteroids. Furthermore, results did not produce asteroids with extremely elongated shapes like Itokawa. This is attributed to the physical tendency for smaller fragments to migrate to low-lying areas on the asteroid's surface during the aggregation process, promoting shapes closer to a triaxial ratio of 1. This trend occurs regardless of the type of size frequency distribution model used for collisional fragments. Conversely, the macroporosity was consistently high, similar to that of Itokawa. With the size frequency distribution models used in this study, which has a relatively gentle slope for the power-low SFD and truncation at the minimum fragment size, it is anticipated that decreasing the macroporosity would require a steeper SFD or a smaller minimum fragment sizes. Further investigations are necessary to identify the conditions under how macroporosity similar to Ryugu and Bennu could be archived.
This study aims to investigate the factors leading to the diversity in overall shape and internal structure of rubble-pile asteroids by reproducing the aggregation process of rubble-pile asteroids using rigid-body physics simulations. We utilized the open-source physics simulation engine, Chrono, to simulate the aggregation and merging processes of collisional fragments constituting rubble-pile asteroids and measured the triaxial ratios and porosity of the formed rubble-pile asteroids. We prepared two types of models: one representing the oligopolistic case where the collisional fragments follow a power-law size frequency distribution (SFD) with an exponent of -2.5 and there are four largest fragments of the same size, and the other representing the monopolistic case where only one largest fragment exists. By repeating simulations of aggregating fragments in random order according to these models, we verified whether the oberved distribution of triaxial ratios and porosity of rubble-pile asteroids could be reproduced, and whether asteroids with extreme shapes like Itokawa could be formed.
It was found that rubble-pile asteroids with triaxial ratios close to 1, similar to Ryugu and Bennu, could be naturally formed in both the oligopolistic and monopolistic cases. However, the range of triaxial ratios obtained across the simulations was biased towards more spherical shapes compared to the distribution range estimated from light curve observations of asteroids. Furthermore, results did not produce asteroids with extremely elongated shapes like Itokawa. This is attributed to the physical tendency for smaller fragments to migrate to low-lying areas on the asteroid's surface during the aggregation process, promoting shapes closer to a triaxial ratio of 1. This trend occurs regardless of the type of size frequency distribution model used for collisional fragments. Conversely, the macroporosity was consistently high, similar to that of Itokawa. With the size frequency distribution models used in this study, which has a relatively gentle slope for the power-low SFD and truncation at the minimum fragment size, it is anticipated that decreasing the macroporosity would require a steeper SFD or a smaller minimum fragment sizes. Further investigations are necessary to identify the conditions under how macroporosity similar to Ryugu and Bennu could be archived.