Rubble pile structures dominate the population of small asteroids, as evidenced by the close-up images of several asteroids. The discrete and low-strength characteristics of this structure raise many unsolved questions that need to be answered to clarify the physical properties and evolution of these bodies. Some analytical and numerical approaches based on continuum medium theory have been developed to interpret the physical properties and dynamical evolution of rubble-pile asteroids. However, these approaches cannot capture the discrete nature of these bodies. To get a better understanding of rubble-pile asteroids, we investigate their structural stability and failure behaviors in response to rotational accelerations using the soft-sphere discrete element modeling. The effects of shape, friction, cohesion and particle size distribution and arrangement are explored. We design a method to evaluate the bulk cohesion of the rubble pile, allowing a direct quantitative comparison with the continuum theory. The results show that the shear strength of a rubble pile depends strongly on both its internal configuration, friction, and cohesion, while its failure mode and mechanism are mainly affected by its shape, internal configuration, and cohesion. The failure mode predicted for the cohesive rubble pile is inconsistent with the analyses obtained by the continuum theory. For a homogeneous cohesive rubble pile, the continuum theory predicts the structure would fail by internal deformation, while our SSDEM results show various failure modes depending on the shape and internal configuration. The reason for this could be due to the cohesion difference in the interior and surface of the discrete rubble-pile model. By analyzing the stress-state variable distributions, we find that the failure mode is related to the pressure and the shear stress of the local failure region in the rubble pile. The stress states at failure are located at different position on the failure envelope from those evaluated by the continuum theory. The sensitivity of a rubble pile’s failure mode to its cohesive strength provides an effective way to interpret the material strength and evolution of actual asteroids.
Acknowledgements: We acknowledge funding from the Université Côte d’Azur Individual grants for young researchers program of IDEX JEDI, from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 870377 (NEO-MAPP project), and from CNES.