Rubble-pile structures dominate the population of small asteroids, as evidenced by the close-up images of several asteroids [1, 2]. The discrete and low-strength structures raise many challenges to understand the physical properties and evolution of these bodies. The YORP thermal torques  have been thought to be an important mechanism affecting the structural evolution of rubble-pile asteroids, such as, forming top-like shapes (often accompanied with small moons) , and disrupting asteroids . Earlier radar observations suggested that about 16% of near-Earth-asteroids may be binary systems with likely top-shaped primary , which could be an evidence for YORP’s effectiveness. Direct measurements of the rotation of the top-shaped asteroid (101955) Bennu have shown that YORP torques can double its spin rate within several millions of years if sustained over that timeframe . However, the geophysical expressions of this effect and the dependency on asteroids’ intrinsic properties are still unclear.
In this study, we conducted numerical experiments using the discrete-element code pkdgrav [8, 9] to model the structural evolution of top-shaped rubble piles undergoing YORP-induced spin-up. Four failure modes are identified based on these experiments, i.e., surface landslides, internal deformation, mass shedding, and tensile disruption. The structural evolution of a rubble pile sensitively depends on its physical properties, e.g., friction angles and cohesion. By analyzing geophysical expressions of different failure modes, we draw implications for the material properties of Bennu. We also compare the results with previous work, which use different numerical or analytical approaches [e.g., 10–12], to review the current understanding of top-shaped rubble-pile evolution under spin-up.
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Acknowledgements: Y.Z. and P.M. acknowledge funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 870377 (NEO-MAPP project), from the Doeblin Federation, and from CNES.