Rubble-pile structural and dynamical evolution under YORP and the pathway to a binary system

Radar observations indicate that about 16% of near-Earth asteroids may be binary systems with likely fast-spinning spheroidal primaries [1]. The formation mechanism of such systems remains uncertain, although it may be linked to rotation-driven structural reconfiguration if these asteroids are rubble piles [2]. On 26 September, 2022, the NASA Double Asteroid Redirection Test (DART) mission successfully demonstrated the use of kinetic impact for planetary defense by colliding with Dimorphos, the moon of the Didymos binary system [3], altering its orbit around the primary Didymos [4, 5]. The DRACO camera onboard the DART spacecraft has provided the first close-up images of the asteroids [3], revealing their physical characteristics that offer valuable insight into the formation and evolution of such binary systems.

In this study, assuming that the primary Didymos is a rubble pile, we employ soft-sphere discrete element modeling to investigate the structural and rotational evolution of Didymos, which allows us to constrain the binary formation scenario. First, by simulating the structural evolution of Didymos given YORP-induced rotational acceleration as a spin-up mechanism, we derived a range of material and structural properties necessary for maintaining its structural stability at the current spin period of 2.26 hr. Our results indicate that Didymos does not require cohesive strength if its constituent granular material has an angle of friction ≥ ~40 deg and its bulk density is ≥ ~2.7 g/cc. Considering a typical friction angle range of dry granular material, e.g., 29–40 deg, a cohesive strength of ~19–33 Pa would be needed for the reported nominal bulk density, i.e., 2.4 g/cc [3]. Then, we tested the structural and dynamical evolution of the derived Didymos models under various conditions, including rotational acceleration and small-scale impact bombardment. Our findings suggest that moon formation via surface mass shedding is more plausible when the body has a relatively high friction angle or a denser interior. Finally, we characterized the morphology and dynamics of the Didymos model and resulting particle systems (if produced) and compared our findings with the geophysical characteristics revealed by the DRACO images. Based on these results, we discuss the possible formation mechanisms for Dimorphos along with the implications for the upcoming Hera mission.

References: [1] Margot et al., Science 296, 1445–1448 (2002). [2] Walsh & Jacobson, in Asteroids IV (2015). [3] Daly et al., Nature, in press (2023). [4] Thomas et al., Nature, in press (2023). [5] Cheng et al., Nature, in press (2023).