The Formation of Troughs on Rubble-Pile Asteroids Ryugu and Dinkinesh

Introduction:  Top-shaped asteroids, characterized by their equatorial ridges resembling spinning tops, are common in the solar system. The Hayabusa2 mission's close-up observations of the top-shaped rubble-pile asteroid Ryugu revealed a more complex morphology than a simple axisymmetric top shape. The western hemisphere of this asteroid, later called the western bulge, is more prominent and less affected by crater bombardment compared to the other side, with longitudinal troughs flanking the bulge, named Horai and Tokoyo Fossae [1]. Chronological analysis further suggests that the western bulge, presumably along with the fossae, formed 2-9 Myr ago, which is much younger than the eastern part, aged 11-18 Myr [2]. Recent observation by the Lucy mission also found longitudinal troughs on another top-shaped asteroid Dinkinesh. Therefore, the troughs may hold clues about the formation and structure of rubble-pile asteroids.

Figure 1. The western bulge and longitudinal troughs of asteroid Ryugu photographed from a distance of about 10 kilometers by Hayabusa2 spacecraft (hyb2_onc_20190724_121159_tvf_l2c). Image credit: JAXA/UTokyo/Kochi U./Rikkyo U./Nagoya U./Chiba Ins. Tech/Meiji U./U. Aizu/AIST.

Results: In this study, we employed the N-body particle simulation code DEMBody [3] to build a discrete element model for the progenitor of asteroids Ryugu and Dinkinesh. We investigated its structural failure modes under YORP (Yarkovsky–O'Keefe–Radzievskii–Paddack) spin-up with different cohesive properties. We identified five failure modes when the asteroid exceeds its critical rotation speed: surface landslides, internal deformation, equatorial shedding, tensile disruption and longitudinal fracturing. Note that this is the first time we discovered the longitudinal fracturing failure mode, which is capable of generating stable linear features on asteroid surfaces. The unique morphology of asteroid Ryugu requires the simultaneous occurrence of surface landslides, internal deformation, and longitudinal fracturing during its evolution. Specifically, internal deformation led to an asymmetric shape with a western bulge, longitudinal fracturing generated the fossa system outlining the western bulge, and the surface landslides resulted in the erosion of craters on the western bulge. We found this distinct failure history could only happen in a layered heterogeneous structure with a loose surface and a much stronger subsurface.

Conclusions: Our work reveals that the western bulge and fossa system of asteroids Ryugu and Dinkinesh are both byproducts of YORP-induced spin-up deformation. We infer that its surface underwent size segregation (Brazil-nut effect) under vibrations during its early history. This leads to the formation of a large reservoir of fine particles in the subsurface, which contributes to stronger bonding and increased strength at depth.

References: [1] Hirabayashi M. et al. (2019) ApJL, 874(1): L10. [2] Cho Y. et al. (2021) JGR Planets, 126(8), e2020JE006572. [3] Cheng B. et al. (2021) Nat. Astron., 5(2), 134-138.