Spin-driven evolution of asteroids' top-shapes at fast and slow spins seen from (101955) Bennu and (162173) Ryugu

Abstract

Proximity observations by OSIRIS-REx and Hayabusa2 provided clues on the shape evolution processes of the target asteroids, (101955) Bennu and (162173) Ryugu. Their oblate shapes with equatorial ridges, or the so-called top shapes, may have evolved due to their rotational conditions at present and in the past. Different shape evolution scenarios were previously proposed; Bennu's top shape may have been driven by surface processing, while Ryugu's may have been developed due to large deformation. These two scenarios seem to be inconsistent. Here, we revisit the structural analyses in earlier works and fill a gap to connect these explanations. We also apply a semi-analytical technique for computing the cohesive strength distribution in a uniformly rotating triaxial ellipsoid to characterize the global failure of top-shaped bodies. Assuming that the structure is uniform, our semi-analytical approach describes the spatial variations in failed regions at different spin periods; surface regions are the most sensitive at longer spin periods, while interiors fail structurally at shorter spin periods. This finding suggests that the shape evolution of a top shape may vary due to rotation and internal structure, which can explain the different evolution scenarios of Bennu's and Ryugu's top shapes. We interpret our results as the indications of top shapes' various evolution processes. Highlights • We reanalyzed results from FEM analyses for asteroids Bennu and Ryugu, the targets of OSIRIS-REx and Hayabusa2, respectively. • We also applied a semi-analytical approach to quantify how the failure modes of top shapes vary with spin. • Top shapes may evolve by two deformation modes: surface processing at low spin and internal failure at high spin.

Hayabusa2 extended mission: New voyage to rendezvous with a small asteroid rotating with a short period

“…3. Ryugu's top shape (a spheroidal shape with an equatorial ridge) implies the violent formation process driven by reaccumulation of fragments after catastrophic disruption of parent bodies and the following evolution process due to fast rotation (Hirabayashi et al, 2020(Hirabayashi et al, , 2019bMichel et al, 2020a;Watanabe et al, 2019).…”

Time-series and Phase-curve Photometry of the Episodically Active Asteroid (6478) Gault in a Quiescent State Using APO, GROWTH, P200, and ZTF

“…The viewing geometry is also much different for our observations in 2020 than from when it was active in 2019. This means that Gault could have a spherical or top-shaped geometry like the near-Earth asteroids Ryugu and Bennu (Hirabayashi et al 2020). It is also worth noting that the near-Earth Asteroid (3200) Phaethon was imaged by Arecibo radar observations and was found to have a round, top-like shape when it passed by Earth in 2017 December (Taylor et al 2019).…”

Long-term Trends of Regolith Movement on the Surface of Small Bodies

“…Besides, we remark that the friction coefficient µ have no influence on the locations of equilibria, because the parameter µ only appears in the scalar coefficient term of Eq. (15). And calculations show that different µ will not change the topological classification of equilibria.…”

“…The migration of regolith materials on asteroids is possible to be initiated by some disturbance factors, including but not limited to global change of the surface dynamical environment due to varying spin state [15], and exogenous excitation, such as impact events which could give rise to stress waves and seismic shaking [16,17]. A slow mechanism that can modify the spin rates and obliquities of small bodies by thermal radiation forces and torques, referred to as Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect [18], plays an important role in driving the surface evolution process of small bodies.…”