Formation of Moons and Equatorial Ridge around Top-shaped Asteroids after Surface Landslide

Top-shaped asteroids have been observed among near-Earth asteroids. About half of them are reported to have moons (on the order of similar to 1 wt.% of the top-shaped primary) and many of them have an equatorial ridge. A recent study has shown that the enigmatic top-shaped figure of asteroids (e.g., Ryugu, Bennu, and Didymos) could result from an axisymmetric landslide of the primary during a fast spin-up near the breakup rotation period. Such a landslide would inevitably form a particulate disk around an asteroid with a short timescale (similar to 3 hr). However, the long-term full dynamical evolution is not investigated. Here, we perform a continuous simulation (similar to 700 hr) that investigates the sequence of events from the surface landslide that forms a top-shaped asteroid and a particulate disk to disk evolution. We show that the disk quickly spreads and produces moons (within similar to 300 hr). The mass of the formed moon is consistent with what is observed around the top-shaped asteroids. We also demonstrate that an equatorial ridge is naturally formed because a fraction of the disk particles re-accretes selectively onto the equatorial region of the primary. We envision that Ryugu and Bennu could once have an ancient moon that was later lost due to a successive moon's orbital evolution. Alternatively, at a top-shaped asteroid that has a moon, such as Didymos, no significant orbital evolution of the moon has occurred that would result in its loss. Our study would also be qualitatively applicable to any rubble-pile asteroids near the breakup rotation period.

Automated summary

Top-shaped asteroids with moons and equatorial ridges have been observed and studied, and it is suggested that their unique shapes are a result of axisymmetric landslides during a fast spin-up. Continuous simulations show that these asteroids initially form a particulate disk which eventually spreads and produces moons. The study also explains the formation of equatorial ridges through the selective re-accretion of disk particles.