  The Japanese spacecraft Hayabusa 2 visited the asteroid (162173) Ryugu and
provided many high-resolution images of its surface, revealing that Ryugu has a
spinning-top shape with a prominent equatorial ridge, much like the shapes
reported for some other asteroids. In this study, through dozens of numerical
calculations, we demonstrate that during a period of fast rotation, ejecta from
craters formed at lower and mid-latitudes can be deposited on the equatorial
ridge. Assuming a rotation period of 3 h, we estimate that an equatorial ridge
with a height of 50 m can be generated in 128(+47 / -27) My for a main-belt
asteroid, or 3.1(+4.2 / -1.2) Gy for a near-Earth asteroid. Therefore, an
equatorial ridge can form within the average mean collisional lifetime of a
km-sized asteroid within the main belt, but not for near-Earth asteroids.
Furthermore, our model may explain why blue (younger) material occurs on the
equatorial ridge.
