Interpreting the Cratering Histories of Bennu, Ryugu, and Other Spacecraft-explored Asteroids

Asteroid crater retention ages have unknown accuracy because projectile-crater scaling laws are difficult to verify. At the same time, our knowledge of asteroid and crater size-frequency distributions has increased substantially over the past few decades. These advances make it possible to empirically derive asteroid crater scaling laws by fitting model asteroid size distributions to crater size distributions from asteroids observed by spacecraft. ForD > 10 km diameter asteroids like Ceres, Vesta, Lutetia, Mathilde, Ida, Eros, and Gaspra, the best matches occur when the ratio of crater to projectile sizes isf similar to 10. The same scaling law applied to 0.3 D < 2.5 km near-Earth asteroids such as Bennu, Ryugu, Itokawa, and Toutatis yield intriguing yet perplexing results. When applied to the largest craters on these asteroids, we obtain crater retention ages of similar to 1 billion years for Bennu, Ryugu, and Itokawa and similar to 2.5 billion years for Toutatis. These ages agree with the estimated formation ages of their source families and could suggest that the near-Earth asteroid population is dominated by bodies that avoided disruption during their traverse across the main asteroid belt. An alternative interpretation is thatf >> 10, which would make their crater retention ages much younger. If true, crater scaling laws need to change in a substantial way betweenD > 10 km asteroids, wheref similar to 10, and 0.3 D < 2.5 km asteroids, wheref >> 10.