Size distribution of asteroids and old terrestrial craters: Implications for asteroidal dynamics during LHB

Recent progress in asteroid surveys has revealed the fine structures down to sub-km in diameter of the size-frequency distributions (SFD) of main belt asteroids (MBAs), as well as near-Earth asteroids (NEAs). These SFDs can be compared with the SFD of lunar and planetary crater projectiles. The SFD of the projectiles that created the oldest craters on the lunar highlands, which are considered a fossil of the Late Heavy Bombardment (LHB) impactors of ~4Ga ago, shows a very good agreement with that of the current MBAs. This fact indicates that the LHB craters were created by the bombardment of ancient asteroids ejected from the main belt by a short-term, size-independent event, such as the radial movement of strong resonances due to the migration of giant jovian planets. On the other hand, the SFD of the projectiles that have created younger craters such as those on Mars is very different from that of the MBAs; instead, it is quite similar to the SFD of NEAs. This newer population of projectiles might be created by a long-term, size-dependent transportation mechanism of asteroids such as the Yarkovsky effect, which preferentially pushes smaller objects into strong resonances.