SIZE DISTRIBUTION OF ASTEROIDS AND OLD TERRESTRIAL CRATERS: IMPLICATIONS FOR ASTEROIDAL DYNAMICS DURING LHB

Abstract

Lunar & Planetary Laboratory, The University of ArizonaTucson, AZ 85712{0092, USARecent progress in asteroid surveys has revealed the nestructures downto sub-km in diameter of the size-frequency distributions (SFD) of mainbelt asteroids (MBAs), as well as near-Earth asteroids (NEAs). TheseSFDs can be compared with the SFD of lunar and planetary craterprojectiles. The SFD of the projectiles that created the oldest craterson the lunar highlands, which are considered a fossil of the Late HeavyBombardment (LHB) impactors of ˘4 Ga ago, shows a verygood agree-ment with that of the current MBAs. This fact indicates that the LHBcraters were created by the bombardment of ancient asteroids ejectedfrom the main belt by a short-term, size-independent event, such as theradial movement of strong resonances due to the migration of giant jo-vian planets. On the other hand, the SFD of the projectiles that havecreated younger craters such as those on Mars is very di erent fromthat of the MBAs; instead, it is quite similar to the SFD of NEAs. Thisnewer population of projectiles might be created by a long-term, size-dependent transportation mechanism of asteroids such as the Yarkovskye ect, whichpreferentially pushessmaller objects intostrongresonances.