A NEW DYNAMICAL MODEL FOR THE LUNAR LATE HEAVY BOMBARDMENT. J. E. Chambers, NASA Ames

Samples of lunar rock returned by the Apollo and Luna missions show a cluster of impact melt ages between about 3.8 and 4.0 billion years (Gyr) ago. Several of the large basins on the moon appear to have formed at aound this time, including Imbrium with a diameter> 1000 km. The lunar highlands which predate Imbrium are covered with smaller craters. Con-versely, the lunar maria, which formed more recently than 3.8 Gyr ago, contain relatively few impact craters, and it appears that no large basins have formed within the last 3.8 Gyr. These data have been interpreted in two ways. Either 3.8 Gyr ago marked the end of an era of large impacts which had been de-clining steadily since the terrestrial planets formed 4:5 Gyr ago. Or there was a sudden increase in the impact rate around 3.9 Gyr ago (sometimes referred to as the “lunar cataclysm”), with relatively quiet periods before and afterwards