Bombardment History of the Moon: What We Think We Know and What We Don't Know

The absolute impace history of the moon and inner solar system can in principle be derived from the statistics of radiometric ages of shock-heated planetary samples (lunar or meteoritic), from the formation ages of specific impact craters on the moon or Earth; and from agedating samples representing geologic surface units on the moon (or Mars) for which crater densities have been determined. This impact history, however, is still poorly defined. The heavily cratered surface of the moon is a testimony to the importance of impact events in the evolution of terrestrial planets and satellites. Lunar impacts range in scale from an early intense flux of large objects that defined the surface geology of the moon, down to recent, smaller impacts that continually generate and rework the lunar regolith. Densities of larger craters on lunar surface units of dated age define a projectile flux over time that serves as the basis for estimating surface ages on other solid bodies, particularly Mars. The lunar cratering history may address aspects of Earth s evolution, such as the possible role of early intense impacts on the atmosphere and early life and possible periodicity in large impact events in the more recent past. But, much about the lunar impact history remains unknown.. On Earth approximately 172 impact craters up to ~300 km in diameter and up to ~2 Gyr in age are recognized. Although these data suggest greater relative numbers of younger craters, possibly suggesting a recent increase in projectile flux, both the diameters and especially the ages of most terrestrial crates are so poorly known that the differential terrestrial impact flux over time is uncertain. For the moon, densities of craters on some mare surfaces and crater ejecta deposits, for which we have measured or estimated formation ages, suggest an approximately constant lunar impact rate of larger projectiles over the past ~3.5 Gyr. However, the data are cumulative in nature and limited. Questions exist as to how accurately dated samples correlate with surfaces having measured crater densities. Studies of ages of many tiny impact-melt beads from Apollos 12 and 14 soils show a decrease in the number of beads with age from ~4 Gyr ago to ~0.4 Gyr ago, followed by a significant increase in beads with age <0.4 Gyr (2). These authors concluded that the projectile flux had decreased over time, followed by a significant flux increase more recently.