Time-variable cratering rates?

Using stepwise laser-heating methods, Culler et al. (1) determined the Ar/Ar formation ages of 155 lunar impact melt beads from a soil sampled during the Apollo 14 mission. Because relatively old ages of .3 billion years (Gy) and fairly young ages of ,0.4 Gy dominate, Culler et al. concluded that the cratering rate in the inner solar system decreased by a factor of 2 to 3 during the past 3 Gy and then intensified again during the most recent 0.4 Gy. Petrographic analysis of lunar soils reveals irregular, dark-colored melt particles, termed agglutinates, that compositionally resemble the fine soil fraction (2), and regularly shaped melt beads that are distinctly colored and have bulk compositions akin to those of local surface rocks (3). Small-scale impacts into powdered soil apparently produce agglutinates, and similar impacts into competent rock produce melt beads (4). The particles analyzed by Culler et al. seem to be of the bead type. The agglutinate content may reach 50% by volume in any lunar soil, whereas beads rarely exceed 5%, a distribution grossly consistent with the fractional surface areas occupied by fine-grained soil and rocks on the present-day moon. This soil-to-rock ratio evolves systematically with time, however. Any terrain rejuvenated by volcanism or impacts will initially consist of rock, from which even the smallest impact can produce only melt beads. Conceptually, the production of such beads will be controlled by the fractional surface area occupied by rock— initially as genuine bedrock, in later stages as freshly excavated boulders. With increasing regolith thickness the system becomes progressively self-buffering, and larger and larger craters are required to emplace fresh bedrock boulders at the surface (5). These processes cause the production rate of surface rocks to decrease progressively with time, with a corresponding decrease in the production rate of melt beads. This average scenario includes the occasional impact that penetrates deeply into bedrock and produces a substantial boulder field at the surface. Such local stochastic anomalies can lead to distinct spikes in the age distribution of melt beads and could cause the seeming increase in cratering activity at 0.4 billion years ago (Ga) reported by Culler et al. The supposed increase may also relate to bead preservation: any ideal production function for melt beads will be modulated by some destruction function. Recently produced beads may be overrepresented in this population because they have experienced a relatively benign bombardment history compared with that of the average soil. For a constant cratering flux, the selfbuffering effects of the evolving debris layer would cause the absolute growth rate of regolith to decrease by a factor of 3 to 4 over the past 3 Gy (5). The production of melt beads may vary by similar factors. Thus, the distribution of bead ages reported by Culler et al. might be consistent with a constant impactor flux. In addition, lunar soils are the products of stochastic processes, and individual samples could have highly idiosyncratic histories; any one soil sample might not faithfully represent the average cratering history through geologic time. Obviously, additional soils need to be investigated using the methods pioneered by Culler et al.