Ages of large lunar impact craters and implications for bombardment during the Moon's middle age

Standard lunar chronologies, based on combining lunar sample radiometric ages with impact crater densities of inferred associated units, have lately been questioned about the robustness of their interpretations of the temporal dependance of the lunar impact flux. In particular, there has been increasing focus on the “middle age” of lunar bombardment, from the end of the Late Heavy Bombardment (∼3.8 Ga) until comparatively recent times (∼1 Ga). To gain a better understanding of impact flux in this time period, we determined and analyzed the cratering ages of selected terrains on the Moon. We required distinct terrains with random locations and areas large enough to achieve good statistics for the small, superposed crater size-frequency distributions to be compiled. Therefore, we selected 40 lunar craters with diameter ∼90 km and determined the model ages of their floors by measuring the density of superposed craters using the Lunar Reconnaissance Orbiter Wide Angle Camera mosaic. Absolute model ages were computed using the Model Production Function of Marchi et al. (Marchi, S., Mottola, S., Cremonese, G., Massironi, M., Martellato, E. [2009]. Astron. J. 137, 4936-4948). We find that a majority (36 of 40) of our superposed crater size-frequency distributions are consistent with the Model Production Function. A histogram of the original crater floor model ages indicates the bombardment rate decreased gradually from ∼3.8 Ga until ∼3.0 Ga, implying an extended tail to the Late Heavy Bombardment. For large craters, it also preliminarily suggests that between ∼3.0 and 1.0 Ga bombardment may be characterized by long periods (>600 Myr) of relatively few impacts (“lulls”) broken by a short duration (∼200 Myr) of relatively more impacts (“spike”). While measuring superposed craters, we also noted if they were part of a cluster or chain (named “obvious secondary”), and analyzed these craters separately. Interestingly, we observe a wide variety of slopes to the differential size-frequency power-law, which demonstrates that there can be considerable variation in individual secondary crater field size-frequency distributions. Finally, four of the small, superposed crater size-frequency distributions are found to be inconsistent with the Model Production Function; possible reasons are: resurfacing has modified these distributions, unrecognized secondary craters, and/or the Model Production Function has incorrect inputs (such as the scaling law for the target terrain). The degraded appearance of the superposed craters and indications of resurfacing suggest that the first cause is the most likely.