Impact Flux on the Moon

The crust of the Moon records the complete history of collisions by different-sized projectiles from various sources since its early solidification. Planetary bodies in the inner Solar System experienced similar sources of impactors, and the Moon is an ideal witness plate for the impact history. Impact flux on the Moon connects planetary endogenic evolution with orbital dynamics of celestial bodies, and the resulting crater chronology enables remote age estimation for geological units on extraterrestrial bodies. Therefore, defining the lunar impact history has long been a core pursuit in planetary sciences. Ubiquitous impact structures on the Moon and their widespread impact melt deposits are the major agents used to untangle lunar crater chronology. Anchored by 10 successful sample return missions from the Moon, cumulative crater densities were derived for 15 geological units based on their interpreted exposure ages (~3.92 Ga to 25 Ma) and superposed crater densities. Afterword, crater production rates in the entire history of the Moon were constructed on the basis of hypothesized change patterns of impact flux. Following this commonly adapted strategy, it has been a consensus that impact flux in the first billion years of the lunar history was orders of magnitude larger than that afterward, and the latter was not only more or less stable but also punctuated by discrete spikes. However, different versions of lunar crater chronology exist because of insufficient constraints by available anchor points and widespread disagreements on both sample ages and crater densities of existing anchor points. Endeavors from various disciplines (e.g., sample analyses, remote observation, and modeling crater formation and accumulation) are making promising progresses, and future sample return missions with both optimized sampling strategy and analyzing techniques are appealed to fundamentally improve the understanding of lunar impact flux.