Search

Abstract The minimum velocity (v) for impact ejecta to form secondary craters (secondaries) remains enigmatic, but it is a crucial parameter in untangling the fate of impact ejecta on planetary surfaces. By cataloging the distances (L) of the nearest secondaries from centers of various-sized (D) primary craters (primaries) on the Moon, Mars and Mercury, we find that v can be as small as ~25 m/s, and an unified power-law relationship of L = 1.86D 0.93 (both in meters) works for both simple and complex craters, regardless of different surface gravity and target properties. This relationship also successfully predicts occurrences of secondaries formed by craters on Venus. The constraint on v explains the common concurrences of structural disturbances in crater walls and continuous ejecta deposits caused by landing of cogenetic ejecta, suggesting that ejecta forming self-secondaries do not need near-vertical ejection angles and tertiary craters should be abundant on terrestrial bodies.

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.

The Chang’E-6 mission accomplished the first sample return from the lunar farside. Earlier crater population measurements estimated the model age of the landing mare to range from the Eratosthenian to Late Imbrian, both of which are underrepresented by earlier returned samples. Establishing a new calibration point for lunar impact flux based on isotopic ages of the samples is promising, but the representative crater density for the landing mare (i.e., spatial density of craters with D ≥ 1 km; N (1)) is equally important for this purpose, which lacks good constraints. After excluding the effects of background secondaries, crater equilibrium, and observational uncertainties on crater statistics, this work extracts production populations in different diameter ranges (∼200 m–2 km) from multiple subareas of the landing mare. Cross-validation of the production populations verifies that N (1) derived from direct measurements of craters with D ≥ 1 km in sketched areas are reliable, which is (2.01 ± 0.90) × 10 ^−3 and (6.05 ± 2.71) × 10 ^−3 km ^2 for the western and eastern mare, respectively.

Heterogeneous shock impedances of planetary materials cause abrupt changes of ejection angles, forming non-radial ejecta. Interpretations for provenances of surface deposits and ages derived from crater counts are affected by such untrackable ejecta.

Impact craters with layered ejecta deposits are widespread on Mars. Prevailing views suggest that such ejecta were formed due to the involvement of target water and/or water ice in the impact excavation and/or the post-deposition movement of the impact ejecta. The long-runout landslides and lobate debris aprons that are likely formed due to the involvement of water ice are used as analogs to compare roughness at multiple scales, considering that these three landforms share some similarities in their geomorphology. Analog studies of the morphological similarities and differences of layered ejecta deposits with different emplacement mechanisms are an important approach to untangling how layered ejecta deposits might form on Mars and beyond. Earlier morphological comparisons were usually based on qualitative descriptions or one-dimensional topographic roughness characteristics at given azimuths; however, the emplacement processes of layered deposits are recorded in two-dimensional topography and at multiple scales. In this study, we designed a multiwavelet algorithm to characterize the multi-scale topographic roughness of different forms of Martian layered deposits. Our comparisons show that the inner facies of the layered ejecta deposits and long-runout landslides exhibited similar roughness characteristics, and the outer facies of the layered ejecta deposits were more similar in roughness to lobate debris aprons. This study highlights the importance of the spatial resolution of digital terrain models in characterizing fine topographic fluctuations on layered ejecta deposits, providing additional insights into the possible emplacement mechanisms of different parts of layered ejecta deposits.