Abstract
Self-secondaries are secondary craters that are formed on both the continuous ejecta deposits and interior of the parent crater. The possible existence of self-secondaries was proposed in the late 1960s, but their identity, formation mechanism, and importance were not revisited until the new generation of high-resolution images for the Moon have recently became available. Possible self-secondary crater populations have now been recognized not only on the Moon, but also on Mercury, Mars, 1Ceres, 4Vesta, and satellites of the ice giants. On the Moon and terrestrial planets, fragments that form self-secondaries are launched with high ejection angles via spallation during the early cratering process, so that self-secondaries can be formed both within the crater and on the continuous ejecta deposits at the end of the cratering process. Self-secondaries potentially possess profound effects on the widely used age-determination technique using crater statistics in planetary geology, because (1) self-secondaries cause nonuniform crater density across the continuous ejecta deposits, which cannot be solely explained by the effect of different target properties on crater size-frequency distributions; (2) crater chronologies for both the Moon and the other terrestrial bodies are largely based on crater counts on the continuous ejecta deposits of several young lunar craters. The effect of self-secondaries on crater chronology can be well addressed after the spatial distribution, size-frequency distribution, and density evolution of self-secondaries are resolved.
Highlights
High velocity impact between celestial materials is arguably the most fundamental process in the formation and evolution of the Solar System
Impact flux ratios between the Moon and the other solar system bodies, which are predicted by orbital dynamics could be used to translate the lunar crater chronology to the other Solar System bodies (e.g., Hartmann 1977), so absolute model ages for geological units on these bodies could be obtained as well (e.g., Neukum et al 2001)
Within the diameter range of self-secondaries, the production size-frequency distribution (SFD) of primaries on continuous ejecta deposits can be roughly constrained based on their samples ages and the impact flux estimated from newly formed craters (e.g., Daubar et al 2013; Speyerer et al 2016)
Summary
Self‐secondaries on the other planetary bodies The Hokusai crater on Mercury (D = 96 km; 57.8°N, 16.8°E) provides the conclusive evidence that self-secondaries dominate the continuous ejecta deposits of newly formed impact craters (Xiao et al 2016). The target property scapegoat With the unambiguous observational evidence for both the existence of self-secondaries (e.g., Fig. 4) and their dominance in small crater populations on the continuous ejecta deposits of newly formed impact craters (e.g., the Hokusai case on Mercury), there are debates about whether or not self-secondaries are needed to explain the observed crater density differences on Copernican-aged lunar craters.
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