On the importance of self-secondaries

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.