Abstract
Self-secondaries are a population of background secondaries, and they have been observed on top of impact melt and ballistically emplaced ejecta deposits on various planetary bodies. Self-secondaries are formed by impacts of sub-vertically launched ejecta, but the launch mechanism is not confirmed. The potential threat of self-secondaries to the theoretical and applicable reliability of crater chronology has been noted, but not constrained. Hitherto discovered self-secondaries were located around complex impact craters, but their potential existence around simple craters has not been discovered. Here we report the first discovery of self-secondaries around lunar cold spot craters, which are an extremely young population of simple craters formed within the past ~1 million years on the Moon. Self-secondaries are widespread on layers of cascading flow-like ejecta deposits around cold spot craters. The spatial density of self-secondaries dwarfs that of potential primary craters. The spatial distribution of self-secondaries is highly heterogeneous across the ejecta deposits. With respect to the impactor trajectory that formed cold spot craters, self-secondaries formed at the downrange of the ejecta deposits have the largest spatial density, while those at the uprange have the smallest density. This density pattern holds for all cold spot craters that were formed by non-vertical impacts, but self-secondaries do not exhibit other systematic density variations at different radial distances or at other azimuths with respect to the impactor trajectory. Among known mechanics of ejecting materials to the exterior of impact craters, impact spallation is the most likely scenario to account for the required large ejection velocities and angles to form self-secondaries. The production population of self-secondaries is estimated based on the highly diverse crater size-frequency distributions across the ejecta deposits of cold spot craters. For a better understanding of the impact history on the Moon, a systematic investigation for the effect of self-secondaries on lunar crater chronology is required.
Highlights
Impact craters formed by hypervelocity collisions between celestial materials are the most common landforms on planetary bodies [1]
We searched for self-secondaries around lunar cold spot craters using images obtained by the Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC), which have pixel scales better than ~1 m [38]
We investigated the spatial distribution of self-secondaries around cold spot craters based on crater size-frequency distribution (SFD) collected at different azimuths and radial distances with respect to the parent crater
Summary
Impact craters formed by hypervelocity collisions between celestial materials are the most common landforms on planetary bodies [1]. The spatial density of impact craters records age information of planetary surfaces, as older surfaces accumulate more craters than younger surfaces. Crater statistics have been the most-frequently used technique in the remote age determination for planetary surfaces [2,3]. If the formation rates of different-sized craters are known [4], absolute ages of planetary surfaces can be modelled based on the observed crater size-frequency distribution (SFD). The crater production function, which describes the SFD of various-sized impact craters formed in a given time interval, and the crater chronology function, which describes the production rate of a given-sized crater, are the two fundamental parts to construct a crater chronology [5].
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