Arizona State University, School of Earth and Space Exploration, Tempe, Arizona 85287, USA.
Cornell University, Cornell Center for Astrophysics and Planetary Science, Ithaca, New York 14853, USA.
Nature. 2016 Oct 13;538(7624):215-218. doi: 10.1038/nature19829.
Random bombardment by comets, asteroids and associated fragments form and alter the lunar regolith and other rocky surfaces. The accumulation of impact craters over time is of fundamental use in evaluating the relative ages of geologic units. Crater counts and radiometric ages from returned samples provide constraints with which to derive absolute model ages for unsampled units on the Moon and other Solar System objects. However, although studies of existing craters and returned samples offer insight into the process of crater formation and the past cratering rate, questions still remain about the present rate of crater production, the effect of early-stage jetting during impacts and the influence that distal ejecta have on the regolith. Here we use Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) temporal ('before and after') image pairs to quantify the contemporary rate of crater production on the Moon, to reveal previously unknown details of impact-induced jetting, and to identify a secondary impact process that is rapidly churning the regolith. From this temporal dataset, we detected 222 new impact craters and found 33 per cent more craters (with diameters of at least ten metres) than predicted by the standard Neukum production and chronology functions for the Moon. We identified broad reflectance zones associated with the new craters that we interpret as evidence of a surface-bound jetting process. We also observe a secondary cratering process that we estimate churns the top two centimetres of regolith on a timescale of 81,000 years-more than a hundred times faster than previous models estimated from meteoritic impacts (ten million years).
彗星、小行星和相关碎片的随机轰炸形成并改变了月球风化层和其他岩石表面。随着时间的推移，撞击坑的积累对于评估地质单元的相对年龄具有根本意义。撞击坑的数量和返回样本的放射性年龄为推导月球和其他太阳系物体上未采样单元的绝对模型年龄提供了限制。然而，尽管对现有撞击坑和返回样本的研究提供了对撞击坑形成过程和过去撞击率的深入了解，但仍存在一些问题，例如当前撞击坑的产生速率、撞击过程中早期喷射的影响以及远距离喷射物对风化层的影响。在这里，我们使用月球侦察轨道器相机（LROC）窄角相机（NAC）的时间（“前后”）图像对来量化月球上当代撞击坑的产生速率，揭示以前未知的撞击诱导喷射细节，并确定一种快速搅动风化层的二次撞击过程。从这个时间数据集，我们检测到 222 个新的撞击坑，发现比月球标准 Neukum 产生和年代学函数预测的撞击坑数量多 33%（直径至少为 10 米）。我们确定了与新撞击坑相关的广泛反射带，我们将其解释为表面喷射过程的证据。我们还观察到一种二次撞击过程，我们估计在 81000 年的时间尺度上将风化层的前两厘米搅动，比以前从陨石撞击中估计的模型快一百多倍（一千万年）。
