Brown University, Department of Earth, Environmental, and Planetary Sciences, 324 Brook Street, Providence, Rhode Island 02912, USA.
Sandia National Laboratories, P. O. Box 5800, MS 0840, Sandia National Laboratories, Albuquerque, New Mexico 87185, USA.
Nature. 2016 Jul 21;535(7612):391-4. doi: 10.1038/nature18278.
Rimmed grooves, lineations and elongate craters around Mare Imbrium shape much of the nearside Moon. This pattern was coined the Imbrium Sculpture, and it was originally argued that it must have been formed by a giant oblique (~30°) impact, a conclusion echoed by later studies. Some investigators, however, noticed that many elements of the Imbrium Sculpture are not radial to Imbrium, thereby implicating an endogenic or structural origin. Here we use these non-radial trends to conclude that the Imbrium impactor was a proto-planet (half the diameter of Vesta), once part of a population of large proto-planets in the asteroid belt. Such independent constraints on the sizes of the Imbrium and other basin-forming impactors markedly increase estimates for the mass in the asteroid belt before depletion caused by the orbital migration of Jupiter and Saturn. Moreover, laboratory impact experiments, shock physics codes and the groove widths indicate that multiple fragments (up to 2% of the initial diameter) from each oblique basin-forming impactor, such as the one that formed Imbrium, should have survived planetary collisions and contributed to the heavy impact bombardment between 4.3 and 3.8 billion years ago.
环形脊、线性特征和长形陨石坑环绕着雨海，构成了月球近地表面的大部分地貌。这种模式被称为雨海地貌，最初有人认为它一定是由一个巨大的倾斜（约 30°）撞击形成的，后来的研究也证实了这一结论。然而，一些研究人员注意到，雨海地貌的许多元素都不是以雨海为中心的，因此暗示其具有内生或结构起源。在这里，我们利用这些非径向趋势得出结论，认为撞击雨海的物体是一颗原行星（直径为灶神星的一半），曾经是小行星带中一大群原行星的一部分。这些对雨海和其他盆地形成撞击体大小的独立约束，显著增加了在木星和土星轨道迁移导致耗尽之前，小行星带中质量的估计。此外，实验室撞击实验、冲击物理代码和脊宽表明，每个倾斜盆地形成撞击体（如形成雨海的撞击体）的多个碎片（最多占初始直径的 2%）应该在行星碰撞中幸存下来，并促成了 43 亿至 38 亿年前之间的重撞击轰炸。
