Canup R M, Asphaug E
Department of Space Studies, Southwest Research Institute, 1050 Walnut Street, Suite 426, Boulder, Colorado 80302, USA.
Nature. 2001 Aug 16;412(6848):708-12. doi: 10.1038/35089010.
The Moon is generally believed to have formed from debris ejected by a large off-centre collision with the early Earth. The impact orientation and size are constrained by the angular momentum contained in both the Earth's spin and the Moon's orbit, a quantity that has been nearly conserved over the past 4.5 billion years. Simulations of potential moon-forming impacts now achieve resolutions sufficient to study the production of bound debris. However, identifying impacts capable of yielding the Earth-Moon system has proved difficult. Previous works found that forming the Moon with an appropriate impact angular momentum required the impact to occur when the Earth was only about half formed, a more restrictive and problematic model than that originally envisaged. Here we report a class of impacts that yield an iron-poor Moon, as well as the current masses and angular momentum of the Earth-Moon system. This class of impacts involves a smaller-and thus more likely-object than previously considered viable, and suggests that the Moon formed near the very end of Earth's accumulation.
人们普遍认为，月球是由早期地球与一个大型偏心天体碰撞后喷出的碎片形成的。撞击的方向和大小受到地球自转和月球轨道所包含的角动量的限制，这一数量在过去45亿年里几乎一直保持不变。目前对潜在月球形成撞击的模拟已经达到了足以研究束缚碎片产生的分辨率。然而，确定能够产生地月系统的撞击却证明很困难。先前的研究发现，要以适当的撞击角动量形成月球，需要在地球仅形成约一半时发生撞击，这是一个比最初设想更具限制性且存在问题的模型。在这里，我们报告了一类能够产生贫铁月球以及当前地月系统质量和角动量的撞击。这类撞击涉及一个比之前认为可行的天体更小——因此也更有可能存在的天体，并表明月球是在地球积聚接近尾声时形成的。
