经济学人下载:月球的历史

Of course, computer simulations can say only what could have happened, not what did. For that, physical evidence is required. Some suggestive evidence does exist, though. For a start, the crust on the far side of the moon is indeed much thicker than that on the nearside. Also, because the second moon would have been smaller than the main one, it would have cooled and solidified more quickly, meaning that its rocks would be older. Sure enough, different parts of the present moon’s crust, recovered by various missions to the lunar surface, differ in age by up to 200m years, a fact that has long puzzled selenologists but which the impact theory neatly explains. Moreover, such a huge collision on the far side would have shifted much of the magma ocean that then underlay the moon’s surface to the near side, which led to the formation of the maria.

当然，计算机模拟只能估计已经发生的情形，而不是确实发生的情况。为此，实物证据是需要的。虽然，某些提示性的证据确实存在。因为从一开始，较远那侧的月球外壳确实比较近侧的外壳要厚实的多。此外，因为这第二个月球应该比最大的月球要小的多，所以这个月球应该冷却、凝固要快得多。这也意味着这个月球的岩石要年久的多。果真如此，现在月球外壳的不同部分，各次去月球表面的探测任务发现，这些岩石年代的不同达到2亿年之久。一个长期困扰月球学家的客观事实被碰撞理论巧妙地解释了。此外，在月球较远那侧如此巨大的一次碰撞应该迅速移动很多岩浆洋，然后在月球表面一直铺到近地侧，这就导致了月海的形成。

More evidence may surface when the Gravity Recovery and Interior Laboratory mission, an American spacecraft designed to map the moon’s gravitational field, arrives in lunar orbit in around six months’ time. That should yield a detailed map of the lunar crust, and how its thickness varies from place to place. But the best way to check the new hypothesis would be for selenologists to get their hands on rocks from the far side.

当引力恢复和室内实验任务时，更多的证据可能会出现。一架设计用来勘测月球引力场的美国航天器，大约会在六个月内到达月球轨道。这次探测任务应该会产生出一份详细的月球外壳图，以及各个地方的厚度如何变化。但是，对月球学家来说，检查这个新假说的最好方式就是找到月球较远那侧的岩石。

Sadly, that is unlikely to happen soon. Neither America nor Russia has any sample-recovery missions planned for the moon. China does (scheduled for 2017). But, like the American and Soviet missions of the 1960s and 1970s, the Chinese are expected to opt for the safety of a landing on the near side. Selenologists may therefore have to wait a long time before they can test the Jutzi-Asphaug model properly.

可叹的是，这种情况不可能很快发生。无论是美国还是俄罗斯都没有任何计划去月球恢复取样的任务。中国倒有一个（定于2017年）。但是，像20世纪60年代和70年代的美国和苏联的任务一样，中国人选择在近地侧安全着陆。因此，月球学家可能需要等待很长一段时间才能够真正测试彻奇希-阿萨福格模型。