经济学人下载:月球的历史

Selenologists, as those who study the moon call themselves, have kicked around many theories to explain the differences between the moon’s two faces: Earth partly shielding the moon from meteoritic impacts; uneven heating from beneath; the collision that excavated a 2,500km-wide crater at the lunar south pole, one of the biggest in the solar system. None seems satisfactory. But in a paper just published in Nature Martin Jutzi and Erik Asphaug of the University of California, Santa Cruz, examine yet another idea. This is that the mountains of the lunar far side may be debris left over from an even bigger collision than the one at the south pole—a collision that left lumps on the lunar surface rather than digging holes in it.

作为研究月球而称他们自己为月球学家的那些人已经非正式地讨论了许多理论来解释月球的两种截然不同表面的区别：地球部分地保护了月球免受陨石的碰撞；月球内部不均匀的加热；在月球南极空砸出一个2500公里宽的陨石坑---太阳系最大的陨石坑之一---的那次碰撞。好像没人对些理论满意。但是，加州大学圣克鲁斯分校的马丁．彻奇希（Martin Jutzi）和埃里克．阿萨福格（Erik Asphaug）刚刚在《自然杂志》发表的一篇论文检验了迄今为止的另一个猜想。这个猜想就是月球较远那侧的群山甚至可能是比月球南极那次碰撞更大的碰撞所留下的岩屑---这次更大碰撞在月球表面留下了隆起，而非砸坑。

Moonstruck
沉迷于幻想

The most widely accepted theory of how the moon formed suggests it is the result of a cosmic smash-up between Earth and an object roughly the size of Mars that happened about 4.5 billion years ago during the solar system’s turbulent youth. This impact would have sprayed huge quantities of material into space, forming a ring around Earth similar to, but much thicker than, those that now adorn Saturn.

月球如何形成的最广泛接受的理论表明，大约发生在45亿年前的年轻的太阳系动荡期间，地球和一个大约火星大小的物体间的宇宙大碰撞的结果而产生了月球。这次碰撞已将大量的物质散发到了太空，从而形成了环绕地球的光环，类似于那些现在装饰土星的光环，但是要厚得多。

The moon is thought to have condensed rapidly from this ring. Computer simulation of the process suggests, however, that several smaller moons may have formed at the same time. Some would then have collided with Earth or the moon, while others would have been ejected from the system altogether. But one or more of them often end up, according to the simulations, in what are known as Trojan orbits, where they followed the proto-moon endlessly around Earth.

月球被认为是从这个光环的气体凝聚而成。然而，计算机模拟的过程表明，这次大碰撞可能在同一时间形成了几个更小的卫星。然后，有些卫星就与地球或月球相撞，而其它卫星则完全被太阳系喷射出来。但，根据计算机多次模拟，其中的一个或多个卫星往往停留在被称为特洛伊星座的轨道上而终结。这些卫星在那儿伴随着永无休止地围绕地球的原月。

Or, rather, almost endlessly. For the hypothesis of Dr Jutzi and Dr Asphaug is that a large moonlet did eventually catch up and collide with the proto-moon—at a relatively low speed given that they shared an orbit—and produced the highlands on its far side as a result.

或者，更确切些，几乎是没有休止的。因为彻奇希博士（Dr Jutzi）和阿萨福格博士（Dr Asphaug）的假说就是一个大的小卫星最终赶了上来并与这个原月发生了碰撞---考虑到它们共享一条轨道，是以相对低的速度碰撞---结果，就在月球较远的那侧产生了那些群山。

To test their theory, the two researchers simulated such a collision on a computer. They found that, assuming the second moon was indeed smaller than the main one, and was made of similar stuff (which, given that it is assumed to have formed from the same ring of debris, seems reasonable), then a comparatively low-speed collision at around 2.4 kilometres a second would mean that much more material from the moonlet was deposited on the lunar surface than was blasted off by the impact. Most of the excess rock, moreover, would have stayed near the point of impact, covering about half of the moon with a thicker, mountainous layer of crust—just what is seen on the real moon.

为了测试他们的理论，这两位研究者在计算机上模拟了这样一次大碰撞。他们发现，假设第二个月球确实比最大的月球（译者注：指的是原月）要小，并且由相似的物质组成（考虑到这个月球是从岩屑的同样光环形成的，说它们为相似的物质好像也有道理）。然后，以大约每秒2.4公里的相对低速碰撞意味着，小卫星上更多的物质被沉积在月球表面而不是因碰撞而喷射出来。此外，过多的岩石将留落在碰撞点附近，且以更厚、多山的外壳层占了月球约为一半的面积 ---正如真正月球上所见到的一样。