考满分 toefl

1 .Listen to part of a lecture in an astronomy class.

听一段天文学课上的讲座片段

1 .Professor: Okay, the collection of Moon rocks that astronauts brought back to Earth in the early 1970s has provided us with a lot of information about the nature of the Moon, but there's one thing about the rocks that's especially puzzled scientists ever since these missions. Why did so many of them have a magnetic signature, evidence of having been magnetized in the past? After all, we know the Moon, unlike Earth, has no magnetic field today. Well, let's back up a bit here and discuss Earth's magnetic field. It's explained by the Dynamo theory.

教授： 好的，20世纪70年代初宇航员将收集的月球岩石样本带回了地球，为我们提供了大量关于月球性质的信息，但关于这些岩石有一件事尤其让科学家们感到困惑。为什么这么多岩石具有磁性特征，是表明它们在过去曾被磁化过？毕竟我们知道，与地球不同，如今的月球没有磁场。好吧，让我们说回来，讨论一下地球的磁场。它可以用“发电机理论”来解释。

1 .The Dynamo theory explains how Earth generates a long-lived global magnetic field. Now we know Earth has a molten metallic core--that is a core made up largely of hot liquid iron. Well, as the warmer molecules of this iron liquid rise, the cooler portions sink in a circulatory motion. This circulating metallic liquid inside Earth, along with the rotation of the Earth, creates electric currents, and these electric currents in turn create Earth's magnetic field. A molten core that generates a magnetic field is a dynamo, and the action of this dynamo magnetizes rocks on Earth.

发电机理论解释了地球如何产生一个长期存在的全球磁场。我们知道地球有一个熔融的金属核心——即主要由热液态铁组成的核心。当这些铁液中较热的分子上升时，较冷的部分会下沉，形成循环运动。这种在地球内部循环流动的金属液体，加上地球的自转，会产生电流，而这些电流又反过来产生地球的磁场。产生磁场的熔融核心就是“发电机”，而发电机的活动会使地球上的岩石磁化。

1 .But the Moon doesn't have a magnetic field now, and scientists have long believed that it has never had one. Why? Well, for one thing, the Moon is cold, and it was also thought that the Moon was too small to have had a molten core. So it came as a big surprise when astronauts returned with rock samples that had been magnetized. Well, ever since those early missions, scientists have been debating the answer to this question. Some scientists have argued that the shock of the impact of meteorites and asteroids left a magnetic signature on the Moon rocks. See 3.9 billion years ago, relatively soon after the Moon formed, it underwent a period of heavy meteorite and asteroid bombardment. The scientists thought it was the impacts during this period of bombardment that shocked or altered many of the Moon rocks and left a magnetic signature on them. Other scientists, however, have theorized that at one time, soon after the Moon formed, it could, in fact, have had a molten core and a magnetic field like Earth does, and that it was this, not meteor strikes, that was the source of the magnetic signatures on the Moon rocks.

但月球现在没有磁场，而且科学家们长期以来一直认为它从未有过磁场。为什么呢？首先，月球是冷的，而且人们也认为月球太小，不可能有熔融的核心。因此，当宇航员带回被磁化的岩石样本时，人们大吃一惊。自那些早期任务以来，科学家们一直在争论这个问题的答案。一些科学家认为，陨石和小行星撞击产生的冲击，在月球岩石上留下了磁性特征。大约39亿年前，月球形成后不久，它经历了一段密集的陨石和小行星撞击时期。科学家们认为，正是在这段撞击时期的冲击改变了许多月球岩石，并在它们上留下了磁性特征。然而，另一些科学家提出理论认为，月球在形成后不久，可能确实曾有过和地球一样的熔融核心和磁场，而这不是陨石撞击，是月球岩石上磁性特征的来源。

1 .So who's right? Well, the very detailed recent analysis of a particular Moon rock known as the Trocto, may have finally helped to shed some light on this debate. The Trocto is made of Troctolite, a combination of minerals that crystallized very early in the Moon's history. And I mean, very early. This rock is about 4.3 billion years old, and the magnetic signature found on Trocto was very different than the signature on other Moon rocks. It suggests that the rock spent a very long period in a magnetic environment, millions of years. You see magnetization from a meteorite generates a short lived magnetic field, which results in a certain magnetic signature, and the Trocto just doesn't show that. In laboratory tests, scientists found it had been exposed to two stable and intense magnetic events, each followed by a long cooling period. The first event of the one we're most interested in occurred about 4.2 billion years ago, about 300 million years after the Moon formed, the time when the Moon would most likely have had a liquid metallic core. And because of the prolonged nature of the magnetic event, scientists speculate that it was caused by a magnetic field generated by a molten metallic core and not a meteor strike. So even if Trocto had been subjected to that later bombardment that occurred 3.9 billion years ago, it's not the source of the magnetization that scientists found.

那么谁是对的呢？最近对一块名为“Trocto”的月球岩石进行的非常详细的分析，可能终于为这些争论提供了一些线索。Trocto由橄长岩（Troctolite）组成，这是一种在月球历史早期结晶的矿物组合，我的意思是，非常早。这块岩石大约有43亿年的历史，而Trocto上发现的磁性特征与其他月球岩石上的特征非常不同。它表明这块岩石在一个磁场环境中存在了非常长的时间，长达数百万年。要知道，陨石撞击产生的磁场是短暂的，会导致某种特定的磁性特征，而Trocto并没有显示出这种特征。在实验室测试中，科学家发现它曾暴露在两次稳定而强烈的磁事件中，每次事件后都有一段长时间的冷却期。我们最感兴趣的第一个事件发生在大约42亿年前，也就是月球形成后约3亿年，这是月球最有可能拥有液态金属核心的时期。由于这次磁事件的长期性，科学家推测它是由熔融金属核心产生的磁场引起的，而不是陨石撞击。因此，即使Trocto后来经历了39亿年前的那次撞击，那也不是科学家发现的磁化来源。

1 .Well, this lunar Dynamo theory certainly is very appealing to many of us. For one thing, we've been trying to explain the origin of the Moon for a long time, and this theory fits very nicely with the most widely accepted hypothesis of the Moon's origin--the giant impact hypothesis. The giant impact hypothesis also allows for the possibility that the Moon had a small, molten core. We'll talk about that theory and other ways the two theories diverge in a few minutes. Now, admittedly, having only one lunar rock as evidence isn't enough for a lot of scientists, but it's a good start.

好吧，这个“月球发电机理论”对我们许多人来说确实非常有吸引力。首先，我们长期以来一直在试图解释月球的起源，而这个理论与最广泛接受的月球起源假说——“大碰撞假说”——非常吻合。大碰撞假说也允许月球曾有一个小的熔融核心的可能性。我们几分钟后再讨论这个理论，以及这两种理论的其他分歧点。当然，只有一块月球岩石作为证据对许多科学家来说是不够的，但这是一个好的开始。