[00:01.77]Narrator: Listen to part of a lecture.
[00:07.16]Lecturer：Okay, the collection of moon rocks that the 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. [00:28.00]Why did so many of them have a magnetic signature - evidence of having been magnetized in the past? [00:36.36]After all, we know the moon, unlike Earth, has no magnetic field today.
[00:43.10]Well, let’s back up a bit here and discover Earth’s magnetic field. [00:48.22]It’s explained by the dynamo theory. [00:51.20]The dynamo theory explains how Earth generates a long lived global magnetic field. [00:58.51]Now, we know Earth has a molten metallic core. [01:03.03]That is a core made up largely of hot liquid iron. [01:07.81]Well, as the warmer molecules of this iron liquid rise, the core of course, sinks in a circulatory motion. [01:17.77]This circulating metallic liquid inside Earth along with the rotation of the Earth creates electric currents and these electric currents, in turn, creates Earth’s magnetic field. [01:31.35]A molten core that generates a magnetic field is a dynamo, and the action of this dynamo magnetizes life on Earth. [01:41.28]But, the moon doesn’t have a magnetic field now, and scientists have long believed that it has never had one.
[01:50.05]Why? [01:51.05]Well, for one thing, the moon is cold and it’s also thought that the moon is too small to have a molten core. [01:59.20]So it came as a big surprise when astronauts returned with rock samples that had been magnetized. [02:07.29]Well, ever since those early missions, scientists have been debating the answer to this question. [02:14.40]Some scientists have argued that the shock of the impact of meteorites and asteroids wrecked the magnetic signature on the moon rocks. [02:24.38]See, 3.9 billion years ago, relatively soon after the moon formed, it underwent a period of heavy meteorite and asteroid bombardment. [02:36.50]These scientists thought it was the impacts during this period of bombardment that shocked or altered many of the moon rocks and wrecked the magnetic signature.
[02:48.31]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 were the source of the magnetic signatures of the moon rocks. [03:09.10]So, who’s right? [03:10.77]Well, the very detailed recent analysis of a particular moon rock, known as the trocto, may have finally helped shade some light on this debate.
[03:22.16]The trocto, is made of troctolite, a combination of minerals that crystallize very early in the moon’s history and I mean very early. [03:33.08]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. [03:44.73]It suggests that the rock spent a very long period in a magnetic environment, millions of years. [03:52.57]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. [04:05.53]In a laboratory test, scientists have found it had been exposed to two stable and intense magnetic events, each followed by a long cooling period. [04:17.49]The first event, the one we’re most interested in, occurred about 4.2 billion years ago, about 300 million years after the moon formed, at the time the moon was mostly likely to have a liquid metallic core. [04:34.78]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. [04:48.58]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 scientist found.
[05:02.03]Well, this lunar dynamo theory certainly is very appealing to many of us. [05:08.35]For one thing, we’ve been trying to explain the origin of the moon for a long time. [05:14.41]And this theory fits very nicely with the most widely accepted hypothesis of the moon’s origin, the giant impact hypothesis. [05:24.49]The giant impact hypothesis also allows for the possibility that the moon had a small molten core. [05:32.54]We’ll talk about that theory and other ways the two theories jive in a few minutes. [05:38.44]Now, admittedly, having only one lunar rock as evidence isn’t enough for a lot of scientists, but it’s a good start.