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Listen to part of a lecture in an astronomy class.
Professor: Ok, 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 has 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. 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, but the Moon doesn’t have a magnetic field now, and scientists have long believed that it has never had one. Why? Well, we know that 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 believe that the shock of the impact of meteorites and asteroids left a magnetic signature on the Moon rocks. As we know, 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. 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 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, 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 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.
    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 explore that theory and other ways the two theories give 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.