听力原文： The origin of Earth's moon, the largest moon in the solar system, is still something of a mystery. There are some theories about its origin however. Now, keep in mind that a theory of the moon's origin has to be consistent with two important facts. The first fact is that the Earth contains a lot of iron, most of it has an iron core. But the moon contains practically no iron. The second fact is that other than the difference in iron content, the moon and Earth are composed of accentually the same minerals, a similarity not shared with any other planet or moon in our solar system. One of the earliest theories of the moon's origin, I called it the Capture Theory, proposes that the moon was somehow captured by Earth's gravitational force. This theory is improbable, however, because it assumes that the moon and Earth formed in different parts of the solar system. If this was true you would expect the moon's composition to be much different from Earth's composition, just as all the other planets in the solar system are so different from Earth. A second theory of the moon's origin is more promising. It is sometimes referred to as the Mars Theory, because according to this theory, when Earth was still molten, it was struck by a planet about the size of Mars. The impact caused the cores of the two planets to melt together and chunks of Earth's crust to be thrown out into space. These chunks came together to form. the moon. Now, remember, Earth's crust is low in iron, because the iron is in Earth's core, but high in various other minerals. This then accounts for why there is little iron but lots of other minerals on the moon. What is the talk mainly about?