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Topic 13
Reading
How did the Moon from? Scientists hypothesized that a piece of space debris the size of the Earth struck the Earth just after the formation of the solar system, ejecting large volumes of heated material from the outer layers of both objects. A disk of orbiting material was formed, and this matter eventually coalesced to form the Moon. Nevertheless this giant-impact theory is questionable for the following reasons.
First, if the Earth had been smacked hard enough to lose such a big piece of its mantle, the impact would have left a huge mark on the surface of the Earth. Yet so far, no trace of a giant impact has been found on the surface of the Earth. Without evidence proving that a huge collision took place, the validity of the giant-impact theory is in doubt.
Second, if the Moon came from material that once made up the Earth, then the rocks on the Earth and the Moon should be much more similar in composition. However, the make-up of the rocks on the Moon is quite different from that on the Earth. For example, the rocks on the Moon contain few volatile substances such as water. This fact makes it unlikely that the Moon formed directly from the Earth.
Finally, the density of the Moon is much lower than that of the Earth. The Moon has a low density compared to the terrestrial planets. Recent findings even suggest that the moon’s core constitutes only 2-4 percent of its total mass, compared to a terrestrial core with about 30 percent of the Earth’s mass. Therefore, the differences between the Moon and the Earth greatly weaken the giant-impact theory.
Topic 13
Listening
While the giant-impact theory of how the Moon formed has been heavily criticized, I do not find these criticisms convincing. Here is why.
First of all, it is true that so far so trace of a giant impact has been found on the surface of the Earth. Yet, we should remember that the giant crash took place more than four billion years ago. Any trace of the collision might have been erased by the forces of nature. The Earth’s surface is made up of a number of enormous rock plates and they have collided with each other over millions of years, resulting in tremendous change in the map of the planet. Therefore, if there had been any impact between the Earth and a space rock, the impact mark might have disappeared long ago.
Second, it is a fact that rocks on the Moon do not contain water, while those on Earth do. However, as the Moon formed out of the collision of spaces rocks, the heat from the collision made the entire Moon molten. The temperatures on this newly formed Moon were extremely high. With such high temperatures, any water there might have evaporated and gone off into space. So, the absence of water on the Moon is not surprising.
Finally, we know that the density of the Earth’s surface is lower than that of the Earth’s center. Since the Moon was originally part of the Earth’s surface, not the Earth’s core, its density should be consistent with the density of the Earth’s surface. This is exactly what we find. The density of the Moon is consistent with that of the Earth’s surface, which further supports the giant-impact theory of the Moon’s formation.