So I would like to continue our discussion about the moon, more particularly about the origins of the moon and how it was actually created.
We talked about three possible solutions, uh, more like theories, about how the moon was created. Can anyone tell me the name of one of these theories?
Sarah.
Well, I do remember the capture theory, which proposes that the moon was located somewhere else in the galaxy until eventually it was kind of, like, captured, by the Earth’s gravitational pull.
Yes, and I’m glad you started with the capture theory because it’s the easiest one to reject. Its primary drawback is that no one knows of any way that early Earth could have captured such a large moon from elsewhere. One body approaching another cannot go into orbit around it without a serious loss of energy. Furthermore, if such a capture did take place, the captured object would go into a very strange orbit rather than the nearly circular orbit our moon goes through today. Finally, there are too many similarities in composition between Earth and the moon. It’s much more likely than the Earth and the moon were somehow connected at one point in the past.
What was another theory discussed, James?
The fission theory. Like you just said, the moon was once a part of the Earth but somehow separated from it early in their history. But I remember you mentioned some problems with this theory, too.
Yes, the fission theory suggests that the moon separated from the Earth, but modern calculations have shown that this type of splitting is nearly impossible. Furthermore, it is difficult to understand how a moon made out of materials from the Earth could have developed so many chemical differences from our own.
And the third? James again.
Yeah, the last one is the sister theory. It claims that the moon formed together with the Earth, but also remained independent from it. This is what many other astronomers once believed of other moons in the solar system, too.
Yes, the sister theory was the dominant idea accepted by most astronomers in the past, but, like the capture and fission theory, it had some problems, particularly when trying to explain how it could have such a lower density when compared to the Earth.
Now, in an effort to resolve these apparent contradictions, scientists developed a fourth hypothesis for the origin of the moon, one that involves a giant impact early in Earth’s history. This idea, known as the giant impact hypothesis, proposes that Earth was struck by an object approximately one-tenth of Earth’s mass, which is about the size of Mars. This is very nearly the largest impact Earth could experience without being shattered.
Such an impact would disrupt much of Earth and eject a vast amount of material into space, releasing almost enough energy to break the planet apart. Computer simulations indicate that material totaling several percent of Earth’s mass could be ejected in such an impact. Most of this material would be from the stony mantles of Earth and the impacting body, not from their metal cores. This ejected rock would then cool and form a ring of material orbiting Earth. It was this ring that ultimately came together and formed the moon.
While we do not have any current way of showing that the giant impact hypothesis is the correct model of the moon’s origin, it does offer potential solutions to most of the major problems raised by the chemistry of the moon. Most importantly, since the moon’s raw material is from the deep rocks of Earth and the asteroid that hit it, the composition and chemistry of the moon is better understood and explained.