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Return to Lesson 11's Main Page. The implication of the Apollo cargo of lunar samples so far does not clarify how the Moon was formed. Several theoretical models have been proposed to explain the origin of the Moon. The older models fall into three broad categories: To these have been added the giant impact model, which is the strongest contender to date.
In , the astronomer Sir George Howard Darwin, son of the biologist Charles Darwin, proposed that the Moon was once part of Earth and broke or fissioned from it due to forces caused by a fast rotation and solar tides.
The fission model proposed the large basin of the Pacific Ocean as the place from which the Moon was ejected. The difference in density between Earth and the Moon might at first glance seem to rule out the fission theory, but the Earth's crust does have a density close to that of the Moon. If the Moon formed from material from the Earth's crust, we should expect the density to be just as we find it. There is a problem with the fission theory, however. Astronomers have difficulty explaining how an object as massive as the Moon might have torn away from the Earth.
No satisfactory mechanism for this event has been proposed. In addition, the Moon does not orbit in the plane of the Earth's equator as it would if it were ejected from a spinning Earth. Early in the twentieth century, another theory was proposed. It holds that the Moon was originally a separate astronomical body that happened to come near Earth, became bound to the Earth's gravitational field, and subsequequently settled into orbit as the Moon.
This is the capture model. There are problems with the capture model. If one astronomical object comes close to another, each of their paths will be changed by the gravitational force between them, but one will not capture the other unless there is contact between the two or unless a third object is involved, so that the interaction of the three objects results in one of them being slowed down to an orbital speed.
Such a near-collision between three objects is remote in nature. The binary accretion sister or coformation model , which was suggested in the early s, is the oldest.
It holds that as Earth formed from a spinning disk of material, not all of that material coalesced to form Earth. A small part of it was left orbiting Earth and formed into the Moon. This model is totally consistent with the models that explain the formation of the solar system. Nevertheless, a simple comparison of densities seems to rule out this model, for if the Moon formed along with Earth, the two bodies should have about the same density. The Earth's density is 5.
In addition, astronomers have difficulty explaining the orientation of the Moon's orbit in this model. The leftover material would orbit in the plane of the Earth's equator. The Moon does not orbit in this plane as it should if it formed from leftover material.
Refer to your reading assignment for more on these models. The Giant Impact Model. In the s, A. Cameron and William Ward proposed a new model. They suggested that early in the Earth's history, it was struck at a glancing angle by a large object, that the impact resulted in a fusion of the two objects, and that material was thrown off of the two to form the present Moon.
Computer simulation of such a collision shows that if the impacting object has a mass nearly as great as Mars, heat resulting from the collision would vaporize material and eject enough of it into orbit to account for the mass of the Moon, once the material coalesced. The large impact model , as it is called, is able to explain both the similarities and the differences in the compositions of Earth and the Moon. Since the mids, a consensus has been building among astronomers that the large impact model fits the data better than the other three models.
Recent theoretical work on the formation of the planets indicates that without large impacts, Earth would rotate every hours instead of every 24 hours. A glancing impact by a large object explains its present rotation rate. Like all new theories, the large impact model will be tested against both existing data and new data as the years pass.
Refer to your reading assignment for more on the impact theory. Key Terms refer to your text for some these terms. Review Questions refer to your text to answer some of these questions 1. Explain how the Moon could have formed by the fission model. Describe the capture model. What is the binary accretion model?
What are the problems associated with each of the three older models? Astronomers are now favoring the large impact model for the formation of the Moon. Advanced Question refer to your text to help you answer this question. Do you need help with this topic? CCAC offers on-campus and online tutoring services.
Online tutoring is available through "Smarthinking. Choose a members of the Physics and Physical Science Faculty from the "Participating Faculty" drop down list who have agreed to conduct some of their office hours online. A separate window will open with information about the selected faculty member.
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You can also review questions and answers that have been sent to the message board during the past seven days. Do you need more help with this topic? Older Models The implication of the Apollo cargo of lunar samples so far does not clarify how the Moon was formed. For this topic, study the true and false, fill in the blanks self-test, and review questions at the end of the Chapter s of your reading assignment. In addition, learn the key words and answer all questions that follow: Key Terms refer to your text for some these terms fission model binary accretion model coformation theory capture model large impact model.