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Formation of the Moon: Theories

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  • 0:54 The Four Moon…
  • 1:53 Capture Theory
  • 2:23 Fission Theory
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

Explore the four major theories on the formation of the Moon. The theories include the fission theory, capture theory, condensation theory and giant impact theory. Also look at both information supporting and flaws found in these theories.

Four Main Theories of Moon Formation

Remember that song from your childhood? I see the Moon, the Moon sees me...

Since ancient times, humankind has made up stories about the Moon. Ever hear that the Moon was made of green cheese? Or about the man in the Moon? Or, as Native American and Buddhism legends say, that a rabbit lives there? Since ancient times, full moons have been associated with odd or insane behavior, and, in fact, the word 'luna,' as in 'lunatic,' comes from the Roman goddess of the Moon, Luna, who was said to ride her silver chariot across the dark sky each night. Hippocrates, considered the father of modern medicine, wrote in the fifth century B.C. that 'one who is seized with terror, fright and madness during the night is being visited by the goddess of the Moon.'

Despite our fascination with the Moon, no one is completely sure how the Moon formed. Any successful theory of the Moon's formation must account for everything we know about the Moon now as well as make predictions about future observations. All of the current theories have serious flaws with their evidence, and the question is not completely settled, but most scientists lean toward the giant impact theory.

Capture theory proposes that the gravitational field around Earth captured the Moon.
Moon Capture Theory

In general, there are four theories for how the Moon formed.

  1. Capture Theory - The Moon was formed elsewhere in the universe and was captured by the Earth's gravitational field when it came too close.
  2. Fission Theory - When the Earth was young, it spun so fast that a piece of it broke off and was flung into space, where it became the Moon.
  3. Condensation Theory - The Earth and Moon condensed at the same time from the nebula.
  4. Giant Impact Theory - Sometimes called the Colliding or Ejected Ring Theory - The Earth was hit by something huge and a piece of Earth was ejected into space.

Capture Theory

In the capture theory, it is thought that the Moon was formed somewhere else in the solar system. It traveled freely through space until it was captured by Earth's gravitational field. This would explain the Moon's different chemical compositions. Others think this is improbable. It is just not likely that the two bodies would come together if created at a great distance. If the Moon were speeding through the solar system, something would have to slow it down by just the right amount so that the Earth could capture it - again, unlikely.

Fission Theory

The Moon looks very similar to outer layers of the Earth.
Moon Fission Theory

Fission means breaking apart, and the fission theory proposes that the Moon was once part of the Earth and somehow separated from the Earth early in the Earth's history, possibly when a rapidly spinning Earth cast it off. The Pacific Ocean basin is the mostly likely site for the part of the Earth from which the Moon came. This theory was thought possible because, while the Moon doesn't have the same composition as the entire Earth, it does greatly resemble the outer layers of our planet. If this theory were true, though, the present-day Earth-Moon system should contain fossil evidence of this rapid spin, and it does not. Also, for this to be true, the Moon would have to be orbiting the Earth exactly on our orbital plane, and it does not.

Condensation Theory

The condensation theory states that the Moon and the Earth were born individually from the nebula that formed the solar system, and the Moon fell into orbit around the Earth. In other parts of the solar system when this happens, the two objects have similar compositions, but our Moon and Earth are different. The Earth contains significant amounts of metals and heavier elements, and the Moon is decidedly metal-poor. The likelihood that the Moon and Earth could have co-formed but ended up with such vast differences in composition is virtually impossible. Another major problem is that the Moon seems to have formed about 100 million years after the Earth.

Giant Impact Theory

The giant impact theory suggests the Moon is partly made of outer layers of the Earth.
Moon Giant Impact Theory

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