How Was the Moon Formed?

How Was the Moon Formed?

Solving mysteries is one of the great joys of science. Researchers collect evidence, propose theories, test out their ideas, and revise their hypotheses as new evidence is gathered.

How the moon was formed is one of science's great unsolved mysteries. Over the years, scientists have put forth several theories, including (but not limited to):

• Earth capturing a pre-formed moon that originated elsewhere;
• Earth and moon forming together from the primordial accretion disk; and
• The moon being spun off from Earth through fission.

How the Moon was Formed Artists depiction

Artist's depiction of the giant impact that is hypothesized to have formed the Moon (Image Courtesy of NASA)

All these theories sound good, but each one has serious flaws. The Earth's atmosphere never extended far enough out to have captured a large moon. Earth would have had to spin at far too fast of a rate for the Moon to have formed through fission. And the accretion disk theory doesn't explain the depletion of metallic iron in the Moon.

To top it off, none of the theories explains the high angular momentum of the Earth-moon system. Angular momentum is the measure of the spin of the Earth and the Moon and the orbital motion of the Moon around the Earth. The angular momentum is too high to be explained by any of those theories.

In 1975, a new theory of how the moon was formed emerged. Two sets of researchers - William K. Hartmann and Donald Davis of the Planetary Sciences Institute in Arizona, and Alfred G. W. Cameron and William Ward of the Harvard-Smithsonian Center for Astrophysics in Massachusetts - published separate papers suggesting the Moon was formed by a giant impact.

According to this theory, a body roughly the size of Mars slammed into a half-formed Earth more than 4 billion years ago with a force about 100 million times greater than the impact believed to have killed the dinosaurs. The impact melted the outer shell of the Earth, creating a magma ocean on the planet, and threw a ring of hot debris into orbit.

The orbiting debris eventually formed into a moon that featured an ocean of hot magma at least 300 miles (500 km) deep. Large impacts of other bodies continued on both the Earth and Moon, adding more material to each world. Eventually the Moon cooled into a solid world.

The theory was controversial, and many scientists refused to take it seriously despite the problems with the other explanations. However, during a 1984 conference on lunar origin in Hawaii, scientists conducted an in-depth comparison of existing theories. The consensus that emerged was that the giant impact theory best explained how the Moon was formed.

The theory fits well with the way that physicists believe planets form. An impact also helps to explain the high angular momentum in the Earth-Moon system. In addition, the relatively abundance of oxygen on the Moon and the Earth are identical, suggesting the two worlds formed at the same distance from the Sun.

Although a giant impact is the predominant theory for the formation of the Moon, it doesn't explain everything. Critics point to a lack of evidence that Earth ever had a magma ocean as well as differences in the compositions of the Moon and Earth that supposedly rule out the impact.

Until scientists get some definitive proof either way, the questions of how the Moon formed will remain a fun topic for scientists to debate.

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