How Were Rocks on the Moon Created?

Co-formation Theory

• The co-formation theory is also called the binary accretion theory. It was proposed in the early 1800s. It hypothesized that both the earth and the moon were created at the same time during the formation of the solar system. As the cosmic materials spun and combined to form the earth, some of the material remained in orbit around the earth and formed the moon. The main problem with this theory is that the moon should then orbit around the earth's equator, and it does not.

Fission Theory

• Charles Darwin's son, astronomer Sir George Howard, proposed the fission theory in 1879. He believed that the moon broke off the face of the earth while it spun at a high rate of speed. He offered the large underwater Pacific Ocean basin as the moon's origin. The moon does have a similar density to that of the earth's crust. However, it is too large to have "spun off" the earth in this way, and its orbital path is not consistent with such an event.

Capture Theory

• The capture theory was proposed in the early 1900s. It hypothesizes that the moon was on an astral path of its own until it got too close to the earth. The earth's gravitational force "captured" the moon, causing it to orbit earth. There are many inconsistencies in this theory, including the fact that the composition of moon rocks is very similar to rocks from the earth's crust, not a roving asteroid.

Giant Impact Theory

• The Giant Impact Theory has become the most widely supported theory since its proposal in the 1970s. It hypothesizes that the moon definitely may have been created by an enormous chunk of the earth's crust blown off by a collision with a Mars-sized object billions of years ago. Analysis of moon rocks brought back during the Apollo Missions supports this theory. Moon rocks contain elements common to the earth's outer crust.