Alternative Theories of the Moon's Origin

Alternative Theories of the Moon’s Origin

Fission Theory

  • The Fission Theory postulates that the Moon was once part of the Earth and somehow separated from the Earth early in the history of the solar system.
  • The present Pacific Ocean basin is the most popular site for the part of the Earth from which the Moon came.
  • This theory was thought possible since the Moon’s composition resembles that of the Earth’s mantle and a rapidly spinning Earth could have cast off the Moon from its outer layers.

Capture Theory

  • The Capture Theory speculates that the Moon was formed elsewhere in the galaxy and was subsequently trapped by the Earth’s gravitational field.
  • The Moon’s different chemical compositions makes this theory a viable one.
  • However, the capture of such a large body as the Moon would have likely caused significant disruption to the Earth’s orbit and rotation, casting doubt on this theory.

Accretion Theory

  • The Accretion Theory argues that the Moon and the Earth formed together as a double system from the primordial accretion disk of the solar system.
  • The heat of formation of the Moon, evidenced by the large pools of Lunar Magma, argues against this theory since a hot Moon would have been more volatile rich.

Giant Impact Hypothesis

  • The Giant Impact Hypothesis, or Theia Impact suggests the Moon formed out of the debris left over from a collision between Earth and an astronomical body the size of Mars, approximately 4.5 billion years ago.
  • This is the most widely accepted theory as it explains the similarity in the Earth and Moon’s isotopic compositions and accounts for the Moon’s angular momentum.
  • It also explains why the Moon is largely made of lighter silicates and not heavier metals, much like the Earth’s crust.

Summary

  • While the Giant Impact Hypothesis is the most widely accepted, it’s important to appreciate the features of as well as the problems with all alternative theories: Fission, Capture, Accretion and Theia Impact.
  • All these theories are attempts to explain the various intriguing aspects of the Moon, such as it’s size in comparison to Earth, it’s unusual orbit and it’s composition.