This image shows the Earth and Mars

The Formation of the Moon

Any successful theory must account for everything we know about the Moon now, as well as make predictions about future observations. There are three theories about how the moon came to be in place:
  • that the moon came out of the crust of the Earth
  • that the moon was captured by the Earth
  • that the Earth and moon formed together out of the primordial nebula
By whatever means it came to be in place, as the Moon finished forming, about 4 billion years ago, the surface was hit by the remains of the boulders which are thought to have formed the Earth, until the boulders were completely gone. During this time the moon warmed and experienced volcanism. There is abundant evidence of volcanic plains from this time period. Toward the end of the period of bombardment by boulders, scientists think that the Moon was hit by a series of very large boulders. These collisions formed what we know today as the lunar Maria.

Because of its small size, the Moon would have cooled very rapidly compared with the Earth. Soon, the Moon cooled completely to the form we see today. Surface activity in the form of plate tectonics and other forms of surface activity ceased once the Moon cooled. Even the interior of the Moon seems to have cooled to the form of inactivity today.

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