This image shows the Earth and Mars
NASA

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 continued to be bombarded by the remains of planetary material (planetismals) available nearby. This period is the same for all planets, and is called the Period of Late, Heavy Bombardment. During this time the moon warmed, separated into a core and mantle, and experienced volcanism. There is abundant evidence of volcanic plains from this time period. Toward the end of the Period of Late, Heavy Bombardment scientists think that the Moon experienced a series of significant collisions which formed what we know today as the lunar Maria. The impact basins formed by the collisions flooded with darker material from the interior of the Moon, forming smoother, darker plains.

Because of its small size, the Moon would have cooled very rapidly compared with the Earth. When the Period of Bombardment ceased, 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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