6.3 Formation of the Moon

• • The giant-impact theory or Collision: The theory that the Moon was formed when a large object, possibly a planet, crashed into the early Earth. is most widely accepted today. This proposes that the Moon formed during a collision between the Earth and another small planet, about the size of the planet Mars. The debris from this impact collected in an orbit around Earth to form the Moon.
• During this massive collision, nearly all of Earth and Theia melted and reformed as one body, with a small part of the new mass spinning off to become the Moon as we know it.

  Scientists have experimented with modelling the impact, changing the size of Theia to test what happens at different sizes and impact angles, trying to get the nearest possible match
• 'If the two bodies had come from the same place and were made of similar stuff to begin with, this would also explain how similar their composition is.'
• The collision would have caused rocks and dust to break away from

  both planets. The theory states that Earth was formed from the two

  planets joined together. The Moon was formed when gravity pulled the rocks and dust together. These stages are shown in this diagram:

  
• Evidence that supports: 'If you look at the lunar surface, it looks pale grey with dark splodges,' Sara says. 'The pale grey is a rock called anorthosite. It forms as molten rock cools down and lighter materials float to the top, and the dark areas are another rock type called basalt.'
• The mineralogy of Earth and the Moon are so close that it's possible to observe Moon-like landscapes without jetting off into space.
• Evidence that supports: By looking at the Moon we can tell a lot about what the Earth was like four billion years ago.
• One of the most puzzling properties of the Moon is that it has much less iron than does Earth, as evidenced by its much lower mean density, 3.3 times that of water as compared to Earth’s 5.5 times that of water. Another way of saying the same thing is that the Moon is made of material that looks more like rock from Earth’s mantle than like the overall composition of Earth.
• Thus, one piece of evidence in favor of the impact idea is that the impact blew off rocky material from Earth’s mantle (and from the impactor’s mantle) and this debris formed the Moon. In this scenario, the iron cores of both bodies had already formed, and core material did not make it into Earth orbit. This explains the Moon’s lack of iron.

  Another piece of evidence, from the lunar samples, is that the Moon’s composition of certain isotopes exactly matches that of Earth, but it doesn’t match those of bodies from elsewhere in the solar system.
• There is evidence outside the Solar System

  of similar collisions causing rings of rock and

  dust.
• The collision theory fi ts with the theory of

  how the Solar System was formed.
• Evidence that denies:

  The surface of the Earth does not appear to ever have been molten. A collision that formed the Moon would have caused the surface of the Earth to melt. The surface would have later solidified.
• Evidence that contradicts:

  Venus has no moon. Collisions in the early years of the Solar System would have been common and scientists would have expected Venus to have a moon formed in the same way.
• The composition of rocks on the Moon would be expected to be more similar to rocks on Theia than rocks on Earth. In fact, the composition of the Moon is more similar to Earth.