The Moon's Interior

Clues

• density is 3300 kg/m³.
  • this is less than Earth's density of 5500 kg/m³.
  • so there must be more crust and mantle, less core.
  • [This suggests that the Moon didn't just accrete material from same rock supply as Earth.]
• no magnetic field.
  • so no liquid Fe core.
• only very mild moonquakes.
  • so no plate tectonics.
• study of moonquake wave propagation indicates a crust about 65 km thick.
• moonquakes most frequent at new moon and full moon
  • suggests they are from bending and stretching of rocks due to gravity of Earth and Sun.
• centers of moonquakes go down very deep (to 1000 km)
  • so rock is brittle to that depth.

Resulting picture:

• crust (65 km)
• mantle, hard (lithosphere) to 1000 km at least
• mantle, soft (asthenosphere) inward from there
• core less than 700 km in radius (or else it would make the average density too high).

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