Theories for the Formation of the Earth's Moon
- A theory which explains the existence of the Moon must accommodate the following facts:
- The Moon's low density (3.3 g/cc) shows that it does not
have a substantial iron core -- the Earth does. Thus the Earth and Moon have substantially different bulk compositions.
- Moon rocks contain few volatile substances (that is, materials that can be baked out
of a rock, for example water) -- implies extra baking relative to Earth.
- The relative abundance of oxygen isotopes
(16O, 17O, 18O) on Earth and
the Moon are identical.
- Since this isotope ratio appears to be a fingerprint revealing where an object formed in the Solar System, this result suggests that the Earth and Moon formed in the same region.
- Reasonable theories that don't work:
- Co-formation: The Moon formed in orbit about
the Earth via accretion.
- If the Moon formed in the vicinity of the Earth
it should have nearly the same composition -- specifically a
significant iron core.
- Capture: The Moon's different composition
could be explained if it formed elsewhere in the Solar System
and was subsequently captured into Earth orbit.
- Capture into the Moon's present orbit is very
improbable. Something would have to slow it down by just the right
amount at just the right time.
- Also, the oxygen isotope ratio reveals than the
Moon must have formed in the vicinity of the Earth.
- Fission: The Moon's composition resembles that
of the Earth's
A rapidly spinning Earth could have cast off the Moon from its outer layers.
- The Earth-Moon system should contain fossil
evidence of this rapid spin. It does not.
- All of these hypotheses do not address the extra
baking lunar material has received (item 2 above).
- A pretty crazy theory that does work:
- Catastrophic Collision: An object about the
size of the planet Mars struck the Earth, heating and ejecting
the outer layers of both objects. The Moon formed from
this ejected material.
- Explains why the Moon is made mostly of rock
and why that rock was heated excessively.
- The Moon is primarily made up of material that was part of the mantles of the proto-Earth and the impacting object.
- Explains why the oxygen isotope ratio is identical between
Earth and Moon.
- The outer layers of both objects are a mixture of the mantle materials of both objects.
- Catastrophic collisions were common late in the formative stages of the solar system.
- Computer models show that a collision at just the
right speed and angle will produce the right amount of debris in Earth orbit. Bad encounter geometries don't produce a Moon.
- Today, this scenario is widely accepted.
Revised November 7, 2007