1. Theories: Origin of Earth and Moon
Scientists look for
Evidence
and an
Explanation

3. Facts - Evidence Searching for an explanation
Earth has a iron core
Moon rocks do not have iron
Earth’s average density is 5.5 grams/cm
Moon’s average density is 3.3 grams/cm
Earth has a specific isotope of oxygen
Moon has the same unique isotope of oxygen
Earth has a large Moon compared to it’s size

4. Theory #1 Accretion Theory
The moon is a sister world that formed in orbit around Earth at the same time as the Earth formed.

6. Pros
We know there was plenty of material to clump together (accrete)
Cons
Why doesn’t the Moon have iron like Earth has?
Why no other large moons around Mars, Venus, Mercury?

7. Theory #2 Moon Capture Theory
Moon was somewhere in space, and was captured into orbit around Earth

8. Pros Cons Explains why Moon has no iron (it came from somewhere else)
Moon has the same isotope composition as Earth
Capturing a body as big as the Moon takes some tricky physics. Unlikely to happen.

9. Theory #3 Fission
Early Earth spun so fast that it spun off the moon

10. Pros
Explains why the Moon has no iron - it came from just Earth’s mantle. The Moon’s density is similar to the Earth’s outer layers.
Cons
The energy in the system does not add up; Earth would have to spin too fast.
Moon would orbit Earth’s equator - it does not.
Rock density is similar, but composition is not.
Why don’t other planets have big moons too?

11. Theory #4 Giant Impact Hypothesis
A very large body crashed into the Earth. Debris from the impact made a moon.

12. Pros
A collision is possible
Explains the lack of iron core on the Moon
Explains why both have the same isotope of oxygen (Moon rocks came from Earth)
Computer models - show physics is possible
Explains why Earth axis is tilted!
Cons
The physics don’t exactly work out - Roche limit problem (not enough angular momentum in today’s Earth-Moon system).
Why only one moon?

13. Roche Limit
The limit around a body where gravity will rip apart another body.
If a body gets too close to the Roche limit it will break up.
Outside the Roche limit debris tends to clump together.

14. A quiet landscape awaits!
Moon
A quiet landscape awaits!

15. Credits
“The Formation of the Moon”, Dept of Astronomy, Case Western Reserve, February 22, 2006
Hartmann, William K. and Davis, Donald, “Origin of the Moon”, Planetary Science Institute, February 22, 2006
Wikipedia, “Roche limit”, Wikimedia Foundation, Inc., February 20, 2006, February 22, 2006