Theories:Origin of Earth and Moon

Scientists look for

Evidenceand an

Explanation

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

Theory #1Accretion Theory

The moon is a sister world that formed in orbit around Earth at the same time as the Earth formed.

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?

Theory #2Moon Capture Theory

• Moon was somewhere in space, and was captured into orbit around Earth

Pros

• Explains why Moon has no iron (it came from somewhere else)

Cons

• Moon has the same isotope composition as Earth

• Capturing a body as big as the Moon takes some tricky physics. Unlikely to happen.

Theory #3Fission

• Early Earth spun so fast that it spun off the moon

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?

Theory #4 Giant Impact Hypothesis

A very large body crashed into the Earth. Debris from the impact made a moon.

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?

• 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.

Roche Limit

Moon

A quiet landscape awaits!

Credits

• “The Formation of the Moon”, Dept of Astronomy, Case Western Reserve, February 22, 2006

http://burro.astr.cwru.edu/.../SolarSys/lunaform.html

• Hartmann, William K. and Davis, Donald, “Origin of the Moon”, Planetary Science Institute, February 22, 2006

http://www.psi.edu/projects/moon/moon.html

• Wikipedia, “Roche limit”, Wikimedia Foundation, Inc., February 20, 2006, February 22, 2006

http://en.wikipedia.org/wiki/Roche_limit