Theories: Origin of Earth and Moon Scientists look for Evidence and 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 #1 Accretion 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 #2 Moon Capture Theory Moon was somewhere in space, and was captured into orbit around Earth

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.

Theory #3 Fission 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?

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.

A quiet landscape awaits! 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