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Theories: Origin of Earth and Moon

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

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Theories: Origin of Earth and Moon

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

  2. 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

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

  4. 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?

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

  6. 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.

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

  8. 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?

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

  10. 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?

  11. 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.

  12. Moon A quiet landscape awaits!

  13. 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

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