Formation of Moon
We need to explain
- large size of Moon
- special circumstances indicated
- Moon’s orbit 20 degrees from Earth’s equator
- couldn’t have formed through fission of early Earth
- relative Fe poorness
- couldn’t have accreted out of the same material as Earth
- Oxygen isotope ratios similar to earth (but not Mars or asteroids)
- unlikely to have been “captured”
Recognizing
- prevalence of cratering, especially during “heavy bombardment” phase (first billion years of solar system)
- Planetesimal theory of planet formation
- increasingly large bodies accreting to form planets